Increase in skeletal muscle blood flow at onset of exercise is not mediated by β-adrenergic receptors

At the onset of exercise there is a rapid increase in blood flow to active skeletal muscle. Since β-adrenergic receptors are capable of mediating vasodilation in skeletal muscle, the purpose of this study was to determine if β receptors in the skeletal muscle vasculature are responsible for vasodilation at the onset of exercise. Mongrel dogs (n=4) were instrumented with transit time ultrasound flow probes on both external iliac arteries and a catheter in one femoral artery for drug infusion. Continuous measurements of blood pressure (BP) and blood flow (BF) were recorded throughout the experiments. A bolus intraarterial infusion of propranolol (β antagonist, 1 mg) or saline was given 30 sec before starting the treadmill at 3 mph. To test the effectiveness of the block, intraarterial isoproterenol was administered 1 min into exercise. Only one experiment was performed per day with each dog and each condition was performed twice in random order. An immediate rapid increase in iliac BF occurred with initiation of treadmill exercise. Measurements taken within the first 30 seconds of exercise showed peak BF was 578±37 ml/min following saline and 604±33 ml/min following propranolol. Although propranolol did not attenuate the exercise-induced hyperemia, the isoproterenol-induced increases in BF were abolished. There were no effects on HR, BP, or BF in the contralateral limb with either infusion. These results demonstrate that β-adrenergic receptors do not play an essential role in the rapid increase in blood flow to skeletal muscle at the onset of exercise.