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Functional Role of Alpha‐1 Adrenergic Receptors in regulating Cerebral Vascular tone in Healthy Humans during Exercise

The role of the sympathetic nervous system on the cerebral vessels remains equivocal. However, blockade of the á‐1 adrenoreceptors led to impairment of cerebral autoregulation during acute hypotension suggesting a sympathetically mediated influence on the cerebral vasculature. Therefore, we tested the hypothesis that activation and blockade of the á‐1 adrenoreceptors would result in an increased cerebral vascular tone and decreased cerebral vascular tone, respectively. In nine subjects (6 men and 3 women) beat‐to‐beat arterial blood pressure (ABP) and middle cerebral artery blood velocity (MCA V) were determined during rest and moderate workload dynamic exercise achieving a heart rate of 130 bpm (Ex130) on an upright cycle ergometer. Norepinephrine concentrations increased significantly from rest to Ex130 (from 1.8±0.2 to 3.2±0.3 pml [ml plasma]‐1, P< 0.05). An increase in critical closing pressure (CCP) was observed from rest to Ex130 (from 18±3 to 31±4 mmHg, P< 0.05) identifying an increase in cerebral vascular tone. However, 74% blockade of functional á‐1 adrenoreceptors with Prazosin abolished the increases in CCP during Ex130 (from 31±4 to 24±4 mmHg, P< 0.05). These findings identify the functional role of the cerebral vascular á‐1 adrenergic receptors acting in conjunction with the myogenic properties of cerebral vessels in establishing cerebral vascular tone.