pHi-dependent membrane conductance of proximal tubule cells in culture (OK) : differential effects on K+ -and Na+-conductive channels

Confluent monolayers of the established opossum kidney cell line were exposed to NH4Cl pulses (20 mmol/liter) during continuous intracellular measurements of pH, membrane potential (PDm) and membrane resistance (R′m) in bicarbonate-free Ringer. The removal of extracellular NH4Cl leads to an intracellular acidification from a control value of 7.33±0.08 to 6.47±0.03 (n=7). This inhibits the absolute K conductance (gK+), reflected by a decrease of K+ transference number from 71±3% (n=28) to 26±6% (n=5), a 2.6±0.2-fold rise ofR′m, and a depolarization by 24.2±1.5 mV (n=52). In contrast, intracellular acidification during a block ofgK+ by 3 mmol/liter BaCl2 enhances the total membrane conductance, being shown byR′m decrease to 68±7% of control and cell membrane depolarization by 9.8±2.8 mV (n=17). Conversely, intracellular alkalinization under barium elevatesR′m and hyperpolarizes PDm. The replacement of extracellular sodium by choline in the presence of BaCl2 significantly hyperpolarizes PDm and increasesR′m, indicating the presence of a sodium conductance. This conductance is not inhibited by 10−4 mol/liter amiloride (n=7). Patch-clamp studies at the apical membrane (excised inside-out configuration) revealed two Na+-conductive channels with 18.8±1.4 pS (n=10) and 146 pS single-channel conductance. Both channels are inwardly rectifying and highly selective towards Cl−. The low-conductive channel is 4.8 times more permeable for Na+ than for K+. Its open probability rises at depolarizing potentials and is dependent on the pH of the membrane inside (higher at pH 6.5 than at pH 7.8).