Pial vascular reaction to intravenous dihydralazine in the cat.

The diameters of pial vessels in the cat were measured through a closed cranial window after i.v. injection of dihydralazine (250 microgram or 1 mg x kg-1). The intracranial pressure was recorded from a needle in the cisterna magna. In 7 cats given 1 mg x kg-1 dihydralazine, arterial vessels with a resting diameter of 72 +/- 24 micron (SD) dilated by 58 +/- 21% (p < 0.01), with no significant change seen in the veins. The intracranial pressure increased by 95 +/- 30%. The maximum increase in intracranial pressure and arterial diameter was observed before the blood pressure had reached its lowest level. The dilatation far exceeded the autoregulatory dilatation observed at corresponding pressure reductions induced by bleeding. It is concluded that dihydralazine dilates pial arterial vessels.