Effect of diazoxide-induced hypotension on cerebral blood flow in hypertensive rats

The effect on cerebral blood flow of acute diazoxide-induced hypotension was studied in rats with renal and spontaneous hypertension. Diazoxide (5 mg/kg, i.v. bolus), caused arterial pressure to fall rapidly to that of normotensive rats, i.e. c. 75 mmHg. There was a concomitant fall in cerebral blood flow of about 35% (P less than 0.01) in renal hypertensive rats and 25% (P less than 0.05) in spontaneously hypertensive rats; the greater fall in flow in the former corresponded to a greater drop in pressure. Flow remained at these reduced levels during a 2 h observation period. Histological examination revealed small areas of ischaemic damage in the brains of five of the twelve animals. In control hypertensive rats not given diazoxide, cerebral blood flow and blood pressure were stable during a 2 1/2 h period and there was no evidence of ischaemic damage to the brains. The diazoxide-induced reduction in cerebral blood flow was interpreted as being secondary to a blood pressure fall to below the lower limit of cerebral blood flow autoregulation. No evidence was found of direct effects on the cerebral circulation such as seen with ganglionic blockers, alpha-blockers and cerebral vasodilators.