Hyperperfusion and enhanced metabolic state of the brain during sudden lowering of the cutaneous temperature in rats

Forty-eight anesthetized and artificially ventilated rats were pretreated with the ganglion blocking agent, pentolinium (5 mg/kg, i.v.), followed by continuous phenylephrine infusion for maintenance of normotension. The rats were subjected to mock treatment (16 rats), to sudden exposure to thermoneutral water (34-36 degrees C, 16 rats), or to cold water (16 degrees C, 16 rats). The latter treatment lowered the cutaneous temperature to 25-30 degrees C. Neither one of these treatments produced any significant changes in arterial pressure or heart rate. Exposure to cold elicited hyperperfusion (microsphere method) in each of the brain regions studied. The increase in cerebral cortical blood flow (by 59 +/- 8%, mean +/- SEM) was most significant. The cerebral cortical metabolic rate for oxygen was increased by 46 +/- 10%. During the cold exposure, excellent tight coupling between the metabolic rate and blood flow was observed in the cerebral cortex. Mock treatment and exposure to thermoneutral water resulted in only trivial increases in blood flow and in metabolic rate. Thus, during cold exposure, an enhanced metabolic state of the brain represented the primary determinant increasing the brain blood flow, provided that normotension was maintained.