Isoproterenol- and insulin-induced hyperpolarization in rat skeletal muscle

Using conventional microelectrode techniques, we investigated the combined effects of isoproterenol (Iso) and insulin (Ins) on the resting membrane potential (RMP) of isolated rat skeletal muscles. In soleus muscle, Iso (1 microM) and Ins (4 units/L) separately induced a hyperpolarization of 9.2 mV and 4.8 mV, respectively. Combined administration of Iso and Ins induced a hyperpolarization of 12.4 mV, larger than either one separately. A similar observation was made in Na(+)-loaded rewarming experiments. 8-Br-cAMP (1 mM and 3 mM) and forskolin (10 microM, an adenylate cyclase activator) induced a hyperpolarization of 5.3 mV, 8 mV, and 6.0 mV, respectively. This hyperpolarizing action was blocked by ouabain, indicating that the Na-K pump was involved in the hyperpolarization. 8-Br-cGMP (3 mM) had no effect on RMP; however, it blocked or reversed the hyperpolarization caused by 8-Br-cAMP (1 mM). In addition, 8-Br-cGMP partially inhibited the hyperpolarizing effect of Iso (1 microM) by 40% and completely prevented the effect of Ins. The phorbol ester, PMA, (1 microM, a PKC activator) induced a ouabain-inhibitable hyperpolarization. These results suggest that cAMP and PKC are involved in the Iso- and Ins-induced hyperpolarization and that Iso and Ins influence the RMP presumably through regulation of the electrogenic Na-K pump via different mechanisms.