The parathormone (PTH) exerts its rapid calcium resorptive action on by acting on on PTH receptors present on?

Ans. B. Osteocytes. (Ref. Guyton 11th/986; Ganong 23rd/ pg. 390-392) Guyton Physiology 11th/pg. 986............. "PIH has two effects on bone in causing absorption of calcium and phosphate. One is a rapid phase that begins in minutes and increases progressively for several hours.. This phase results from activation of the already existing bone cells (mainly the osteocytes) to promote calcium and phosphate absorption. The second phase is a much slower one, requiring several days or even weeks to become fully developed; it results from the proliferation of the osteoclasts, followed by greatly increased osteoclastic reabsorption of the bone itself, not merely absorption of the calcium phosphate salts from the bone." Parathyroid hormone (PTH) # PTH, secreted by the chief cells of the parathyroid gland, is essential for life. # PTH has a direct effect on the bone to increase bone resorption and mobilize Ca2+; this effect is mediated by increasing intracellular cAMP levels in osteoblasts. # PTH also increases calcium absorption from the gut, although that effect is the result of PTH-mediated increases in renal 1,2,5-dihydroxy-cholecalciferol. PTH has a phosphaturic action due to a decrease in phosphate reabsorption in the proximal tubules. # Thus, The main function of the parathyroid gland is to maintain a constant ionized calcium level in the extracellular fluid. To do this, PTH stimulates increased plasma calcium levels, chiefly by mobilizing calcium from bones, although PTH can also increase renal tubular reabsorption of calcium and intestinal absorption of calcium. # PTH increases Ca2+ reabsorption from the thick ascending limb and the distal convoluted tubule. # Although most of the filtered Ca2+ is reabsorbed in the proximal tubule, the regulation of Ca2+ excretion occurs in the thick ascending limb and the distal convoluted tubule. PTH regulates the reabsorption of HP042? in the proximal tubule. # PTH is released from the parathyroid gland in response to lowered plasma Ca2+ concentrations. In addition to inhibiting the reabsorption of phosphate from the proximal tubule, PTH increases the reabsorption of Ca2+ from the loop of Henle. # The secretion of PTH is inversely related to the circulating levels of ionized calcium.