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

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 mu M), action potentials (APs) and contraction (IC50 3.7 mu M) produced by the muscarinic receptor agonist, methacholine. It also produced a concentration-dependent inhibition of APs and accompanying contraction (IC50 31 mu M) 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 mu M) otilonium bromide enhanced the e.j.p. and. at higher concentrations (IC50 22 mu M and 16 mu M 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, [Sar(9)]substance P-sulphone and suppressed the corresponding contraction (IC50 43 mu M) 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 mu M and 45 mu M, respectively) induced by the tachykinin NK2 receptor agonist [beta Ala(8)]neurokinin A (4-10). When tested in the presence of nifedipine (1 mu M), otilonium bromide had no effect on the membrane depolarization induced by [Sar(9)]substance P-sulphone but inhibited in a concentration-dependent manner the depolarization induced by [beta Ala(8)]neurokinin A (4-10) (IC50 41 mu M). In contrast, the blocker of receptor-operated cation channels, SKF 96365, inhibited with similar potency the depolarization induced by both [Sar(9)]substance P-sulphone and [beta Ala(8)]neurokinin A (4-10) (IC50 60 mu M and 54 mu M, respectively). In radioligand binding experiments otilonium bromide produced a concentration-dependent inhibition of the binding of both an agonist ([I-125]neurokinin A, K-i 7.2 mu M) and an antagonist ([H-3]SR 48968, K-i 2.2 mu M) to membranes of Chinese hamster ovary cells transfected with the human tachykinin NK2 receptor. In conclusion, the present findings demonstrate that, in the mu M 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.