ANALYSIS OF NEUROGENIC CONTRACTIONS INDUCED BY ML-1035 AND OTHER BENZAMIDES IN THE GUINEA-PIG NON-STIMULATED ISOLATED ILEUM

Amino-5-chloro-substituted benzamides have been shown to increase gast ric motility in-vivo and enhance field-stimulated and peristaltic cont ractions in-vitro. The present experiments examined the contractile re sponse to a series of benzamides in the guinea-pig non-stimulated ileu m. Four benzamides elicited contractions in the isolated ileum which w ere expressed as a percentage of the contraction induced by 1 mum acet ylcholine (% acetylcholine response = 12 +/- 2, 19 +/- 3, 26 +/- 2, 51 +/- 3, n = 13, 8, 17, and 21, with EC50 values of 0.85, 1-8, 5.7, and 14.2 mum for cisapride, zacopride, metoclopramide, and ML-1035 inyl)- ethoxy)-N-(2-(diethylamino)-ethyl)-benzamide hydrochloride), respectiv ely). ML-1035 contractions were completely blocked by atropine and tet rodotoxin, while ganglionic blockade with hexamethonium was ineffectiv e. Metoclopramide has been reported to sensitize postjunctional muscar inic receptors, however, ML-1035 did not enhance acetylcholine-induced contractions. Tropisetron (ICS 205-930, 1 mum), caused a parallel rig htward shift in the concentration-response curve for both ML-1 035 and zacopride (EC50 = 14.2 +/- 1.3 and 1.8 +/- 0.8 muM in the absence, an d 26 +/- 2.7 and 6-9 +/- 2.3 mum in the presence of tropisetron for ML -1035 and zacopride, respectively) with apparent-pK(B) values of 5.9 a nd 6.0 for the respective compounds. 5-Hydroxytryptaminergic receptor desensitization by 2-methyl-5-hydroxytryptamine (5-HT3) and 5-methoxyt ryptamine (5-HT4), attenuated the response to ML-1035. We also examine d the effect of the benzamides on [H-3]acetylcholine release from long itudinal muscle myenteric plexus preparations; however, these compound s had little effect on basal [H-3]acetylcholine release. Thus, the pha rmacological data indicate that the benzamides can elicit neurogenic c ontractions in the non-stimulated ileum by activating postganglionic, cholinergic neurons which is independent of an effect on smooth muscle .