HORMONAL-REGULATION OF ENACS - INSULIN AND ALDOSTERONE

Although a variety of hormones and other agents modulate renal Na+ tra nsport acting by way of the epithelial Na+ channel (ENaC), the models) , pathways, and their interrelationships in regulation of the channel remain largely unknown. It is likely that several hormones may be pres ent concurrently in vivo, and it is, therefore, important to understan d potential interactions among the various regulatory factors as they interact with the Na+ transport pathway to effect modulation of Na+ re absorption in distal tubules and other native tissues. This study repr esents specifically a determination of the interaction between two hor mones, namely, aldosterone and insulin, which stimulate Na+ transport by entirely different mechanisms. We have used a noninvasive pulse pro tocol of blocker-induced noise analysis to determine changes in single -channel current (i(Na)), channel open probability (P-o), and function al channel density (N-T) Of amiloride-sensitive ENaCs at various time points following treatment with insulin for 3 h of unstimulated contro l and aldosterone-pretreated A6 epithelia. Independent of threefold di fferences of baseline values of transport caused by aldosterone, 20 nM insulin increased by threefold and within 10-30 min the density of th e pool of apical membrane ENaCs (N-T) involved in transport. The very early (10 min) increases of channel density were accompanied by relati vely small decreases of i(Na) (10-20%) and decreases of P-o (28%) in t he aldosterone-pretreated tissues but not the control unstimulated tis sues. The early changes of i(Na), P-o, and N-T were transient, returni ng very slowly over 3 h toward their respective control values at the time of addition of insulin. We conclude that aldosterone and insulin act independently to stimulate apical Na+ entry into the cells of A6 e pithelia by increase of channel density.