ELECTROPHYSIOLOGY OF ADRENAL CORTICAL ENDOCRINE TISSUES

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There are three major physiological aldosterone secretagogues which act on cells of the zona glomerulosa of the adrenal cortex: ACTH, angiotensin II (AII), and potassium. Extracellular calcium ion is a requirement for activation by each of these agents, however, recent studies suggest that the locus of its involvement differs for each case. Exposure of isolated glomerulosa cells to increased concentrations of all three of these secretagogues is associated with membrane depolarization, but our preliminary data demonstrate that the ionic mechanism by which this occurs in each case is different. We propose studies to examine the ionic mechanisms and kinetics of the electrophysiological correlates of secretion resulting from treatment of glomerulosa cells by these agents. Experiments will be performed on enzymatically isolated cells from adrenal glands of rats. These studies will employ classical electrophysiological methods as well as the recently developed techniques of single-electrode voltage clamp and patch clamp. The following experimental questions will be addressed: (1) We will examine the basal electrophysiological properties of glomerulosa cells. Specifically, the resting membrane permeabilities to Na+, K+, C1- will be examined as well as the voltage dependency of K+ and Ca++ conductances. (2) We will investigate the electrophysiological events underlying modulation of hormonal secretion by K+. (3) We will investigate the electrophysiological events accompanying modulation of secretion by the secretagogues AII and ACTH as well as those underlying the mechanisms of their antagonists (i.e. dopamine, atrial natriuretic factor, etc.) in these cells. In normal man and normotensive rats, sodium restriction and/or dietary potassium loading both increase the responsiveness of the glomerulosa cell to its secretagogues. The mechanisms underlying this change in sensitivity with changes in dietary intake are unknown. We will perform experiments to see if electrophysiological correlates of these sensitivity changes can be demonstrated under these conditions. Nearly 50% of patients with essential hypertension appear to lack this sodium mediated change in responsiveness to AII. A genetically hypertensive strain of rats (SHR) has a similar abnormality. We will perform comparative studies on normotensive (Sprague Dawley) and spontaneously hypertensive (SHR) rats designed to see if changes in dietary sodium or potassium intake modify the basic electrophysiological properties of the glomerulosa cell.