IS ACTIVE SKELETAL-MUSCLE FUNCTIONALLY VASOCONSTRICTED DURING DYNAMICEXERCISE IN CONSCIOUS DOGS

We investigated whether the increase in hindlimb blood flow and vascul ar conductance in conscious dogs during graded dynamic exercise is fun ctionally restrained by the sympathetic nervous system. Dogs were chro nically instrumented to monitor terminal aortic blood flow (TAQ) as an index of hindlimb skeletal muscle blood flow and mean arterial pressu re (MAP). The extent of functional sympathetic tone was assessed by me asuring the increase in TAQ and terminal aortic vascular conductance ( TAG, calculated as TAQ/MAP) in response to intra-arterial infusion of the alpha-adrenergic antagonist prazosin (PZ; 50 mu g/kg) into the hin dlimbs at rest and during steady-state dynamic (treadmill) exercise ra nging from mild (3.2 km/h, 0% grade) to moder ately heavy (8 km/h, 15% grade) workloads. This dose of PZ completely abolished the large hind limb vasoconstrictor response to phenylephrine (1 mu g/kg ia). At rest , PZ increased TAQ by 0.10 +/- 0.02 l/min and TAC by 1.85 +/- 0.53 ml . min(-1). mmHg(-1). During exercise, as workload increased and the co ntrol levels of TAQ and TAC rose progressively, Delta TAQ and Delta TA C with PZ infusion also increased. At the highest workload, PZ increas ed TAQ by 0.41 +/- 0.07 l/min and TAC by 4.81 +/- 0.38 ml . min(-1)mmH g(-1). The increases in TAQ and TAC with PZ were linearly related to t he control level of TAQ, indicating that as workload increases, progre ssively greater restraint of muscle vasodilation by the sympathetic ne rvous system occurs. We conclude that during dynamic exercise in consc ious dogs the sympathetic nervous system progressively restrains the n ormal vasodilation in active skeletal muscle, thereby limiting skeleta l muscle perfusion.