Rat infarct model of myocardial infarction and heart failure

Journal of Cardiac Failure Vol. 1 No. 2 1995

Rat Infarct Model of Myocardial Infarction and Heart Failure

S T E V E N G O L D M A N , M D , T H O M A S E. R A Y A , M D

Tucson, Arizona

Abstract: This review outlines the development and current use of the rat coronary artery ligation model of heart f~ilure. The techniques to ligate the left coronary artery and to obtain morphologic/hemodynamic measurements are described. The authors show how the pathol- ogy seen in this mode~ relates to clinical ischemic heart disease. An effort is made throughout the review to relate the changes that occur in this model to clinically relevant observations. For example, the progression to heart failure in these rats is similar to what happens when a patient sustains a large myocardial infarction, survives, but goes on to develop heart failure without another ischemic insult. In both rats and people with large infarctions, the noninfarcted myocardium, even though not damaged at the time of the infarct, cannot compensate sufficiently to prevent the eventual development of heart failure. In addition to being a good approximati on of human disease, the responses to pharmacologic interventions, like angiotensin converting enzyme inhibitors, in rats has proved useful in predicting what will happen in humans given the same treatment. More recent data on the molecular control of ventricular remodeling emphasizes how this model will provide important information in the study of integrated physiology by examining biochemical, pharmacologic, and physio- logic changes in the same tissue. Key words: congestive heart failure, coronary artery ligation, ventricular remodeling after infarction.

Although we have made major advances in the treatment of congestive heart failure, this disease remains an important public health problem. It has been estimated that 2.5 to 3 million people in the United States have congestive heart failure. This represents about 1% of the population, and the prevalence of heart failure is actually increasing in this country. It has been estimated that there is an annual incidence of heart failure of 3 per 1,000, with new diagnoses in over 750,000 people each year in the United States. I'2 The treatment of heart failure is a major economic problem in this country. In 1991, it was estimated that heart failure accounted for 6.5% of the total

From the Tucson VA Medical Center and University of Arizona Heart CenteT; Tucson, Arizona.

Supported by grants from the Veterans Administration, the National Institute of Health (RO 1 HL-48163), Arizona Disease Control Research Commission (82-0697), and the Arizona Affiliate of the American Heart Association.

Manuscript received September 13, 1994; revised manuscript received November 8, 1994; accepted November 8, 1994.

Reprint requests: Dr. Steven Goldman, Cardiology Section, l l lC, Tucson VA Medical Center, Tucson, AZ 85723.

healthcare budget and resulted in $5.5 billion in healthcare financing administration costs, more than double the cost of cancer hospitalizations. 3

To better understand the pathophysiology and to develop new treatment regimens for heart failure, investigators have developed animal models that simu- late human disease. The choice of the animal model is important, because while it is possible to induce cardiac hypertrophy and heart failure with a variety of interventions in animals, to be useful, the pathophysiology must approximate the clinical condition as closely as possible. The need to have an animal model that is clinically relevant, in part, explains the attractiveness of the coronary artery ligation model of heart failure in rats.

The first description of creating experimental myocardial infarction by coronary artery occlusion in small animals was in 1954. 4 Data on infarct size and ventricular function in rats after coronary artery ligation appeared in the late 1970s and early 1980s. s-7 Based on this work, laboratories explored the structural, functional, and biochemical changes that occurred in rats after coronary artery ligation. The rat coronary artery ligation model has

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170 Journal of Cardiac Failure Vol. 1 No. 2 March 1995

generated even more enthusiasm in recent years because the model has-many pathophysiologic and clinical char- acteristics that are similar to the clinical syndrome of heart failure after anterior wall myocardial infarction in people. Perhaps the most clinically relevant observation is that the progression to heart failure in these rats is similar to what happens when a patient sustains a large myocardial infarction, survives, but goes on to develop heart failure without another ischemic insult. In both rats and people with large infarctions, the noninfarcted myocardium, even though not damaged at the time of the infarct, cannot compensate sufficiently to prevent the eventual development of heart failure. In addition to being a good approximation of human disease, the responses to pharmacologic interventions in rats has proved useful in predicting what will happen in people given the same treatment. Lastly, the relatively low costs of the rat infarct model make it economically attractive.

Materials and Methods

Although different methods have been used to ligate the left coronary artery of rats, the basic approach is to perform a left thoracotomy and secure a ligature around the intramyocardial portion of the artery that lies just ventral to the left atrium. Both male and female rats have undergone coronary artery ligation, and while the most common species is adult Sprague-Dawley rats, our laboratory has also infarcted Fischer-344 and Brown Norway Fischer-344 cross rats. The approach used in our laboratory is as follows. After induction of anesthesia with acepromazine maleate 50 mg/kg, xylazine 5 mg/kg, and ketamine He1 50 mg/kg intraperitoneally, a left anterior thoracotomy is performed under sterile conditions. The heart is expressed through the incision and a 7-0 syn- thetic ligature is secured snugly around the proximal left anterior coronary artery. The lungs are inflated to reduce the pneumothorax, and the muscle layer and skin are closed separately. Postoperative analgesia is provided with acetaminophen (67 rag/L) in the drinking water. An acute survival rate of approximately 50% is generally achieved. Other variations on this basic approach are to use endotracheal intubation with ventilator support so as to have more time to perform the ligation and to treat rats with perioperative lidocaine to decrease the incidence of ventricular tachycardia and fibrillation.

As opposed to patients with infarctions, it is possible to use electrocardiographic (ECG) screening to stratify rats with respect to the size of the infarction. To obtain an ECG, rats are anesthetized with a short-acting inhaled anesthetic, and a nine-lead ECG with six limb leads and three chest leads is performed. With the use of criteria described previously, 8 rats with evidence of large myocardial infarctions can be selected with high specificity. Our ECG criteria for a large infarction are the presence of Q waves (>1 mV) in the limb leads (I or aVL) and the sum of R waves in the precordial leads less than 10 mV. ffthe additional criteria of

absence of an R wave in leads I and aVL are added, the specificity for large myocardial infarction is greater than 97% (unpublished data).

While ECG screening is useful as a screening measure, especially when rats are entered into pharmacologic treatment protocols, most investigators ultimately quantify infarct scar size after the animal is sacrificed and the heart is excised. The most common method of histologic quantification of infarct size is to dissect the separated left ventricle plus septum into four transverse slices from apex to base, immersion-fix the sections in 10% formalin, and embed them in paraffin. Thin sections are stained with Masson's trichrome, and infarct size is measured by tracing the outline of the infarcted and noninfarcted regions of the left ventricle at each of the four levels. Infarct size is reported as the mean percentage of epicardial and endocardial circumference occupied by scar tissue for the four sections.

Another method of obtaining quantitative measurements of infarct size is to determine the volume of infarcted and noninfarcted tissue. These tissue volumes can be calculated by summing the areas of viable and nonviable tissue from each slice of tissue to compute the fractional volumes of each type of tissue. 9 The fractional percentage is then multiplied by the total ventricular volume. In brief, this has been done by staining the tissue with methylene blue and safranine. The number of myocyte nuclei per unit area of myocardium sectioned transversely is determined and divided by the average nuclear length to calculate the number of nuclei per unit volume of myocardium. From the product of total ventricular volume of noninfarcted myocardium and myocyte nuclei per unit volume, the total number of nuclei in the ventricle can be determined. The total number of myocyte nuclei still present in the infarcted ventricle is divided by the total number of myocyte nuclei calculated in the control-operated animals to determine percentage of nuclei spared and lost (% infarct).

Structural Changes

The pathologic changes that occur after coronary artery ligation in the rat have been extensively studied. Initially, there is thinning and distension of both the infarcted and the noninfarcted myocardium. Subse- quently, the noninfarcted myocardium hypertrophies, but left ventricular weight and left ventricular weight/body weight do not change, because the previously normal left ventricle is replaced by scar.

The ventricular remodeling that occurs after infarction in the rat is similar to what happens in patients. The infarcted wall thins with resultant scar formation, while the noninfarcted myocardium hypertrophies in response to the increased stress. After ligation, hypertrophy of surviving myocytes occurs in proportion to infarct size for infarctions involving 0-20% of the ventricle. There is little additional hypertrophy in larger infarctions. 1° In

rats with infarction of less than 20%, there are minimal, if any, changes in hemodynamics or peak pumping capacity of the heart. However, rats with large infarctions develop increased left ventricutar end-diastolic pressures and a rightward shift of the left ventricular pressure-volume relation. H,12 Progress:.ve left ventricular dilatation occurs up to 3-4 months postinfarct; cardiac output begins to fall at 6 months. ~3

Although the temporal relation between these functional changes and myocyte size has not been studied, myocyte hypertrophy is seen as early as 3 days after infarction. At 3 weeks after infarction, myocyte cross-sectional area is increased by about 30%. Side-to-side slippage of cells is present 2 days after ligation and this persists up to 1 month, when the healing process is completed irL the rat infarcted ventricle. 9,~4 The end result is a change ir the shape of the heart from ellipsoidal to cylindrical.~S

After a large infarction, coronary blood flow and coronary flow reserve are depressed in the noninfarcted myocardium. ~6 These studies showed a correlation between minimal coronary resistance, myocyte cross- sectional area, and left ventricular end-diastolic pressure, suggesting that the depression in coronary reserve is related in part to the elevated preload. Vascular flow capacity measured in a hindquarter preparation in this model showed that regional flow capacities were reduced in soleus and red gastrocnemius muscles but not in white gastrocnemius muscles. ~7 These studies have been used to support the argument that changes in blood flow contribute to decreases in exercise tolerance in pai:ients with heart failure. The rat coronary artery ligation model is ideal for these investigations because the argument cannot be made that the changes in perfusion are due to residual coronary stenoses or insufficient collateral flow. That is, abnor malities in coronary and regional flow :eserve must be directly related to compromised left ventricular function.

F u n c t i o n a l C h a n g e s

After large myocardial infarction, muscle function in the noninfarcted myocardium is augmen:ed initially and then eventually deteriorates without another ischemic insult. It is thought that a mechanical stimulus (ie, an increase in wall stress) is probably responsible for the progressive deterioration in muscle funci:ion (Fig. 1). In the rat infarct model, the functional changes are reflected in mortality data. The 6-month survival for all infarct rats is generally 50%; for rats with large infarcts it is 35%. 8

Muscle Dysfunction After Myocardial Infarction

While changes in organ function (ie, ventricular hemodynamics) are well documented after myocardial infarction, there are fewer data on alterations in rauscle function. In clinical studies, it is difficult to separate changes in organ

Rat Infarct Model • Goldman and Raya 171

200 - Muscle function

100 . --.

~" 75

0 3 6 1

weeks weeks year Infarct

Fig. 1. The concept of the time course and how changes in diastolic wall stress are the primary determinants of changes in biochemical markers and muscle function after myocardial infarction are shown. In the rat heart, diastolic wall stress increases immediately after infarction. This results in alterations in biochemical markers that proceed the deterioration in muscle function. Although muscle function decreasing from a baseline value after infarction is depicted here, it is possible that immediately after infarction, muscle function is initially enhanced prior to deterioration.

and muscle function because hemodynamic data, like measurements of pressure and flow, represent a summation of changes in the infarctedJnoninfarcted myocardium as well as alterations in ventricular-arterial coupling. Changes in the peripheral circulation also affect left ventricular function because a damaged ventricle is afterload dependent. Thus, in the setting of myocardial infarction and heart failure, experimental studies in the laboratory, as opposed to clinical investigations, offer a better opportunity to separate changes in organ and muscle function.

Hypertrophy of the noninfarcted myocardium is a compensatory mechanism that initially may even result in hyperfunction of the left ventricle. This augmentation is transient, and at some point the hypertrophied noninfarcted myocardium fails. An initial preliminary report showed minimal changes in muscle function and no change in muscle stiffness 3 weeks after large infarction. TM Data from our laboratory showed that at 6 weeks after large infarction there was contractile dysfunction with increased muscle stiffness, myocyte hypertrophy, and increased collagen content in the residual noninfarcted myocardium.19

Changes in Left Ventricular Diastolic Function

Changes in diastolic function have been well documented in the rat coronary artery ligation model.


Changes in the passive-elastic properties of the ventricle have been described by measuring the passive pressure-volume relationship in the isolated left ventricle. The fact that changes in the left ventricular pressure- volume relationship are time-dependent after infarction has not been well appreciated. Initially the left ventricular pressure-volume curve is shifted to the left toward the pressure axis at 24 hours (Fig. 2). By 1 week, the curve shifts back away from the pressure axis such that by 3 weeks, the pressure-volume relationship is displaced rightward with large increases in operating end-diastolic volume. 11 The left ventricle continues to dilate with documentation of changes in the pressure-volume relationship up to 1 year after infarction.

Although the active components of diastole have not been well studied in the rat coronary artery ligation model, relaxation of the noninfarcted left ventricle is thought to be slowed after infarction as a result of compensatory hypertrophy. Whether this change in relaxation represents a change in muscle function is not clear, because when isolated papillary muscle function is examined after infarction, a measure of relaxation, the time of 50% decline from maximal tension was not altered. 19 These data have to be contrasted with the observation that tan or the time constant of left ventricular relaxation is prolonged after infarction. 2°

30,

a-

1

0 .2 ,4 .6 .8 1.0 1.2 1A 1.6 1,8 2.0 2,2 2.4 2.6 2.8

Vol (ml/kg)

Fig. 2. Left ventricular pressure-volume relationship in control rats ([Z), and in rats 3 hours (C)), 24 hours (A), 3 days (m), 5 days (O), and more than 22 days (A) after infarction. With acute infarction, there a shift of the pressure-volume relationship to the left. In the healing stages, there is progressive rightward displacement of the pressure-volume relationship. From Raya et al. al With permission.

Alterations in the Peripheral Circulation

In addition to the contractile changes that have been defined in the rat coronary artery ligation model of heart failure, it is clear that this model accurately reflects changes that occur in the peripheral circulation. For example, decreases in cardiac output and mean arterial pressure result in an increase in peripheral vascular resistance. This increase in afterload has been documented by defining increases in input impedance and wall s t r e ss . 21

In addition to changes in afterload, systemic venoconstriction has been documented after infarction, resulting in an increase in mean circulatory filling pressure, a decrease in venous compliance, and an increase in circu- lating blood volume. 22 These changes in the venous circulation result in an increase in central blood volume and are thought to represent a compensatory response de- signed to improve cardiac output in the failing heart.

Studies of endothelium-derived relaxing factor show that its activity is impaired in the rat infarct model, which suggests that a defect in endothelium-derived relaxing factor activity may contribute to changes in vasomotor tone in heart failure. 23 This endothelial dysfunction is a progressive time-dependent process with little effect early after infarction) 4 Endothelium-mediated dilation in response to acetylcholine is attenuated in a hindquarter preparation from rats after myocardial infarction, confirming that this may be an additional mechanism responsible for impaired vasodilatory capacity in heart failureY

Recent studies from our laboratory have shown that arterial remodeling occurs in the rat coronary artery ligation model. 26 This remodeling is defined by alterations in the passive mechanical properties, water permeability, and structure of large arteries. Support for the important role of the renin angiotensin system in this model of heart failure comes from data that show that captopril but not hydralazine in part reverses this arterial remodeling in heart failure.

Neurohumoral Control

The rat coronary artery ligation model of heart failure has also been used to examine neurohumoral control in heart failure. Because this model results in such a homogeneous population of rats with similar hemodynamics, serial studies can be done in different rats in the same subset. Earlier studies that first described neurohumoral activation in this model concentrated on changes in the sympathetic nervous system and the renin-angiotensin systemY Our laboratory reported that atrial natriuretic peptide (ANP) levels progressively increased over time as heart failure developed after coronary artery ligationY Other investigators reported similarly increased ANP levels, but no salt retention or changes in plasma renin activity or plasma aldosterone concentrations. 29 When the molecular control of ANP synthesis was examined, ANP

was increased in the atria, but the mRNA for ANP was increased in infarcted rats in both the right and left ventricles, suggesting that specific ANP gene expression occurs in the ventricle. 3°

Our laboratory examined the interaction between ANP and the renin-angiotensin system 3~,32 and the role of argi- nine vasopressin. 33 Recently we showed that baroreflex control of heart rate was altered, without a change in baroreflex sensitivity in rats with heart failure. 34 Treatment with captopril altered baroreflex control of heart rate independent of changes in syn~tpathetic tone.

Biochemical and Molecular Changes

The rat coronary artery ligation model has been used to study biochemical changes after infarction, with most of the recent interest in the model directed at examining molecular control of left ventricular remc,deling. The ear- liest studies in this model showed that myosin heavy- chain composition was altered in rats after infarction, with an increase in the beta-myosin and a decrease in the alpha-myosin heavy-chain component? 7 This change in myosin composition was partially reversed with capto- pill, suggesting that the increase in t'ae beta-myosin isoenzyme was responsible for the improvement in the speed of muscle contraction seen with treatment. Dobutamine has also been reported to increase myosin adenosine triphosphatase activity and alpha-myosin heavy-chain composition. 35

Other examples of how the rat infarct model has been used to examine biochemical changes include the observation that regional differences in left ventilcular collagen accumulation and mature cross-linldng have been reported in rats after infarction. 36 Activa~:ion of myocardial ANP has also been examined as an a.tempt to define the control of the signaling mechanism that predicts biochemical changes in heart failure? 7

Investigators have used the rat infarct model to examine the role that tissue angiotensin lI play.~ in ventricular remodeling. Since tissue angiotensin II levels are difficult to measure, investigators have focused on defining the importance of myocardial angiotensin-co~verting enzyme levels. 37 Interestingly, these same investigators showed differential effects on serum, lung, and aortic converting enzyme activities when rats are treated with captopril versus enalapril. 38 When angiotensin II conten: was measured, levels were not increased in the noninfarcted fight or left ventricles, but were increased in the infarcted scar. 39 Other investigators have examined the role that angiotensin II plays in influencing the inotropic state of noninfarcted ventricular muscle after infarction, emphasizing that angiotensin II exerts a positive inotropic effect at muscle lengths on the ascending limb of the Starling curve, but a negative inotropic effect at muscle lengths normally asso- ciated with maximum force development. 4° Within 5 days


of an infarction, angiotensinogen mRNA levels in the noninfarcted left ventricle were elevated, but renmaed to control values by 25 days. 4~

Because of the important role that changes in calcium handling play in the development of heart failure, investigators have used the rat infarct model to examine calcium transport. Decreases in sarcolemmal Na+-depen - dent Ca 2+ uptake, with no changes in Ca2+pump activity, have been found in rat ventricles after infarction. 42 Studies of intracellular calcium handling have shown that decreases in muscle function correlate well with prolongation of the time to peak Cai2+.43 Treatment with captopril resulted in partial normalization of papillary muscle function and Cai 2+ handling, suggesting that chronic load reduction with converting enzyme inhibition can partially reverse the defect in excitation- contraction coupling that occurs in heart failure.

Although it is not clear what the stimulus is for the biochemical and molecular changes, we feel that change in diastolic wall stress is the primary physical stimulus that is transduced via molecular signaling mechanisms into changes in muscle function (Fig. 1). The precise sequence of events has not been worked out, but our concept is that diastolic wall stress increases immediately after infarction. This results in alterations in biochemical markers and signaling mechanism(s) that precede the deterioration in muscle function. The immediate effect of infarction on the noninfarcted myocardium has not been studied, but it is possible that immediately after infarction, muscle function is initially enhanced prior to when the combination of pressure and volume overload leads to deterioration in muscle function.

Responses to Pharmacologic Intervention

The rat coronary artery ligation model has been a very useful heart failure model to study responses to therapy as a predictor of outcomes in patients after myocardial infarction. Perhaps the best example of this is the response of the infarcted rat to treatment with converting enzyme inhibition where the changes in hemodynamics and mortality in rats accurately predicted what occurred clinically. TM Interestingly, the mortality data in rats showed that only rats with moderate-sized infarctions, as opposed to those with small and large infarctions, had an improvement in survival with captopril. 8 The improvement in mortality that was documented in these rats with captopril provided the basis for the large clinical trials (CONSENSUS, SAVE, SOLVD, and V-HeFT), which proved that patients in heart failure benefited from treatment with converting enzyme inhibition. 45-48

Converting Enzyme Inhibition

While it is commonly believed that the beneficial effects of captopril are due to improvement in ventricular


loading, this explanation may be too simplistic. The rat infarct model has enabled investigators to explore this question. In ischemic left ventricular dysfunction, failure of the heart at the organ level may be due to increases in afterload, loss of contractile units, or dysfunction of indi- vidual contractile units. The loss of functioning myocardium is an irreversible process, but abnormalities of ventricular loading and dysfunction of surviving myocardium may potentially be prevented or improved by appropriate treatment. To define the effects of converting enzyme inhibitors on both muscle and organ function, it is necessary to study animal models of heart failure.

Data from our laboratory showed that in the rat coronary artery ligation model of heart failure, captopril dilates both the arterial and venous circulations and decreases blood volume. 44 This results in a decrease in left ventricular end-diastolic pressure and volume. We concluded from these studies that the converting enzyme inhibitors were both arterial and venous vasodilators as opposed to an agent like hydralazine that is predominantly an arterial dilator. In spite of the improvement in hemodynamics, this change in loading conditions resulted in only a modest improvement in papillary muscle function (ie, there are no changes in developed tension or passive stiffness). Time to peak tension is the only parameter of isometric function that was improved during captopril treatment. 49

We initially concluded that since converting enzyme inhibition acts mainly through effects on loading conditions, it is possible that some of the benefit from these agents is derived from blocking the growth effects of angiotensin II on the heart and large arteries. Since angiotensin II is a growth factor, it has been proposed that some of the cardiac hypertrophy that occurs after infarction is related to the trophic effects of angiotensin II. After infarction, myocyte size increases and then with captopril there is a decrease, but collagen content and myocardial stiffness remain abnormal. 49 Thus, captopril improves global cardiac performance. Probably most of this benefit is due to its effects on the peripheral circulation, but it is possible that converting enzyme inhibition has a direct effect on the heart itself.

Supportive evidence for the direct effect of angiotensin II on the heart comes from studies where captopril and hydralazine were given to rats after infarction and interstitial DNA and collagen content were measured, s° These studies showed that captopril reduced the increase in DNA synthesis and collagen content where hydralazine did not affect DNA synthesis but reduced collagen content.

To determine the most efficacious time to initiate treatment after myocardial infarction, Gay 51 created infarctions in rats and then randomized the rats to treatment started immediately after coronary ligation versus treatment started 3 weeks later. He found no differences in

hemodynamics or end-diastolic volumes at 4 months, suggesting that the timing of starting treatment was not a critical factor in defining the long-term therapeutic out- come. Support for this comes from other studies that showed that administering captopril to rats early after infarction did not improve cardiac output during follow- up tests, s2 Recent data show that using captopril after late reperfusion in the rat heart failure model results in improvements in hemodynamics, causes decreases in left ventricular cavity size and left and right ventricular weights, and alters the left ventricular pressure-volume relationship, s3

Angiotensin II Receptor Blockade

The development of nonpeptide specific angiotensin II receptor blockers resulted in studies that examined the rel- ative efficacy of these agents compared to converting enzyme inhibitors in the rat coronary artery ligation model. Our laboratory reported that similar hemodynamic changes occurred when infarcted rats were treated with captopril or losartan, that is, there were similar decreases in both preload and afterload with both agents, s4 Both converting enzyme inhibition and receptor blockade increased venous compliance. Other investigators have suggested that captopril exerted its effects mainly through reducing load, while losartan affected myocardial function. 55 Recently, studies have shown that both converting enzyme inhibition and AT 1 receptor blockade reduce cardiac hypertrophy, restore minimal coronary vascular resistance in postinfarction reactive hypertrophy, and attenuate the development of myocardial interstitial fibrosis in the noninfarcted myocardium. 56 These data argue that inhibition of the gen- eration of angiotensin II and AT1 receptor blockade are equally effective in altering ventricular remodeling after myocardial infarction.

Beta-adrenergic Receptor Blockade

The first investigations to define the effects of beta- adrenergic blockade in rats after myocardial infarction were pathologic studies, which showed that propranolol treatment resulted in decreased myocyte dimensions and increased left ventricular cavity dilatation. 57 Data from our laboratory showed that propranolol given to rats after infarction resulted in no change in systolic function but a shift in the left ventricular pressure-volume relationship to the right, away from the pressure axis. 58 There was a decrease in the number of beta-adrenergic receptors in association with a decrease in adenylyl cyclase activity in these heart failure rats. 59 In this model, where we showed that baseline muscle function in the noninfarcted myocardium is compromised, 19 propranolol improved basal isometric muscle function and increased beta-adrenergic receptor density, but did not improve adenylyl cyclase activity or isoproterenol-stimulated muscle function.

Other Pharmacologic Interventions

The rat coronary artery ligation model of heart failure has provided a means to test other pharmacologic interventions. For example, survival studies in rats after infarction showed that milrinone improved survival 6° and a calcium channel blocking agent, anipamil, decreased survival. 6I Our laboratory used the rat coronary artery ligation model of heart failure to study changes in fatty acid oxidation and the use of thyroid hormone analogs to treat heart failure.

Two-tetradeclycidic Acid and Ventricular Remodeling

In the initial studies, we found that an inhibitor of fatty acid oxidation, 2-tetradeclycidic acid (TDGA), increased myocyte size and left ventricular mass, but did not alter systolic function in normal rats. 62 When TDGA was administered to rats after infarction myocyte size increased and there was improved systol c function and a decrease in left ventricular end-diastolic pressure and volume while stroke volume was maintained. The left ventricular pressure-volume relationship was shifted leftward, toward the pressure axis. We concluded that induction of myocardial hypertrophy after infarction prevented left ventricular dilatation. 63 It is iEteresting to note that interventions with opposite effects on left ventricular mass (ie, converting enzyme inhibition, which decreases myocyte size, and TDGA, which increases myocyte size) both prevent left ventricular dilatation after infarction.

Thyroid Hormone Analogs as Treatment for Heart Failure

To understand how thyroid hormone alters gene expression and hemodynamics after myocardial infarction, we treated rats in heart failure with short-term (3 days) and longer (10 days) administration of thyroid hormone. 64,6s These studies showed that with acute administration of T4, end-diastolic pressure decreased and heart rate did not change. After 10 days, end-diastolic pressure remained elevated and heart rate increased, suggesting that thyroid hormone was not a useful agent to treat heart failure. Because of the lack of efficacy of the native hormone, our laboratory began exploring analogs of thyroid hormone that could selectively upregulate gene expression in the failing heart. We recently reported that an analog of thyroid hormone, 3,5-diiodothyropropionic acid (DITPA), in combination with captopril proved effective in rats with heart failure after coronary artery ligation. 2° This analog binds to nuclear receptors, ,alters transcrip- tion of T 3 responsive genes, and increases +dP/dt in hypothyroid rats with substantially less effect on heart rate and metabolism than thyroid hormone. 66 The combination of DITPA and captopril improved cardiac output,


increased -dP/dt, and increased the rate of left ventricular relaxation to a greater extent than captopril treatment alone. In rabbits with heart failure after circumflex coronary artery ligation, treatment with DITPA alone decreased left ventricular end-diastolic pressure and left ventricular relaxation and increased maximum positive and negative dP/dt. 6v These beneficial effects of thyroid hormone analogs in animal models of heart failure suggest that altering selective thyroid hormone responsive genes may represent a new class of agents to treat heart failure.

Conclusion

The rat coronary artery ligation model of heart failure has become a useful model for investigators. The attractiveness of this model is related to its accurate reflection of human pathophysiology and the fact that, for the most part, responses to therapy in rats seem to predict what will occur when patients with heart failure are treated with the same agents. The wide experience with the model, the observation that similar sized infarctions cre- ate a homogeneous population with stable heart failure, and the fact that the molecular biology techniques can be applied to study integrated pathophysiologic responses means that the rat infarct model will continue to be used well into the future.

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28. Lee RW, Gay R, Moffett C, Johnson DG, Goldman S: ANP levels during development of chronic heart failure in rats. Life Sci 1987;40:2025

29. Hodsman GR Kohzuki M, Howes LG, Sumithran E, Tsunoda K, Jonhston CI: Neurohumoral responses to chronic myocardial infarction in rats. Circulation 1988;78:376-81

30. Drexler H, Hanze J, Finckh M, Lu W, Just H, Lang RE: Atrial natriuretic peptide in a rat model of cardiac failure: atrial and ventricualr ventricular mRNA, atrial content, plasma levels, and effect of volume loading. Circulation 1989;79:620-33

31. Lee RW, Gay RG, Goldman S: Atrial natriuretic peptide reverses angiotensin induced venoconstriction in dogs. Am J Physiol 1989;257:H1062-7

32. Raya TE, Lee RW, Westhoff T, Goldman S: Captopril restores hemodynamic responsiveness to atrial natriuretic peptide in rats with heart failure. Circulation 1989;80: 1893-902

33. Raya TE, Gay RG, Goldman S: Selective vasopressin inhibition in rats with heart failure decreases afterload and results in venodilatation. J Pharmacol Exp Ther 1990;255:1015-20

34. Deck CD, Raya TE, Gaballa MA, Goldman S: Baroreflex control of heart rate in rats with heart failure after myocardial infarction: effects of captopril. J Pharmacol Exp Ther 1992;263:1424-31

35. Buttrick R Perla C, Malhortra A, Geenen D, Lahorra M, Scheuer J: Effects of chronic dobutamine on cardiac mechanics and biochemistry after myocardial infarction in rats. Am J Physiol 1991;29:H473-9

36. McCormick RJ, Musch TI, Bergman BC, Thomas DP: Regional differences in LV collagen accumulation and mature cross-linking after myocardial infarction in rats. Am J Physiol 1994;35:H35d-9

37. Hirsch AT, Talsness CE, Schunkert H, Paul M, Dzau VJ: Tissue-specific activation of cardiac angiotensin converting enzyme in experimental heart failure. Circ Res 1991;69:475-82

38. Hirsch AT, Talsness CE, Schunkert H, Smith AD, Schunkert H, Ingelfinger JR, Dzau VJ: Differential effects of captopril and enalapril on tissue renin-angiotensin sys- tems in experimental heart failure. Circulation 1992;86:1566-74

39. Yamagishi H, Kim S, Nishikimi T, Takeuchi K, Takeda T: Contribution of cardiac renin-angiotensin system to ventricular remodeling in myocardial-infarcted rats. J Mol Cell Cardio1 1993;25:1369-80

40. Peng L, Sonneblick EH, Anversa P, Capasso JM: Length- dependent modulation of ANG II inotropism in rat myocardium: effects of myocardial infarction. Am J Physiol 1994;266:H779-86

41. Lindpainter K, Lu W, Niedermajer N, Schieffer B, Just H, Ganten D, Drexler H: Selective activation of cardiac

angiotensinogen gene expression in post-infarction ventricualr remodeling in the rat. J Mol Cell Cardiol 1993;25:133-43

42. Dixon Ira, Hata T, Dhalla NS: Sarcolemmal calcium transport in congestive heart failure due to myocardial infarction in rats. Am J Physiol 1992;262:H1387-94

43. Litwin SE, Morgan SE: Captopril enhances intracellular calcium handling and beta-adrenergic responSiveness of myocardium from rats with postinfarctien failure. Circ Res 1992;71:797-807

44. Raya TE, Gay RG, Aquirre M, Goldman S: The importance of venodilatation in the prevention of LV dilatation after chronic large myocardial infarction in rats: comparison of captopril and hydralazine. Circ Res 1989;64:330-8

45. The CONSENSUS Trial Study Group: Effects of enalapril on mortality in severe congestive heart failure: results of the Cooperative North Scandinavian Enalapril Survival Study (CONSENSUS). N Engl J Med 1!)87;316:1429-35

46. Cohn JN, Johnson G, Ziesche S, Co~b F, Francis G, Tristani F, Smith R, Dunkman WB, Loeb H, Wong M, Bhat G, Goldman S, Fletcher RD, Doherty J, Hughes CV, Carson P, Cintron G, Shabeti R, Haakenson C: A comparison of enalapril with hydralazine-isosorbide dinitrate in the treatment of chronic congestive heart failure. N Engl J Med 1991;325:303-10

47. Pfeffer MA, Braunwald EB, Moye LA, Basta L, Brown EJ, Cuddy TE, Geltman EM, Goldman S, Flaker GC, Klein M, Lamas GA, Packer M, Rouleau J, Rouleau JL, Rutherford J, Wertheimer JH, Hawkins M, and S~,VE investigators: The effect of captopril on mortality and morbidity in patients with left ventricular dysfunction following myocardial infarction. N Engl J Med 1992;327:669-77

48. The SOLVD Investigators: Effect of enalapril on survival in patients with reduced left ventricular ejection fractions and congestive heart failure. N Engl v Med 1992;327: 669-77

49. Litwin SE, Raya TE, Anderson, PG, Litwin CM, Bressler R, Goldman S: Induction of myocardial hypertrophy after coronary ligation in rats decreases vewxicular dilatation and improves systolic function. Circalation 1991;84: 1819-27

50. van Krimpen C, Smits JFM, Cleutjens JPM, Debets JJM, Schoemaker RG, Struyker Boudier HAJ, Bosman FT, Daemen MJAP: DNA synthesis in the non-infarcted cardiac interstitium after left coronary artery ligation in the rat: effects of captopril. J Mol Cell Cardiol 1991;23: 1245-53

51. Gay RG: Early and late effects of captopril treatment after large myocardial infarction in rats. J Am Coll Cardiol 1990; 16:967-77

52. Schoemaker RG, Debets JJM, Struyker-Boudier HAJ, Smits JFM: Delayed but not immediate captopril therapy improves cardiac function in conscious rats, following myocardial infarction. J Mol Cell Cardiol 1991;23:187-97

53. Jain R Giridhar K, Mallavarapu C, Sikand V, Lillis O, Cohn PF: Effects of captopril therapy after late reperfusion on left ventricular remodeling after experimental myocardial infarction. Am Heart J 1994; 127:756-63


54. Raya TE, Fonken S J, Lee RW, Daugherty S, Goldman S, Wong PC, Timmermans PB, Morkin E: Hemodynamic effects of direct angiotensin blockade compared to converting enzyme inhibition in rat model of heart failure. Am J Hypertens 1991;4:334S-40S

55. Capasso JM, Li R Meggs LG, Herman MV, Anversa P: Efficacy of angiotensin-converting enzyme inhibition and AT~ receptor blockade on cardiac pump performance after myocardial infarction in rats. Cardiovasc Pharmacol 1994;23:584-93

56. Schieffer B, Wirger A, Meybrunn M, Seitz S, Holtz J, Riede UN, Drexler H: Comparative effects of chronic angiotensin- converting enzyme inhibition and angiotensin II type 1 receptor blockade on cardiac remodeling after myocardial infarction in the rat. Circulation 1994;892273-82

57. Fishbein MC, Li-Quan L, Rubin S: Long-term propranolol administration alters myocyte and venticular geometry in rat hearts with and without infraction. Circulation 1988;78:369-75

58. Gay RG, Raya TE, Goldman S: Left ventricular systolic function is preserved during chronic propranolol treatment after large myocardial infarction in rats. J Cardiovasc Pharmacol 1990;6:529-37

59. Warner AL, Bellah KL, Raya TE, Roeske WR, Goldman S: Effects of beta-adrenergic blockade on papillary muscle function and the beta-adrenergic receptor system in noninfarcted myocardium in compensated left ventricular dysfunction. Circulation 1992;86:1584-95

60. Sweet CS, Ludden CT; Stabilito II, Emmert SE, Heyse JF: Beneficial effects of milrinone and enalapril on long-term survival of rats with healed myocardial infarction. Eur J Pharmacol 1988;147:29-37

61. Gaudron R Blumrish M, Ertl G: Aggravation of left ventricular dilatation and reduction of survival by a calcium channel blocker in rats with chronic myocardial infarction. Am Heart J 1993;125:1226-33

62. Litwin SE, Raya TE, Gay RG, Bedotto JB, Bahl JJ, Anderson PG, Goldman S, Bressler R: Chronic inhibition of fatty acid oxidation: new model of diastolic dysfunction. Am J Physiol 1990;27:H51-7

63. Litwin SE, Raya TE, Gay RG, Bedotto JB, Bahl JJ, Anderson PG, Goldman S, Bressler R: Chronic inhibition of fatty acid oxidation: new model of diastolic dysfunction. Am J Physiol 1990;27:H51-7

64. Gay RG, TA Gnstafson, S Goldman, E Morkin: Effects of L-thyroxine in rats with chronic heart failure after MI. Am J Physiol 1987;253:H341-6

65. Gay RG, Graham S, Aquirre M, Goldman S, Morkin E. Effects of 10-12 days treatment with L-thyroxine in rats with myocardial infarction. Am J Physiol 1988;255: H801-6

66. Pennock GD, Raya TE, Bahl JJ, Goldman S, Morkin E: Cardiac effects of 3,5-diiodothyroproprionic acid, a thyroid analog with inotropic selectivity. J Phmanacol Exp Ther 1992;263:163-9

67. Mahaffey KW, Raya TE, Pennock GD, Morkin E, Goldman S: Left ventricular performance and remodeling in rabbits after myocardial infarction: the effects of a thyroid hormone analogue. Circulation (in press)

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Rat infarct model of myocardial infarction and heart failure

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