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Departments of Anesthesiology and Physiology, Medical College of Wisconsin and Veterans Affairs Medical Center, Milwaukee, Wisconsin 53295
The purpose of this study was to determine
whether the autonomic nervous system is involved in skeletal muscle
vasodilation at the onset of exercise. Mongrel dogs
(n = 7) were instrumented with flow
probes on both external iliac arteries. Before treadmill exercise at 3 miles/h, 0% grade, hexamethonium (10 mg/kg) and atropine (0.2 mg/kg)
or saline was infused intravenously. Ganglionic blockade increased
resting heart rate from 87 ± 5 to 145 ± 8 beats/min (P < 0.01) and reduced mean arterial
pressure from 100 ± 4 to 88 ± 5 mmHg
(P < 0.01). During steady-state
exercise, heart rate was unaffected by ganglionic blockade (from 145 ± 8 to 152 ± 5 beats/min), whereas mean arterial pressure was
reduced (from 115 ± 4 to 72 ± 4 mmHg;
P < 0.01). Immediate and rapid
increases in iliac blood flow and conductance occurred with initiation
of exercise with or without ganglionic blockade. Statistical analyses
of hindlimb conductance at 5-s intervals over the first 30 s of
exercise revealed a statistically significant difference between the
control and ganglionic blockade conditions at 20, 25, and 30 s
(P < 0.01) but not at 5, 10, and 15 s of exercise. Hindlimb conductance at 1 min of exercise was 9.21 ± 0.68 and 11.82 ± 1.32 ml · min
1 · mmHg1
for the control and ganglionic blockade conditions, respectively. Because ganglionic blockade did not affect the initial rise in iliac
conductance, we concluded that the autonomic nervous system is not
essential for the rapid vasodilation in active skeletal muscle at the
onset of exercise in dogs. Autonomic control of skeletal muscle blood
flow during exercise is manifested through vasoconstriction and not vasodilation.
conductance; ganglionic blockade; hexamethonium; vasodilation; dogs
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