Effect of theophylline treatment on the functional hyperaemic and hypoxic responses of cerebrocortical microcirculation.

The purpose of the present study was to elucidate the importance of extracellular adenosine (ADO) in the regulation of cerebrocortical microcirculation during rest, hypoxia, and brain activation. Cerebrocortical microcirculation and fluorescence of reduced nicotinamide adenine dinucleotide (NADH) were measured by surface fluororeflectometry through a cranial window. Arterial hypoxia and brain activation were produced by respirating the animals with a gas mixture containing 6-7% O2 and by injecting 4-6 mg/kg metrazol into the lingual artery, respectively. These reactions were used as test before and after theophylline (THEO) treatment. In some of the experiments only the cortical area beneath the cranial window was treated with THEO (10(-4) M), in others 2 X 10(-4) mol/kg THEO was injected intraperitoneally. Potency of THEO in antagonizing the cerebral blood flow (CBF) increasing effect of topically applied ADO was also tested. It was found that superfusion of the brain cortex with artificial cerebrospinal fluid (mock CSF) containing 10(-4) M THEO does not alter resting CBF, but inhibits the CBF increasing effect of 10(-6) M and 10(-5) M ADO by approximately 70% and 40%, respectively. Intraperitoneally injected THEO increased CBF by approximately 60%, which has been attributed mostly to its action on the systemic circulation. Under control conditions, arterial hypoxia and epileptic seizures increased CBF by approximately 150% and 300%, respectively. Since neither topical nor systemic THEO treatment altered the vasodilatory and CBF increasing potency of arterial hypoxia and attenuated these effects of epilepsy slightly, it was concluded that extracellular ADO is not a critical factor in the regulation of cerebrocortical microcirculation.