Excessive oxygen or glucose supply does not alter the blood flow response to somatosensory stimulation or spreading depression in rats

We investigated the influence of hyperoxia (arterial pO₂ 446 ± 43 mmHg) and hyperglycemia (blood glucose 19.4 mmol/l) on somatosensory stimulation (whisker deflection) employing laser Doppler flowmetry (LDF). Our aim was to test the hypothesis that a possible substrate-sensing mechanism for glucose and oxygen contributes to the coupling between cortical activity and regional cerebral blood flow (rCBF) in order to match increased demand with substrates. In addition, we looked at the influence of hyperglycemia (blood glucose 17.9 mmol/l) and hypercapnia (arterial pCO₂ 62 mmHg) on rCBF (LDF) and regional cerebral blood oxygenation changes (rCBO) in the even stronger metabolic stimulus of cortical spreading depression (CSD). For the latter we employed the new non-invasive technique of near infrared spectroscopy (NIRS). All experiments were done using chloralose/urethane-anesthetized rats. Somatosensory stimulation increased rCBF by about 20% of baseline, in the case of both norm- and hyperoxia as well as both normo- and hyperglycemia. The blood-flow response to CSD consisted of a temporary sharp increase in rCBF to more than 400%. At the same time, the concentration of oxyhemoglobin [HbO₂] increased, while deoxyhemoglobin [Hb] decreased, indicating excessive oxygenation. Hyperglycemia altered neither the rCBF nor the rCBO response. Preexisting hypercapnia, however, produced reductions in both hyperperfusion (rCBF) and hyperoxygenation (rCBO) during CSD. We found that, for experimental hyperglycemia, i.v. may be superior to i.p. application of glucose because of the latter's side effects in connection with blood flow. Our findings cannot support the hypothesis of a substrate sensing mechanism in coupling.