In this study we tested the hypothesis that insulin may differentially affect isolated arterioles from red (RGM) and white gastrocnemius muscles (WGM) because of their differences in function and metabolic profile. We also determined whether the responses of these arterioles are endothelium dependent and mediated by either prostaglandins or nitric oxide (NO). Arterioles were isolated, pressurized to 85 mmHg, equilibrated in Krebs bicarbonate-buffered solution (pH 7.4) gassed with 10% O2 (5% CO2-85% N2), and studied in a no-flow state. Control diameters for first-order arterioles from RGM averaged 77 +/- 8 micrometers and from WGM averaged 77 +/- 5 micrometers. Cumulative dose-response curves to insulin (10 microU/ml, 100 microU/ml, 1 mU/ml, and 10 mU/ml) were obtained in arterioles before and after endothelium removal or administration of either indomethacin (Indo, 10(-5) M) or NG-nitro-L-arginine (L-NNA, 10(-4) M). Insulin evoked concentration-dependent increases in control diameter of intact RGM and WGM arterioles of 6-26% and 9-28%, respectively. Indo was without any effect on insulin-induced dilation in RGM and WGM arterioles. Insulin-evoked dilation in both RGM and WGM arterioles was completely inhibited and converted to vasoconstriction by endothelium removal and administration of L-NNA. These results indicate that in endothelium-intact arterioles from RGM and WGM, insulin evokes an endothelium-dependent dilation that is equivalent and mediated by NO. In contrast, in the absence of a functional endothelium, insulin evokes arteriolar constriction. The finding that insulin can constrict arterioles, at physiological concentrations, suggests that insulin may play a more significant role in the regulation of vascular tone and total peripheral resistance than previously appreciated.