Role of protein kinase C in insulin activation of the Na-K pump in cultured skeletal muscle

Administration of insulin to preparations of skeletal muscle causes an increase in Na⁺-K⁺ pump activity within 15-30 min. Although several mechanisms have been proposed, such as promotion of Na⁺ influx and translocation of pumps from intracellular to membrane sites, the early events involved in this effect remain unknown. We have investigated the possibility that activation of protein kinase C (PKC) may be an initial event in Na⁺- K⁺ pump activation in primary cultures of rat skeletal muscle. Insulin (80- 100 mU/ml) and tumor-promoting phorbol esters (10-100 nM) increased Na⁺-K⁺ pump activity as determined by measurements of ouabain-suppressible ⁸⁶Rb uptake, electrogenic pump component of membrane potential, and specific [³H]ouabain binding. These effects were not reduced by treatment of myotubes with amiloride, which blocks Na⁺-H⁺ exchange, or with tetrodotoxin, which blocks voltage-dependent Na⁺ channels. Effects of insulin and phorbol esters were not additive, suggestive of a common mechanism. Effects of both phorbol esters and insulin were significantly reduced by staurosporine (50-100 nM) and by downregulation of PKC (by pretreatment of myotubes with phorbol ester for 24 h). The findings suggest that insulin may stimulate Na⁺-K⁺ pump activity in skeletal muscle by activation of PKC.