Role of alpha-adrenoceptors in the control of the cerebral blood flow response to hypoxia.

This study assessed the role of vascular and central alpha-adrenoceptors in the regional cerebral blood flow response to moderate hypoxia. Studies were conducted in 21 rabbits using radioactive microspheres under normoxic and hypoxic (10% O2 in N2) conditions. Animals were divided into three groups and administered either saline, N-methyl chlorpromazine, or phenoxybenzamine. During normoxia, there were regional differences in cerebral blood flow distribution in the saline- and N-methyl chlorpromazine-treated rabbits which were eliminated by phenoxybenzamine. In control, hypoxia significantly increased average cerebral blood flow from 57 +/- 22 to 132 +/- 52 ml/min per 100 g. Flow to the hindbrain increased to a significantly greater extent than to the mid- or forebrain during hypoxia. The increase in average cerebral blood flow during hypoxia was significantly reduced to 97 +/- 34 ml/min per 100 g by phenoxybenzamine. Both alpha-adrenoceptor antagonists prevented the significantly greater increase in hindbrain flow during hypoxia. The greater flow responsiveness of the hindbrain to hypoxia appears to be related at least in part to alpha-adrenoceptors found in the cerebral vasculature.