Adenosine receptors involved in the inhibitory control of non-adrenergic non-cholinergic neurotransmission in guinea-pig atria belong to the A1 subtype.

We have previously shown that endogenous adenosine inhibits non-adrenergic, non-cholinergic (NANC) neurotransmission in isolated guinea-pig atria. In the present study the effect of adenosine analogues, such as N6 cyclopentyladenosine (CPA), 5' N-ethylcarboxamide adenosine (NECA), 2 chloroadenosine (2-CADO), R- and S-phenylisopropyladenosine (R- and S-PIA) on the cardiac response to transmural nerve stimulation has been tested in order to characterize the subtype of adenosine receptor involved in the inhibitory control of NANC neurotransmission. The effect of the adenosine antagonist 8-phenyltheophylline (8-PT) was then tested against CPA and NECA. The prototypical A-1 selective agonist CPA was the most active agonist, reducing the response to the stimulation of NANC nerves with an IC50 value of 2.8 nM; R-PIA, NECA and 2-CADO showed IC50 values of 9.5, 13.7 and 35 nM respectively. S-PIA was the least active agonist, showing an IC50 value (306 nM) about 30-fold greater than that of R-PIA (9.5 nM). None of the agonists tested was able to modify cardiac response to exogenous CGRP. Furthermore, 8-PT competitively antagonized the effect of CPA and NECA with very close pA2 values (6.77 +/- 0.01 and 6.63 +/- 0.08 respectively). From these findings we concluded that prejunctional inhibitory adenosine receptors on capsaicin sensitive sensory nerves of cardiac tissue belong to the A-1 subtype.