Vasoconstrictor responsiveness in contracting human muscle: influence of contraction frequency, contractile work, and metabolic rate.

PURPOSE: The aim of this study was to examine whether independent effects exist between contractile work and metabolic demand (VO2m ) on vasoconstrictor responsiveness (i.e., functional sympatholysis) under different contraction durations matched for total contractile work in exercising human skeletal muscle.

METHODS: Ten young men performed rhythmic forearm contractions at 10 and 15% of maximum voluntary contraction (MVC) which consisted of muscle contractions using the same duty cycle but altering the duration of the contraction-relaxation cycles of exercise and included: 1) fast frequency contractions at 10% MVC (FFC10% ) using a contraction relaxation cycle at 1:2 s; 2) slow frequency contractions at 10% MVC (SFC10% ) at 2:4 s; and 3) SFC at 15% MVC (SFC15% ) at 2:4 s. Lower body negative pressure (LBNP) was applied to increase sympathetic vasoconstriction during forearm exercise. Brachial artery diameter and blood velocities (measured via Doppler ultrasound) determined forearm blood flow (FBF), and forearm vascular conductance (FVC) was calculated from FBF (ml min-1 ) and mean arterial blood pressure.

RESULTS: Results revealed that steady-state indices of FBF, FVC, and VO2m were greater (P < 0.05) in FFC10% and SFC15% vs. SFC10% . In addition, the magnitude of vasoconstriction (percent reduction in FVC) in response to reflex increases in sympathetic activity during LBNP was greater with SFC10% vs. FFC10% (-20.6 ± 3.0 vs. -11.1 ± 2.0%; P < 0.05), whereas there was no difference with FFC10% vs. SFC15% (-11.1 ± 2.0 vs. -11.8 ± 1.8%; P = 0.91).

CONCLUSIONS: Our data indicate that faster work-matched muscle contractions increase blood flow and metabolism, leading to improved functional sympatholysis as compared to slower work-matched muscle contractions in humans.