Effect of 4-hydroxypyrazolo (3,4-d) pyrimidine (allopurinol) on postirradiation cerebral blood flow: implications of free radical involvement.

In an attempt to elucidate mechanisms underlying the irradiation-induced decrease in regional cerebral blood flow (rCBF) in primates, hippocampal and hypothalamic blood flows of rhesus monkeys were measured by hydrogen clearance, before and after exposure to 100 Gy, whole body, gamma irradiation. Systemic blood pressures were monitored simultaneously. Compared to control animals, the irradiated monkeys exhibited an abrupt decline in systemic blood pressure to 35% of the preirradiation level within 10 min postirradiation, falling to 12% by 60 min. A decrease in hippocampal blood flow to 32% of the preirradiation level was noted at 10 min postirradiation, followed by a slight recovery to 43% at 30 min and a decline to 23% by 60 min. The hypothalamic blood flow of the same animals showed a steady decrease to 43% of the preirradiation levels by 60 min postirradiation. The postradiation systemic blood pressure of the allopurinol treated monkeys was not statistically different from the untreated, irradiated monkeys and was statistically different from the control monkeys. However, the treated, irradiated monkeys displayed rCBF values that were not significantly different from the nonirradiated controls. These findings suggest the involvement of free radicals in the postirradiation decrease in regional cerebral blood flow but not necessarily in the postirradiation hypotension seen in the primate.