Carotid and aortic baroreflexes of the rat: II. Open-loop frequency response and the blood pressure spectrum

To determine the relationship between blood pressure (BP) variability and t he open-loop frequency domain transfer function (TF) of the baroreflexes, w e measured the pre- and postsinoaortic denervation (SAD) spectra and the ef fects of periodic and step inputs to the aortic depressor nerve and isolate d carotid sinus of central nervous system-intact, neuromuscular-blocked (NM B) rats. Similar to previous results in freely moving rats, SAD greatly inc reased very low frequency (VLF) (0.01-0.2 Hz) systolic blood pressure (SBP) noise power. Step response-frequency measurements for SBP; interbeat inter val (IBI); venous pressure; mesenteric, femoral, and skin blood flow; and d irect modulation analyses of SBP showed that only VLF variability could be substantially attenuated by an intact baroreflex. The -3-dB frequency for S BP is 0.035-0.056 Hz; femoral vascular conductance is similar to SBP, but m esenteric vascular conductance has a reliably lower and IBI has a reliably higher -3-dB point. The overall open-loop transportation lag, of which less than or equal to0.1 s is neural, is approximate to1.07 s. Constrained alge braic solution, over a range of frequencies, of the pre- and postSAD endoge nous noise spectra and the independently determined relative frequency and absolute lag measurements was used to calculate the absolute gain for the o pen-loop TF. The average gain at 0.02 Hz, the frequency of maximum sensitiv ity, was 1.47 (95% confidence interval = +/-0.48), which agrees well with e stimates for the dog reversible sinus. We found that, in the NMB rat, the e ffects of SAD on the BP noise spectrum were accounted for by the open-loop properties of the baroreflex.