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

To characterize the baroreflex in central nervous system-intact neuromuscul ar-blocked rats, we measured the vascular and cardiac responses and compare d direct stimulation of the aortic depressor nerve (ADN) with a capacitance electrode (differentially activating either A or A + C fibers) to carotid sinus pressure with a micro-balloon (SINUS). One-thousand-two-hundred-ninet y-seven open-loop measurements of systolic blood pressure (SBP), heart rate , venous pressure (VBP), and mesenteric (msBF), femoral (fmBF), and skin (s kBF) blood flow were completed; the linear range of the effects was determi ned for each response and stimulus mode. The rats were sinoaortic denervate d (SAD). The open-loop stimulation effect was very stable; e.g., the mean e ffect of 790 ADN stimulations during > 7 days was -9.8 mmHg, with an averag e drift of +0.001 mmHg/h. In contrast, there was large variability of the S BP baseline (e.g., SD = +/-10.9), which was due to SAD (+/-6.3 to +/- 6.3 m mHg, t = -13.9, df = 4, P < 0.0002) and was reversed by ganglionic block (/-10.8 to +/- 2.9 mmHg, t = -12.9, df = 3, P < 0.001). The ADN stimuli prod uced larger depressor responses than sinus stimuli (-66 vs. -45 mmHg); all component responses paralleled the magnitude of the SBP effect, except inte rbeat interval (IBI), for which the ADN Delta IBI was approximate to 10 tim es that of SINUS. For all stimuli, fmBF increased and msBF did not. Mesente ric 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 Delta IBI, there was an orderly, sub stantially linear, relationship between stimulus strength and response magn itude.