Stimulation of pyruvate dehydrogenase activity in intact rat adipocytes by insulin mediator from rat skeletal muscle.

This study compared the effects of insulin and insulin mediator from skeletal muscle of control and insulin-treated rats on intact adipocyte pyruvate dehydrogenase. Increasing insulin concentrations stimulated pyruvate dehydrogenase activity in a biphasic manner with a maximal stimulation at 100 microU/ml which was 2-fold and sustained for up to 1 h. The mediators from control or insulin-treated rats also stimulated pyruvate dehydrogenase of intact adipocytes with the effect increasing in a linear manner up to a 1:10 final dilution. The latter mediator had twice the stimulatory activity as the former. Peak stimulation of pyruvate dehydrogenase by the mediators was attained within 10 min of incubation. The enzyme activity rapidly declined thereafter, with the stimulation by mediator from control rats decreasing at a faster rate than that due to mediator from insulin-treated rats. The stimulatory effect of the mediators on adipocyte pyruvate dehydrogenase was found to be additive to that of insulin. This study demonstrates: 1) that insulin mediator can act on mitochondrial pyruvate dehydrogenase of intact, functional adipocytes as it does on isolated intact or broken mitochondria; 2) that the mediator is degraded by the adipocyte; and 3) that the amount of mediator generated by insulin probably limits the stimulation of pyruvate dehydrogenase by insulin. These findings further substantiate the physiological relevance of this putative insulin second messenger.