Hyperglycemia Activates Glucose Transport in Rat Skeletal Muscle Via a Ca²⁺-Dependent Mechanism

We investigated the acute effect of hyperglycemia on 3-O-methylglucose transport in isolated rat epitrochlearis muscles. High levels of glucose (20 mmol/1) induced an ∼ twofold increase in the rate of glucose transport when compared with muscles exposed to a low level of glucose (8 mmol/1) (P < 0.001). The hyperglycemic effect was additive to the effects of both insulin and exercise on the glucose transport rates. Dantrolene (25 αmol/l), a potent inhibitor of Ca²⁺ release from the sarcoplasmic reticulum, blocked the ability of hyperglycemia to increase glucose transport by 73% (P < 0.01). Although dantrolene had no effect on the non-insulin-stimulated or the insulin-stimulated glucose transport rates during normoglycemic conditions, the effect of exercise was completely blocked in the presence of dantrolene (P < 0.01). Inhibition of phosphatidylinositol (PI) 3-kinase by wortmannin (500 nmol/l) had no effect on the activation of glucose transport by hyperglycemia, whereas the insulin-stimulated glucose transport was completely abolished (P < 0.001). These findings suggest that hyperglycemia activates glucose transport by a Ca²⁺- dependent pathway. The signaling system for this Ca²⁺- dependent activation of glucose transport does not involve the activation of PI 3-kinase and is separate from the mass-action effect of glucose on glucose transport.