Set-point For Secretion Research Articles

Alterations in parathyroid glandular sensitivity to calcium may contribute to the hypersecretion of PTH in hyperparathyroidism. Since the cytosolic calcium concentration may mediate the effects of extracellular calcium on PTH release, we have employed the calcium-sensitive intracellular dye QUIN-2 to examine the relationship between extracellular calcium, cytosolic calcium, and PTH secretion in adult, neonatal, and cultured bovine as well as pathological human parathyroid cells. PTH release was measured using C- and N-terminal radioimmunoassays. Neonatal bovine parathyroid cells showed a greater set-point for secretion (the Ca++ concentration causing half of the maximal inhibition of PTH release) than adult cells (1.27 +/- 0.11 vs. 1.06 +/- 0.11 mM extracellular calcium, P less than 0.01), and a slightly higher extracellular calcium was necessary to raise the cytosolic calcium concentration to a given level in neonatal than in adult bovine parathyroid cells. In individual neonatal and adult cell preparations, there was a close correlation between the set-point for secretion and the "set-point" for cytosolic calcium (r = 0.832, P less than 0.001). In cells from five human parathyroid adenomas, which had an increase in set-point for secretion, the extracellular calcium concentration necessary to raise the cytosolic calcium concentration to a given level was slightly greater than in the neonatal cells. In four preparations of human parathyroid cells there was a significant correlation between the set-points for secretion and cytosolic calcium (r = 0.856, P less than 0.01). Because neonatal bovine and pathological human parathyroid glands show cellular hyperplasia, we studied the temporal relationship between cellular proliferation and the regulation of PTH release and cytosolic calcium concentration in cultured bovine parathyroid cells. Cellular proliferation, estimated by 3H-thymidine incorporation, increased significantly in culture from 104 +/- 10.1 counts/well on day 1 (first 24 h in culture) to 588 +/- 188 and 6,156 +/- 649 counts/well on days 2 and 4, respectively. In cultured cells on day 1, highly Ca++ (2-3 mM) inhibited maximal PTH release by 58.8 +/- 3.2%, which decreased significantly (P less than 0.001) to 38.2 +/- 1.9 and 17.1 +/- 3.7% on days 2 and 4, respectively. The cytosolic calcium observed at 3 mM calcium on day 1 was 701 +/- 43 nM, which declined to 466 +/- 60 and 314 +/- 14 nM on days 2 and 4 (P less than 0.05). There was a close correlation between this progressive decrease in maximal inhibition of PTH release and the cytosolic calcium at high extracellular calcium in cultured cells (r = 0.99, P < 0.001). Thus, during active proliferation of cultured cells, there is an alteration in the regulation of cytosolic calcium at a given extracellular calcium concentration, and changes in the regulation of PTH release and cytosolic calcium by extracellular calcium may be related to enhanced cellular proliferation.