SPHINGOMYELINASE STIMULATES 2-DEOXYGLUCOSE UPTAKE BY SKELETAL-MUSCLE

The effects of sphingomyelinase, phosphorylcholine, N-acetylsphingosin e (C2-ceramide), N-hexanoylsphingosine (C6-ceramide) and sphingosine o n basal and insulin-stimulated cellular accumulation of 2-deoxy-D-gluc ose in rat soleus muscles were investigated. Preincubation of muscles with sphingomyelinase (100 or 200 m-units/ml) for 1 or 2 h augmented b asal 2-deoxyglucose uptake by 29-91%, and that at 0.1 and 1.0 m-unit o f insulin/ml by 32-82% and 19-25% respectively compared with control m uscles studied at the same insulin concentrations. The sphingomyelinas e-induced increase in basal and insulin-stimulated 2-deoxyglucose upta ke was inhibited by 91% by 70 mu M cytochalasin B, suggesting that it involves glucose transporters. Sphingomyelinase had no effect on the c ellular 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 m u M phosphorylcholine, 50 mu M C2-ceramide or 50 mu M C6-ceramide. Pre incubation of muscles with 50 mu M sphingosine augmented basal 2-deoxy glucose transport by 32%, but reduced the response to 0.1 and 1.0 m-un it of insulin/ml by 17 and 27% respectively. The stimulatory effect of sphingomyelinase on basal and insulin-induced 2-deoxyglucose uptake w as not influenced by either removal of Ca2+ from the incubation medium or dantrolene, an inhibitor of Ca2+ release from the sarcoplasmic ret iculum. This demonstrates that Ca2+ does not mediate the action of sph ingomyelinase on 2-deoxyglucose uptake. Sphingomyelinase also had no e ffect on basal and insulin-stimulated activities of insulin receptor t yrosine kinase and phosphatidylinositol 3-kinase. In addition, 1 and 5 mu M wortmannin, an inhibitor of phosphatidylinositol 3-kinase, faile d to inhibit the sphingomyelinase-induced increase in 2-deoxyglucose u ptake. These results suggest that sphingomyelinase does not increase 2 -deoxyglucose uptake by stimulating the insulin receptor or the initia l steps of the insulin-transduction pathway. The data suggest the poss ibility that sphingomyelinase increases basal and insulin-stimulated 2 -deoxyglucose uptake in skeletal muscle as the result of an unknown po st-receptor effect.