Sympathetic regulation of the human cerebrovascular response to carbon dioxide

Although the cerebrovasculature is known to be exquisitely sensitive to CO(2), there is no consensus on whether the sympathetic nervous system plays a role in regulating cerebrovascular responses to changes in arterial CO(2). To address this question, we investigated human cerebrovascular CO(2) reactivity in healthy participants randomly assigned to the α(1)-adrenoreceptor blockade group (9 participants; oral prazosin, 0.05 mg/kg) or the placebo control (9 participants) group. We recorded mean arterial blood pressure (MAP), heart rate (HR), mean middle cerebral artery flow velocity (MCA(V mean)), and partial pressure of end-tidal CO(2) (Pet(CO(2))) during 5% CO(2) inhalation and voluntary hyperventilation. CO(2) reactivity was quantified as the slope of the linear relationship between breath-to-breath Pet(CO(2)) and the average MCAv(mean) within successive breathes after accounting for MAP as a covariate. Prazosin did not alter resting HR, Pet(CO(2)), MAP, or MCA(V mean). The reduction in hypocapnic CO(2) reactivity following prazosin (-0.48 ± 0.093 cm·s(-1) · mmHg(-1)) was greater compared with placebo (-0.19 ± 0.087 cm · s(-1) · mmHg(-1); P < 0.05 for interaction). In contrast, the change in hypercapnic CO(2) reactivity following prazosin (-0.23 cm · s(-1) · mmHg(-1)) was similar to placebo (-0.31 cm · s(-1) · mmHg(-1); P = 0.50 for interaction). These data indicate that the sympathetic nervous system contributes to CO(2) reactivity via α(1)-adrenoreceptors; blocking this pathway with prazosin reduces CO(2) reactivity to hypocapnia but not hypercapnia.