Alpha-adrenergic blockade with phenoxybenzamine enhances cerebrovascular CO2 reactivity

The influence of sympathetic nervous activity on cerebral circulation and cerebrovascular CO2 reactivity was investigated by an alpha-adrenergic blockade with phenoxybenzamine (PBZ). Cerebral oxygen and carbon dioxide tension (BrPo2, BrPco2) and arterial blood pressure were continuously recorded before, during and after intracarotid infusion of 5 mg/kg of PBZ. The effects of 5% CO2 inhalation were measured before and after the infusion of PBZ. Following the intracarotid infusion of PBZ, BrPo2 and BrPco2 did not change significantly. After the alpha-adrenergic blockade the degree of the increase in BrPo2 during 5% CO2 inhalation was significantly enhanced. The increase in the cerebrovascular CO2 reactivity produced by low dose PBZ suggests that the sympathetic nervous system modifies cerebrovascular CO2 reactivity.