Role of hyperglycaemia-related acidosis in ischaemic brain damage

Although previous results have shown unequivocally that pre-ischaemic hyperglycaemia aggravates brain damage due to transient ischaemia, several questions have remained unanswered. First, is the effect of hyperglycaemia due to a further fall in intra-and extracellular pH? Second, is aggravation of damage a step function or a continuous function of plasma glucose concentration or of pH? Third, which are the mechanisms responsible for aggravation of damage, notably for the transformation of selective neuronal damage to infarction, for oedema development, and for post-ischaemic seizures? Recent results have provided new information on all of these issues. Thus, normoglycaemic animals with superimposed hypercapnia showed a similar, albeit not identical, aggravation of ischaemic damage, suggesting that acidosis is one major mediator. Furthermore, experiments with graded increase in plasma glucose concentration revealed a threshold effect at values of 10-12 mM, while microelectrode measurements showed a narrow extracellular pH range (6.4-6.5) for post-ischaemic seizure development. These results suggest that aggravation of damage due to excessive acidosis is due to mechanisms with a steep pH dependence. Finally, results are now at hand suggesting that the effect of acidosis is not mediated by a further perturbation of cell calcium metabolism. The more likely mediators are free radicals. Thus, acidosis is known to enhance iron-catalysed production of reactive oxygen species, probably by releasing iron from its bindings to transferrin, ferritin and other proteins.