The gastrointestinal motor effect of benzamide derivatives is unrelated to 5-HT3 receptor blockade.

The prokinetic properties of a number of 5-HT3 antagonists containing the benzamide moiety (metoclopramide, cisapride, BRL 24924, zacopride) were compared with those of the chemically unrelated antagonist, ICS 205-930. Their 5-HT3 antagonistic potency was evaluated using the Bezold-Jarisch test. All compounds accelerated gastric emptying of beads in the rat, with potencies comparable to those found for inhibiting the Bezold-Jarisch reflex. Metoclopramide, cisapride and zacopride potentiated the twitch contraction of guinea pig ileum and contracted the isolated guinea pig colon in a concentration-dependent manner. Furthermore, they increased the contractility of the gastric Heidenhain pouch in the conscious dog. In contrast, ICS 205-930 was devoid of agonist or antagonist activities in all models except gastric emptying in the rat. Two findings, (1) that benzamide derivatives showed high efficacy in all models of gastrointestinal motility in contrast to ICS 205-930 that was active only to increase gastric emptying, and (2) the different potency order of benzamides in the Bezold-Jarisch test as compared to the in vitro motility tests, indicate that 5-HT3 receptors are involved in gastric emptying, whereas a different receptor operates in the other models used.