CALCIUM AND PHOSPHATE HOMEOSTASIS

Organs:

Parathyroid Four oval masses on posterior of thyroid gland Develops from the 3 rd and 4 th 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 D 3 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 CHIEF CELL OF PARATHYROID SERUM Ca ++ BONE OSTEOCLASTS KIDNEY DISTAL C T SERUM Ca ++ PTH Ca ++ RELEASE Ca ++ REABSORPTION PTH VIT. D

Regulation of Calcitonin THYROID PARAFOLLICULAR CELLS SERUM Ca ++ BONE OSTEOCLASTS KIDNEY DISTAL C T SERUM Ca ++ CT Ca ++ RELEASE Ca ++ REABSORPTION CT

Regulation of Vitamin D SKIN UV LIGHT LIVER KIDNEY CHOLECALCIFEROL 25-OH-D 3 PTH GH/PRL ESTROGEN 1,25-OH-D 3 SERUM PHOSPHATE TANNING

Regulation of Ca ++ SERUM Ca mg/dl SERUM Ca ++ SERUM Ca ++ BONE KIDNEY PARAFOLLICULAR CELLS CT  Ca ++ REABSORPTION  OSTEOCLAST  OSTEOBLAST PARATHYROID GLAND PTH  Ca ++ REABSORPTION  OSTEOCLAST

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