Regulation of Parathyroid Hormone Secretion*

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Abstract
I. Introduction PARATHYROID hormone (PTH) maintains calcium homeostasis through its effects on target tissues, bone, and kidney. The parathyroid cell is unusual in that decreasing concentrations of calcium stimulate secretion of PTH, and increasing concentrations of calcium inhibit secretion of PTH. Thus, this cell, which is responsible for the critical maintenance of ionized blood calcium levels, is, in turn, regulated by calcium itself. The inverse relationship between extracellular calcium concentration and PTH secretion has been well established experimentally. However, the cellular mechanisms responsible for this relationship are poorly understood. This review will focus on several questions relating to the regulation of PTH secretion. Because of the physiological importance of the interaction between serum calcium and PTH, we will emphasize regulation of PTH by calcium. The role of other regulators will also be considered. The main questions we will address are: 1. Does intracellular or extracellular calcium regulate PTH secretion? 2. What are the mechanisms that allow extracellular calcium to influence the intracellular calcium concentration? 3. What is the mechanism for the inverse dependence of secretion on calcium concentration? 4. What is the role of cAMP and other second messengers in regulating PTH secretion?

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