In normotensive rats, an increase in dietary salt leads to decreased arteriolar responsiveness to acetylcholine (ACh) because of suppressed local nitric oxide (NO) activity. We evaluated the possibility that generation of reactive oxygen species in the arteriolar wall is responsible for this loss of NO activity. Arteriolar responses to iontophoretically applied ACh were examined in the superfused spinotrapezius muscle of Sprague-Dawley rats fed a low-salt (LS; 0.45%) or high-salt diet (HS; 7%) for 4-5 wk. Responses to ACh were significantly depressed in HS rats but returned to normal in the presence of the oxidant scavengers superoxide dismutase + catalase or 2,2,6, 6-tetamethylpiperidine-N-oxyl (TEMPO) + catalase. Arteriolar responses to the NO donor sodium nitroprusside were similar in HS and LS rats. Arteriolar and venular wall oxidant activity, as determined by reduction of tetranitroblue tetrazolium, was significantly greater in HS rats than in LS rats. Exposure to TEMPO + catalase reduced microvascular oxidant levels to normal in HS rats. These data suggest that a high-salt diet leads to increased generation of reactive oxygen species in striated muscle microvessels, and this increased oxidative state may be responsible for decreased endothelium-dependent responses associated with high salt intake.

Endothelium-derived nitric oxide (NO) attenuates arteriolar constriction in the rat small intestine during periods of increased sympathetic nerve activity. This study was undertaken to test the hypothesis that a flow-dependent fall in arteriolar wall PO(2) serves as the stimulus for endothelial NO release under these conditions. Sympathetic nerve stimulation at 3-16 Hz induced frequency-dependent arteriolar constriction, with arteriolar wall O(2) tension (PO(2)) falling from 67 +/- 3 mmHg to as low as 41 +/- 6 mmHg. Arteriolar responses to nerve stimulation were enhanced after inhibition of NO synthase with N(G)-monomethyl-L-arginine (L-NMMA). Under a high-O(2) (20%) superfusate, the fall in wall PO(2) was significantly attenuated, arteriolar constrictions were increased by 57 +/- 9 to 66 +/- 12%, and these responses were no longer sensitive to L-NMMA. The high-O(2) superfusate had no effect on vascular smooth muscle responsiveness to NO (as judged by arteriolar responses to sodium nitroprusside) or on arteriolar wall oxidant activity (as determined by the reduction of tetranitroblue tetrazolium dye). These results indicate that a flow-dependent fall in arteriolar wall PO(2) may serve as a stimulus for the release of endothelium-derived NO during periods of increased sympathetic nerve activity.

Previous reports by this laboratory have indicated that a flow-dependent fall in arteriolar wall PO₂ may be a stimulus for the sustained release of endothelial nitric oxide (NO) during sympathetic vasoconstriction in the superfused rat intestine. In this study, we tested the hypothesis that locally formed adenosine serves as the link between the fall in local PO₂ and NO synthesis under these conditions. Adenosine applied via pressurized micropipettes directly onto the wall or at a distance of 25 µm from the wall of first-order arterioles (resting diameter = 54 ± 1 µm) elicited dose-dependent dilations of 15–46% that were significantly reduced by the NO synthase inhibitor N^G-monomethyl-L-arginine (L-NMMA, 10^–4M). Arteriolar responses to sympathetic nerve stimulation were enhanced by 57–66% in the presence of L-NMMA or when tissue PO₂ was prevented from falling under a high O₂ superfusate. Adenosine deaminase (2.0 U/ml) or the selective A₁ receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (4 × 10^–4M) completely blocked the enhancing effect of L-NMMA on sympathetic constriction. These results are consistent with the hypothesis that the fall in arteriolar wall and/or tissue PO₂ that accompanies sympathetic arteriolar constriction in the rat intestine can lead to local adenosine production, which in turn preserves endothelial NO release.

We have previously reported that adenosine formed in response to reduced arteriolar and/or tissue PO(2) preserves endothelial nitric oxide (NO) synthesis during sympathetic vasoconstriction in the rat intestine. To more precisely identify the site and mechanism of adenosine formation under these conditions, we tested the hypothesis that ATP released in response to reduced O(2) levels serves as a source of adenosine. Direct application of ATP to the wall of first-order arterioles elicited dose-dependent dilations of 15-33% above resting diameter that were reduced by 71-80% by the 5'-ectonucleotidase inhibitor alpha,beta-methyleneadenosine 5'-diphosphate (AOPCP, 4.5 x 10(-5) M) and completely abolished by N(G)-monomethyl-L-arginine (L-NMMA, 10(-4) M). Under control conditions, sympathetic nerve stimulation at 3 and 8 Hz induced arteriolar constrictions of 11 +/- 1 and 19 +/- 1 microm, respectively. These responses were enhanced by 58-69% in the presence of L-NMMA or when local PO(2) was maintained at resting levels. However, in the presence of AOPCP, the enhancing effects of L-NMMA and the high O(2) superfusate on sympathetic constriction were preserved. These results suggest that, although exogenously applied ATP can stimulate arteriolar NO release in the intestine largely through its sequential extracellular hydrolysis to adenosine, this process does not contribute to adenosine formation and sustained NO release during sympathetic constriction in this vascular bed.

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