Alpha-adrenoreceptor antagonists and pial vessel diameter during hypercapnia and hemorrhagic hypotension in the cat.

It has been proposed that sympathetic activation may prevent maximum dilatation of extra-parenchymal cerebral vessels during hemorrhagic hypotension and hypercapnia. In the present study, the effect of alpha-adrenoreceptor antagonists (phenoxybenzamine 1.5 mg kg-1 or phentolamine 8 mg kg-1) on pial vessel diameter was studied in the cat during hypotension and hypercapnia alone or in combination. Two levels of hypercapnia were used (mean PaCO2 74 and 122 mm Hg respectively). Pial arterial vessels (resting diameter 40--184 micrometers) were observed by means of a closed cranial window technique using a Leitz intravital microscope, an image splitting eyepiece, and a videoangiometer, the latter giving continuous data on changes of the inspected vessel diameters. Alpha-adrenoreceptor antagonists did not further increase the arterial diameter in any of the situations studied.