Dynamic exercise attenuates sympathetic responsiveness of canine vascular smooth muscle

The phenomenon of reduced responsiveness of the skeletal muscle arterial va sculature to sympathetic activation during exercise (sympatholysis) remains controversial. The purpose of this study was to examine the vascular effec ts of sympathoactivation in dynamically exercising skeletal muscle. Mongrel dogs (19-24 kg) were instrumented chronically with transit-time ultrasonic flow probes on the external iliac arteries. After pretreatment with atropi ne (0.2 mg/kg), an intravenous bolus (4 mug/kg) of a nicotinic ganglion sti mulant [1,1-dimethyl-4-phenylpiperazinium iodide (DMPP)] was given at rest and during treadmill exercise at graded intensities. Administration of DMPP was associated with prompt reductions in iliac blood flow and increases in arterial pressure under all conditions. There were significant reductions (P < 0.05) in iliac vascular conductance of 58 +/- 4 (SE), 48 +/- 3, 36 +/- 5, and 16 +/- 3% at rest, 3 miles/h and 0% grade, 6 miles/h and 0% grade, and 6 miles/h and 15% grade, respectively. These data demonstrate that acti vation of postganglionic sympathetic nerves with DMPP caused vasoconstricti on in the skeletal muscle vasculature at rest and during exercise. Addition ally, the magnitude of vasoconstriction was inversely related to exercise i ntensity. These results support the concept of exercise sympatholysis.