Obesity is related to insulin resistance and hypertension, but the underlying mechanisms are unclear. Insulin exerts both vasodilator and vasoconstrictor effects on muscle resistance arteries, which may be differentially impaired in obesity. Objectives To investigate whether vasodilator and vasoconstrictor effects of insulin are impaired in muscle resistance arteries of obese rats and the roles of Akt and endothelial NO synthase (eNOS). Methods/Results Effects of insulin were studied in resistance arteries isolated from cremaster muscles of lean and obese Zucker rats. In arteries of lean rats, insulin increased activity of both NO and endothelin (ET-1), resulting in a neutral effect under basal conditions. In arteries of obese rats, insulin induced endothelin-mediated vasoconstriction (-15+/-5 percent at 1 nM, P<0.05 vs. lean). Insulin induced vasodilatation during endothelin receptor blockade in arteries of lean rats (20+/-5 percent at 1 nM), but not in those of obese rats. Inhibition of NO synthesis increased vascular tone (by 12+/-2 percent) and shifted insulin-mediated vasoreactivity to vasoconstriction (-25+/-1 percent at 1 nM) in lean rats but had no effect in arteries of obese rats, indicating reduced NO activity. Protein analysis of resistance arteries revealed that insulin-mediated activation of Akt was preserved in obese rats, whereas expression of eNOS was markedly decreased. Conclusions Vasodilator but not vasoconstrictor effects of insulin are impaired in muscle resistance arteries of obese rats, and this selective impairment is associated with decreased protein levels of eNOS. These findings provide a new mechanism linking obesity to insulin resistance and hypertension.