Effects of insulin on vascular responses to mental stress and norepinephrine in human forearm

Essential hypertension is frequently associated with insulin resistance and hyperinsulinemia. In vitro, insulin has vasodilator actions, but its possible hemodynamic effect on muscular vascular beds in humans is a matter of controversy. We investigated the effects of local hyperinsulinemia on the vascular responses to norepinephrine and physiological vasodilation during mental stress in the perfused-forearm model. Nine glucose-tolerant, normotensive, nonobese men (aged 22 to 36 years) participated. Forearm perfusion studies (venous occlusion plethysmography) were performed during randomized, double-blind intrabrachial artery infusions of insulin (to raise plasma insulin 100 microU/mL) or placebo for 2 hours. A mental stress test and stepwise intra-arterial infusion of norepinephrine (6 to 1200 ng/min) were performed during each infusion. Insulin infusion increased venous plasma insulin to 98.4 microU/mL and increased net glucose uptake threefold. Insulin had a gradual vasodilator effect (P < .05 by ANOVA), and after 90 minutes blood flow was 36 percent units higher relative to the control arm than during placebo (P = .005). During mental stress, forearm blood flow increased by 81% (t test, P = .006) and 92% (P = .01) in the study arm during insulin and placebo infusions, respectively (insulin versus placebo, P = NS). An increased forearm blood flow was maintained throughout the mental stress test during insulin infusion (ANOVA, P = .03). Forearm glucose uptake increased during stress, reflecting forearm hyperperfusion since fractional glucose extraction was unaffected by stress. The increased blood flow was maintained throughout the five norepinephrine dose steps (ANOVA, P < .04).(ABSTRACT TRUNCATED AT 250 WORDS)