AMILORIDE-SENSITIVE NA+ TRANSPORT ACROSS CULTURED RENAL (A6) EPITHELIUM - EVIDENCE FOR LARGE CURRENTS AND HIGH NA-K SELECTIVITY

Electrical techniques were used to determine the Na:K selectivity of the amiloride-sensitive pathway and to characterize cellular and paracellular properties of A6 epithelium. Under control conditions, the mean transepithelial voltage (VT) was -57±5 mV, the short-circuit current (Isc) averaged 23±2 μA/cm2 and the transepithelial resistance (RT) was 2.8±0.3 k Ωcm2 (n=13). VT and Isc were larger than reported in previous studies and were increased by aldosterone. The conductance of the amiloride-sensitive pathway (Gamil) was assessed before and after replacement of Na+ in the mucosal bath by K+, using two independent measurements: (1) the slope conductance (GT), determined from current-voltage (I-V) relationships for control and amiloride-treated tissues and (2) the maximum amiloride-sensitive conductance (Gmax) calculated from the amiloride dose-response relationship. The ratio of Gamil in mucosal Na+ solutions to Gamil for mucosal K+ solutions was 22±6 for GT measurements and 15±2 for Gmax data. Serosal ion replacements in tissues treated with mucosal nystatin indicated a potassium conductance in the basolateral membrane. Equivalent circuit analyses of nystatin and amiloride data were used to resolve the cellular (Ec) and paracellular (Rj) resistances (∼5 k Ωcm2 and 8-9 k Ωcm2, respectively). Analysis I-V relationships for tissues depolarized with serosal K+ solutions revealed that the amiloride-sensitive pathway could be described as a Na+ conductance with a permeability coefficient (PNa)=1.5±0.2× 10-6 cm/s and the intracellular Na+ concentration (Nai)=5±1 mM (n=5), similar to values from other tight epithelia. We conclude that A6 epithelia are capable of expressing large amiloride-sensitive currents which are highly Na+ selective. © 1990 Springer-Verlag.