DIVERSE EFFECTS OF CALCIUM-CHANNEL BLOCKERS ON SKELETAL-MUSCLE GLUCOSE-TRANSPORT

Verapamil, a calcium channel blocker, inhibited the insulin-stimulated glucose transport rate in isolated rat epitrochlearis muscle in a dose-dependent manner (1-200-mu-M) without affecting basal glucose transport rate. Verapamil's inhibition was rapid in onset and disappearance; changes in glucose transport rate were detectable when verapamil was added to or removed from the incubation medium 15 min prior to measurement of glucose transport. Verapamil also inhibited the stimulation of muscle glucose transport caused by hypoxia, indicating that the effect was not limited to insulin action. Although the optical isomers of verapamil vary considerably in their potency as Ca2+ channel blockers, they were equally effective inhibitors of insulin-stimulated glucose transport rate. Nifedipine (10-200-mu-M), a more potent blocker of skeletal muscle Ca2+ channels than verapamil, was less effective as an inhibitor of insulin-stimulated glucose transport. Furthermore, nifedipine (10-mu-M) did not inhibit hypoxia-stimulated glucose transport. Diltiazem (200-mu-M), another Ca2+ channel blocker, did not reduce insulin-stimulated glucose transport.