Carotid and aortic baroreflexes of the rat: I. Open-loop steady-state properties and blood pressure variability

loop steady-state properties and blood pressure variability. Am J Physiol Regulatory Integrative Comp Physiol 279: R1910–R1921, 2000.—To characterize the baroreflex in central nervous system-intact neuromuscular-blocked rats, we measured the vascular and cardiac responses and com-pared direct stimulation of the aortic depressor nerve (ADN) with a capacitance electrode (differentially activat-ing either A or A 1 C fibers) to carotid sinus pressure with a micro-balloon (SINUS). One-thousand-two-hundred-ninety-seven open-loop measurements of systolic blood pressure (SBP), heart rate, venous pressure (VBP), and mesenteric (msBF), femoral (fmBF), and skin (skBF) blood flow were completed; the linear range of the effects was determined for each response and stimulus mode. The rats were sino-aortic denervated (SAD). The open-loop stimulation effect was very stable; e.g., the mean effect of 790 ADN stimula-tions during.7 days was 29.8 mmHg, with an average drift of 10.001 mmHg/h. In contrast, there was large vari-ability of the SBP baseline (e.g., SD 5 610.9), which was due to SAD (66.3 to 616.3 mmHg, t 5 213.9, df 5 4, P, 0.0002) and was reversed by ganglionic block (610.8 to 6 2.9 mmHg, t 5 212.9, df 5 3, P, 0.001). The ADN stimuli produced larger depressor responses than sinus stimuli (266 vs. 245 mmHg); all component responses paralleled the magnitude of the SBP effect, except interbeat interval (IBI), for which the ADN DIBI was ’10 times that of SINUS. For all stimuli, fmBF increased and msBF did not. Mesenteric and femoral vascular conductance both increased, whereas VBP decreased and skBF followed SBP. We found that for all baroreflex response components, with the exception of SINUS-elicited DIBI, there was an orderly, substantially linear, relationship between stimulus strength and response magnitude. baroreceptors; aortic depressor nerve; carotid sinus; sinoaor-tic denervation; noise; baroafferent stimulatio