Neural 5-HT 4 receptors in the human isolated detrusor muscle: Effects of indole, benzimidazolone and substituted benzamide agonists and antagonists

1. In strips of human isolated detrusor muscle, the 5-hydroxytryptamine (5-HT) receptor (5-HT ₄) that mediates facilitation of neuromuscular cholinergic transmission was further characterized by using 5-HT and a series of ligands known for their 5-HT ₄ agonist (5-methoxytryptamine: 5-MeOT, cisapride, (R,S)-zacopride, BIMU 8) or antagonist (RS 23597, GR 125487, DAU 6285) properties. 2. In the presence of methysergide (1 μM) and ondansetron (3 μM) to isolate pharmacologically the 5-HT ₄ receptors, 5-HT (0.3 nM-1 μM), 5-MeOT (10 nM-30 μM), BIMU 8 (10 nM-3 μM), cisapride (0.1-10 μM) and (R,S)-zacopride (0.1-30 μM) potentiated cholinergic contractions to electrical held stimulation in a concentration-dependent manner. RS 23597 (10 nM-10 μM), a competitive 5-HT ₄ receptor antagonist in other systems, also showed agonist properties. The following rank order of potency as an agonist was obtained: 5-HT (pEC ₅₀ = 8.0) > RS 23597 (7.0) = BIMU 8 (6.9) ≤ cisapride (6.6) > 5-MeOT (6.0) ≤ (R,S)-zacopride (5.7). Relative to 5-HT (intrinsic activity = 1), 5-MeOT acted as a full agonist (1.03), while BIMU 8 (0.76), (R,S)-zacopride (0.61), RS 23597 (0.60) and cisapride (0.41) behaved as partial agonists. 3. The potentiation by 5-HT was competitively antagonized by the selective 5-HT ₄ receptor antagonist GR 125487 (0.3-3 nM) with a pA ₂ estimate of 9.75 (Schild slope of 1.09), and by DAU 6285 (1 μM; pK(B) = 6.45). Additionally, GR 125487 (3 nM) antagonized the responses to 5-MeOT (pK(B) = 9.72) and reversed the potentiation induced by RS 23597. As an antagonist, RS 23597 (10, 30 and 100 nM) inhibited the response to 5-HT. In addition, 30 and 100 nM RS 23597 reduced the 5-HT response maximum by 30 and 50%, respectively. The pK(B) value calculated at 10 nM was 8.0. 4. Thus, in the human isolated detrusor muscle, the 5-HT ₄ receptors mediating facilitation of cholinergic neuromuscular transmission are activated by indoleamines (5-HT, 5-MeOT), substituted benzamide (cisapride, (R,S)-zacopride), benzoate (RS 23597) and benzimidazolone (BIMU 8) derivatives. The activities (in terms of both potency and efficacy) of most agonists, as well as the affinity estimates of the antagonists GR 125487 and DAU 6285, are comparable to those found in other peripheral tissues. Exceptions are RS 23597, which acted either as a partial agonist or as an antagonist of the response to 5-HT, and 5-MeOT that showed an unusually low potency. The latter findings may be ascribed to differences in the efficiency of receptor coupling mechanisms and/or in the molecular structure (i.e. splice variants) of the 5-HT ₄ receptor.