Abnormal calcium-regulated PTH release in normal parathyroid tissue from patients with adenoma.

The regulation of parathyroid hormone (PTH) secretion by calcium was studied in normal and abnormal parathyroid tissue from five patients with a parathyroid adenoma. Dispersed cells were prepared from the adenoma and from a portion of a normal parathyroid gland and were incubated for two hours with varying concentrations of calcium. PTH release as a function of the concentration of calcium was determined by radioimmunoassay (C-terminal). Cells from the normal glands showed a lower set-point for calcium (the concentration of calcium causing half of the maximal inhibition of PTH release) than those from the adenomas in four of five cases. Moreover, both set-point and maximal PTH release at low concentrations of calcium were significantly lower in normal glands from patients with an adenoma than in normal glands from patients with normal calcium homeostasis (0.77 +/- 0.04 [SEM] versus 0.99 +/- 0.03 mM calcium and 3.4 +/- 0.43 versus 10.1 +/- 0.78 ng/10(5) cells/hr, respectively). These observations may explain, in part, the transient hypocalcemia frequently seen in patients after removal of a parathyroid adenoma. In addition, they suggest that the set-point for calcium and maximal PTH release in normal parathyroid tissue may be altered by prior exposure to chronic hypercalcemia or other physiologic variables. Finally, the "normal" set-point that we have noted previously in parathyroid tissue from some patients with primary parathyroid hyperplasia may be inappropriately high for the hypercalcemia seen in those cases.