Which of the following is false regarding PTH secretion (AIIMS NOV.2011)
|A||Non ionic calcium is most important stimulus for PTH secretion|
|B||Mg stimulates PTH secretion in the same way as Ca|
|C||Hypercalcemia in cancer patients is due to parathormone related protein.|
|D||Parathyroid senses calcium through calcium sensing receptor|
I. Circulating ionized calcium acts directly on the parathyroid glands in a negative feedback fashion to regulate the secretion of PTH. The key to this regulation is a cell membrane Ca2+ sensing receptor, CaR.
II. Activation of this G-protein coupled receptor leads to phosphoinositide turnover in many tissues. In the parathyroid, its activation inhibits PTH secretion.
III. In this way, when the plasma Ca2+ level is high, PTH secretion is inhibited and the Ca2+ is deposited in the bones. When it is low, secretion is increased and Ca2+ is mobilized from the bones.
IV. Mg2+ works in the similar fashion and when it is low, PTH secretion is increased
V. Parathyroid hormone-related protein (or PTHrP) is a protein member of the parathyroid hormone family. It is occasionally secreted by cancer cells (breast cancer, certain types of lung cancer including squamous cell carcinoma).
VI. PTHrP acts as an endocrine, autocrine, paracrine, and intracrine hormone. It regulates endochondral bone development by maintaining the endochondral growth plate at a constant width.
VII. It also regulates epithelial-mesenchymal interactions during the formation of the mammary glands.
VIII. PTHrP is related in function to the "normal" parathyroid hormone. When a tumor secretes PTHrP, this can lead to hypercalcemia. As this is sometimes the first sign of the malignancy, hypercalcemia caused by PTHrP is considered a paraneoplastic phenomenon.
IX. PTHR1 is responsible for most cases of humoral hypercalcemia of malignancy