Activation of glucose transport in skeletal muscle by phospholipase C and phorbol ester. Evaluation of the regulatory roles of protein kinase C and calcium

It has been hypothesized on the basis of studies on BC3H-1 myocytes that diacylglycerol generation with activation of protein kinase C (PKC) is involved in the stimulation of glucose transport in muscle by insulin (Standaert, M.L., Farese, R.V., Cooper, R.D., and Pollet, R.J. (1988) J. Biol. Chem. 263, 8696-8705). In the present study, we used the rat epitrochlearis muscle to evaluate the possibility that PKC activity mediates the stimulation of glucose transport by insulin in mammalian skeletal muscle. Phospholipase C from Clostridium perfringens (PLC-Cp), which generates diacylglycerol from membrane phospholipids and 4β-phorbol 12β-myristate 13α-acetate (PMA) induced increases in glucose transport activity (assessed using 3-O-methylglucose transport) that were ~80 and ~20% as great, respectively, as that induced by a maximal insulin stimulus. PLC-Cp and PMA both caused a ~2-fold increase in membrane-associated PKC activity. In contrast, insulin did not affect PKC activity. These findings argue against a role of diacylglycerol-mediated PKC activation in the stimulation of skeletal muscle glucose transport by insulin. They also show that the BC3H-1 myocyte is not a good model for studying regulation of glucose transport in skeletal muscle. Neither the submaximal nor maximal effects of PLC-Cp and insulin on glucose transport were additive, suggesting that PLC-Cp interferes with insulin action. The maximal effects of PLC-Cp and hypoxia or muscle contractions were also not additive. However, the submaximal effects of hypoxia and PLC-Cp were completely additive. These findings raise the possibility that PLC-Cp stimulates glucose transport by the exercise/hypoxia-activated, not the insulin-activated, pathway in skeletal muscle. Exposure to PLC-Cp activated glycogen phosphorylase and potentiated twitch tension in response to electrical stimulation, providing evidence that PLC-Cp increases cytoplasmic Ca²⁺ concentration. Dantrolene, an inhibitor of Ca²⁺ release from the sarcoplasmic reticulum, completely blocked both the activation of phosphorylase and the stimulation of glucose transport by PLC-Cp. These findings provide evidence that an increase in cytoplasmic Ca²⁺ concentration is involved in the activation of glucose transport in skeletal muscle by PLC-Cp.