Neither a purine nor VIP is the mediator of inhibitory nerves of opossum oesophageal smooth muscle.

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Abstract
Effects of stimulation of intramural nerves in the circular smooth muscle layer of the body of the esophagus of the opossum (D. marsupialis) were studied, simultaneously measuring the membrane potential of muscle cells using the sucrose-gap technique and contractions of the muscle. Electrical field stimulation of the preparation, superfused with Krebs solution at 27.degree. C, induced a transient hyperpolarization of the smooth muscle cell membrane (ijp) followed by a transient depolarization in which muscle action potentials were often superimposed. The muscle did not develop active tension spontaneously; it therefore did not relax during the ijp, but often contracted during the off polarization. The ijp and the responses following it were characterized as mediated by intramural, non-adrenergic, non-cholinergic (nanc) nerves. The ijp amplitude was reduced by raising the external K concentration. When Cl was replaced by isethionate, a small hyperpolarization of the smooth muscle cell membrane ensued along with a comparable small reduction of the ijp amplitude. The off activity following the ijp disappered completely in Cl-free medium. Apamin (10-7-10-5 M) did not influence this preparation nor the ijp. Adenosine and its related adenine nucleotides in concentrations up to 10-3 M hardly affected the preparation. Prolonged superfusion with adenosine and 6-chloradenosine revealed a gradually increasing attenuation of the ijp. Exogenously applied vasoactive intestinal polypeptide (VIP) induced rhythmic depolarizations of the smooth muscle cell membrane and spontaneous contractions, which were insensitive to neurotoxins but absent in Cl-free media. Field stimulation in the presence of VIP caused an i.j.p. which transiently interrupted the VIP-induced contractile responses. The inhibitory mediator of the intramural nanc nerves present in the circular smooth muscle layer of the opossum esophagus is neither a purine nor VIP. The ijp may result from a selective increase in K permeability of the smooth muscle cell membrane, the off depolarization may involve an increase in the Cl ion conductance. Evidently, the release of neurotransmitter from intramural nanc nerves is modulated presynaptically via Pi-purinoceptors and VIP is a likely candidate for an excitatory transmitter, released simultaneously with the inhibitory transmitter.