Pre- and postsynaptic effects of p-tyramine and p-octopamine in the prostatic portion of the rat vas deferens

Summary The effects of p-tyramine and p-octopamine on the twitch responses of the prostatic portion of the rat vas deferens to electrical stimulation (0.025 Hz) were compared with the effects of noradrenaline. In tissues with normal monoamine oxidase (MAO) activity, the three amines increased the height and duration of the twitch contractions. When MAO activity was inhibited by pargyline (10 mol/l), p-tyramine and p-octopamine had mixed excitatory-inhibitory effects on the twitches, while noradrenaline had mostly excitatory effects along the whole range of concentrations assayed (0.158–15.8 mol/l). Selective blockade of ₁- and ₂-adrenoceptors, by corynanthine and yohimbine, respectively, showed that the excitatory effect of the amines depended on the activation of ₁-adrenoceptor and that the inhibitory action was related to the activation of ₂-adrenoceptors. Pretreatment with reserpine (5 mg/kg, 24 h; 2.5 mg/kg, 2 h before the experiment) largely prevented the effects of p-tyramine and p-octopamine, but the amines still modified the twitch responses to field stimulation. The addition of corynanthine and yohimbine to the bathing fluid revealed a considerable activation of ₁-excitatory and ₂-inhibitory adrenoceptors. Cocaine (10 mol/l) did not antagonize, but rather enhanced the inhibitory effects of p-tyramine and p-octopamine in tissues with normal contents of noradrenaline. Moreover, cocaine did not antagonize the inhibition caused by p-tyramine, and enhanced the inhibition induced by p-octopamine in the prostatic portion of the vasa deferentia from reserpine-pretreated animals. These results suggest that in this tissue, at least when MAO activity is inhibited, p-tyramine and p-octopamine behave similarly. The effects of both amines on the twitch contractions depend on the noradrenaline-releasing action of the compounds and, in addition, the compounds seem to activate adrenoceptors directly. Send offprint requests to S. M. Celuch at the above addressCareer Investigator on leave of absence from the Consejo de Investigaciones Científicas y Técnicas, ININFA, Junín 956, 5°P, RA-1113 Buenos Aires, Argentina