The effect of three calcium antagonists (verapamil, diltiazem, and nifedipine) on insulin effects was investigated in isolated rat soleus muscles. Soleus muscles were incubated in the presence of insulin (100 microU/ml), a concentration that stimulates the rates of lactate formation and glycogen synthesis half-maximally and with and without a calcium antagonist. A decrease (48%; P < 0.001) was noted in the insulin-mediated rate of glycogen synthesis by verapamil at 100 microM; no effect was observed at lower concentrations of verapamil. Diltiazem decreased the insulin-mediated rates of glycogen synthesis by 36 (P < 0.001), 64 (P < 0.001), and 73% (P < 0.001) at 1, 10, and 100 microM, respectively. Nifedipine decreased the insulin-mediated rates of glycogen synthesis by 37% at 0.1 microM (P < 0.001), 36% at 1 microM (P < 0.001), 21% at 10 microM (P < 0.05), and 72% at 100 microM (P < 0.001). Verapamil at 100 microM decreased lactate formation by 48% (P < 0.001). However, diltiazem increased the rate of lactate formation by 22 (P < 0.01), 43 (P < 0.001), and 61% (P < 0.001) at 1, 10, and 100 microM, respectively. In contrast, nifedipine increased the insulin-mediated rate of lactate formation by 45% only at 100 microM (P < 0.01). The increased rate of lactate formation was probably caused by an increased rate of glycogenolysis, because high concentrations of all the calcium antagonists significantly decreased muscle glycogen content. The insulin-stimulated rate of 3-O-methyl-D-glucose transport or cAMP content was not affected by diltiazem at 1 or 10 microM. The results suggest that the calcium antagonists work by a mechanism, possibly by activating a calcium channel or an extracellular receptor, to influence markedly insulin-mediated intracellular glucose metabolism in skeletal muscle.