This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Endocrine System, Subject name: Physiology
[{'content': 'Which of the following exe maximum effect on parathyroid hormone secretion? \nSelect and explain. \nA. Calcitonin, B. Plasma calcium, C. Plasma phosphate, D. 1,25-dihydroxycholecalciferol', 'role': 'user'}, {'content': 'Answer: B \nExplanation: 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.', 'role': 'assistant'}]