Limitation of arteriolar myogenic activity by local nitric oxide: segment-specific effect of dietary salt

The purpose of this study was to determine if local nitric oxide (NO) activ ity attenuates the arteriolar myogenic response in rat spinotrapezius muscl e. We also investigated the possibility that hypertension, dietary salt, or their combination can alter any influence of local NO on the myogenic resp onse. Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR) fed low-salt (0.45%, LS) or high-salt (7%, HS) diets were enclosed in a ventil ated airtight box with the spinotrapezius muscle exteriorized for intravita l microscopy. Mean arterial pressure was unaffected by dietary salt in WKY but was significantly higher and augmented by dietary salt in SHR. In all e xperiments, elevation of microvascular pressure by box pressurization cause d a 0-30% decrease in the diameter of large (arcade bridge) arterioles and a 21-27% decrease in the diameter of intermediate (arcade) arterioles. Inhi bition of NO synthase with N-G-monomethyl-L-arginine (L-NMMA) significantly enhanced myogenic responsiveness of arcade bridge arterioles in WKY-LS and SHR-LS but not in WKY-HS and SHR-HS. L-NMMA significantly enhanced the myo genic responsiveness of arcade arterioles in all four groups. Excess L-argi nine reversed this effect of L-NMMA in all cases, and arteriolar responsive ness to the NO donor sodium nitroprusside was not different among the four groups. High-salt intake had no effect on the passive distension of arterio les in either strain during box pressurization. We conclude that 1) local N O normally attenuates arteriolar myogenic responsiveness in WKY and SHR, 2) dietary salt impairs local NO activity in arcade bridge arterioles of both strains, and 3) passive arteriolar distensibility is not altered by a high -salt diet in either strain.