Contribution of regional vascular responses to whole body autoregulation in conscious areflexic rats

We designed studies to evaluate the autoregulation response during volume expansion in three major circulation regions (intestine, kidney, and hind limb) during simultaneous determination of whole body autoregulation in conscious areflexic rats. Cardiac output was measured with chronically implanted electromagnetic flow probes on the ascending aorta. Regional blood flow velocity was measured with pulsed Doppler flow probes on the superior mesenteric (n = 7), left renal (n = 7), and right iliac (n = 7) arteries. Doppler flow probes were calibrated in situ in each rat to determine regional blood flow values. Neurohumoral reflex control of pressure was removed pharmacologically, and blood pressure and cardiac output were returned to resting control values with intravenous norepinephrine infusion, which was maintained at that constant level throughout the study. Hemodynamic changes were measured in response to blood volume expansion with infusion of 0.9 ml blood over 6 minutes. This small change in blood volume resulted in significant increases in vascular resistance of 15% in the whole body, 8% in the intestine, 18% in the kidney, and 15% in the hind limb. The pressure-flow slope, used as an index of autoregulation (slope = 0, perfect autoregulation; slope = 1, rigid vasculature), averaged 0.34 in the whole body, 0.52 in the intestine, 0.19 in the kidney, and 0.39 in the hind limb. When compared with the whole body, blood flow autoregulation was less in the intestine, greater in the kidney, and the same in the hind limb.