Transient metabolic and vascular volume changes following rapid blood pressure alterations which precede the autoregulatory vasodilation of cerebrocortical vessels.

It has been shown by surface fluoro-reflectometry that stepwise decrease of arterial blood pressure causes a biphasic cerebrocortical vascular volume response. After the arterial blood pressure decrease the vascular volume first decreased and later increased. In both parts of the biphasic reflectance change, the cerebrocortical NAD-NADH redox state shifted considerably towards reduction and there was no reoxidation after the onset of cortical vasodilatation. Since a very rapid NADH reduction occurred during the first 30 secs. of the arterial hypotension in parallel with the vascular volume decrease, it is suggested that in the transient phase of arterial hypotension cerebral hypoxia may occur. Furthermore it is suggested that anaerobic tissue metabolites or some unknown NAD-NADH dependent process might dilate the cerebrocortical arterial network during the autoregulatory adjustment of CBF. The participation of the sympathetico-adrenal system in transient brain hypoxia caused by bleeding is a possibility since both the early vasoconstriction and the steep NADH reduction were prevented by the administration of phenoxybenzamine (1 mg/kg) before bleeding.