This grant proposal focuses on the cascade of ionic events involved in acute and chronic regulation of aldosterone secretion. The role of cytosolic Ca2+ in adrenal glomerulosa cells during stimulation by angiotensin II and external K+ has been well established. Less well studied are the effects of cytosolic Na+ and pH during agonist-stimulated aldosterone secretion. Our preliminary data show that adrenal glomerulosa cells have a low basal Na+ permeability which is markedly enhanced by both agonists. Angiotensin II and external K+ activate Na+ entry and H+ efflux via Na+ -H+ exchange, although through different mechanisms. This enhanced Na+ influx results in an elevation of cytosolic Na+ and activation of Na+ efflux coupled to K+ influx via the Na+ -K+ pump. Inhibition of either the Na+ H+ exchanger or the Na+ -K+ pump blocks secretagogue-stimulated aldosterone production, indicating that cytosolic Na+ regulation is critical to steroidogenesis and that changes in cytosolic Na+ may be significant events during agonist- stimulated aldosterone secretion. These findings led us to the central hypothesis of this proposal: angiotensin II and external K+ induce changes in cytosolic Na+ mediated by activation of Na+ -H+ exchange, which are critical for signal transduction. One mechanism by which these changes in cytosolic Na+ may participate in agonist-stimulated aldosterone secretion is the regulation of the enzymatic steps in aldosterone synthesis located in the mitochondria through control of mitochondrial Ca2+. An alternative but not necessarily exclusive corollary to this hypothesis proposes that activation of Na+ -H+ exchange is necessary for regulation of cytosolic pH in response to the cellular acidifying effects of increased Ca2+ extrusion and metabolic activity during stimulation. A major factor controlling the amount of aldosterone secretion by adrenal glomerulosa is the state of Na+ balance in vivo. Steroid production by adrenal glomerulosa cells is enhanced when dietary Na+ intake is restricted and appears to be mediated, in part, by an increase in the enzymatic activity of the late pathway for aldosterone synthesis. Our preliminary findings on the effects of dietary salt show salt restrict increased the basal activity of Na+-H+ exchange and its stimulation by angiotensin II, implying that signal transduction through changes in cytosolic Na+ is also modified by dietary salt restriction. These data suggest that increased adrenal responsiveness may also result from the impact of these ionic transduction changes on the late pathway of aldosterone synthesis. To address this issue, modifications of these ionic pathways will be examined and their effect on the steps leading to aldosterone synthesis. These studies will provide a better understanding of the ionic transduction pathways involved in stimulation of adrenal glomerulosa cells and help in clarifying the mechanisms controlling aldosterone secretion in vivo.
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