Autonomic influence on skeletal muscle blood flow at the onset of dynamic exercise

At the onset of exercise there is a rapid increase in blood flow to active skeletal muscles. There are conflicting data regarding the role of the autonomic nervous system in regulating skeletal muscle blood flow during exercise. The purpose of this study was to determine if the autonomic nervous system influences skeletal muscle vasodilation at the onset of dynamic exercise. Mongrel dogs (n=5) were instrumented with transit time ultrasound flow probes on both external iliac arteries and catheters in surgically exteriorized carotid arteries. Continuous measurements of blood flow and blood pressure were recorded at rest and for the first 30 sec of exercise on a treadmill at 3 mph. One experiment was performed under control conditions and one after ganglionic blockade by iv infusion of hexamethonium (10 mg/kg) and atropine (0.2 mg/kg). With initiation of exercise, immediate increases in iliac blood flow were observed under both conditions. The initial increases in hindlimb vascular conductance over the first 5 sec of exercise were similar for control (6.85±0.50 ml/min/mmHg) and ganglionic block conditions (7.25±0.58 ml/min/mmHg). At 30 sec, vascular conductances were 9.27±0.88 and 12.30±0,87 ml/min/mmHg, respectively. Since autonomic blockade did not attenuate the initial increase in hindlimb conductance, we conclude that the autonomic nervous system does not contribute to skeletal muscle vasodilation at the onset of exercise. On the other hand, the elevated hindlimb conductance at 30 sec suggests substantial autonomic restraint of skeletal muscle blood flow during exercise.