EXCITATORY AND INHIBITORY EFFECTS OF NORADRENALINE ON THE ISOLATED GUINEA-PIG VAS DEFERENS

SUMMARY 1. Contractions of guinea-pig isolated vasa deferentia produced by noradrenaline were found to consist of several phases. The sustained part of the contraction was very small compared with maximal contractions produced by stimulation of the postganglionic nerves. The sustained phase was usually preceded by a rapid phasic contraction and in some experiments was followed by contraction on washing out the noradrenaline.
2. Contractions in response to noradrenaline were blocked by the α-adrenoceptor antagonist, phentolamine.
3. Noradrenaline-induced contractions were inhibited in the presence of low conditioning doses of noradrenaline, phenylephrine, oxymetazoline or methoxamine. This effect may provide an explanation for the observation that noradrenaline inhibits contractions produced by sympathetic nerve stimulation. Although it was presumably mediated via α-adrenoceptors the inhibitory effect was not due to depolarization alone since it could not be reproduced by increasing the potassium concentration of the Krebs solution.
4. The α-receptor agonists inhibited responses to noradrenaline or phenylephrine, but augmented potassium and acetylcholine contractions. This specificity of action eliminates the possibility that a distinct group of α-receptors solely mediating a relaxant effect were activated in these experiments but suggests receptor desensitization.
5. Receptor-specific desensitization was also found to be a property of the acetylcholine receptors in the vas.