RESUMEN
The isolated right rat atrium was used to investigate the chronotropic effects of dl-tranylcypromine and its d- and l-isomers. The concentration-effect curves of the three compounds were similar. The response of the preparation to the three drugs was completely blocked by pretreatment of the animal with reserpine, thus indicating an indirect effect of the drugs by the release of the natural mediator. Cocaine present in the bathing fluid partially antagonized the effect of dl-tranylcypromine and its isomers. It is concluded that the accumulation of the different forms of the drug by neural structures and the subsequent release of the neurotransmitter are not stereospecific processes. Furthermore it is suggested that the efflux of the neurotransmitter in the rat atrium may be carrier-mediated, and that this process is inhibited by cocaine.