Involvement of alpha-adrenoceptors in the excitatory effect of the A2 adenosine receptors agonist 5'-N-ethylcarboxamidoadenosine (NECA) on cardiac automaticity in the isolated right ventricle of the rat.

The effects of the non-selective A2 adenosine receptor agonist 5'-N-ethyl-carboxamidoadenosine (NECA) were studied on ventricular automaticity induced by a local injury in the isolated right ventricle of the rat. In concentrations ranging from 0.1 to 100 nM, NECA significantly increased ventricular automaticity. This effect was not apparent when the nonselective alpha-adrenoceptor blocker phenoxybenzamine was present at a concentration of 10 microM, which antagonizes both alpha 1- and alpha 2-adrenoceptors, as well as when rats were pretreated with reserpine. In non-reserpinized rats, the excitatory effect of NECA was also abolished in the presence of the selective alpha 1-adrenoceptor antagonist prazosin, but not in the presence of the alpha 2-adrenoceptor antagonist idazoxan. In reserpinized rats, the excitatory effect of NECA was restored in the presence of the non specific alpha-adrenoceptor agonist phenylephrine as well as in the presence of the selective alpha 1-adrenoceptor agonist amidephrine but not in the presence of the selective alpha 2-adrenoceptor agonist clonidine. These results suggest that the excitatory effect of NECA on ectopic ventricular automaticity is dependent on endogenous catecholamines and that alpha-adrenoceptors of type 1 are, in some way, involved in this effect.