Sphingomyelinase stimulates 2-deoxyglucose uptake by skeletal muscle

The effects of sphingomyelinase, phosphorylcholine, N-acetylsphingosine (C2-ceramide), N-hexanoylsphingosine (C6-ceramide) and sphingosine on basal and insulin-stimulated cellular accumulation of 2-deoxy-D-glucose in rat soleus muscles were investigated. Preincubation of muscles with sphingomyelinase (100 or 200 m-units/ml) for 1 or 2 h augmented basal 2-deoxyglucose uptake by 29-91%, and that at 0.1 and 1.0 m-unit of insulin/ml 32-82% and 19-25% respectively compared with control muscles studied at the same insulin concentrations. The sphingomyelinase-induced increase in basal and insulin-stimulated 2-deoxyglucose uptake was inhibited by 91% by 70 microM cytochalasin B, suggesting that it involves glucose transporters. Sphingomyelinase had no effect on the cellular accumulation of L-glucose, which is not transported by glucose transporters. The sphingomyelinase-induced increase in 2-deoxyglucose uptake could not be reproduced by preincubating the muscles with 50 microM phosphorylcholine, 50 microM C2-ceramide or 50 microM C6-ceramide. Preincubation of muscles with 50 microM sphingosine augmented basal 2-deoxyglucose transport by 32%, but reduced the response to 0.1 and 1.0 m-unit of insulin/ml by 17 and 27% respectively. The stimulatory effect of sphingomyelinase on basal and insulin-induced 2-deoxyglucose uptake was not influenced by either removal of Ca2+ from the incubation medium or dantrolene, an inhibitor of Ca2+ release from the sarcoplasmic reticulum. This demonstrates that Ca2+ does not mediate the action of sphingomyelinase on 2-deoxyglucose uptake. Sphingomyelinase also had no effect on basal and insulin-stimulated activities of insulin receptor tyrosine kinase and phosphatidylinositol 3-kinase. In addition, 1 and 5 microM wortmannin, an inhibitor of phosphatidylinositol 3-kinase, failed to inhibit the sphingomyelinase-induced increase in 2-deoxyglucose uptake. These results suggest that sphingomyelinase does not increase 2-deoxyglucose uptake by stimulating the insulin receptor or the initial steps of the insulin-transduction pathway. The data suggest the possibility that sphingomyelinase increases basal and insulin-stimulated 2-deoxyglucose uptake in skeletal muscle as the result of an unknown post-receptor effect.