Characterization of apical and basolateral membrane conductances of rat inner medullary collecting duct

Initial segments of the inner medullary collecting duct of the rat were perfused in vitro, and the electrophysiological properties of the apical and basolateral membranes were examined with KCl-filled microelectrodes. The fractional resistance of the apical membrane (FRa = Ra/Ra + Rbl) and the transepithelial resistance (RT) were estimated by cable analysis. In control tubules the transepithelial voltage (VT) averaged -2.2 mV, and the voltage across the basolateral membrane (Vbl) averaged -51.1 mV. RT was 11.9 k omega.cm (72.8 omega.cm2), and FRa was 0.94. Pretreatment of the rats with deoxycorticosterone (DOC)-pivalate for 7-10 days did not alter these electrophysiological properties. In control tubules, amiloride in the lumen (10(-5) M) changed VT from -3.0 to +1.4 mV and increased Vbl from -49.4 to -53.8 mV, RT from 12.5 to 13.6 k omega.cm, and FRa from 0.92 to 0.98. Thus the apical membrane is conductive to Na+. An increase of the bath K+ concentration from 4 to 15 mM caused an 18.8 mV depolarization of Vbl: barium in the bath also depolarized Vbl. A fivefold decrease in the [HCO3-] in the bath depolarized Vbl by 13.1 mV. 4,4'-Diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) blocked this depolarization. Thus the basolateral membrane is conductive to K+ and HCO3-. Experiments with ouabain revealed a Na+-K+-ATPase in the basolateral membrane. Taken together, the results support a model in which electrogenic Na+ absorption is driven by the Na+-K+-ATPase in the basolateral membrane, with passive movement of Na+ occurring through an amiloride-sensitive conductive pathway in the apical membrane.