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. 1 -2 These studies revealed that in the absence of the rapidacting reflex controllers of blood pressure, small increases in blood volume (5%) caused significant increases in total peripheral resistance (TPR) (22%). Whole body autoregulation has been defined to be a result of the summation of all the regional autoregulatory responses. These responses have been proposed to be responsible for the rise of TPR in volume-dependent forms of hypertension.
-7 The contribution of the regional autoregulatory responses in major flow regions to TPR in volume-dependent hypertension has remained uncertain. The three major flow regions evaluated in this study, which represent at least 75% of the cardiac output (CO), are the skeletal muscle, intestinal, and renal circulations. Given the potential importance of the responses of these regional circulations during changes of blood volume and CO in normal and hypertensive states, the present studies were designed 1) to characterize the autoregulation response to an increase in blood volume in the skeletal muscle, intestinal, and renal circulations in conscious rats, and 2) to compare the relative contributions of the mesenteric, renal, or hind limb circulations to the whole body autoregulation response in the same animal.
Methods
Surgical InstrumentationThree surgical procedures were performed to instrument the rats with regional Doppler flow probes, an aortic electromagnetic flow probe, and arterial and venous catheters. In the first operation, male Sprague-Dawley rats weighing 300-350 g were anesthetized with a mixture of acepromazine (5 mg/kg i.m.) and ketamine (50 mg/kg i.m.). Doppler flow probes with various lumen sizes were constructed in our lab according to the method of Haywood et al 8 using 20-MHz piezoelectric crystals (DBF-120A, Crystal Biotech Inc, Holliston, Mass.) and insulated silver-plated copper wire leads (Cooner Wire, C...