Carotid Baroreflex Regulation Of The Cerebral Vasculature In Humans During Rest And Dynamic Exercise

Abstract

Recently, a number of studies have indicated that sympathetic activity influences cerebral vascular tone during exercise and hypotension. In humans, at rest the peripheral vasculature is controlled by the autonomic nervous system and the carotid baroreflex plays an important role in beat-to-beat blood flow regulation and arterial blood pressure. PURPOSE: To test the hypothesis that arterial baroreflex control of sympathetic nerve activity reflexly regulates the cerebral vasculature at rest and during dynamic exercise in humans. METHODS: In seven subjects (5 men and 2 women) pulsatile stimulation of the carotid baroreceptors was performed using neck pressure (NP) at + 40 mmHg and neck suction (NS) stimuli at - 40 mmHg at a predetermined frequency of 0.1 Hz to entrain the responses of the mean arterial blood pressure (MAP), middle cerebral artery blood velocity (MCA V) and cerebral tissue oxygenation (ScO2). The pulsatile NP/NS stimuli were performed during rest and two workload (WL) intensities; low WL at 10W and moderate WL achieving heart rates of 84 bpm (Ex84) and 130 bpm (Ex130), respectively, on an upright bicycle ergometer without and with Prazosin (72% blockade of the α-1 adrenoreceptors achieved). Measurements of beat-to-beat ABP, MCA V and ScO2 were obtained during rest and each exercise WL. RESULTS: The NP entrainment of the carotid baroreceptors augmented the power spectral density (PSD) of the MAP (P = 0.048), MCAV (P = 0.02) and ScO2 (P = 0.006) at rest and during exercise compared to no neck collar stimulation. The PSD for MCA V and ScO2 with NP stimulation was markedly greater than the PSD of MAP mediated by NP induced sympathoexcitation during rest and exercise. However, Prazosin blockade decreased the PSD of the NP stimuli of MAP, MCA V and ScO2 during rest and exercise. The percentage change of PSD from baseline measurements of MCA V and ScO2 compared to that of the MAP indicates the presence of another factor in addition to the changes in myogenic tone. CONCLUSIONS: These data strongly suggest that in addition to the myogenic responses to the pulsatile changes in perfusion pressure, the NP/NS induced alterations in sympathetic neural activity are involved in the beat-to-beat regulation of the cerebral vasculature.