9. Current thinking is that in addition to promoting the breakdown of bone, para
ID: 3475344 • Letter: 9
Question
9. Current thinking is that in addition to promoting the breakdown of bone, parathyroid hormone also stimulates calcium absorption by stimulating vitamin D production. Researchers hypothesize that parathyroid hormone activates a receptor on certain kidney cells that in turn activates the adenylate cyclase pathway in those cells. a.) Back in the day, Joe Schmoe Biomedical researcher thought that parathyroid hormone promoted vitamin D synthesis by activating a type of tyrosine kinase receptor-enzyme. What simple experiment and result might have convinced Joe that the tyrosine kinase receptor-enzyme is NOT involved in vitamin D synthesis. b.) What is one simpe experiment and corresponding result that would support the hypothesis that parathyroid hormone stimulates vitamin D synthesis via activation of the cyclic AMP pathway.
Explanation / Answer
Active form of vitamin D formation : Cholecalciferol formed in the skin as a result of irradiation of 7-dehydrocholesterol, a substance normally present in the skin, by ultraviolet rays from sun. Cholecalciferol enter circulation and is converted to 25-hydroxycholecalciferol in the liver . 25-hydroxycholecalciferol is converted to 1,25-dihydroxycholecalciferol (active form of vitamin-D) in the kidney tubules due to action of Parathyroid hormone on the renal tubules. This formation of active form of vitamin D increases renal tubular reabsorption of calcium and so decreasing Calcium excretion by the kidneys.
The simple experiment to suggest that Vitamin D formation in renal tubular cells by Parathyroid hormone action is not due to tyrosine kinase receptor enzyme because : Parathyroid hormone in vivo produces a prompt increase in urinary excretion of cyclic 3,5 AMP. The effect of hormone on urinary cyclic AMP can be regarded as the physiologic consequence of direct activation of adenylate cyclase in the kidney by Parathyroid hormone.
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