Inhibition of potassium (KATP) channels reduces the short-circuit current response of rat colonic mucosa to acetylcholine

Distinct subclassification of DRG neurons innervating the distal colon and glans penis/distal urethra based on the electrophysiological current signature

Spinal sensory neurons innervating visceral and mucocutaneous tissues have unique microanatomic distribution, peripheral modality, and physiological, pharmacological, and biophysical characteristics compared with those neurons that innervate muscle and cutaneous tissues. In previous patch-clamp electrophysiological studies, we have demonstrated that small- and medium-diameter dorsal root ganglion (DRG) neurons can be subclassified on the basis of their patterns of voltage-activated currents (VAC). These VAC-based subclasses were highly consistent in their action potential characteristics, responses to algesic compounds, immunocytochemical expression patterns, and responses to thermal stimuli. For this study, we examined the VAC of neurons retrogradely traced from the distal colon and the glans penis/distal urethra in the adult male rat. The afferent population from the distal colon contained at least two previously characterized cell types observed in somatic tissues (types 5 and 8), as well as four novel cell types (types 15, 16, 17, and 18). In the glans penis/distal urethra, two previously described cell types (types 6 and 8) and three novel cell types (types 7, 14, and 15) were identified. Other characteristics, including action potential profiles, responses to algesic compounds (acetylcholine, capsaicin, ATP, and pH 5.0 solution), and neurochemistry (expression of substance P, CGRP, neurofilament, TRPV1, TRPV2, and isolectin B4 binding) were consistent for each VAC-defined subgroup. With identification of distinct DRG cell types that innervate the distal colon and glans penis/distal urethra, future in vitro studies related to the gastrointestinal and urogenital sensory function in normal as well as abnormal/pathological conditions may be benefitted.

Serosal application of Ba(2+) 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 (Ba(2+)), a potassium (K(+)) channel inhibitor, caused oscillatory increases in short-circuit current (I(sc)). Mean values of the size and frequency of I(sc) were 369.1 microA cm(-2) and 2.3 min(-1). The oscillatory I(sc) induced by the low concentrations of Ba(2+) was blocked by either higher concentrations of Ba(2+) (2-5 mM) or other K(+) channel inhibitors, such as tetraethylammonium (TEA) (1 mM) and quinine (20 mM). The Ba(2+)-induced oscillatory I(sc) was also inhibited by tetrodotoxin (TTX) and atropine. In a nominally Ca(2+) free solution plus serosal addition of 0.1 mM ethylene glycol-bis (beta-aminoethyl ether) N,N,N',N'-tetraacetic acid (EGTA), a Ca(2+) chelator, the oscillatory I(sc) slowed and diminished. Further, the Ba(2+)-induced oscillatory I(sc) was partially inhibited by apical addition of 100 microM 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 H(2) receptor antagonist, or hexamethonium, a nicotinic antagonist, to the serosal side did not affect the Ba(2+)-induced oscillatory I(sc). In conclusion, the Ba(2+)-induced oscillatory I(sc) is the transepithelial Cl(-) current which is stimulated by activation of cholinergic neurones in submucosal plexus of guinea-pig distal colon.

Imidazolines inhibit secretory responses of rat colonic mucosa to calcium-dependent but not cyclic AMP-dependent secretagogues

The purpose of this study was to investigate whether imidazolines have an anti-secretory action on intestinal epithelial cells. Muscle-stripped preparations of rat colon and monolayers of T84 human colonic epithelial cells were set up in Ussing chambers for measurement of short-circuit current. In rat colon acetylcholine, histamine, vasoactive intestinal polypeptide and forskolin elicited secretory responses which were recorded as increases in short-circuit current. Secretory responses to acetylcholine were inhibited in a concentration-dependent manner by the imidazolines phentolamine, idazoxan and clonidine. The effect of clonidine was not reversed by pre-incubation of mucosal preparations with yohimbine. Secretory responses to vasoactive intestinal polypeptide were unaffected by the three imidazolines. Phentolamine reduced responses of colonic mucosa to histamine but had no effect on responses to forskolin. Responses to vasoactive intestinal polypeptide and forskolin were significantly reduced in the presence of barium. In T84 cell monolayers phentolamine significantly reduced responses to acetylcholine. Three imidazolines, two with alpha-adrenoceptor-antagonist properties and one with alpha-agonist properties, have anti-secretory effects in rat colonic mucosal preparations. The anti-secretory action appears to discriminate between calcium-dependent and cyclic AMP-dependent secretagogues, inhibiting the former but not the latter.