Flow-dependent arteriolar dilation in normotensive rats fed low- or high-salt diets

Ingestion of a high-salt diet has previously been shown to suppress the endogenous influence of nitric oxide (NO) on arteriolar tone in hypertension-resistant, salt-resistant Dahl (SR/Jr) rats. Because luminal blood flow can be an important stimulus for endothelial NO release, this study was undertaken to determine whether high salt intake can also lead to a deficit in the direct flow-dependent regulation of arteriolar diameter. The spinotrapezius muscle microvasculature was studied by in vivo microscopy in SR/Jr rats fed low (0.45%)- or high (7%)-salt diets for 2 wk, and arcade arteriole responses to increased luminal flow (via parallel vessel occlusion) were studied in both dietary groups. There was no significant difference between groups in arterial pressure or in resting arteriolar diameters, volume flows, or wall shear rates. In low-salt SR/Jr, a 36% increase in luminal flow produced an average arteriolar dilation of 38% that was significantly reduced by the NO synthase inhibitor NG-monomethyl-L-arginine (L-NMMA). In high-salt SR/Jr, a similar flow increase produced an average dilation of only 16% (P < 0.05 vs. low-salt SR/Jr), and this response was unaffected by L-NMMA. Inhibition of cyclooxygenase activity with meclofenamate had no effect on this response in either group. These findings suggest that NO release mediates a portion of flow-dependent arteriolar dilation in rat spinotrapezius muscle and that high salt intake, in the absence of hypertension, can attenuate this response via a suppression of NO activity.