[On the mode of action of serotonin in the isolated atrium of the hamster. I. Serotonin: amine of mixed action (author's transl)].

: 5-hydroxytryptamine (serotonin) produces a positive chronotropic effect in the isolated atrium of the hamster. The characterization of this effect using reserpine pretreatment, propranolol, pargyline, 6-hydroxydopamine or the combination of some of those drugs enable us to conclude that serotonin is an amine of mixed action in the isolated atrium of the hamster, that is, its positive chronotropic effect has a direct component, which is obtained with lower concentrations of agonist and an indirect component through the liberation of endogenous noradrenaline from the sympathetic nerve terminals. The highest concentrations of serotonin are responsible of that component. Pargyline potentiates the indirect component of 5-HT, because inhibition of intraneuronal monoamine oxidase (MAO) avoids the degradation of the noradrenaline released, but it does not potentiate the direct component, indicating that postsynaptic cardiac MAO has little importance in the metabolism of serotonin administered exogenously.