Evidence that ATP is a neurotransmitter in the frog heart.

In the presence of atropine, electrical field stimulation of frog atria produced a positive inotropic response. This response had two components. The initial phase was blocked by selective desensitization of P2-purinoceptors with alpha, beta-methylene ATP. The second phase, which was not always clearly separated from the initial phase, was blocked by the beta-adrenoceptor antagonist propranolol. The present results suggest that ATP may be a cotransmitter released with adrenaline from nerve terminals of the vagosympathetic trunk supplying the frog atria.