Reflex versus autoregulatory control of hindlimb blood flow during treadmill exercise in dogs.

To evaluate the competition between local autoregulation and reflex neurohumoral control of hindlimb blood flow (HLBF), the hindlimb vascular pressure-flow relationship was determined in nine dogs in response to a 10% decrease in mean arterial pressure (AP) imposed during both low (3.0 km/h, 0% grade) and high (5.5 km/h, 14% grade) intensities of treadmill exercise. HLBF was measured with a Doppler flow probe on the left external iliac artery, and AP was controlled with a gravity reservoir connected to the left carotid artery. A 10 +/- 2% reduction in AP for 25 min caused HLBF to decrease 25 +/- 2% during low-exercise intensity but only 10 +/- 2% during high-exercise intensity. The corresponding closed-loop gains (Gc) of HLBF regulation [Gc = 1 - (% delta hindlimb blood flow/% delta hindlimb perfusion pressure) were -1.6 +/- 0.4 and -0.06 +/- 0.2 during low- and high-exercise intensity, respectively. Autonomic ganglionic blockade (hexamethonium) increased the Gc during low-intensity exercise to 0.07 +/- 0.2. Antagonism of adenosine receptors (aminophylline) decreased the Gc of HLBF regulation during high-intensity exercise to -0.57 +/- 0.3. These data demonstrate that in response to an imposed decrease in AP, autonomic vasoconstriction overrides autoregulatory vasodilatory mechanisms during low-intensity exercise. HLBF regulation increases at a higher exercise intensity, in part due to adenosine, but autoregulation does not predominate over arterial pressure regulating mechanisms.