Selective desensitization of the 5-HT4 receptor-mediated response in pig atrium but not in stomach

The time dependency of the effect of 5-HT4 receptor agonists depends on many specific regulatory mechanisms, which vary between tissues. This has important implications with regard to the effects of endogenous 5-HT, as well as to the clinical use of 5-HT4 receptor agonists, and might contribute to tissue selectivity of agonists. The progression and desensitization of 5-HT4 receptor-mediated responses were evaluated in an organ bath set-up using two, clinically relevant, porcine in vitro models: gastric cholinergic neurotransmission and atrial contractility. Exposure of gastric tissue to 5-HT or to the selective 5-HT4 receptor agonists prucalopride and M0003 results in a sustained non-transient effect during exposure; after washout, the response to a subsequent challenge with 5-HT shows no clear desensitization. Incubation of left atrial tissue with 5-HT resulted in a transient response, leading after washout to a marked desensitization of the subsequent response to 5-HT. The selective 5-HT4 receptor agonists prucalopride and M0003 induce only very weak atrial responses whereas they are very effective in desensitizing the atrial response to 5-HT. The observations also suggest that the properties of prucalopride and M0003 to bind to and/or activate the 5-HT4 receptor differ from those of 5-HT. This difference might have contributed to the observed desensitization. The high potency of prucalopride and M0003 in desensitizing the response to 5-HT together with their low efficacy in the atrium emphasizes the cardiac safety of this class of 5-HT4 receptor agonists.