Regulation of parathyroid hormone secretion.

Calcium is the most important physiological regulator of PTH secretion. Peak PTH secretion occurs at an intracellular calcium concentration of about 200 nM, regardless of the extracellular calcium concentration. We suggest, therefore, that intracellular calcium concentration is a regulator of PTH secretion that maintains calcium homeostasis. Other factors may be responsible for modulation of the intracellular calcium concentration, ultimately modulating PTH secretion. The "paradoxical" nature of the dependence of PTH secretion on the calcium concentration may be explained by considering PTH secretion to be unusual in a quantitative, rather than a qualitative, fashion. A possible mechanism for the control of PTH secretion by intracellular calcium, which involves calcium-activated potassium channels, is proposed. The parathyroid cell plasma membrane contains several sensors or channels by means of which the cell senses extracellular calcium. It is not clear whether these entities are coupled to each other or whether they function independently. Guanine nucleotide regulatory proteins are transducers of extracellular signals, including calcium. Several other second messengers that influence PTH secretion have also been described, but possible interactions between these messengers have not yet been determined.