1. Field stimulation or vasoactive intestinal polypeptide (VIP) relaxed lower oesophageal sphincter (LOS) from North American opossum. Pretreatment with carbachol in Cl-ion-containing or Cl-ion-free Krebs solution or with 10(-3) M 9-aminoacridine abolished or markedly reduced relaxation due to VIP applied exogenously but not that elicited by field stimulation of non-adrenergic, non-cholinergic nerves. 2. Inhibitory junction potentials (7.5 +/- 1.2 mV, n = 5) could be recorded in LOS strips with the sucrose gap technique. They lacked significant after-depolarizations but were accompanied by decreased membrane resistance (61 +/- 6%, n = 3). In these strips, VIP (10(-6) M) produced small hyperpolarizations (2.1 +/- 1.1 mV, n = 5) sometimes followed by membrane potential oscillations but no change in conductance. 3. Removal of external chloride depolarized the membranes (7.6 +/- 1.7 mV) but did not prevent the hyperpolarization to VIP or the occurrence of inhibitory junction potentials. Restoration of external chloride repolarized the cells. It appears that an appreciable chloride conductance may be present in sphincter muscle cells and this may cause them to be more depolarized than non-sphincter muscle. 4. We conclude that it is very unlikely that VIP is the inhibitory NANC neurotransmitter since it does not mimic the inhibitory junction potential.

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