Responses of pial arterioles after prolonged hypercapnia and hypoxia in the awake rabbit.

The changes in the responsiveness of pial arterioles to CO2 and in the composition of cortical cerebrospinal fluid bathing these vessels were studied in the awake rabbit before and after 6 days exposure to hypercapnia (7% CO2) or hypoxia (10% O2). The vasodilator response of pial arterioles to inhalation of 3--10% CO2 was diminished after prolonged hypercapnia and enhanced after prolonged hypoxia. After both hypoxia and hypercapnia, pial arteriolar responsiveness to CO2 was immediately returned toward control levels by washing the brain surface with normal artificial cerebrospinal fluid. The bicarbonate concentration of cerebrospinal fluid bathing the pial vasculature showed a significant decrease after hypoxia and a significant increase after hypercapnia, whereas CSF pH remained unaltered. We conclude that the alteration in responsiveness of pial arterioles to CO2 is due to a change in the chemical composition of the CSF bathing these vessels, involving an adjustment in the concentration of bicarbonate ions.