Sensitization by chronic diazepam treatment of A2A adenosine receptor-mediated relaxation in rat pulmonary artery.

The effects of a 10-day i.p. treatment of rats with diazepam on responses to subtype selective adenosine receptor agonists were studied 3 h, 2 and 8 days after termination of diazepam treatment in isolated cardiovascular tissues possessing distinct adenosine receptors. After long-lasting diazepam exposure, the relaxation elicited by the specific A2A receptor agonist CGS 21680 was enhanced in rat main pulmonary arteries (a tissue containing A2A adenosine receptors). The increased sensitivity of A2A receptors observed 3 h and 2 days after withdrawal of diazepam was completely restored by the 8th day of the wash-out period. N6-cyclopentyladenosine (CPA)-induced suppression in mechanical activity of electrically stimulated rat atrial myocardium (a tissue containing A1 adenosine receptors) was not altered following diazepam treatment. In order to reveal the possible role of inhibition of membrane adenosine transport in the effects of diazepam (a moderate inhibitor of membrane adenosine transport), the action of a 10-day treatment with dipyridamole or S-(p-nitrobenzyl)-6-thioinosine (NBTI; prototypic adenosine uptake inhibitors) was also studied. Dipyridamole or NBTI treatment, like diazepam, increased the responsiveness of rat pulmonary artery to CGS 21680, but did not influence the cardiodepressive effect of CPA in electrically driven left atrial myocardium. The CGS 21680-induced relaxations were significantly antagonized by 10 nM ZM 241385 (a selective A2A adenosine receptor antagonist) in vessels of diazepam-treated rats. The relaxation responses to verapamil were unaltered in pulmonary arteries obtained from animals chronically treated with diazepam, dipyridamole or NBTI. These results suggest that chronic diazepam treatment is able to enhance the A2A adenosine receptor-mediated vascular functions, but does not modify the responses mediated via A1 receptors of rat myocardium, where nucleoside transport inhibitory sites of membrane are of a very low density. It is possible that sensitization of A2A adenosine receptor-mediated vasorelaxation is due to a long-lasting inhibition of membrane adenosine transporter during diazepam treatment.