EFFECTS OF ALDOSTERONE ON THE IMPEDANCE PROPERTIES OF CULTURED RENAL AMPHIBIAN EPITHELIA

The cultured renal amphibian cell line A6 has proven advantageous for studies of Na+ transport regulation. In the present study, the effects of aldosterone action on the transepithelial electrical properties of this epithelium were assessed. Specifically, the time course of aldos terone action was determined and the effects of chronic (10-18 day) al dosterone elevation were assessed using transepithelial equivalent cir cuit methods and impedance analysis techniques. Short-term (< 4 hr) ex posure to aldosterone (0.1 muM) stimulated the amiloride-sensitive sho rt-circuit current (I(sc)) by over twofold and increased the transepit helial conductance (G(T)) by approximately 12%. The increases in I(sc) and G(T) were maintained in epithelia subjected to chronic aldosteron e exposure. In contrast to previous reports, paracellular resistance ( R(j)) was not altered by aldosterone. This difference may be related t o the longer time of exposure or different basal Na+ transport rates i n the present study. The apical membrane conductance was significantly increased for aldosterone-treated epithelia compared to aldosterone-d epleted (i.e., serum-deprived) controls. Apical membrane area (capacit ance) was not significantly affected. This finding is consistent with a higher density (number of channels per membrane area) of conducting Na+ channels in this membrane following aldosterone stimulation. Basol ateral membrane properties were not significantly altered for aldoster one-treated tissues compared to serum-treated control tissues. In cont rast, basolateral membrane-specific conductance (i.e., basolateral mem brane conductance normalized to basolateral membrane capacitance) was significantly lower for serum-deprived epithelia than for serum-treate d controls or aldosterone-treated tissues. The effects of chronic aldo sterone exposure were also evaluated for the A6 subclonal cell line, 2 F3. Similar to A6 epithelia, I(sc) was essentially doubled following a ldosterone stimulation while R(j) and cellular driving force (E(c)) we re not affected. Apical membrane conductances under control conditions for 2F3 epithelia were higher than those for A6, but were not signifi cantly different from A6 following aldosterone exposure or serum depri vation. These findings suggest possible differences in the regulation of apical membrane Na+ channels for 2F3 and A6 epithelia.