<prompt> Which of the following exe maximum effect on parathyroid hormone secretion?CalcitoninPlasma calciumPlasma phosphate1,25-dihydroxycholecalciferol  <response>: The primary regulator of PTH secretion is ionized plasma Ca2. The PTH-secreting cells in the parathyroid gland have a sensor in their membranes - Ca2-sensing receptor CaSR. CaSR is a member of the GPCR superfamily; it spans the membrane 7 times. CaSR is also stimulated by high levels of magnesium; high magnesium inhibits PTH secretion. PTH would then exe its actions on bone, kidneys, and intestine to increase the Ca2 level. Calcitonin has a receptor on osteoclasts; it decreases the bone resorption by these cells and thereby causes Ca2 deposition on bone. The effect is opposite to that of PTH; it does not influence PTH levels directly. 