1. The pre- and post-junctional α-adrenoceptor blocking agent, phentolamine, was given by intravenous infusion to eight healthy volunteers during rest, forearm exercise and post-exercise.
2. Phentolamine produced a sustained increase in heart rate. The diastolic blood pressure decreased slightly whereas systolic and mean blood pressures remained unchanged. Phentolamine caused a marked increase in the forearm blood flow and a decrease in vascular resistance at rest and post-exercise, but did not influence the blood flow or vascular resistance in the exercising forearm.
3. There was a small increase in arterial blood glucose concentration, and a decrease in blood alanine concentration during drug infusion. Blood lactate was not affected by phentolamine. The arterial concentrations of free fatty acids and glycerol increased, and the concentration of triglycerides decreased during phentolamine infusion.
4. Forearm exchange of glucose, lactate, alanine, glycerol, free fatty acids, triglycerides and forearm oxygen consumption were not changed significantly.
5. These findings corroborate the concept that α-adrenoceptor induced vasoconstriction plays a subordinate role in exercising skeletal muscle. All of the metabolic findings might be explained as secondary to an increased noradrenaline release during phentolamine infusion.