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The overall aim of this study was to determine if endogenous nitric oxide (NO) and/or cyclooxygenase products can attenuate the arteriolar response to increased sympathetic nerve activity, and if so, to determine if these factors are endothelial in origin. Intravital microscopy was used to examine the effect of (1) NO synthase and cyclooxygenase inhibition and (2) functional disruption of the endothelium, on arteriolar responses to sympathetic nerve stimulation in the superfused rat small intestine. Sympathetic nerve stimulation induced frequency-dependent constriction of small feed arteries (SFA), first-order arterioles (1A) and second-order arterioles (2A) and these constrictions were completely abolished by the {dollar}\\alpha{dollar}-adrenoceptor antagonist phentolamine. In the presence of the NO synthase inhibitor N{dollar}\\sp{lcub}\\rm G{rcub}{dollar}-monomethyl L-arginine (10{dollar}\\sp{lcub}-4{rcub}{dollar} M L-NMMA), both the magnitude and rate of sympathetic constriction were significantly increased in all vessel types. The effect of L-NMMA on sympathetic constriction was completely reversed by the additional presence of excess L-arginine (5 {dollar}\imes{dollar} 10{dollar}\\sp{lcub}-3{rcub}{dollar} M) in the superfusate. The cyclooxygenase inhibitor meclofenamate (3 {dollar}\imes{dollar} 10{dollar}\\sp{lcub}-5{rcub}{dollar} M) completely abolished the dilator responses to topically-applied arachidonic acid, but had no effect on the magnitude or rate of constriction in any vessel type. Based on the above finding that endogenous NO is an important modulator of sympathetic neurogenic constriction whereas cyclooxygenase products are not, the focus of the second series of experiments was to determine if the primary source of NO is endothelial in origin. To suppress endothelial function, CO{dollar}\\sb2{dollar} was directly infused into the 1A lumen. This transient CO{dollar}\\sb2{dollar} microembolization significantly attenuated endothelium-dependent dilation to acetylcholine without altering endothelium-independent dilation to sodium nitroprusside. After CO{dollar}\\sb2{dollar}-induced suppression of endothelial function, sympathetic neurogenic constriction was significantly increased. Subsequent addition of L-NMMA had no further effect on the magnitude of vasoconstriction induced by nerve stimulation. Similar results were seen when embolization was achieved with N{dollar}\\sb2{dollar}, and CO{dollar}\\sb2{dollar} embolization had the same effect on norepinephrine-induced constriction as it did on sympathetic neurogenic constriction. Taken together, the above findings suggest that inhibition by the endothelium of sympathetic vasoconstriction is achieved primarily through the release of NO and not cyclooxygenase products.