Apical and basolateral conductance in cultured A6 cells

Abstract

Confluent monolayers of the cultured renal distal tubule cell line (A6) were impaled with microelectrodes under short-circuit conditions. Specific membrane conductances were calculated from equivalent circuit equations. Transport properties of the apical and basolateral membranes were investigated during control conditions and short-term increases in basolateral potassium concentration [K⁺] from 2.5 to 20 mmol/l, with or without 0.5 mmol/l Ba²⁺ at the basolateral side. As in most other epithelia, the apical membrane represents the major resistive barrier. Transcellular, apical and basolateral membrane conductances (g _c, g _o and g _i respectively), obtained from 22 acceptable microelectrode studies, averaged 61, 80 and 292 μS/cm², respectively. There was a highly significant correlation between short-circuit current (I _sc) and g _o, whereas g _i was unrelated to I _sc. The I _sc, which averaged 4.1 μA/cm², was almost completely blocked by amiloride. This was associated with fast hyperpolarization; the intracellular potential (V _sc) increased from −69 to −83 mV and the fractional apical resistance rose to nearly 100%. Using the values of V _sc during amiloride at normal and high [K⁺], an apparent transference number for K⁺ at the basolateral membrane of 0.72 can be calculated. This value corresponds with the decrease in g _i to about 25% of the control values after blocking the K⁺ channels with Ba²⁺. The nature of the remaining conductance is presently unclear. The cellular current decreased during high [K⁺] and Ba²⁺, in part resulting from reduction of the electrochemical gradient for apical Na⁺ uptake due to the depolarization. In addition, g _o decreased to less than 40%, which is considerably lower than predicted by the constant-field equation; this might indicate voltage sensitivity of the apical Na⁺ permeability.