Stimulation of Na + ‐alanine cotransport activates a voltage‐dependent conductance in single proximal tubule cells isolated from frog kidney

1 The swelling induced by Na + ‐alanine cotransport in proximal tubule cells of the frog kidney is followed by regulatory volume decrease (RVD). This RVD is inhibited by gadolinium (Gd 3+ ), an inhibitor of stretch‐activated channels, but is independent of extracellular Ca 2+ . 2 In this study, the whole cell patch clamp technique was utilized to examine the effect of Na + ‐alanine cotransport on two previously identified volume‐ and Gd 3+ ‐sensitive conductances. One conductance is voltage dependent and anion selective ( G VD ) whilst the other is voltage independent and cation selective ( G VI ). 3 Addition of 5 mM L‐alanine to the bathing solution increased the whole cell conductance and gave a positive (depolarizing) shift in the reversal potential ( V rev , equivalent to the membrane potential in current‐clamped cells) consistent with activation of Na + ‐alanine cotransport. V rev shifted from ‐36 ± 4·9 to +12·9 ± 4·2 mV ( n = 15 ). 4 In the presence of alanine, the total whole cell conductance had several components including the cotransporter conductance and G VD and G VI . These conductances were separated using Gd 3+ , which inhibits both G VD and G VI , and the time dependency of G VD . Of these two volume‐sensitive conductances, L‐alanine elicited a specific increase in G VD , whereas G VI was unaffected. 5 The L‐alanine‐induced activation of G VD was significantly reduced when cells were incubated in a hypertonic bathing solution. 6 In summary, in single proximal tubule cells isolated from frog kidney, on stimulation of Na + ‐alanine cotransport G VD is activated, while G VI is unaffected. Taken with other evidence, this suggests that G VD is activated by cell swelling, consequent upon alanine entry, and may play a role as an anion efflux pathway during alanine‐induced volume regulation.