Serosal application of Ba²⁺ induces oscillatory chloride secretion via activation of submucosal cholinergic neurones in guinea-pig distal colon

The enteric nervous system regulates ion and fluid secretion in the mammalian intestine at both resting and stimulated conditions. To determine the type and activation mechanism of neurones involved, mucosa-submucosa sheets isolated from guinea-pig distal colon were studied in vitro in Ussing chambers. Serosal addition of 0.5-1 mM barium (Ba²⁺), a potassium (K⁺) channel inhibitor, caused oscillatory increases in short-circuit current (/_sc). Mean values of the size and frequency of /_sc were 369. 1 μA cm^-2 and 2.3 min^-1. The oscillatory /_sc induced by the low concentrations of Ba²⁺ was blocked by either higher concentrations of Ba²⁺ (2-5 mM) or other K⁺ channel inhibitors, such as tetraethylammonium (TEA) (1 mM) and quinine (20 mM). The Ba²⁺-induced oscillatory /_sc was also inhibited by tetrodotoxin (TTX) and atropine. In a nominally Ca²⁺ free solution plus serosal addition of 0.1 mM ethylene glycol-bis (β-aminoethyl ether) N, N, N′, N′-tetraacetic acid (EGTA), a Ca²⁺ chelator, the oscillatory /_sc slowed and diminished. Further, the Ba²⁺-induced oscillatory /_sc was partially inhibited by apical addition of 100 μM 5′-nitro-2-(3-phenylpropylamino)benzoic-acid (NPPB), a Cl^- channel inhibitor, and completely disappeared in a low Cl^- solution (11 mM) on both sides. On the other hand, application of either cimetidine, a histamine H₂ receptor antagonist, or hexamethonium, a nicotinic antagonist, to the serosal side did not affect the Ba²⁺-induced oscillatory /_sc. In conclusion, the Ba²⁺-induced oscillatory /_sc is the transepithelial Cl^- current which is stimulated by activation of cholinergic neurones in submucosal plexus of guinea-pig distal colon.