Midazolam selectively potentiates the A(2A) - but not A1- receptor--mediated effects of adenosine: role of nucleoside transport inhibition and clinical implications.
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 A1-adenosine (negative dromotropy) and A2A-adenosine (coronary vasodilation) receptor-mediated effects.


METHODS
In membranes from porcine striatum and guinea pig ventricle, competition binding assays to displace [3H]nitrobenzylmercaptopurine riboside ([3H]NBMPR) from nucleoside transporter were performed using alprazolam, chlorodiazepoxide, diazepam, flurazepam, and midazolam. The augmentation by the most potent benzodiazepine of A1- and A2A-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 [3H]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:pKi = 5.22+/-0.41, Ki = 6 microM). In isolated hearts, midazolam (5, 10, 20 microM) significantly augmented coronary flow in a concentration-dependent manner in the presence of adenosine (30 nM), an effect reversed by ZM 241385, a selective A2A-receptor antagonist. In contrast, midazolam did not increase the effect of adenosine (30 nM) on atrioventricular conduction. Similarly, midazolam potentiated A2A- but not A1-receptor-mediated effects of endogenous adenosine released during hypoxia.


CONCLUSIONS
Structurally distinct benzodiazepines inhibit nucleoside transporter to different degrees. Midazolam selectively augments A2A- but not A1-receptor-mediated effects of adenosine by inhibiting nucleoside transporter.
Seubert
C N
CN
Department of Anesthesiology, University of Florida College of Medicine, Gainesville 32610-0254, USA.
Morey
T E
TE
Martynyuk
A E
AE
Cucchiara
R F
RF
Dennis
D M
DM
D000241:Adenosine / Q000378:metabolism / Q000494:pharmacology*; D018686:Anesthetics, Intravenous / Q000494:pharmacology*; D000818:Animals; D001283:Atrioventricular Node / Q000187:drug effects; D003326:Coronary Circulation / Q000187:drug effects; D004357:Drug Synergism; D018757:GABA Modulators / Q000494:pharmacology*; D006168:Guinea Pigs; D006321:Heart / Q000187:drug effects; D006329:Heart Conduction System / Q000187:drug effects; D006352:Heart Ventricles / Q000187:drug effects; D066298:In Vitro Techniques; D008566:Membranes / Q000187:drug effects / Q000378:metabolism; D008874:Midazolam / Q000494:pharmacology*; D017072:Neostriatum / Q000187:drug effects / Q000378:metabolism; D009705:Nucleosides / Q000378:metabolism; D058906:Purinergic P1 Receptor Agonists; D043705:Receptor, Adenosine A2A; D013552:Swine
D018686:Anesthetics, Intravenous; D018757:GABA Modulators; D009705:Nucleosides; D058906:Purinergic P1 Receptor Agonists; D043705:Receptor, Adenosine A2A; D000241:Adenosine; D008874:Midazolam