Prejunctional inhibition mediated via alpha 2-adrenoceptors in excitatory transmission of canine small intestine.

Noradrenaline(NA) inhibited the contractile responses of longitudinal and circular muscles of canine small intestine to field stimulation (20 Hz, 1 ms, supramaximal voltage for 5 s). The purpose of this study was to see if NA-induced inhibitory action is mediated via alpha 1- or alpha 2-receptors and whether the inhibitory action involved prostaglandins. In both muscle layers the alpha 2-agonist, clonidine inhibited the contractile response to field stimulation more potently than did the alpha 1-agonists phenylephrine or methoxamine. It is suggested that NA-mediated prejunctional inhibition is mediated via alpha 2-receptors. Yohimbine, an alpha 2-adrenoceptor blocking drug antagonized NA-induced prejunctional inhibition, while E-643, an alpha 1-blocker, did not affect the inhibition. These results suggest that the inhibitory action of NA is mediated via alpha 2-receptors in both muscle layers. Indomethacin, an inhibitor of prostaglandin synthesis, did not affect NA-induced prejunctional inhibition in circular muscle, while augmenting it in longitudinal muscle. It is thought that NA-induced prejunctional inhibition is not mediated via prostaglandin release.