Role of calcium in the regulation of adenohypophysial hormone release.

It is now accepted by most investigators that the initial action of most peptide hormones involves an interaction with a specific receptor on (or in) the plasma membrane of the target cell. A cascade of intracellular events results and culminates in the physiological response characteristic of the interaction of the particular hormone with its target cell. The regulation of hormone release from the adenohypophysis by the hypothalamic releasing hormones is presumed to occur via a similar process. The nature of the interaction at the cell surface as well as the details and sequence of the subsequent intracellular events are largely unknown. We do know, however, that two of the key factors regulating the intracellular secretory machinery in most cells are 1) the adenylate cyclase — cyclic AMP — protein kinase system and 2) the divalent cation, calcium. Since there have been several recent reviews (1–3) which have covered the role of the cyclic nucleotides in pituitary hormone secretion, this discussion will be restricted to a consideration of the regulatory role played by calcium.As was the case with tissues, the early work regarding calcium and the adenohypophysis followed the pattern of determining the ability of secretagogues to release pituitary hormones subsequent to various manipulations designed to remove what was often implicity considered to be extracellular calcium.