Signal transduction mechanism for the stimulation of the sarcolemmal Na⁺-Ca²⁺ exchanger by insulin

The signal transduction pathway for insulin-mediated activation of sarcolemmal Na⁺-Ca²⁺ exchange was examined. Insulin stimulated Na⁺-Ca²⁺ exchanger activity in a dose-dependent manner, with the EC50 being about 0.7 U/l. The insulin effect was blocked by the protein kinase inhibitor, staurosporine, indicating possible involvement of a protein kinase in insulin action. Also, the relationship between the insulin effect and activation of a G protein, was examined by testing the effects of 5′ guanylyl imidodiphosphate (Gpp(NH))p) on Na⁺-Ca²⁺ exchange in, the presence and absence of insulin. When exchanger activity was assayed at a calcium concentration of 40 μM, insulin alone had no effect whereas ATP and Gpp(NH)p increased exchanger activity. However, insulin responsiveness was restored in vesicles preloaded with either ATP or Gpp(NH)p, suggesting that insulin may act through a combination of G protein coupling and protein phosphorylation to enhance Na⁺-Ca²⁺ exchanger activity. We conclude that calcium overload in the diabetic heart may involve a defect in acute activation of the exchanger by insulin.