Regulation of System A amino-acid transport activity by phospholipase C and cAMP-inducing agents in skeletal muscle: modulation of insulin action.

The present study was designed to investigate the effect of phospholipase C and compounds known to promote synthesis of cAMP on System A transport activity under basal and insulin-stimulated conditions in the incubated muscle. In parallel, we also examined the effect of these agents on muscle glucose transport activity. Phospholipase C caused marked stimulation of alpha-(methyl)-aminoisobutyric acid (MeAIB--a System-A-specific analogue) uptake uptake and that of 3-O-methylglucose by the incubated muscle. In contrast, the activatory effect of insulin on System A was largely inhibited by phospholipase C. The effects of phospholipase C on transport processes differed from the effects provoked by phorbol esters (TPA), indicating that they are not just a consequence of TPA-sensitive protein kinase C activation. Agents such as isoproterenol, cholera toxin or forskolin, known cAMP inducers, caused glycogen depletion and stimulation of lactate production in the incubated muscle. However, these agents did not alter basal or insulin-stimulated MeAIB uptake. Isoproterenol and cholera toxin did not affect maximal stimulation of 3-O-methylglucose uptake caused by insulin. Our data indicate that System A transport is activated by phospholipase C in skeletal muscle, and that this effect is not due simply to activation of TPA-sensitive isoforms of protein kinase C. The effect of insulin on System A is reduced by either phospholipase C or TPA, which suggests the mediation of protein kinase C. On the basis of the lack of effect of cAMP-inducing agents on insulin-stimulated System A and glucose transport activities, we conclude that cAMP-dependent protein kinase does not cause any generalized blockade of insulin action in skeletal muscle, in contrast to what has been reported in other cell types.