Midazolam selectively potentiates the A2A- but not A1-receptor-mediated effects of adenosine

Background: Inhibition of adenosine metabolism offers a unique approach to harness the cardioprotective properties of adenosine in a site- and event-specific manner. Benzodiazepines inhibit adenosine metabolism by blocking nucleoside transporter. Therefore, the authors studied the binding affinities of structurally different benzodiazepines to nucleoside transporter and benzodiazepine-induced potentiation of A(1)-adenosine (negative dromotropy) and A(2A)-adenosine (coronary vasodilation) receptor-mediated effects. Methods: In membranes from porcine striatum and guinea pig ventricle, competition binding assays to displace [H-3]nitrobenzylmercaptopurine riboside ([H-3]NBMPR) from nucleoside transporter were performed using alprazolam, chlorodiazepoxide, diazepam, flurazepam, and midazolam. The augmentation by the most potent benzodiazepine of A(1)- and A(2A)-adenosine receptor-mediated responses, elicited by exogenous administration of adenosine or brief periods of global hypoxia, was subsequently studied in guinea pig Langendorff-perfused hearts. Results: All benzodiazepines completely displaced [H-3]NBMPR in a concentration-dependent manner with Hill coefficients not significantly different from unity in both striatal and ventricular membranes. Midazolam was the most potent inhibitor of nucleoside transporter (ventricle: pK(i) = 5.22 +/- 0.41, K-i = 6 mu M). In isolated hearts, midazolam (5, 10, 20 mu M) significantly augmented coronary flow in a concentration-dependent manner in the presence of adenosine (30 nM), an effect reversed by ZM 241385, a selective A(2A)-receptor antagonist. In contrast, midazolam did nor increase the effect of adenosine (30 nM) on atrioventricular conduction. Similarly, midazolam potentiated A(2A)- but not A(1)-receptor-mediated effects of endogenous adenosine released during hypoxia. Conclusions: Structurally distinct benzodiazepines inhibit nucleoside transporter to different degrees. Midazolam selectively augments A(2A)- but not A(1)-receptor-mediated effects of adenosine by inhibiting nucleoside transporter.