7 The stimulus for release of parathyroid hormone A PRH B lo

7. The stimulus for release of parathyroid hormone A) PRH. B) low levels of calcium ions in the blood. is C) TSH, D) nerve impulses from the hypothalamus. E) calcitonin

Solution

Anwer B: The stimulus for release of parathyroid hormone is low levels of calcium ions in the blood

Explanation: Parathyroid hormone is released when there is low concentration of ionized calcium ions in serum and this follows negative feedback pathway. Parathyroid cells contain receptors that are sensed by calcium. This follows the phospholipase C pathway, the latter hydrolyzes phosphatidylinositol 4,5-bisphosphate (PIP2) to release IP3 and diacylglycerol (DAG) and the latter two result in a release of calcium from intracellular stores. Contrariwise to the principle that other secretory cells use, the higher cytoplasmic calcium levels inhibits the fusion of vesicles containing granules of preformed PTH with the membrane of the parathyroid cell, and thus inhibits release of PTH.