Question

Hello, I was studying calcium and phosphate regulation on two textbooks, but they are in contradiction. Which is the right version? (I wrote in capitals the contradicting words)

Book 1: The conversion of cholecalciferol into calcitriol is regulated indirectly by an INCREASE in plasma Ca2+ concentration, which triggers secretion of PTH, or directly by a DECREASE in circulating phosphates, which stimulates activity of the enzyme responsible for conversion of cholecalciferol into calcitriol.

Book 2: When extracellular fluid calcium concentration falls below normal, the parathyroid glands are directly stimulated by the LOW calcium levels to promote increased secretion of PTH. Moreover, INCREASED plasma concentration of phosphate stimulates PTH, which increases calcium reabsorption by renal tubules.

Answer 1

The parathyroid glands (4 in number) secrete parathyroid hormone (PTH) or collip`s hormone, which increases the level of calcium in the blood. Bone tissue acts as a storage reservoir for calcium and PTH stimulates the removal of calcium from the bone to increase levels in the blood; therefore it stimulates osteoclastic activity. It increases the reabsorption of calcium by the renal tubules of the kidney so that less is lost in urine but at the same time it stimulates the loss of phosphate in the urine. PTH also stimulates kidney to secrete calcitriol which, in turn, increases calcium absorption from the digested food in the gut.

Based on the above discussion, book 2 is the right version.