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Study Guide for Calcium and Phosphate Metabolism
• The most important first messengers for Dental Biochemistry include parathyroid hormone, calcitonin, insulin, and glucagon
• Name common second messengers and how they operate
• Define calcium balance
• Describe the properties and mechanism of action of parathyroid hormone (PTH) including G-protein function
• Review the composition of bone, and know what bone mineral is
• What are the two main functions of parathyroid hormone in kidney?
• Describe the metabolic activation of vitamin D
• What are the actions of vitamin D?
• What is osteomalacia? Rickets? Hyperparathyroidism? Pseudohypoparathyroidism?
• What is osteoporosis?
• Describe physiological ways to establish and maintain maximum bone density
• What foods are good sources of dietary calcium?
Elements of the Body (Table 2-1)
OverviewCalcium (Ca2+) Characteristic Phosphate (HPO4
2-)
99% Body total in bone/teeth 85%
1.3 mM (ionized) Plasma concentration of ionic form
1.3 mM
Carefully controlled Normal variation of ionic form
Within 50%
104-105 [extracellular]/[intra-cellular]
10
Cell signaling and 2nd messenger
Neurotransmitter and hormone release
Exocytosis of proteins
Muscle contraction
Blood clotting
Biomineralization
Regulatory roles
Important conclusion, calcium is an
important regulator
Nil
Regulation of Calcium Metabolism• Minerals; serum concentration
– Calcium (Ca2+); 2.2-2.6 mM (total)– Phosphate (HPO4
2-); 0.7-1.4 mM– Magnesium (Mg2+); 0.8-1.2 mM
• Organ systems that play an import role in Ca2+ metabolism– Skeleton– GI tract– Kidney
• Calcitropic Hormones– Parathyroid hormone (PTH)– Calcitonin (CT)– Vitamin D (1,25 dihydroxycholecalciferol)– Parathyroid hormone related protein (PTHrP)
Sutherland Second Messenger Hypothesis
Understand this key concept
Second Messengers
Fig. 19-4
More Second Messenger
Three Forms of Circulating Ca2+
Calcium Balance• Intake = output• Negative calcium
balance: Output > intake– Neg Ca2+ balance
leads to osteoporosis
• Positive calcium balance: Intake > output– Occurs during
growth
• Calcium is essential, we can’t synthesize it
Calcium and the Cell
• Translocation across the plasma membrane• Translocation across the ER and mitochondrion;
Ca2+ ATPase in ER and plasma membrane
Anatomy and Feedback Inhibition
Parathyroid Hormone Structure• Synthesized in
the 4 para-thyroid glands
• PreProPTH • 115 aa precursor
giving a 90 aa prohormone
• Cleaved at -6/-7 84 residues in the mature peptide
• Regulator of Ca2+ homeostasis
Parathyroid Hormone Biosynthesis
Regulation of PTH Secretion and Biosynthesis
• Extracellular Ca 2+ regulates secretion of PTH– Low Ca 2+ increases– High Ca 2+ decreases
• Ca2+ also regulates transcription
• High levels of 1,25 dihydroxyvitamin D3 inhibit transcription
Calcium Sensing Receptor (CaSR)
• Parathyroid chief cells contain a Ca2+ sensing receptor (CaSR)– 7 transmembrane segments (We will see a lot of 7 TM receptors)– mM affinity for Ca2+
– GPCR of the GPLC and GI varieties• Generates inositol 1,4, 5-trisphosphate which increases
intracellular Ca2+
• There are two paradoxes– The receptor responds to decreasing concentrations of
agonist– Low extracellular Ca2+ increases intracellular Ca2+
– Also found in thyroid C cells (calcitonin), kidney, and brain
Circulating Forms of PTH
• Intact, active PTH of 84 aa• Inactive carboxyterminal fragments lack the 1-34 active
domain• PTH t1/2 (half life) is 2-3 min• Liver (2/3rds) and kidney (1/3rd) are major sites of
fragmentation
Actions of Parathyroid Hormone• Fine tunes Ca2+ levels in blood
– It increases Ca2+
– It decreases Pi
• Parathyroid hormone acts directly on bone to stimulate resorption and release of Ca2+ into the extracellular space (slow)– Gs protein-coupled receptors in osteoblasts increase cAMP and activate PKA– Inhibits osteoblast function– This occurs when PTH is secreted continuously; the opposite occurs when it is
given once daily by injection
• Two effects in kidney– Parathyroid hormone acts directly on kidney to increase calcium reabsorption
and phosphate excretion (rapid)• Gs protein-coupled receptors• Parathyroid hormone acts on distal tubule • Calcitonin inhibits
– Stimulates transcription of 1-alpha hydroxylase for Vitamin D activation in kidney
• Vitamin D increases calcium and phosphate absorption
Parathyroid Hormone Receptor• 7 TM
• GPCR
G-Protein Cycle (Fig. 19-10)
Regulation of Adenylyl Cyclase(Fig. 19-11)
7 and 12 TM segments
Cyclic AMP Metabolism (Fig. 19-12)
Know each step involved in the generation of cAMP by PTH (words, not structures)
Bone• Inorganic (67%)
– Hydroxyapatite 3 Ca10(PO4)6(OH)2
– There is some amorphous calcium phosphate
• Organic (33%) component is called osteoid– Type I collagen (28%)– Non-collagen structural proteins (5%)
• Proteoglycans• Sialoproteins• Gla-containing proteins (gamma carboxyglutamate)• Phosphoproteins• Bone specific proteins: osteocalcin, osteonectin
– Growth factors and cytokines (Trace)
• Bone undergoes continuous turnover or remodeling throughout life– About 20% of bone is undergoing remodeling at any one time
Bone Composition
Calcium and the Skeleton
• A, absorption is stimulated by Vit D; S, secretion• GF, glomerular filtration; TR, tubular reabsorption
of Ca2+ is stimulated by PTH
Osteoblast and Osteoclast Function
• Osteoblasts
• Bone formation
• Synthesis of matrix proteins– Type I collagen
– Osteocalcin
– Others
• Mineralization
• Activation of osteoclasts via RANKL production
• Osteoclasts• Bone resorption
– Degradation of proteins by enzymes
– Acidification
• RANK is activated by RANKL, and this leads to cells differentiation to osteoclasts
Bone Remodeling• Osteoclasts dissolve
bone– Large multinucleated
giant cells
• Osteoblasts produce bone– Have receptors for PTH,
CT, Vitamin D, cytokines, and growth factors
– Main product is collagen
• When osteoblasts become encased in bone, they become osteocytes
PTH and Osteoblastogenesis
Osteoclast Mediated Bone Resorption
Osteoclastogenesis: RANKL, RANK, and OPG
• Osteoblasts activate osteoclasts, formation of a multinuclear cell• The molecular participants in this pathway are the membrane-associated
protein named RANKL (receptor activator of nuclear factor kappa B ligand,) a member of the tumor necrosis factor family of cytokines
• Its cognate receptor is RANK; TRAF, TNF receptor associated factors– Mediates activation of NF-kappa-B by unknown mechanism
• OPG (osteoprotegerin) is a soluble "decoy" receptor for RANKL• RANKL is expressed on the surface of osteoblastic stromal cells • By binding to RANK, its receptor, on osteoclast precursors, RANKL
enhances their recruitment into the osteoclastogenesis pathway in the physiology of bone metabolism
• RANKL also activates mature osteoclasts to resorb bone• RANKL is a factor through which osteoblasts regulate osteoclasts, and
bone formation is coupled to bone resorption
RANK and RANKL
Osteoclastogenesis
PTH and Kidney
PTH acts on the distal tubule
Calcitonin• Product of
parafollicular C cells of the thyroid
• 32 aa
• Inhibits osteoclast mediated bone resorption – This decreases serum
Ca2+
• Promotes renal excretion of Ca2+
Calcitonin
• Probably not essential for human survival
• Potential treatment for hypercalcemia
• 7 transmembrane segment receptor
• Stimulates cAMP production in bone and kidney
Vitamin D Metabolism
Transport and Metabolic Sequence of Activation of Vitamin D
Proposed Mechanism of Action of 1,25-DihydroxyD3 in Intestine
Vitamin D-dependent Ca2+ Absorption
• Duodenum>jejunun>ileum
• Absorption is greater at low pH– The pH of the stomach is about 2– Peak absorption at the beginning of the
duodenum
Vitamin D Deficiency: Rickets• Inadequate intake and absence of sunlight• The most prominent clinical effect of Vitamin D
deficiency is osteomalacia, or the defective mineralization of the bone matrix
• Osteoblasts contain the vitamin D receptor• Vitamin D deficiency in children produces rickets• A deficiency of renal 1α-hydroxylase produces
vitamin D-resistant rickets– Sex linked gene on the X chromosome
– Renal tubular defect of phosphate resorption
– Teeth may be hypoplastic and eruption may be retarded
Rickets
Vitamin D-Resistant Rickets
• Above: Hypoplastic teeth
• Below: Minimal caries can produce pulpitis; periapical abscesses are thus common
• Lack 1-hydroxylase in kidney• Rx: Respond well to 1, 25-
dihydroxyD3
PTHrP; Parathyroid Hormone related Protein
• It is synthesized as 3 isoforms as a result of alternative splicing (139, 141, 173 aa)
• Can activate the PTH receptor• Plays a physiological role in lactation, possibly as a
hormone for the mobilization and/or transfer of calcium to the milk
• May be important in fetal development• May play a role in the development of hypercalcemia of
malignancy– Some lung cancers are associated with hypercalcemia– Other cancers can be associated with hypercalcemia
PTHrP; Parathyroid Related Protein
Causes of HypocalcemiaHypoparathyroid Nonparathyroid PTH Resistance
Postoperative Vitamin D deficiency
Pseudo-hypoparathyroidism
Idiopathic Malabsorption
Post radiation Liver disease
Kidney disease
Vitamin D resistance
Sequence of Adjustments to Hypocalcemia
Pseudohypoparathyroidism• Symptoms and signs
– Hypocalcemia– Hyperphosphatemia– Characteristic physical appearance: short stature, round face, short
thick neck, obesity, shortening of the metacarpals– Autosomal dominant
• Resistance to parathyroid hormone• The patients have normal parathyroid glands, but they fail to respond
to parathyroid hormone or PTH injections• The rise in urinary cAMP after parathyroid hormone fails to occur• The cause of the disease is a 50% deficiency of Gs in all cells• Symptoms begin in children of about 8 years
– Tetany and seizures– Hypoplasia of dentin or enamel and delay or absence of eruption
occurs in 50% of people with the disorder• Rx: vitamin D and calcium
Pseudohypoparathyroidism
Elfin facies, short stature, enamel hypoplasia
Signs and Symptoms of Hypercalcemia
• Neurologic– Lethargy, drowsiness, depression, confusion– Can lead to coma and death
• Neuromuscular– Muscle weakness, hyptonia, decreased reflexes
• Cardiac– Arrhythmias
• Bone– Ache, pain, fracture
Causes of Hypercalcemia
Common Uncommon
Malignant disease, e.g. some lung cancers
Renal failure
Hyperparathyroidism Sarcoidosis
Vitamin D toxicity (excessive intake)
Multiple myeloma
Causes of Hypercalcemia• Primary hyperparathyroidism
– Most people are asymptomatic
– Classically affects skeleton, kidneys, and GI tract
• Triad of complaints: bones, stones, and abdominal groans
– Renal stones are most common single presenting complaint
– Usually due to an adenoma (tumor)
Hyperparathyroidism• The disorder is characterized by hypercalcemia,
hypercalcuria, hypophosphatemia, and hyperphosphaturia
• Parathyroid hormone causes phosphaturia and a decrease in serum phosphate
• Calcium rises and it is also secreted in the urine• Most common complication are renal stones made of
calcium phosphate– Stone chemistries: calcium, phosphate, urate
• Most serious complication is the deposition of calcium in the kidney tubules resulting in impaired renal function
Primary Hyperparathyroidism• Calcium excretion > calcium intake• Large regions of bone are replaced by connective
tissue• Two lesions: maxilla and forehead
Hyperparathyroidism
• Left: Giant Cell Granuloma
• Right: Loss of lamina dura, pathognomonic oral change in hyperparathyroidism
Lamina Dura
• Tooth sockets are bounded by a thin radiopaque layer of dense bone• Lamina dura: “hard layer”
Congenital Hypoparathyroidism
• Hypoplasia of the teeth, shortened roots, and retarded eruption
Hypercalcemia of Malignancy
• Treatment improves quality of life when Ca2+ is elevated but not yet life threatening
• Treat with bisphosphonates– Inhibits osteoclastic activity
• When serum Ca2+ > 3.00 mM treat with NaCl IV
Bisphosphonates
Osteoporosis
• Osteoporosis is characterized by a significant reduction in bone mineral density compared with age- and sex-matched norms
• There is a decrease in both bone mineral and bone matrix
• Osteoporosis is the most common metabolic bone disease
• Affects 20 million Americans and leads to 1.3 million fractures in the US per year
• Women lose 50% of their trabecular bone and 30 % of their cortical bone
• 30% of all postmenapausal women will sustain an osteoporotic fracture as will 1/6th of all men
• The cost of health care and lost productivity is $14 billion in the US annually
Normal and Osteoporotic Bone
Factors that Affect Peak Bone Mass• Gender (M>F), males have greater PBM than females• Race (Blacks >Whites)• Genetics (osteoporosis runs in families and this may be the
predominant factor)– Estrogen receptor gene
– Type I collagen gene
– Vitamin D receptor gene
• Gonadal steroids (estrogen and testosterone increase bone mass)
• Growth hormone (increases bone mass)• Calcium intake (supplements work)• Exercise (increases bone mass)
Sequelae of Osteoporosis
Osteo-porosis
Bone Density as a Function of Age
FDA Approved Rx’s for Osteoporosis• Bisphosphonates (alendronate and risedronate),
calcitonin, estrogens, parathyroid hormone and raloxifene are approved by the US Food and Drug Administration (FDA) for the prevention and/or treatment of osteoporosis
• The bisphosphonates (alendronate and risedronate), calcitonin, estrogens and raloxifene affect the bone remodeling cycle and are classified as anti-resorptive medications
• Teriparatide, a form of parathyroid hormone, is a newly approved osteoporosis medication. It is the first osteoporosis medication to increase the rate of bone formation in the bone remodeling cycle
Treatments (Continued)• Exercise, activity
• Calcium intake should be 1000-1500 mg/day
– Postmenapausal women take in less than 500 mg/day
– Males and females should take in 1000-1500 mg/day
– All adults greater than 65 years should take 1500 mg/day
– Three glasses of milk or three cups of yogurt per day provide 1000-1500 mg/day
• Estrogen treatment
– Estrogen inhibits osteoclastic activity
– Bone density increases 3-5% per year for the first three years after menopause
– This therapy needs to be individualized• Estrogen may increase the incidence of breast cancer, heart attacks, stroke, blood
clots
• That it may exacerbate cardiovascular disease is controversial
– All the data are not in yet, and estrogen treatment is under review; for more information go to http://www.fda.gov/bbs/topics/NEWS/2003/NEW00863.html
Treatments (Continued)• Raloxifene (Brand name Evista) is a selective estrogen receptor
modulator• Decreases in estrogen levels after menopause lead to increases in bone
resorption and bone loss. Bone is initially lost rapidly because the compensatory increase in bone formation is inadequate to offset resorptive losses. This imbalance between resorption and formation is related to loss of estrogen, and may also involve age-related impairment of osteoblasts or their precursors
• Raloxifene reduces resorption of bone and decreases overall bone turnover. These effects on bone are manifested as increases in bone mineral density (BMD)
• Raloxifene’s biological actions, like those of estrogen, are mediated through binding to estrogen receptors. This binding results in the modulation of expression of multiple estrogen-regulated genes in different tissues
Treatments (Continued)• Bisphosphonates inhibit osteroclasts
– Alendronate (Brand name Fosamax)
– Risedronate (Brand name Actonel)
• Calcitonin (Brand name Miacalcin )
– From salmon
– Given intranasaly
– Probably least effective Rx
• Vitamin D
– Most Americans consume less than recommended amount
– 800 IU per day seems safe and not enough to cause vitamin D toxicity
Treatments (Continued)• Parathyroid hormone (Brand name Forteo)
– Teriparatide, a form of parathyroid hormone, is approved for the treatment of osteoporosis in postmenopausal women and men who are at high risk for a fracture
– Chronically elevated PTH leads to bone loss; however, intermittent PTH (once daily bolus injection) leads to new bone synthesis
– Must be injected daily, a major disadvantage– Cost about $7000 per year
• Future Rx’s• Sodium fluoride
– Considered a possibility for years– Adoption seems unlikely
• Strontium ranelate– NEJM 350 (2004) 459-468.
Calcium Content of Foods
• http://www.nal.usda.gov/fnic/foodcomp/Data/SR16/wtrank/wt_rank.html
Selected Web Sites
Properties of Parathyroid Hormone• It is a small protein hormone• It acts on its 7 transmembrane segment receptor
– Gs protein-coupled receptors in osteoblasts increase [cAMP] and activates protein kinase A
– Parathyroid hormone acts directly on bone to stimulate resorption and release of Ca2+ into the extracellular space (slow)
– Parathyroid hormone acts directly on kidney to increase calcium reabsorption and phosphate excretion (rapid)
– Parathyroid hormone acts on distal tubule and stimulates calcium resorption
– Parathyroid hormone stimulates transcription of 1-alpha hydroxylase for vitamin D activation in kidney
• Know how 7 transmembrane segment receptors activate or inhibit adenylyl cyclase via the G-protein cycle
• Know the pathway for the formation of active vitamin D
Transport and Metabolic Sequence of Activation of Vitamin D
PTH Biosynthesis
• PTH is co-secreted with chromogranin A, a protein; significance unknown
Sequence of Adjustments to Hypocalcemia