CRITICAL CELLULAR EVENTS IN ALDOSTERONE SYNTHESIS

Aldosterone secretion from the adrenal glomerulosa cell is regulated by a number of secretagogues. Most of these agents probably mediate their effects, at least in part, via the calcium messenger system. In the case of AII, it is thought that upon hormone binding there is an activation of a membrane phosphodiesterase which breaks down phosphatidylinositol 4,5 bisphosphate to inositol 1,4,5 trisphosphate and diacylglycerol. The former causes intracellular calcium release, thus activating a calmodulin-dependent protein kinase, while the latter is thought to activate membrane bound, protein kinase C. Key events take place at three levels: 1) hormone-receptor interaction; 2) membrane-associated events, and 3) intracellular events. The goal of this study is to more fully characterize the events occurring at each of these levels and integrate them into a comprehensive model of hormone action. The studies on hormone- receptor interaction will first focus on whether there is a guanine nucleotide binding protein linking the receptor to the phosphodiesterase and, if so, we will attempt to isolate and characterize this protein. Next, the cellular substrates and temporal patterns of phsophatidylinositol turnover will be determined with radioactive labeling and chromatographic techniques. What role these products may have as second messengers will also be evaluated. The studies on membrane- associated events will focus on determining if there is a Na+/H+ exchanger in glomerulosa cells and if there is, what role it plays in signal transduction. The role of the membrane associated C- kinase will be evaluated by using immunocytochemical localization techniques. The studies on intracellular events will focus on what role protein kinase (CaM/PK II, C-kinase, M-kinase) may play in signal propagation. Using 32P or 35S labeling, the existence of a "labile" protein will be documented, and a determination will be made as to whether or not it is the link between the changes in protein kinase activity and actual synthesis and secretion of aldosterone. The importance of these studies is two-fold. First, aldosterone is involved in salt and water homeostasis and thus plays a role in multiple pathological states. By understanding the mechanisms of regulation of aldosterone secretion, we may be able to favorably alter their course. Second, the hormone-receptor interactions being studied take place not only in the adrenal glomerulosa cell but in a variety of cells throughout the body. Thus, principles derived from these studies may also apply to the hormonal regulation of cellular responses in these other tissues.