Some properties of the junctional and extrajunctional receptors in the vas deferens of the guinea-pig

1. Pharmacological properties of the neuromuscular junction in the isolated hypogastric nerve-vas deferens preparation of the guinea-pig were studied using the sucrosegap technique.
2. Earlier observations concerning the ineffectiveness of α-adrenergic blocking agents on transmission were confirmed.
3. Noradrenaline exerted a dual influence on the nerve-mediated contraction, i.e. inhibitory at a low concentration (10^?8 to 10^?7 g/ml) but potentiating at a higher concentration.
4. Noradrenaline invariably reduced the size of the excitatory junction potential (EJP) without altering the resting potential. This effect was not blocked by β-adrenergic blocking agents. The inhibitory action of a low concentration of noradrenaline may be accounted for by this EJP-suppressing effect.
5. In the presence of an α-adrenergic blocking agent, noradrenaline reduced the height of nerve-induced contraction even at a high concentration. This is explained by the selective blocking of the potentiating effect without altering the EJP-depression.
6. Tyramine reduced the height of nerve-mediated contraction.
7. These experimental results can consistently be explained by assuming the presence of two different kinds of adrenergic receptors in the vas deferens, ‘junctional’ and ‘extrajunctional’. While the extrajunctional receptors are typical α-type receptors, the junctional receptors are resistant to both α- and β-blocking agents, and seem to be desensitized by noradrenaline.
8. The K⁺-contracture was also reduced in size by the combination of dibenamine and noradrenaline. The assumption that the transmitter released from depolarized nerve-endings would contribute to the height of K⁺-contracture was substantiated by experiments using reserpinized vas deferens.