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

Abstract

The effect 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 mumol/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 mumol/l). Selective blockade of alpha 1- and alpha 2-adrenoceptors, by corynanthine and yohimbine, respectively, showed that the excitatory effect of the amines depended on the activation of alpha 1-adrenoceptor and that the inhibitory action was related to the activation of alpha 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 alpha 1-excitatory and alpha 2-inhibitory adrenoceptors. Cocaine (10 mumol/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.(ABSTRACT TRUNCATED AT 250 WORDS)