Microvascular responses to oxygen and muscle contraction in hypertensive Dahl rats

We evaluated the possibility that increased arteriolar tone in salt-sensitive hypertension could be partially due to an altered vascular responsiveness to oxygen and/or tissue metabolites. The microvasculature of the superfused spinotrapezius muscle was studied with fluorescence microscopy in Dahl salt-sensitive rats fed low (0.45%) or high (7%) salt diets for 4 weeks. High salt intake produced hypertension (mean arterial pressure = 162 +/- 5 mm Hg vs 130 +/- 4 mm Hg for low salt rats, p < 0.05), and increased vascular tone at the level of the arcade arterioles but not in the smaller transverse or distal arterioles. Arcade arteriole constriction induced by increasing superfusate oxygen content (from 0% to 10% O2) was 60% greater in hypertensive rats than in normotensive rats, whereas oxygen-dependent constriction of transverse and distal arterioles was similar in the two groups. The oxygen-induced reduction in capillary perfusion (the fraction of time during which flow was observed in each vessel) was also greater in hypertensive than in normotensive rats. Arcade arteriole dilation during 2 Hz muscle contraction was significantly greater in hypertensive than in normotensive rats, but there were no differences between groups in the dilation of any other vessel type or in the capillary flow increases accompanying 2 or 8 Hz contraction. These results suggest that a hyperresponsiveness to the actions of blood-borne or tissue oxygen could contribute to increased arcade arteriolar tone in the spinotrapezius muscle of Dahl rats with salt-induced hypertension. The enhanced dilation of arcade arterioles during muscle contraction also suggests a localized hyperresponsiveness to tissue metabolites in this form of hypertension.