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CALCIUM AND PHOSPHATE HOMEOSTASIS Organs: Parathyroid • Four oval masses on posterior of thyroid gland • Develops from the 3rd and 4th pharyngeal pouches • Cell Types – Chief – Oxyphil Parathyroid hormone (PTH) • Protein/peptide • Metabolized in liver and kidney • Receptor: G protein associated receptor Thyroid (Parafollicular cells) • Cells located outside the thyroid follicles • Derived embryonically from the ultimobranchial body • Make up about 0.1% of the mass of the thyroid • Calcitonin – 32-amino acid peptide Vitamin D • Sterol hormone • Production requires several organs • Production: – 7-dehydrocholesterol – Cholecalciferol – 25 Hydroxy Cholecalciferol – 1,25-Dihydroxy Cholecalciferol • No dietary source of Vitamin D3 • Binds to an intracellular receptor Actions of the Hormones Parathyroid hormone (PTH) • Increases serum Ca++ concentration • Kidney – Ca++ reabsorption – phosphate reabsorption • Bone • Intestine Calcitonin • • • • • Decreases serum Ca++ concentration Bone Kidney Question of its physiological role Clinical usefulness Dihydroxy-Vitamin D • Increases intestinal absorption of Ca++ • Stimulate bone matrix protein production by osteoblasts • Activate osteoclasts • Increase phosphate reabsorption by kidney • Maintain Ca++ reabsorption by the kidney • Inhibits PTH production & secretion Regulation of PTH SERUM Ca++ SERUM Ca++ Ca++ REABSORPTION CHIEF CELL OF PARATHYROID PTH KIDNEY VIT. D PTH BONE DISTAL CT Ca++ RELEASE OSTEOCLASTS Regulation of Calcitonin SERUM Ca++ SERUM Ca++ Ca++ REABSORPTION THYROID PARAFOLLICULAR CELLS CT KIDNEY CT BONE DISTAL CT Ca++ RELEASE OSTEOCLASTS Regulation of Vitamin D UV LIGHT TANNING SERUM PHOSPHATE SKIN CHOLECALCIFEROL 1,25-OH-D3 LIVER ESTROGEN 25-OH-D3 GH/PRL KIDNEY PTH Regulation of Ca++ REABSORPTION SERUM Ca++ OSTEOCLAST KIDNEY BONE PTH PTH PARATHYROID GLAND SERUM Ca++ 4.64-5.28 mg/dl ++ Ca PARAFOLLICULAR CELLS CT BONE CT KIDNEY OSTEOCLAST OSTEOBLAST SERUM Ca++ Ca++ REABSORPTION Other Hormones ++ Affecting Ca • Estrogen – Inhibits Osteoclasts – Stimulates Calcitonin – Stimulates Vitamin D • GH/IGF-1 • Glucocorticoids • Thyroid Hormone Pathologies • Hypercalcemia – Hyperparathyroidism • Hypocalcemia – Hypoparathyroidism • Osteoporosis – Role of estrogen • Paget’s Disease • Osteomalacia • Rickets
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