Endocrinology, lecture on Calcium and Phosphate Balance
USD Department of Biology
Endocrinology
Summers
Hormones
Endocrine Glands
Receptors
Homeostasis
2nd Messengers
Nuclear Receptors
Genetic Regulation
Hormone Synthesis
Receptor Regulation
Hypothalamo-Hypophysial Communication
Tropic Hormones
Neurohypophysial Nonapeptides
Thyroid Axis
Steroids
Adrenal Axis
Adrenal Medulla
Osmotic-Pressure Balance
Reproductive Endocrinology
Prolactin
Somatic Axis
Growth Factors
Immune Factors
Ca++, PO4 Homeostasis
Pancreatic Hormones
GI Hormones
Guts 'n Brains
Brain Hormones
Neurosteroids
Neuropeptides
Endocrine Evolution
Figures for Endocrionology
text:Vertebrate Endocrinology5th Edition - David O. Norris:
Read pages 501-527 for this lecture
acronyms    end
XXV. Calcium and Phosphate Homeostasis 			video


	A. Hormones involved are: Calcitonin, Parathormone, & Vitamin D


		1. organs involved in regulation of homeostasis are:
		   bone, kidney, & gastrointestinal tract


			a. but Ca++ participates in the function of most,
			   if not all, cells


				i. myocontraction, nerve transmission, coagulation of blood
				   or milk, activation or inhibition of enzymes, hormone
				   function, exocytosis, cell-cell interaction, cell
				   duplication, and 2nd messenger function


	B. Parathormone or Parathyroid hormone (PTH)


		1. 84 aa (1-34 necessary for full potency; preproPTH = 115 aa)


			a. homologous PTHrP (PTH-related protein) found in
			   mammary, bladder & uterus


		2. secreted by the chief cells of the parathyroid glands


			a. parathyroids embedded within the thyroid glands


				i. oxyphil cells have no known function (appear after puberty)


			b. low plasma Ca++ levels stimulates synthesis
			    and release of PTH via exocytosis


				i. ususally high [Ca++] stimulates exocytosis
				   in endocrine or exocrine glands


		3. PTH elevates plasma Ca++ levels


			a. dissolving bone salts


				i. activates osteoclasts in bone


					(1) induces lysosomal enzymes


			b. preventing renal excretion


				i. stimulates Ca++ reabsorption


				ii. promotes PO4 and bicarbonate excretion


		4. 2nd messengers are cAMP and IP3 /PLC/ PKC					


			a. may act directly on osteoblasts (where receptors are found)
			   and indirectly activate osteoclasts


	C. Calcitonin (Ct)


		1. 32 aa peptide (136 aa prepro) made in parafollicular or C cells
		   in the thyroid gland


			a. between or partially embeded in thyroid follicles


			b. brain makes CGRP (calcitonin gene related peptide)
			   from the same precursor


			c. high [Ca++] stimulates synthesis and secretion


		2. Ct is a PTH antagonist


			a. both work on the osteoclast


				i. calcitonin directly; receptors on osteoclasts


				ii. inhibits bone resorption,  \ protects bone mass


			b. Ct promotes PO4 transport into cells; \ ¯ plasma levels


		3. Ct / CT-R also works through cAMP and IP3/ PLC/ PKC


			a. does not directly inhibit PTH action


			b. results in a decrease in intracellular Ca++


	D. Vitamin D (1,25-DHCC = 1,25-dihydroxycholecalciferol or cholecalciferol) and calcitriol


		1. made from 7-dehydrocholesterol (or ergosterol from plants)


			a. some must be absorbed from diet


			b. UV aromatizes at skin, hydroxylated in the liver 


				i. Vitamin D is the prohormone


			c. calcitriol is the active steroid


				ii. hydroxylated from Vitamin D at kidney


		2. calcitriol makes Ca++ absorption
		    in the digestive tract possible


			a. induces a Ca++-binding peptide (CaBP)


				i. a member of the calmodulin family


				ii. PTH may also stimulate CaBP


			b. diet is the main source of Ca++


				i. PTH may stimulate calcitriol in intestines


			c. promotes absorption of  PO4 also


		3. mimics and synergizes with PTH in bone


		4. stimulates Ca++ reabsorption from the
		   distal convoluted tubule of the kidney

	
	E. Other hormones


		1. thyroid hormones   plasma Ca++, accelerate
		   replenishment of cellular Ca++


			a. but inhibit intestinal absorption of Ca++


		2. glucocorticoids reduce osteoblastic function and slow growth


			a. may lead to osteoporosis


			b. reduce  intestinal absorption of Ca++


		3. estrogens reduce the response of bone tissue to PTH


			a. directly stimulate osteoblasts


			b. potent inhibitor of osteoporosis


		4. PRL, GH, & estrogens stimulate the production of calcitriol

XXVI. Pancreatic Hormones