Inosine transiently decreases coronary flow but potentiates vasodilation by adenosine

To test whether inosine interferes with the action of adenosine, we investigated the effects of intracoronary administration of inosine, adenosine, 8-phenyltheophylline, and adenosine deaminase on isolated rat heart. Inosine did not change heart rate or developed tension but increased the effluent adenosine concentration. Inosine also decreased exogenous adenosine uptake and breakdown. After the start of the inosine infusion (20 microM), a transient 30% decrease of coronary flow took place within 0.5 min. The nucleoside caused sustained vasodilation, dependent on the concentration (30-400 microM). After the infusion we observed a transient vasodilation. A nonvasoactive inosine concentration (10 microM) combined with an adenosine concentration that increased flow by 60% (0.1 microM) raised vasodilation by another 60%. Infusion of adenosine, adenosine deaminase, or 8-phenyltheopylline did not influence the inosine-induced transient decrease of flow, suggesting that this decrease is independent of adenosine and its receptor. We conclude that inosine 1) potentiates the vasodilation induced by adenosine and 2) is a coronary vasodilator but probably also a vasoconstrictor.