1,2-Diacylglycerol and Ceramide Levels in Rat Skeletal Muscle and Liver in Vi00

Studies on BC3H-1 myocytes suggest that the insulin-induced increase in cellular diacylglycerol level mediates the insulin-stimulated glucose transport in these cells (Standaert, M. L., Farese, R. V., Cooper, D. R., and Pollet, R. J. (1988) J. Biol. Chem. 263,86968705). The present study tested whether diacylglycerol could mediate the insulin-induced and exerciseinduced increases in glucose uptake by rat skeletal muscle in uiuo. Glucose uptake by calf muscles of the rat was assessed by measuring cellular 2-deoxyglucose uptake in uiuo. Diacylglycerol and ceramides in muscles frozen in situ were assayed with diacylglycerol kinase. Intravenous injection of 0.1 unit of insulin/rat resulted in a 6-fold increase in muscle 2-deoxyglucose uptake during the subsequent 25-min period. In contrast, no statistically significant changes in muscle diacylglycerol or ceramide levels were observed at 2, 5, 10, and 25 min after insulin injection. When calf muscles of the hindlimb were exercised in viva for 25 min by electrical stimulation inducing one contraction/s, 2deoxyglucose uptake by muscles was increased 15-fold. However, no statistically significant changes in muscle diacylglycerol or ceramide content were observed at 5,10,15, and 25 min of exercise. Although the findings do not exclude the possibility of a compartmentalized increase in diacylglycerol level, the present data suggest that diacylglycerol is not a mediator of the insulininduced or exercise-induced augmentation of glucose uptake by skeletal muscle in uiuo. Since interruption of nerve supply to the muscles makes the muscles insulin resistant (Turinsky, J., (1987) Am. J. Physiol. 252, R531-R637), the effect of denervation on diacylglycerol and ceramide levels in calf muscles of the rat was also examined. The denervation resulted in 2 1, 51, and 117% increases in muscle diacylglycerol levels at 3, 16, and 32 days after denervation, respectively. No statistically significant changes in muscle ceramide levels were observed at any postdenervation interval. Finally, the measured lipids were studied in muscles and livers of rats infused with supraphysiological doses of vasopressin (86 pmol/ min). In controls, diacylglycerol concentrations of the muscles and liver did not significantly differ, but the liver exhibited a 5-fold higher level of ceramides than the muscles. Infusion of vasopressin for 5 min did not have a statistically significant effect on diacylglycerol concentration of the liver but continuation of the same infusion for 10 min resulted in a 63% increase in liver