Effect of inosine on pial arterioles: potentiation of adenosine-induced vasodilation

We examined the cerebral vasoactivity of inosine and its effect on pial arteriolar vasodilation induced by adenosine. Pial circulation was observed through cranial windows implanted in rats anesthetized with halothane. No significant change in venous or arteriolar diameter was apparent when inosine (10(-6) to 10(-3) M) was superfused. In contrast, adenosine in concentrations of 10(-7) and 10(-6) M dilated pial arterioles by 9.0 +/- 1.2 and 17.7 +/- 1.7%, respectively. Addition of 10(-5) M inosine had no effect, whereas 10(-4) M inosine enhanced the vasodilation induced by 10(-7) M adenosine to 19.4 +/- 1.7% and that by 10(-6) M adenosine to 23.3 +/- 2.3%. When theophylline (5 X 10(-5) M) was perfused together with 10(-7) M adenosine and 10(-4) M inosine, the vasodilation was almost completely abolished. The present study indicates that inosine alone does not affect pial vessel diameter but potentiates the response of pial arterioles to exogenous adenosine. This potentiating action of inosine may be attributed to an increase in perivascular adenosine and may be explained by the action of inosine as an adenosine uptake inhibitor.