The mechanisms underlying the association between obesity and hypertension are unclear, but increased sympathetic neural drive to skeletal muscle circulation and kidney has been implicated. This insight has been derived from direct intraneural recordings of sympathetic nerve activity and regional norepinephrine turnover. It has been assumed that the increases in sympathetic nerve activity (SNA) directly translate into elevated sympathetic vasoconstrictor tone, but these assumptions have not been rigorously tested. We have recently obtained preliminary data using maximal alpha-adrenergic receptor blockade of the forearm vasculature that demonstrates no increase in sympathetic vascular tone in normotensive obese humans despite increased muscle SNA. This suggests that there is dissociation between sympathetic neural outflow and sympathetically mediated vasoconstrictor tone. Interestingly, we also have preliminary data indicating that in obese hypertensive humans, increased muscle SNA is associated with increased sympathetic vasoconstrictor tone. This suggests that a predisposition to hypertension interacts with obesity to permit the increased SNA to be expressed as increased sympathetic vascular tone. This project will test the hypothesis that the vasoconstrictor effects of SNA are attenuated in obesity but preserved in obese humans with a predisposition to hypertension, with these specific aims: 1) What are the effects of hypertension, obstructive sleep apnea and visceral adiposity on the resistance vessel expression of elevated SNA? 2) Is endothelial nitric oxide generation responsible for attenuated effects of SNA on vascular tone in obese compared to lean normotensive humans, and is the modulating influence of nitric oxide attenuated in obese hypertensive humans? 3) What are the effects of different dietary components on SNA and sympathetically mediated vasoconstrictor tone in obese normotensive and hypertensive subjects? Sympathetic nerve traffic to skin and skeletal muscle will be assessed by microneurography. Sympathetic vasoconstrictor tone will be assessed by the vasodilator response of skin (laser Doppler flux) and skeletal muscle (oxygenation measured by near infra-red oximetry) to intra-arterial infusion of an alpha-adrenergic antagonist. These studies should provide new insights into the cardiovascular effects of obesity and the factors that predispose to obesity-related hypertension.