The reactivity of canine cerebral arteries to O2 and CO2 in vitro

The responses of canine middle cerebral arteries to changes in pCO2 and pO2 were tested in vitro. It was found that there was no response to changes in pCO2 from 38.1 mm. Hg to 26.6 mm. Hg, but there was some constriction of the vessels with lowering of the pCO2 below 26.6 mm. Hg and there was minimal dilatation of the vessels when the pCO2 was increased from 38.1 mm. Hg to 87.2 mm. Hg. There was no response to changes in pO2 from more than 55 mm. Hg to 59.6 mm. Hg, but when pO2 was lowered below 50 mm. Hg there was a sudden, massive constriction of the arteries tested. It is postulated that this constriction is due to build-up of a substance (substances) during a period of hypoxia (pO2 less than 50 mm. Hg). The significance of the results obtained are discussed.