Autoregulation of hind-limb blood flow in conscious dogs

We evaluated the efficiency of blood flow autoregulation of the hind-limb vascular bed of eleven conscious dogs during: resting conditions; graded levels of treadmill exercise; and increases in oxygen consumption produced by the administration of 2,4-dinitrophenol (DNP). Blood flow to the left hind limb was measured with an electromagnetic flow probe on the left external iliac artery. Hind-limb perfusion pressure was measured from a catheter in the deep femoral artery and was controlled via an externally inflatable occlusion cuff positioned just distal to the flow probe. Arterial pressure was measured in the abdominal aorta. Experiments were performed 5-16 days after instrumentation. Hind-limb pressure-flow (P-F) relationships were evaluated by decreasing hind-limb perfusion pressure in 4-5 small sequential 'square-wave' steps of 10-15 mmHg each while measuring flow. Each step decrease in perfusion pressure was maintained for 2 min. The efficiency of autoregulation was quantified by calculating the closed-loop gain of flow regulation (Gc) at each decrement in perfusion pressure utilizing the equation: Gc = 1-[(F0-Fn/F0)/(P0-Pn/P0)] where F0 and P0 are the starting (control) flows and pressures prevailing prior to decreasing perfusion pressure, and Fn and Pn are the new flows and pressures at each decrement in perfusion pressure. A Gc value less than 0 indicates a predominantly passive P-F relationship, while a Gc of 1 is perfect autoregulation of flow. When the dogs were at rest, decrements in hind-limb perfusion pressure were accompanied by almost equivalent decreases in flow, i.e. no autoregulation occurred, and Gc averaged -0.177 +/- 0.044 over the pressure range from 100-40 mmHg. During all levels of treadmill exercise (on gradients of 0, 7, or 21%), however, positive Gc values were found that averaged from 0.258 +/- 0.046 at a gradient of 0% to 0.392 +/- 0.041 at a gradient of 21% and were significantly different from Gc values found during rest at perfusion pressure ranges from 90-40 mmHg. The administration of DNP directly into the hind-limb circulation increased hind-limb blood flow from 199 to 481 ml/min. In the presence of DNP, Gc values were positive over perfusion pressure ranges from 100-40 mmHg and averaged 0.473 +/- 0.054. These data demonstrate that hind-limb blood flow is not autoregulated in resting dogs, but that significant autoregulation is manifest during conditions that increase oxygen consumption.(ABSTRACT TRUNCATED AT 400 WORDS)