Apical and basolateral conductance in cultured A6 cells

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 S/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 A/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.