In rat atrium the characteristics of purinergic receptors were investigated by observing the effects of some purinergic receptor agonists and antagonists on action potential and contractile force. The statistically significant effects of ATP(10<sup>-6</sup> ~ 10<sup>-3</sup>M) and adenosine (10<sup>-6</sup> ~ 10<sup>-3</sup>M) on normal action potential characteristics were a dose-dependent shortening of action potential duration (APD<sub>90</sub>) by both agents and hyperpolarization by ATP(10<sup>-4</sup>,10<sup>-3</sup>M). CAP(10<sup>-8</sup> ~ 10<sup>-4</sup>M), an A<sub>1</sub> adenosine receptor agonist, shortened (APD<sub>90</sub>) markedly in a dose-dependent manner and these effects were almost abolished by DPCPX (10<sup>-6</sup> M), an A<sub>1</sub>, adenosine receptor antagonist, but not affected by DMPX(2 X 10<sup>-6</sup> M), an A<sub>2</sub> adenosine receptor agonist. On the other hand, CGS 21680(10<sup>-7</sup> ~ 10<sup>-4</sup>M), an A<sub>2</sub> adenosine receptor agonist, elicited a slight shortening of (APD<sub>90</sub>) and these effects were inhibited by DPCPX but persisted in the presence of DPMX. Adenosine (10<sup>-6</sup> ~ 10<sup>-4</sup> M) decreased the basal contraction of atrial muscle in a dose-dependent manner and these effects were not inhibited by DMPX but by DPCPX. These results suggests that purinergic receptor agonists depress the cardiac activity by a short ening of action potential duration and this effect is mostly mediated by A<sub>1</sub> adenosine receptors in rat atrium.
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