These findings indicate that NO is involved in the central regulation of sympathetic outflow in humans and suggest that both neuronal and endothelial NO synthesis may contribute to the regulation of vasomotor tone.

Haemodynamic and sympathetic effects of inhibition of NO synthase by infusion of N(G)-monomethyl-L-arginine into humans are dose dependent. At higher doses, N(G)-monomethyl-L-arginine exerts sympathoinhibitory effects that are comparable to those evoked by a non-specific vasoconstrictor drug, whereas at lower doses, it exerts sympatho-excitatory effects.

Obesity, a major health problem in industrialized societies, is associated with a high incidence of cardiovascular complications such as hypertension, ischemic heart disease and stroke. However, the underlying mechanism relating obesity and these cardiovascular events is not clear. In lean subjects even slight elevations in plasma insulin concentration exert marked effects on the cardiovascular system, and these effects are directly related to insulin (rather than to insulin-induced stimulation of intermediary metabolism). Moreover, insulin's vascular effects are mediated both by the sympathetic nervous system and the L-arginine nitric oxide pathway. Obesity is characterized by sustained sympathetic activation (possibly related to chronic hyperinsulinemia) and an impaired vasodilator responsiveness to insulin. Although, undoubtedly many factors contribute to the increased incidence of cardiovascular complications in overweight subjects, sympathetic activation could be one important mechanism and either trigger acute events or--possibly in conjunction with an impairment in insulin-induced vasodilation--contribute to sustained elevation of arterial pressure.

Insulin-induced stimulation of blood flow and sympathetic nerve activity in skeletal muscle tissue is impaired in obesity, but the underlying mechanism is unknown. To determine whether insulin resistance alters sympathetic and vasodilatory responses to euglycemic hyperinsulinemia, in eight healthy subjects we measured calf blood flow and muscle sympathetic nerve activity (MSNA) (n = 5) during insulin/glucose infusion (euglycemic hyperinsulinemic [6 pmol.kg-1.min-1] clamp) performed alone and performed during concomitant fat emulsion infusion, a maneuver designed to induce insulin resistance. The major new finding is that fat emulsion infusion, which attenuated insulin-induced stimulation of carbohydrate oxidation by 39 +/- 7% (P < 0.01), did not have any detectable effect on insulin-induced vasodilatory and sympathetic responses: at the end of the 2-h clamp, blood flow and MSNA had increased by 35 +/- 6% (P < 0.01) and 152 +/- 58% (P < 0.01), respectively, during insulin infusion alone and by 35 +/- 7% (P < 0.01) and 244 +/- 90% (P < 0.01), respectively, during insulin infusion superimposed on free fatty acid infusion. These observations in lean healthy subjects indicate that induction of resistance to the stimulatory effects of insulin on carbohydrate metabolism does not attenuate muscle blood flow and MSNA responses evoked by acute euglycemic hyperinsulinemia. These findings provide further evidence that hyperinsulinemia per se is the primary stimulus that triggers stimulation of muscle blood flow and MSNA during insulin/glucose infusion in humans and suggest that the impaired insulin-induced vasodilation in obese subjects is not related primarily to impaired stimulation of muscle carbohydrate metabolism.