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 (G(c)) of HLBF regulation [G(c) = 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 G(c) during low-intensity exercise to 0.07 +/- 0.2. Antagonism of adenosine receptors (aminophylline) decreased the G(c) 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.