Lecture XXXIIIntroduction to Block IIIBasically, this is a simple section on how the heart cell fails. This will introduce the heart cells physiology through the case study of a disease that is well-known to many, particularly congestive heart failure. This is where normal heart function eventually transitions into severe heart dysfunction. This block will gather understanding of:1. Anatomy of the Heart Cell2. Physiology of Cardiac Muscle Contraction3. Intracellular Signaling in Heart Cells4. Congestive Heart Failure when things go bad

The recurring themes in this course is simply the following:1. Cellular Organization: Molecule Cell Complex Tissue2. Cellular Specialization: Development Divergence3. Cellular Structure and Function: Generic Cell Cardiac Cell4. Cellular Homeostasis: The reversible transition from normal to abnormal (Normal Abnormal)5. Cellular Failure: Cell Failure Whole Heart Failure

Understanding the Heart is based on the following:1. Model System: Using a simulation to solve non-model problems, such as in heart failure.2. Non-Intuitive: Use of research and facts to deduce the answer.3. Empirical Data: Data produced from observation and experiments.4. Cant Get Enough: Well, that..sounds..like an addiction?

With this we can organize this into the several recurring themes:1. Cellular Organization: Chemical, Cellular, Tissue, Organs, Organ Systems, Organism2. Cellular Specialization: From a generic cell to a rod cell, but can also entail the development of a cell to blood vessels or the muscular heart. The specialized cell can have a specific function. Cardiac cell specialization has the following properties: (1) contractile, (2) conservation, (3) electrical, (4) energetic, (5) intrinsic, and (6) extrinsic. 3. Cellular Structure and Function: Understanding cellular structure and function involves knowing about the various roles of the cells and organs of the organ system. For example the speed of contraction correlates with the ATPase activity of myosin. This property can arise from its components: (1) striated pattern, (2) sarcomeres, (3) myofilaments, (4) T-tubules, (5) sarcoplasmic reticulum, (6) Z-disk, and (7) mitochondria. These components play a role in adaptation of the cardiac cell.4. Cellular Homeostasis: Cardiac cells exhibit various forms of regulation, through (1) negative feedback and positive feedback which are (2) acute or long-term. They can be (3) physiological or pathological, exhibiting (4) extrinsic or intrinsic properties. They can arise from (5) non-genetic or genetic causes, which then spurs the goal or either reaching (6) homeostasis or steady-state. This will play a major role in adaptation. Homeostasis is the relative constancy of the internal environment.5. Cellular Failure: This can arise from the failure of homeostasis, which are attributed to the previously stated causes.

Lecture XXXIIICase Study: Congestive Heart FailureDistinguish between CHF as syndrome compared to a disease.Congestive heart failure (CHF) can be defined in two different manners: as a (1) syndrome, and as a (2) disease. Generally, it is referred to the inability of the heart to supply sufficient blood flow to meet the needs of the body, causing shortness of breath, leg swelling, and exercise intolerance. Congestive Heart Failure can be considered a syndrome mainly because the kidneys can maintain a salt-avid state. However, other organs can contribute to its progressive nature, such as the failure of the liver.

The disease aspect involves the failure of the cardiac cell. An initial insult (genetic or non-genetic) can cause a production of factors (cellular signaling) and changes in the gene profile and heart remodeling (cellular remodeling) which expands to the level of the organ (organ remodeling). Eventually, there will be regulation (feedback control) and will be (in a longer-term) will be of concern as the body attempts to reset itself. This begins a downward spiral with maladaptation, due to remodeling and eventual ventricular dysfunction.

All in all, this can be diagnosed in the clinic as well as in the hospital setting. A patient typically enters the clinic or hospital and the physician will identify structural or functional abnormalities with a (1) full history and physical examination, (2) non-invasive imaging and functional analysis, (3) invasive imaging or functional analysis, and (4) laboratory parameters.

A full history and physical examination takes into account the initial complaint (not the main complaint), symptoms in context or normal activities, and obvious physical signs. The main ones in consideration in the physical examination are: (1) reduction in effort tolerance, (2) syndrome of fluid retention, such as edema, and/or (3) no symptoms of cardiac disorder.

Noninvasive imaging will involve the use of imaging tools such as echocardiography, magnetic resonance imaging, and ultrasound. This can allow a general idea of function parameters and a better idea of what to fix. Echocardiography along a single dimension can allow understanding of the diameter of the chamber size in contraction (systole) and relaxation (diastole). Noninvasive imaging can allow understanding of two variables: (1) fractional shortening and (2) ejection fraction. Fractional shortening is the change in the diameter in systole and diastole. Ejection fraction is the volume ejected in every single beat. These variables allow functional characterization of the heart.

Invasive imaging techniques can be done to further define the extent of the diagnosis. Tools often include the transesophageal echocardiography and right heart catheterization. There has been no established role in periodic invasive or noninvasive hemodynamic measurements in the management of heart failure. The tranesophageal echocardiography involves insertion of an imaging probe into the esophagus, giving it a direct view of the heart. The image, though, can be perturbed by obesity. Right heart catheterization involves insertion of the probe into the blood vessels and attaining measurements. It2 is used to measure pressures, sample oxygen concentrations, and determine the functional capabilities of the heart chambers. It is a good assessment of the functional parameters, because it can show true pressure measures in the chambers of the heart.

Other confirmations can involve laboratory testing, particularly for serum electrolytes, serum brain natriuretic peptide, and other tests for renal function. This confirms the multisystemic considerations in congestive heart failure. The heart wont completely give out, but the kidney or the liver will. From these factors we can help determine a mode of treatment suitable for the patient.

Treatment modalities and course of patient care are in the form of (1) pharmacological, (2) surgical, and (3) transplant efforts. Most drugs utilized for the treatment of heart failure are prescribed on the basis of their ability to improve symptoms or survival rather than their effect on hemodynamic variables. There are several different stages, according to the New York Heart Association, of heart failure, with recommended therapy based on each stage. Class I: Patients with cardiac disease but without resulting limitation of physical activity. Ordinary physical activity does not cause undue fatigue, palpitation, dyspnea, or anginal pain. No objective evidence of cardiovascular disease Class II: Patients with cardiac disase resulting in slight limitation of physical activity. They are comfortable at rest. Ordinary physical activity results in fatigue, palpitation, dyspnea, or anginal pain. Objective evidence of minimal cardiovascular disease. Class III: Patients with cardiac disease resulting in marked limitation of physical activity. They are comfortable at rest. Less than ordinary activity causes fatigue, palpitation, dyspnea, or anginal pain. Objective evidence of moderately severe cardiovascular disease. Class IV: Patients with cardiac disease resulting in inability to carry on any physical activity without discomfort. Symptoms of heart failure or the angina syndrome may be present even at rest. If any physical activity is undertaken, discomfort is increased. Objective evidence of severe cardiovascular disease.

Prognosis can often be predicted by with computer programs, but therapy is ultimately going to be focused on improving prognosis if not stabilizing the patient.List at least three causes for CHF.There are several causes for congestive heart failure, on a pathological aspect:1. Hypertension: high blood pressure can impede blood flow to the heart2. Myocardial Infarction and Atherosclerosis: Blockage to specific parts of the heart can causes coronary events and damage to the myocardium.3. Diabetes/Obesity: Elements of the metabolic syndrome can increase the risk for heart attack or disease.4. Viral Myocarditis: Viral infection to the myocardium can perturb function by causing a reduction to an abnormal state.Define cardiac remodeling.Cardiac remodeling refers to the histological and structural changes in size, shape, and eventual function of the heart after injury. They can be attributed to anything to can perturb function along the cardiovascular system, such as changes in pressure and volume as well as injury.Identify cause of Familial Hypertrophic Cardiomyopathy/Hypertrophic Cardiomyopathy.Familial Hypertrophic Cardiomyopathy is typically caused by mutation in one of the genes known to encode different components of the sarcomere, characterized by left ventricular hypertrophy in the absence of predisposing cardiovascular conditions. It can be manifested in the form of progressive heart failure to sudden cardiac death with variation among each individual. Changes in the following genes and expressed proteins can play a role in hypertrophic cardiomyopathy.

The mutation of particular interest in this block will be a mutation in myosin S1. Mutations have been pointed out throughout the myosin head and cause a long-term progression. Perturbations do not imply decrease in function. They can often enhance function but eventually cause later hypertrophy and dilation. One study observed the relationship of gender to overall hypertrophic cardiomyopathy (HCM)-related mortality and progression to New York Heart Association (NYHA) functional classes III and IV, or heart failure (HF) or stroke death (right panel). In general, women with hypertrophic cardiomyopathy were under-represented, older, and more symptomatic than men with a higher risk of progression.

Lecture XXXIVCardiac Muscle Structure and UltrastructureIdentify the 4 chambers of the heart.The four chambers of the heart is as follows: the (1) right atrium, (2) right ventricle, (3) left atrium, and (4) the left ventricle. An atrium is the chamber of the heart that receives blood, either from the systemic circulation (as in the right atrium) or the pulmonary circulation (as in the left atrium). The ventricles are mainly the pumping muscle of the heart, responsible for the ejection of blood either to the lungs (as in the right ventricle) or to the systemic circulation (as in the left ventricle). The left ventricle is much larger in size in comparison to the right ventricle due to its function of pumping a large volume to the bodys system. The two ventricles are separated by the interventricular septum, further enforcing a closed circulation. The left ventricle is typically of interest because any remodeling that decreases the size of the left ventricles wall can consequently decrease the function of the ventricle.Trace the path of blood through the heart and throughout the body.The best way to illustrate a path of blood through the heart and through the body is through a flow chart, which follows this statement:

List the 3 components of ventricular remodeling/adaptation.Ventricular remodeling is deifned in a particular sequence of events. Physicians and scientists monitor the progression of disease from the initial insult (acutely) to the long-term (chronically). As part of that progression of disease, there are changes in the structure of the heart, starting with the ultrastructure. Under hypertensive conditions, it grows in a different way relative to exercise conditions. With the cellular modeling, there will be systemic remodeling. The heart resets itself at every moment of the human life, changing constantly in response to external or internal stimuli. The genetic mutation in familial hypertrophic cardiomyopathy will be the initial insult under study in this block.

Disease progression is characterized by adaptation and remodeling. Remodeling refers to the changes in size, shape, and function of the heart after insult to the ventricles. There are three parts of this remodeling that spiral towards the disease state:1. Cellular/Biochemical: Within that cell, there is chemical signaling occurring and the heart will remodel by signaling. It will change in certain aspects.2. Ultrastructural/Histological: Observing the fact that there is going to be a structure change in the heart. The ultrastructure and histology are altered. Along with the cellular remodeling, you get structural remodeling.3. Genetic/Molecular: Cellular signaling will affect gene expression. As the disease progresses, some are activated while others are deactivated. Ultimately, there will be changes in the molecular level. Cellular signals will impact cell in terms of gene expression, which feed back into the cellular/biochemical signals.

Initially, remodeling is occurring at a rapid pace in the early stages of heart development, and continue to remodel until the end of life. In the early stages, the bilateral heart fields form after progenitor cells have migrated from the primitive streak (shown as A1). Fusion of the bilateral heart fields at the midline creates a cardiac crescent (A2). Fusion of the cardiac crescent leads to the formation of the linear heart tube (A3). Disintegration of the dorsal mesocardim enables the heart tube to undergo its characteristic D-loop (A4). The tubular heart at A3 is suspended from the rest of the embryo by the dorsal mesocardium (B). After disintegration of the dorsal mesocardium in the mid-section, the heart tube remains attached to the embryo by means of its dorsal mesocardial connections at the arterial and venous poles (C). By embryonic day (E) 8.5-9 in a mouse, the respective components of the embryonic heart still aligned in series (D).

In the human, the heart continues to develop to the point where the fetus is born, and consequently spurs physiological growth.Differentiate between fiber orientations in the endocardium compared to the epicardium.The heart has a very specific orientation, which differs based on body size. In the context of disease, the heart will growth, and the heart will be of different size because the body is pumping a different volume. Understanding orientation is an important thing in diagnosing the patient. It is important to first understand that the heart has certain ultrastructures. One of them is the pericardium. The pericardium will have a dramatic impact due to its pericardial fluid. The fluid reduces friction of the heart. The myocardium has a strong distinction from the pericardium and endocardium, and there are specific orientations of these fibers in order to allow the heart to function better. Observing the heart, one can note that the fibers are spiraling from the apex up to the base. They are starting to swirl from the apical point. This allows contraction to contract blood, wringing out the entire blood volume from the chamber. The fiber orientation shows that the endocardium fans out but decreases in the spread as it approaches the midwal. However, what is also of interest is that as one approaches the epicardium, there is also fanning out in the opposite direction. Once again this is significant to allow a wringing motion of the heart with more effective packing. There is regional variation in these cavities, and the fiber orientation plays a role in its contractility and this is the significant variable that is diminished in disease states. If fiber orientation is disrupted, there may be significant pumping action.List the major non-myocyte cell type and describe its function.The heart of course, is made of myocyes. However, if you look at the populations of cells in the heart, the majority actually consists of cells known as non-myocytes. What that means is that myocytes are the contracting factor, and are the majority of the volume (meaning that most of the weight of the heart is the myocytes). Fibroblasts are one example of a non-myocyte. Most of the non-myocytes are fibroblasts. They play a role in not only collagen but also in signaling. It is an amorphous structure, and can respond to external signals as well. Their major function is the production of collagen.

Collagen is produced in the rough endoplasmic reticulum, where the ribosomes are located. It is assembled in the following manner:1. Preprocollagen2. Procollagen (N-terminal removal)3. Hydroxylation of Prolines and Lysines4. Glycosylation of Lysines5. Protein Disulfide Isomerase Enzyme that allows proper orientation towards the structural integrity path. It will target specific amino acids and allow formation of the triple helix in a specific manner.6. Triple Helix Formation7. Movement of Golgi for Secretion8. Proteolytic Cleavage (outside the cell). The protein is claved.9. Covalent Cross-linking: Main element of covalent modification.10. 3D structure

Covalent modification is catalyzed by lysyl oxidase. Lysyl oxidase catalyzes the formation of covalent bonds between the ends of collagen molecules.Describe the function of the extracellular matrix in the heart and how it relates to collagen.The extracellular matrix in the heart has several functions:1. Provision of structural integrity: It is one of the more important structural components. An individual heart cell can be torn apart. However, the whole heart cannot be torn apart because of collagen.2. Maintenance in the alignment of myofibrils: It plays a role in how the structure is laid down initially. It is known that having the matrix allows myofibrils to sit in a very specific orientation. In short, the extracellular matrix dictates orientation.3. Reservoir for bioactive molecules/signaling components: Signaling is occurring between the myocardial cells and the extracellular matrix. The cells that make matrix proteins are high in number, and have an important role in signaling. There is this autocrine effect that is attained from the dictation of cellular processes.4. Maintenance in the structural interaction with the vascular system: It has to feed itself. The extracellular matrix has a role in maintaining the interaction with the vascular system. It keeps the vessels in this region. The role of the matrix is not just to provide a framework for the heart cell, but designate where different components go.

Now this begs the question as to the relevance of collagen. There are a number of vessels that have a network of proteins. It is made of these collagen fibers. The matrix is consists of several different components, but the major one is collagen fibers. It is used structurally throughout the body. If they are put in a specific orientation, they are extremely rigid and have a lot of structural integrity. Each of these fibers are in the triple helix. As they form the triple helix, the structure becomes remarkably stable. It is extremely versatile and can be used for different components. There are different types of collagen fibers in the heart, and distributed throughout the body. In the heart, though, the predominant collagen fibers are Type I and Type III. The ratio of these fibers is associated with disease progression. An increase in Type III is associated with disease states, and observations in the transition from Type I to Type III can allow inferences on disease progression. This can also allow the inference that a transition from a normal to disease state in cells will allow inference of the abnormal ultrastructure of the heart.

Two proteins that are abnormal are fibrin and laminin. Fibronectin is a glycoprotein of the extracellular matrix that binds to receptor proteins called integrins, and also plays a role in the binding of extracellular matrix components such as collagen and fibrin. Its peptide structure consists of certain domains and areas which can bind to the cell. If there is a genetic abnormality, you can have a gene that can make multiple proteins, and muscle has many different types of myosin. Each chain is almost 2,500 amino acid residues and each chain is folded into five or six rodlike domains connected by flexible polypeptide segments. The individual domains are specialized for binding to a particular molecule or to a cell. Fibronectin interacts at the C-terminus, and has a specific amino acid sequence that can interact with the cell. When that fibronectin molecule interacts with that cell, they have interaction through a receptor and allow a signaling pathway to occur.

Laminin is another protein that can have multiple interacting sites. Laminins are trimeric proteins that are secreted and incorporated into cell-associated extracellular matrices. It is vital in the maintenance and survival of tissues, and defects in laminins can cause poor muscular development. From there, you can infer that laminins can alter the pathway of the development of the extracellular matrix. It is multidomain glycoprotein that is composed of three polypeptides (A, B1, and B2) that are disulfide bonded into an asymmetric crosslike structure. Three types of chains, three types of B1, and two types of B2 chains have been identified to allow association to form 18 different laminin isoforms.

As a whole, there are collagen molecules that are interacting with laminin molecules, and they can combine to form a rigid matrix, with structure and orientation. This rigid structure is produced and rapidly modified during a severe disease stimulus, and remodeling that occurs at the matrix level is rapid. There are a number of proteins involved in the regulation of this organization, but the main ones of concern are fibronectin and laminin.

Lecture XXXV Cardiac Muscle Structure and function: The Cardiac CycleTrace the path of blood through the heart and throughout the body.Remember that the major function of the heart is to move blood through the circulation by the application of pressure. This is accomplished through the work that the heart does during each cardiac cycle. This is a passive phenomenon.

So remember, the path of the heart is as follows:

Compare and contrast diastole and systole.Remember that a complete sequence of diastole (relaxation) and systole (contraction) is called a cardiac cycle. The diastole is the period of relaxation when a chamber if filling with blood, and systole is the period of contraction when a chamber is ejecting blood. The heart spends the majority of its time in relaxation or diastole, and when a situation that goes away two-thirds of the time, there can be deviations from the normal.

The cardiac cycle can be dissected into a series of major events: (1) ventricular filling and (2) ventricular emptying. It can be further expanded according to the following steps: (1) passive filling during ventricular and atrial diastole (where blood is returned to the heart), (2) atrial contraction (the point where contraction helps filling), (3) isovolumic ventricular contraction (where the heart has filled but is generating pressure, and no net movement of blood), (4) ventricular ejection, and (5) isovolumic ventricular relaxation. The whole cardiac cycle occurs about once every second. The left and right sides of the heart undergoes the same series of events at the same time.Define pressure gradients across heart valves and isovolumic states during the cardiac cycle.With the structural components called valves, you can allow a pressure gradient. Valves separate the pressure gradients along these chambers. Thus, opening and closing of valves is controlled by pressure. When pressure is greater behind the valve, there will be opening and a permission of flow in that direction. However, when pressure is greater in front of the valve, the valve remains closed and does not allow opening and flow in the opposite direction.

Blood flow through valves during the cardiac cycle is dependent on the pathway. Atrioventricular valves open when the pressure is greater in the atria than in the ventricles. As a one-way valve, you need a rigid structure to enforce the one way flow, which explains the role of the papillary muscles and chordae tendinae function It is passive, but at the end of the contraction, the pressure in the ventricles is very low. Semilunar valves open when the pressure is greater in the ventricles than in the aorta or pulmonary trunk.

When the valves are closed, there cannot be any change in the blood volume contained in it. Thus this is in an isovolumic state, or a state of constant volume. This allows changes in pressure without changes in volume. When the valves are open, however, there is a change in blood volume. There is movement of blood at these very specific stages. When valves are open, ventricular blood volume can change. When the semilunar valves open, there is a decrease in ventricular volume (emptying), and when the AV valves open, there is an increase in ventricular volume (filling).Identify the Wiggers Diagram.To understand how blood is pumped in an organized fashion through the heart, it is essential to know the sequential changes in pressure and volume that occur in the heart chambers over time during the cardiac cycle. This can be observed with a Wiggers diagram. The Wiggers diagram is a plot of an electrocardiogram, chamber pressures, chamber volumes, and heart sounds all along the same time scale. There is cyclic generation of high pressure in the ventricle, and the pressure is used to move blood out of the ventricle, resulting in cyclic changes in ventricular volume. Each phase is explained in the following.

Late diastole is when the left atrial pressure slightly exceeds the left ventricular pressure. The left atrioventricular valve is open and blood passively fills the left ventricle. Left ventricular volume increases slowly by passive filling.

Atrial systole occurs when the contraction of the atrium causes an increase in atrial pressure and blood is actively forced into the ventricle to complete filling. Left ventricular volume increases more rapidly during the active filling phase. This occurs because of the atrial kick, where the blood pressure in the atria increases and consequently forcing more blood into the ventricles. The atrial kick provides enough force to where there will be maximal filling, and elevates pressure in proportion to increase in the volume.

In isovolumic ventricular contraction, the left ventricular pressure increases rapidly during systole. Left ventricular volume remains constant because all of the valves are closed.

Ventricular ejection occurs when the left ventricular pressure exceeds aortic pressure and the aortic valve opens. The left ventricular volume drops rapidly as contractions continues and blood is ejected into the aorta. It rapidly declines and stops as soon as the left ventricular pressure is less than the aortic pressure.

Isovolumic ventricular relaxation occurs when the left ventricular pressure drops below the aortic pressure and the aortic valve closes. The left ventricular volume remains constant because all the valves are closed. Contraction is going to end at which the pressure external to the valve is going to be greater than pressure inside the ventricle. The aortic valve is going to shut, and relaxation will occur.We can further summarize this in the following table:PhaseLate DiastoleAtrial SystoleIsovolumic Ventricular ContractionVentricular EjectionIsovolumic Ventricular Relaxation

Descrip-tionLeft AV Valve is open and blood fills the left ventricle. Flow occurs by passive filling.Blood is actively forced into the ventricle to complete filling by an atrial kick.No net change in volume because valves are closed, but the left ventricular pressure increases rapidly.Ejection of blood into the aorta, causing a drop in left ventricular volume.Drop in left ventricular pressure and aortic valve closure.

Pres-sures

Calculate stroke volume, ejection fraction, and cardiac output.From the Wiggers diagram, we can make collect some quantitative information. Two variables of importance are the (1) end-diastolic volume (EDV) and the (2) end-systolic volume (ESV). End-diastolic volume (EDV) is the volume of blood in the ventricle at the end of diastole, or the maximal amount that the ventricle will contain. The end-systolic volume (ESV) is the volume of blood in the ventricle at the end of systole, or the minimal amount that the ventricle will contain. The difference between these volumes is referred to as the stroke volume, the amount of blood that is pumped out of each ventricle with each contraction . Stroke volume is necessarily a strong determinant of cardiac function because stroke volume can be increase with an increase in heart size. A better determinant would be the ejection fraction, which is the quotient of the stroke volume and end-diastolic volume . Stroke volume is also one of the determinants of cardiac output, which is the product of heart rate and stroke volume . Wiggers diagram can be used to look at the relationship of the ECG to the cardiac cycle (timing of events).

Relate the Wiggers Diagram to a pressure-volume relationship of the left ventricle.Another way to view changes in pressure and volume during the cardiac cycle is by plotting ventricular pressure versus ventricular volume. Ventricular pressure vs. volume must be shown in two plots, known as active and passive curves. Active curves are pressures developed in a purely isovolumic contraction, with no semilunar valves to open. The passive curve is pressure developed by filling during diastole. Isovolumic contraction is contraction in which there is no change in the volume. There is contraction but no change in the volume (going from the passive to the active curve). Isovolumic relaxation is relaxation in which there is no change in the volume (going from the active to the passive curve). A pressure-volume loop is a plot of ventricular pressure versus ventricular volume for a single cardiac cycle. A pressure-volume loop illustrates the sequential dynamic changes in a single cardiac cycle, and there are four distinct phases in this loop:1. Filling Phase: the ventricle is relaxed and is being filled with blood through the open AV valve.2. Isovolumic contraction: the ventricle contracts while all the valves are closed. The volume stays constant as the pressure goes up.3. Ejection phase: the aortic valve opens and blood is ejected from the ventricle as the ventricle continues to contract. Volume in the ventricle falls as blood is ejected.4. Isovolumic relaxation: the aortic valve closes and the ventricle relaxes. All valves are closed so the volume is constant while pressure falls.

Stroke Volume can be read off of the P-V loop. At the same time, there are four distinct valve events associated with the PV loop: (1) AV Valve opening, (2) AV Valve closure, (3) Aortic Valve opening, and (4) Aortic Valve closure. P-V loops can be used to measure cardiac work.

Pressure-volume loops have a variety of application. The total area within the P-V loop is equivalent to the stroke work performed by the heart . Thus, they can be utilized as potential clinical tools. Changes in the cardiac function with heart disease can show up in pressure-volume loop measurements.

Lecture XXXVICardiac Muscle Structure and Function: Pressure-Volume RelationshipDraw the cardiac cycle in terms of the pressure-volume relationship.It is important to note that the pressure-volume loop is a plot of ventricular pressure versus ventricular volume for a single cardiac cycle, and that there are two curves, or boundaries, that the pressure volume loop must abide by: the (1) isovolumic systolic curve or active curve and the end-diastolic curve or passive curve. It is also important to remember the four distinct phases and events that occur.

The four phases:PhaseNamePressureVolumeDiagramDescription

1FillingConstantIncreasesVentricle is relaxed and being filled with blood through the open AV valve. In an ideal situation, there is an atrial kick.

2Isovolumic ContractionIncreasesConstantThe ventricle contracts while all the valves are closed. The volume stays constant as the pressure goes up. The atrial valve shuts, and there is no net movement of volume.

3EjectionExceeds pressure of the valveSharply decreasesThe aortic valve opens and blood is ejected from the ventricle as the ventricle continues to contract.

4Isovolumic RelaxationDecreasesConstantThe aortic valve closes and the ventricle relaxes.

Stroke volume can be read from the PV loop, which is the difference between the end-diastolic and end-systolic volumes.Identify key points on the pressure-volume loop.There are four key points on the pressure volume loop:1. AV Valve opens (end of isovolumic relaxation)2. AV Valve closes (beginning of isovolumic contraction)3. Aortic Valve Opens (end of isovolumic contract/beginning of ejection)4. Aortic Valve Closes (start of isovolumic relaxation)Compare and contrast isovolumic relaxation and contraction.Isovolumic contraction and relaxation are two terms that assign changes in pressure while under constant volume, because all the valves are closed. Isovolumic contraction is exhibited by an increase in pressure under constant volume and occurs prior to ejection, while isovolumic relaxation is exhibited as a decrease in pressure under constant volume, occurring after ejection. List the parameters that determine cardiac output.Cardiac output is the volume of blood pumped by each ventricle per minute. In order to get at determinants of cardiac performance, we can break down cardiac output into its determinants. Cardiac output is the product of heart rate (HR) and stroke volume (SV) or . Cardiac output can be altered by changing either heart rate or stroke volume (or even both).Calculate cardiac output and describe and how it can be altered.Remember that cardiac output is the product of heart rate and stroke volume . For example, if a patient has a heart rate of 70 and a stroke volume of 70 mL/beat, we can then calculate cardiac output by the following: .Relate changes in contractility, preload, and afterload to the ventricular pressure-volume relationship.Heart rate is one of the 4 determinants of cardiac performance, but is dictated by external stimuli, particular sympathetic or parasympathetic stimulation. The other three fall under the variable of stroke volume. Stroke volume can be altered in three different ways: (1) changing contractility, (2) changing the preload, and (3) changing the afterload.

Cardiac contractility is the strength of contraction of the cardiac muscle at a given end-diastolic volume, or a given amount of filling volume. Heart can change its ability to contract when there is a change in preload. When contractility is increased for a given end-diastolic volume, the ventricular pressure developed during isovolumic contraction is greater. There is a shift in the pressure-volume relationship to the left in increased contractility, and to the right in decreased contractility.

The other two determinants of cardiac performance (preload and afterload) have to do with the workload placed on the heart either before or after contraction begins. Preload is the workload imposed on the heart before contraction begins (end-diastolic volume). Afterload is the workload imposed on the heart after contraction begins (having to do with the end-systolic volume). Remember that end-diastolic volume has to do with the volume of blood in the ventricle at the end of diastole, or the maximum amount that the ventricle will contain. End-Diastolic volume determines how stretched out the cardiac muscle fibers are. On pressure-volume curves (on the whole heart), an increase in preload leads to an increase in stroke volume upon contraction, based on the Frank-Starling Relationship. Whatever comes into the heart must affect the relationship. If you increase filling, you increase the end-diastolic volume. It needs to undergo a contraction that imposes a load against it.

For the whole heart, afterload is related to the arterial blood pressure because that is the pressure that the heart must work against. Remember that in order for ejection of blood to occur, the left ventricular pressure must be greater than the aortic pressure, and the left ventricular pressure must increase until ejection occurs. An increase in afterloads leads to a decrease in stroke volume upon contraction. The heart will continue to generate pressure until ejection can occur. The heart will handle this increase in afterload by elevating pressure and eventually reach a different end-systolic pressure volume relationship. Eventually, the heart will shift to a different pressure volume loop. If there is no external stimulus, it will shift to the right to maintain stroke volume. Stroke volume is different before increase. It will shift to increase stroke volume to match the stroke volume that was there before. This can be done by intrinsic mechanisms. The heart can reset itself without any external input.

How does this all fit together? Remember that P-V loops can be used to measure cardiac work. The total area within the P-V loop equals the stroke work performed by the heart. The dynamic properties of the whole heart are based on the fundamental dynamics of an individual sarcomere.

Lecture XXXVIICardiac Muscle Structure and Function: Components of Cardiac MuscleDifferentiate between skeletal muscle cells from cardiac cells.Cardiac cells and skeletal muscle cells rather distinct in terms of function and histology, giving distinct orientations to the fibers. The skeletal muscle cell is long, multinucleated and striated. The cardiac muscle has one nucleus, striated, and branched. It contains intercalated discs and gap junctions. This is important later for the cardiac muscle for cell-cell communication and the all-or-none activity observed in the cardiac muscle.

Skeletal muscle cells are distinctly different from cardiac cells by the following:CharacteristicSkeletal MuscleCardiac Muscle

Diagram

MorphologyLong, multinucleated, striatedBranched, central nucleus, striated

Fiber Diameter (m)10-10010-20

Fiber Length (m)100-3000,000,00050-100

Junctions between fibersNoneIntercalated Discs

Contraction SpeedFastModerate

Diagram a sarcomere during diastole and systole.What is necessary to understand is that the dynamic properties of the whole heart are based on the fundamental dynamics of an individual sarcomere. This is important because each sarcomere is going to contribute to the ultrastructure of the heart. If one was to take a cross-section of a sarcomere, they can note that there are proteins interdigitating with one another in a specific orientation in an ordered, regular pattern. The sarcomere has four landmarks of interest: I band, also known as the Isotropic band A band, also known as the Anisotropic band H zone, which is bright M line, or Middle Line. The M line is found in the H zone.

In a state of diastole, or a relaxed state, the sarcomere as a whole would lengthen. In a state of systole (or contraction), the sarcomere as a whole would decrease in length. These mechanisms are to allow maximal or minimal interactions of two specific fibers, known as actin and myosin. Distinguish between the thin filaments from the thick filaments.There are two types of fibers that are found in the sarcomere, known as thin filaments and thick filaments. Thin filaments are polymers that consist of actin monomers, while thick filaments consist of myosin. Both these two contribute to the sliding filament theory of contraction. By experimentation, scientists found that if the relative force between actin and myosin is generated at each of a series of points in the region of overlap in each sarcomere, then the tension should be proportional to the number of these points, and therefore to the width of the overlap. They also found that a possible driving force for contraction in this model might be the formation of actin-myosin linkages when adenosine triphosphate (ATP), having previously displaced actin from myosin, is enzymatically split by the myosin head. In short, these proteins, when overlapping, dictate the contraction. It should also be noted that the speed of contraction of a muscle is directly proportional to the speed of contraction of the sarcomere.

One can observe the structure and function by x-ray crystallography, a technique used to closely examine the structure of muscle. This involves the energy of electrons. When electrons accelerate, the movement of the electrons (in the presence of deflecting agents such as magnets), can allow the release of X-rays from the electron. One can take the electron and undulate it extremely fast, generating high-intensity X-rays and direct it towards a muscle. The diffraction will reflect based on the orientation of the molecules. From the information, calculations can be done based on the image produced. If a muscle is forced into action by activation and then analyzed by X-ray diffraction, there are noticeable changes in the diffraction pattern. Based on these changes of the diffraction pattern, scientists can gather information at the molecular level. They can look specifically at the pattern of extrusions, discerning what is happening at the different heads, and can produce a structure of a muscle. Closer examination will show that the extrusions play a role in contraction.Draw a schematic of the functional domains of myosin.The myosin head consists of three function sites: (1) ATP binding pocket, (2) actin binding domain, (3) regions of association (RLC and ELC). The myosin head consists of the S1 region. The hinge domain is considered the S2 region. The hinge domain is critical to allow flexure of the myosinhead, which plays a role in the structure and function of the muscle. The myosin head, though the smallest, is considered the majority in terms of amino acid amounts.

Myosin exists as a compilation of molecules in the thick filament. Myosin can associate with another myosin to make a coiled coil. It is a heterohexamer, consisting of 2 myosin and 4 light chains. There are different types of myosin (alpha and beta myosins arising from different genes), and like myosins do not have to associate with one another. In cardiac tissue, humans have both alpha and beta. In humans, the majority of myosin is beta-myosin. Alpha myosin is more prominent in smaller animals, and beta is more prominent in large animals. In disease states, there is a higher concentration of beta myosins. Intuitively, there is a shift that is measurable and indicative of disease. Coiled-coil dictates packaging of the filament. Though it does exist as a heterohexamer, the x-ray diffraction cannot distinguish between one head or another. The cause has not been addressed fully. However, it should be noted that myosins have a very specific orientation and polarity. What can be examined is how those thick filaments associated with these thin filaments, consisting of actin.

Binding occurs very specifically between actin and myosin. There are proteins associated with the thick and thin filaments. Actin monomers have to be assumed to create the thin filament. This process is tightly regulated. There are a number of events to occur in the heart to force actin and myosin together in the sarcomere. It is able to get into that sarcomere and align itself in that filament. It has a specific, constant length, with the main goal is to form a long chain from actin monomers. This is also tightly regulated. There are three steps involved in actin assembly:1. Nucleation: Requirement to have more than one monomer together to make the filament (at least three or more).2. Growth3. Elongation: Using the monomers to create the polymer of a specific length.

All in all, this is an ATP-dependent process. The only difference between the actin and myosin structure is really in the difference in size. These proteins are turning over regulary. Once the length has been established, there are processes that will put monomers on different ends. Actin can actually bind to ATP, and when ATP isbound to actin it readily associates with other monomers. ATP allows association with other monomers. However, ATP hydrolysis destabilizes the system on one end compared to the other, regulated by certain proteins that regulate length of growth and filament length.

Lecture XXXVIIICardiac Muscle Structure and Function: Components of the SarcomereList and describe the 4 levels of protein organization.Remember that there are four levels of protein organization:1. Primary the amino acid sequence that is yielded from translation at the ribosomes.2. Secondary conformational changes of the backbone either into an alpha helix or to a beta sheet.3. Tertiary three-dimensional arrangement (or motifs) of secondary structural elements. This is taking into account the protein is sitting in real space, and needs to fold into a specific interaction. If may require regulatory proteins in order to accomplish the tertiary structure.4. Quaternary Formation of multiple subunits from the motifs. Proteins and subunits associate with one another, combining into dimers. Channels are typically multiple subunits of different genes.

We do have proteins arranged in a very specific order. It does not happen spontaneously and has a specific arrangement.Compare the structural organization of the thick and thin filament in a sarcomere. Differentiate between actin filament structure and myosin structure.FilamentThickThin

ComponentMyosinActin

Diagram

Main Form of ArrangementCoiled-CoilDouble Helix Monomers

ArrangementIt is important to remember that myosin exists as a heterohexamer, confirmed by x-ray crystallography. It is in coiled coil, and in ordered packaging in the thick filament.Requires an ATP-dependent process that involves individual actin units to accumulate in a specific manner. They can interact with other molecules and regulatory units to ensure assembly, disassembly, regulation of length, and stability.

Diagram titin and troponin structure within a sarcomere.ProteinTitinMyomesinTroponin

Placement in Sarcomere

LocationFound in I-band. M-line, and Z-line of sarcomere.Found in the M-band of muscle sarcomeres in association with M-protein.Found in the sarcomere.

FunctionMolecular spring responsible for the passive elasticity of muscle.Found in both slow and fast fibers. Molecular spring that protects the sarcomere and stabilizes during intense or sustained stretching.Attached to the protein tropomyosin and lies on actin units.

Charac-teristicsLargest known protein.Involved in anchoring thick filaments to other filaments, specifically titin.Various types depending on function.

It should be noted that there is profound interest in the troponins, which impart regulation on the actin filament. The crossbridge interaction can be acting in a controlled manner. Troponin C is a calcium-binding protein. Troponin I links troponin C to entire complex (as an inhibitory protein). Troponin T binds to tropomyosin, which links the whole complex together. Overall this complex acts as a functiona unit, and each part cannot be separated and analyzed in a vacuum.

In the Z-disk, there are a whole lot of proteins. What is known is that there are various defined signaling pathways, as there are for other cells. Thought of having these proteins lead to signaling of one another. It is a highly regulated system in which there are proteins to do their job. It is clearly a complicated structure, and goes beyond just a structural entity where it holds the filaments together. There are intracellular signals going back and forth as well. There are a lot of proteins associated with the Z-disk. This constants yields a recurring them that the primarily structural proteins have an important role in the signaling of the molecule. Based on the structures in the proteins, scientists were able to piece together the concept of the sliding filament theory.

Dystrophin is another example of a protein involved in structure and signaling, and plays a role in the muscle cells association with the matrix. It occurs through very specific proteins. It dos have the ability to link the extracellular matrix. Not only this can provide structural support but plays a role in signaling.

Lecture XXXIXCardiac Muscle Structure and Function: Mechanism of Muscle Contraction IState the sliding filament theory of muscle contraction.The sliding filament theory of muscle contraction describes the mechanism by which muscles contract. Basically, scientific observations showed that in periods of extension and contraction, there were signs of cross-bridging occurring within the sarcomere. Myosin acts as the molecular motor (such as a ratchet) and actin acts as the thin filaments that transmit the force generated by myosin to the ends of the muscle. The mechanism is in the following:1. Myosin heads binding to passive actin filaments at the myosin binding sites.2. Upon strong binding, myosin and actin undergo isomerization (myosin rotation) extending an extensible region in the neck of the myosin head.3. Shortening occurs when the extensible region pulls the filaments across each other, while myosin remains attached to the actin.4. The binding of ATP allows myosin to detach from actin. ATP hydrolysis then occurs to allow the myosin head to reset, allowing binding of actin.5. The collective bending of numerous myosin heads in the same direction to combine and move the actin filament relative to the myosin filament, resulting in muscle contraction.Draw the length-tension relationship for skeletal muscle and cardiac muscle.The length-tension relationship is the relationship relates length of the fiber and the force the fiber produces at the length. It essentially has two curves: an (1) ascending curve (that exhibits an increase in tension with an increase in length) and a (2) descending curve (with a decreased tension with an increasing length). The length refers to length of the isolated fiber and dependent upon position of actin and myosin filaments. When a muscle fiber is stretched to the minimal overlap and stimulated by an electrode, the measured contraction force is minimal. There is a reach point where there are optimal myosin heads, and at that point the amount of heads are decreasing as the length is increased, essentially stretching the actin filaments beyond the thick filament.. The amount of heads interacting with the actin filament is decreasing as you increase the length, stretching the actin filaments beyond the thick filament. This leads to important characteristics of the sarcomere. They have a specific range of length. If they measure all the filaments, they are not individually the same length, but as a whole can be the same length.

The reasoning of the curve is that length is equivalent to volume. There are myocytes surrounding the volume. With the volume, there is a generated force on the wall. If you take an elastic structure, there is a limit to that elasticity. But, as you increase the volume of that elastic structure, the pressure goes up. There is a limit to that. As one approaches the end of the elastic properties of that structure, pressure has to go down. What can also be noted is that velocity also increases with the decreasing force, mainly because there is a greater change in the number of actin-myosin interactions. The difference between the curvilinear relationships for skeletal and cardiac muscle is mainly due to the different proteins that are found in the cardiac muscle.

Even though they are both muscles, they have different proteins. They may exhibit the same basic relationship, but cardiac tissue has a steeper curve that has a greater shift to the right, giving cardiac muscle a greater change in tension as a function of sarcomere length. What is important is that force is proportional to the number of strongly bound cross-bridges.Compare the length-tension relationship for cardiac muscle and the pressure-volume relationship of the heart.The length tension relationship can be related to the heart because there is a dramatic increase in tension development with small changes in length. There is a huge reserve in order to attain larger volumes. There is a lot being learned about the pressure-tension relationship that can actually determine and dictate disease states. Diastolic dysfunction can lead to heart disease. This is the passive length-tension relationship. One can infer that with changes in length that there are going to be changes in tension. Remember in the pressure-volume relationship, that P-V loops can be used to measure cardiac work. The total area within the P-V loop equals the stroke work performed by the heart.

Experimentally, studying the movements of an intact twitching muscle of a frog can do this. A microscope apparatus will often utilize a force transducer and a motor arm that adjusts the length finely, and with light diffraction, measure the changes in length and observe the force generated. This can be done because muscle is striated, so shining a light yields a diffraction pattern. Remember that light diffraction can allow measurement of the specimen under study by allowing refraction of light to a larger medium.

Cardiac contraction requires energy. The myocardial O2 consumption goes up with increasing work performed. One can measure blood going into the heart and blood coming out. As the external work is increased, myocardial oxygen consumption goes up. This implies that something is being used to generate work.Differentiate between a crossbridge and myosin.Cross bridges is simply are globular heads of a myosin molecule that project from a myosin filament in muscle and in the sliding filament hypothesis of muscle contraction area held to attach temporarily to an adjacent actin filament and draw it to the A band of a sarcomere between the myosin filaments. Myosin is the molecular motor to do work. In the context of the heart, it is in the form of contraction. Motors bind stably to a support or cargo and transiently to a cytoskeletal fiber (actin filament or microtubule). Energy liberated by ATP hydrolysis prouces force to stretch an elastic element somewhere in the physical connection between the cargo and the cytoskeletal fiber. The resulting motion depends on whether the force in the spring exceeds the resistance of the fiber or the cargo.

Scientists have been able to use various techniques to observe as extended and contracted conditions, which exhibit extensions from the thick filament onto the thin filament. What they were able to identify was cross-bridge formation.Diagram the ATPase cycle of the myosin crossbridge.What should be known is that myosin can be utilized as an ATPase. Myosin in the presence of actin rapidly hydrolyses ATP. If a scientist placed myosin alone with ATP, hydrolysis will occur. Addition of actin will allow hydrolysis to occur at a greater rate. Removal of actin will yield a slower reaction rate.

What also should be remembered is that myosin has 3 parts:1. Myosin S1: Takes up most of the amino acids in the whole sequence of the molecule2. Neck/Hinge Region (S2): Exhibits flexibility, which plays a role in the myosins ability to interact with actin. Decreases in flexibility can affect interaction between myosin and actin.3. LMM

In the presence of ATP, there is a presence of actin and myosin, and allows a conformation change in the myosin molecule. With addition of troponins and tropomyosin, there is now regulation of the system. If myosin can activate ATP faster, there is possible hindrance to the actin and myosin relationship. When going from a relaxed to a contracted state, there is a shift in the molecular mass to a different area. Once the muscle is activated, limited scattering occurs and forces a concentration shift into the central region. It essentially allows conglomeration to a single mass. Actin and myosin interact in a very specific way, arranging itself in a barbed end and a pointed end. Myosin binds to actin with a given polarity. There is a specific interaction in order for a contracted and relaxed state to occur, and specific orientation of the head required relative to the molecule. There must be a change in the myosin conformation to allow it to shift towards the middle of the sarcomere. There are two states of the myosin head, known as the ADP-Pi and the ADP/nucleotide-free. This shows that there must be a release of the inorganic phosphate in order to have the conformational change. This reaction shift eventually combines to form what is known as the cross-bridge cycle. This basically establishes the mechanism that actin and myosin interact and permit muscle contraction.

The cross-bridge cycle is acts by the following steps:1. Resting fiber; cross bridge is not attached to actin2. Cross bridge binds to actin3. Inorganic phosphate is released, causing conformational change in myosin.4. Power stroke causes filaments to slide, and ADP is released.5. A new ATP binds to the myosin head, allowing it to release from actin.6. ATP is hydrolyzed, causing cross bridge to return to its original orientation.Identify the powerstroke of the crossbridge cycle.The powerstroke of muscle contraction occurs when actin rebinds to myosin, spurring the release of the terminal phosphate group of ATP, which allows the myosin molecule to reverse the conformational change while bound to actin. This is important because the conformation change is required in order for the interaction and consequential contraction.

Based on the illustration, the myosin heads are in the ADP-Pi state and catalytic cores bind weakly to actin. One the myosin head docks to an actin subunit, the inorganic phosphate is released. Phosphate release results in an increase in the affinity of the myosin head for actin and allows the myosin to the post-stroke, ADP state. The exact distance may vary from cycle to cycle depending upon the initial prestrike vinding configuration of the myosin on actin. After the execution of the stroke, ADP dissociates and ATP binds to the empty active site, causing the catalytic core to detach from actin. The lever arm then re-cocks the myosin head back to its pre-stroke state.

Lecture XLCardiac Muscle Structure and Function: Mechanism of Muscle Contraction IIList major thin filament regulatory components (proteins).The major thin filament regulatory components are (1) troponin C, (2) troponin I, and (3) troponin T. They ultimately will play a role in the formation of a regulatory complex and move the tropomyosin to allow exposure of the myosin binding sites on actin.Define the steric hindrance model of muscle contraction.Another model that is under discussion is the steric-hindrance model, which is an extension of the sliding filament model of muscle contraction. It simply states that the interaction of actin and myosin can be physically blocked, and requires a conformational change in order to allow the attachment of myosin to actin. There is a blockage of atin and myosin that the troponins and tropomyosins are responsible for. This blockage is essentially what is responsible for the selectivity. ATP hydrolysis is required to initiate the myosin head pivoting and allow interaction. Steric hindrance of the contractile cycle can occur by tropomyosin and troponin. Tropomyosin filaments periodically studded with troponin run along the actin double helix, blocking the myosin-binding site. With an influx of calcium, calcium binds to troponin (specifically troponin C) to allow a shift of tropomyosin complex towards actin filament, and consequently allows exposure of the actin binding site and spur muscle contraction. Approximately 60% of mutations can occur in the myosin-binding protein C and troponins, which are implicated in familial hypertrophic cardiomyopathy.

A better question to ask though is how the troponins and tropomyosin play a role in the regulation of actin-myosin interactions. Troponins C, T, and I are formed together to yield a regulator complex along actin. Troponin C binds calcium ions and initiates a macromolecular rearrangement. Cardiac troponin I is different than in sketal muscle. When TnC binds to TnI, it allows an opening up of the region. This molecule will open up slightly. There will be amino acids that will have tnI bound, and a hydrophobic pocket will be yielded from the movement of tropomyosin. In states of diastole, the tropomyosin will be down, overlapping the binding sites. In states of systole, the regulatory complex will rearrange and move the tropomyosin and expose the myosin-binding sites.Differentiate between troponin subunits.Troponin SubunitDescription

IBinds to actin in thin myofilaments to hold the actin-tropomyosin complex is place. Because of it, actin can bind to myosin in relaxed muscle.

CContains four calcium binding EF hands. It also contains an N lobe and a C lobe. The C lobe serves a structural purpose and binds to N domain of Troponin I. The C lobe can bind either to calcium or magnesium ions. The N lobe, which only binds to calcium ions, is the regulatory lobe and binds to the c domain of Troponin I after calcium binding.

TIt binds to tropomyosin, interlocking to for a troponin-tropomyosin complex. It helps orient it on actin.

Identify two of three putative mechanisms for cooperativity.Muscle has very important cooperative functions. Cooperative means that tension is greater than number of calcium binding sites. This implies that there are more binding sites. Cooperativity is defined as tension development is greater than the number of calcium binding sites, in the case of cardiac muscle. Proposed mechanisms is as follows:1. Cross bridge formation between actin and myosin increases the affinity of troponin for calcium ions. There is an increase of troponin C for calcium. I you have a movement of proteins that is transmitted to the next troponin complex does not explain the different amounts of activation even though there is the same amount of calcium.2. Binding of a cross bridge increases the rate of formation of neighobring cross bridges and that multiple cross bridges can activn activation even in the absence of calcium ions. Potentially, you can move the tropomyosin to the neighboring region to allow myosin to act even if calcium is absent.3. End-to-end interactions between adjacent troponin and tropomyosin.

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