Effects of sex and exercise training on β-adrenoreceptor mediated opposition of evoked sympathetic vasoconstriction in resting and contracting muscle of rats.

This study investigated the hypothesis that β-adrenoreceptor mediated inhibition of sympathetic vasoconstriction would be enhanced in female compared to male rats, and that exercise training would augment β-adrenoreceptor inhibition of sympathetic vasoconstriction in male and female rats. Sprague-Dawley rats were randomized into sedentary (Male: n=7; Female: n=8) and exercise trained (Male: n=9; Female: n=9) groups. Following 4 weeks of exercise training or sedentary behavior, rats were anesthetized and surgically instrumented for stimulation of the lumbar sympathetic chain, muscle contraction and measurement of arterial blood pressure and femoral artery blood flow (FBF). Femoral vascular conductance (FVC) was calculated as FBF/mean arterial pressure. The percentage change of FVC in response to sympathetic stimulation delivered at 2 and 5 Hz was measured at rest and during contraction of the triceps surae muscles before and after β-adrenoreceptor blockade (Propranolol;0.075 mg·kg-1, IV). We found that, at rest, β-adrenoreceptor blockade decreased (main effect of drug, 2Hz: P <0.001; 5Hz: P<0.001) sympathetic vasoconstriction. During contraction, sympathetic vasoconstrictor responsiveness was lower (main effect of sex, 2Hz: P=0.001; 5Hz: P=0.023) in female compared to male rats, and sympatholysis was enhanced (main effect of sex, 2 Hz: P=0.001; 5Hz: P<0.001) in female rats. β-adrenoreceptor blockade decreased (main effect of drug, 2Hz: P=0.049; 5Hz: P<0.001) evoked sympathetic vasoconstriction in contracting muscle. The present study demonstrated that β-adrenoreceptors do not blunt sympathetic vasoconstriction in resting or contracting skeletal muscle of male or female rats. Sympatholysis was enhanced in female rats, however, this was not attributable to β-adrenoreceptor mediated blunting of sympathetic vasoconstriction.