Effect of insulin on intracellular pH in frog skeletal muscle fibers

The effect of insulin on intracellular pH (pHi) and membrane potential was studied in frog semitendinosus muscle fibers, using recessed-tip pH-sensitive glass microelectrodes and conventional 3 M KCl-filled microelectrodes. After a lag period of approximately 20 min, insulin [1 mU/ml, 0.1% bovine serum albumin (BSA)] produced a slow hyperpolarization of 2-5 mV and an alkalinization of 0.05-0.10 pH unit, which were both completed within 1 h and were not reversed by washing in insulin-free solution for 1 h. The effect of insulin on the pHi recovery rate from CO2-induced acidification was examined at various membrane voltages. At normal membrane voltage, insulin (400 mU/ml, no BSA) slightly increased the slow pHi recovery (from 0.01 to 0.04 delta pH/h). In fibers depolarized in 15 mM K to about -50 mV, insulin nearly tripled the recovery rate (from 0.05 to 0.13 delta pH/h). This insulin-induced recovery was abolished by 1 mM amiloride, a Na-H exchange inhibitor. The increased pHi recovery in 15 mM K thus represents an increased Na-H exchange, which may be due to an interaction between insulin and either membrane depolarization, per se, or increased intracellular Ca. In fibers depolarized in 50 mM K to about -25 mV, insulin did not affect recovery (0.28 delta pH/h). This lack of insulin effect might be due to fiber swelling or to the difference in the time course of elevation of intracellular Ca at -25 and -50 mV. These results are consistent with an alkalinizing effect of insulin in frog muscle mediated by Na-H exchange.