Antimuscarinic, calcium channel blocker and tachykinin NK2 receptor antagonist actions of otilonium bromide in the circular muscle of guinea-pig colon

Abstract

We have analyzed, by the sucrose gap method, the action of otilonium bromide, a quaternary ammonium derivative in use for the symptomatic therapy of irritable bowel syndrome, on the electrical and mechanical responses initiated by different stimuli in the circular muscle of the guinea-pig proximal colon. Otilonium bromide produced a concentration-dependent inhibition of membrane depolarization (IC₅₀ 4.1 µM), action potentials (APs) and contraction (IC₅₀ 3.7 µM) produced by the muscarinic receptor agonist, methacholine. It also produced a concentration-dependent inhibition of APs and accompanying contraction (IC₅₀ 31 µM) produced by KCl (30 mM), and had a biphasic effect on the cholinergic excitatory junction potential (e.j.p.) produced by single pulse electrical field stimulation: at low concentrations (0.1–0.3 µM) otilonium bromide enhanced the e.j.p. and, at higher concentrations (IC₅₀ 22 µM and 16 µM toward depolarization and contraction), produced a concentration-dependent inhibition. Otilonium bromide eliminated the APs superimposed on the depolarization induced by the tachykinin NK₁ receptor agonist, [Sar⁹]substance P-sulphone and suppressed the corresponding contraction (IC₅₀ 43 µM) but had little effect on the sustained membrane depolarization induced by this agonist. On the other hand, otilonium bromide produced a similar inhibitory effect on both membrane depolarization and contraction (IC₅₀ 38 µM and 45 µM, respectively) induced by the tachykinin NK₂ receptor agonist [βAla⁸]neurokinin A (4–10). When tested in the presence of nifedipine (1 µM), otilonium bromide had no effect on the membrane depolarization induced by [Sar⁹]substance P-sulphone but inhibited in a concentration-dependent manner the depolarization induced by [βAla⁸]neurokinin A (4–10) (IC₅₀ 41 µM). In contrast, the blocker of receptor-operated cation channels, SKF 96365, inhibited with similar potency the depolarization induced by both [Sar⁹]substance P-sulphone and [βAla⁸]neurokinin A (4–10) (IC₅₀ 60 µM and 54 µM, respectively). In radioligand binding experiments otilonium bromide produced a concentration-dependent inhibition of the binding of both an agonist ([¹²⁵I]neurokinin A, K _i 7.2 µM) and an antagonist ([³H]SR 48968, K _i 2.2 µM) to membranes of Chinese hamster ovary cells transfected with the human tachykinin NK₂ receptor. In conclusion, the present findings demonstrate that, in the µM range of concentrations, otilonium bromide acts as a muscarinic and tachykinin NK₂ receptor antagonist and as a calcium channel blocker. The latter property is likely to account for its ability to suppress contraction initiated by the tachykinin NK₁ receptor agonist. Therefore multiple mechanisms of action account for the ability of otilonium bromide to reduce stimulated motility of intestinal smooth muscle.