Abstract

This study examines the possibility that changes of cerebral extracellular pH (PH e) or adenosine concentration may provide coupling mechanisms of a general nautre, adjusting cerebral blood flow (CBF) to metabolic demands. Although there is considerable indirect evidence that CBF varies inversely with pHe, results obtained during the last few years indicate that large increases in flow may occur in the absence of a fall in pHe. Thus, induction of hypoxia or epileptic seizures leads to maximal increase in CBF before pHe falls or even when there is initial alkalosis due to concomitant hypocapnia. Furthermore, CBF increases in hypoglycaemia and after administration of amphetamine, two conditions unassociated with tissue acidosis. The possibility that adenosine may be a coupling factor was examined in hypoxia and during epileptic seizures in rats. In both conditions a four- to fivefold increase in CBF occurs in spite of the fact that tissue adenosine concentrations remain at or below 1 mumolkg-u. It is concluded that adenosine accumulates first when there is a perturbation of cerebral energy state with a rise in AMP concentration. It seems unlikely that adenosine, formed by breakdown of AMP, acts as a general coupling factor.