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Frequency response characteristics of whole body autoregulation of blood flow in rats

Department of Integrative Physiology, The University of Iowa, Iowa City, Iowa

Submitted 5 December 2008 ; accepted in final form 20 February 2009

Previously, we demonstrated that very low-frequency (VLF) blood pressure variability (BPV) depends on voltage-gated L-type Ca²⁺-channels, suggesting that autoregulation of blood flow and/or myogenic vascular function significantly contributes to VLF BPV. To further substantiate this possibility, we tested the hypothesis that the frequency response characteristic of whole body autoregulation of blood flow is consistent with the frequency range of VLF BPV (0.02–0.2 Hz) in rats. In anesthetized rats (n = 11), BPV (0.016–0.5 Hz) was induced by computer-regulated cardiac pacing while blood pressure, heart rate, and cardiac output (CO) were recorded during control conditions (NaCl, 1 ml/h iv) and during ₁-adrenergic receptor stimulation (phenylephrine, 1 mg·ml^–1·h^–1 iv) that has been reported to facilitate myogenic vascular function. Baroreceptor-heart rate reflex responses were elicited to confirm a functional baroreflex despite anesthesia. During control conditions, transfer function analyses between mean arterial pressure (MAP) and CO, and between MAP and total vascular conductance (CO/MAP) indicated autoregulation of blood flow at 0.016 Hz, passive vascular responses between 0.033 and 0.2 Hz, and vascular responses compatible with baroreflex-mediated mechanisms at 0.333 and 0.5 Hz. Stimulation of ₁-adrenergic receptors extended the frequency range of autoregulation of blood flow to frequencies up to 0.033 Hz. In conclusion, depending on sympathetic vascular tone, whole body autoregulation of blood flow operates most effectively at frequencies below 0.05 Hz. This frequency range overlaps with the lower end of the frequency band of VLF BPV in rats. Baroreceptor reflex-like mechanisms contribute to LF (0.2–0.6 Hz) but not VLF BPV-induced vascular responses.

cardiac pacing; blood pressure variability; myogenic vascular function