Passive leg movement-induced vasodilation and exercise-induced sympathetic vasoconstriction.

The role of nitric oxide (NO) as a modulator of functional sympatholysis has been debated in the literature, but the preponderance of evidence suggests that the magnitude of NO-mediated dilation is restrained by sympathetic vasoconstriction. Therefore, we hypothesized that passive leg movement (PLM)-induced vasodilation, which is predominantly NO-mediated, would be attenuated by an exercise-induced increase in muscle sympathetic nerve activity (MSNA). To test this hypothesis, MSNA, leg blood flow (LBF), and mean arterial blood pressure (MAP) were measured and leg vascular conductance (LVC) calculated in 9 healthy subjects (30 ± 3 yr), during PLM with and without sympathoexcitation evoked by arm-cranking exercise (ACE), at 25, 50, and 75% of maximal capacity. During this incremental intensity ACE, MSNA increased significantly (26 ± 2, 34 ± 3, and 41 ± 5 bursts/100 HB, respectively). LVC during PLM fell markedly (~1.2 ml/min/mmHg) with each increase in ACE intensity, and there was a strong relationship (r = 0.92; p < 0.05) between ∆MSNA and ∆Peak LVC induced by the three intensities of ACE. Thus, as anticipated, this study reveals that the, NO-mediated, PLM-induced vasodilation, is significantly and proportionally attenuated by exercise-induced MSNA. This finding highlights the dominant role of MSNA in regulating skeletal muscle vascular conductance.