Mechanism of insulin-induced activation of Na⁺-K⁺-ATPase in isolated rat soleus muscle

Insulin augments Na⁺-K⁺-ATPase activity in skeletal muscles. It has been proposed that the sequence of events is activation of Na⁺-H⁺ antiporter, increased intracellular Na⁺ concentration ([Na⁺](i)), and stimulation of Na⁺-K⁺ pump. We have used isolated rat soleus muscles to test this hypothesis. Insulin increased the ouabain-suppressible K⁺ uptake in a dose- and time-dependent manner. The maximal effect was observed at 50-100 mU/ml insulin. Stimulation of K⁺ uptake was accompanied by increased specific [³H]ouabain binding and lowered [Na⁺](i). The ionophore monensin, which promotes Na⁺-H⁺ exchange, also increased the rate of ouabain-suppressible K⁺ uptake in soleus muscle, with a maximal effect obtained at 10-100 μM ionophore. However, this increase was accompanied by an elevation of [Na⁺](i). In the presence of 10-100 μM monensin, addition of 100 mU/ml insulin further increased K⁺ uptake but reduced [Na⁺](i). The effect on K⁺ uptake was additive. Ouabain (10^-3 M) completely suppressed the effect of insulin on [Na⁺](i). Insulin had no effect on the ouabain-resistant K⁺ uptake. Inhibition of the Na⁺-H⁺ antiporter by amiloride reduced the [Na⁺](i) but had no effect on the magnitude or the time course of insulin stimulation of K⁺ uptake. Thus equal stimulation of Na⁺-K⁺-ATPase by insulin was observed when [Na⁺](i) was elevated (under monensin) or lowered (under amiloride). These data suggest that activation of Na⁺-K⁺-ATPase in soleus muscle by insulin is not secondary to stimulation of Na⁺-H⁺ antiporter.