1 The pharmacological pro®le was studied of MEN 11420, or cyclo{[Asn(b-D-GlcNAc)-Asp-Trp-PheDap-Leu]cyclo(2b-5b)}, a glycosylated derivative of the potent, selective, conformationally-constrained tachykinin NK 2 receptor antagonist MEN 10627 (cyclo(Met-Asp-Trp-Phe-Dap-Leu)cyclo(2b-5b)). and ion channels. 4 In the rabbit isolated pulmonary artery and rat urinary bladder MEN 11420 potently and competitively antagonized tachykinin NK 2 receptor-mediated contractions (pK B =8.6+0.07, n=10, and 9.0+0.04, n=12; Schild plot slope=71.06 (95% c.l.=71.3; 70.8) and 71.17 (95% c.l.=71.3; 71.0), respectively). MEN 11420 produced an insurmountable antagonism at NK 2 receptors in the hamster trachea and mouse urinary bladder. However, in both preparations, the eect of MEN 11420 was reverted by washout and an apparent pK B of 10.2+0.14, n= 9, and 9.8+0.15, n=9, was calculated in the hamster trachea and mouse urinary bladder, respectively. 5 MEN 11420 showed low anity (pK B 56) at guinea-pig and rat tachykinin NK 1 (guinea-pig ileum and rat urinary bladder) and NK 3 (guinea-pig ileum and rat portal vein) receptors. On the whole, the anities (potency and selectivity) showed by MEN 11420 for dierent tachykinin receptors, measured either in binding or in functional bioassays, were similar to those shown by the parent compound, MEN 10627. ) and intraduodenal (100 ± 300 nmol kg 71 ) administration of MEN 11420. MEN 11420 was more potent (about 10 fold) and longer lasting than its parent compound MEN 10627, possibly due to a greater metabolic stability. 7 A dose of MEN 11420 (100 nmol kg 71 , i.v.), that produced potent and long lasting inhibition of the contraction of the rat urinary bladder induced by challenge with the NK 2 selective receptor agonist [bAla 8 ]neurokinin A(4 ± 10) (10 ± 300 nmol kg 71 ), was without eect on the responses produced by the NK 1 receptor selective agonist [Sar 9 ]substance P sulphone (1 ± 10 nmol kg 71 ). 8 These ®ndings indicate that MEN 11420 is a potent and selective tachykinin NK 2 receptor antagonist. The introduction of a sugar moiety did not produce major changes in the anity pro®le of this antagonist as compared to MEN 10627, but markedly improved its in vivo potency and duration of action. With these characteristics, MEN 11420 is a suitable candidate for studying the pathophysiological signi®cance of tachykinin NK 2 receptors in humans.

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 microM), action potentials (APs) and contraction (IC50 3.7 microM) produced by the muscarinic receptor agonist, methacholine. It also produced a concentration-dependent inhibition of APs and accompanying contraction (IC50 31 microM) 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 microM) otilonium bromide enhanced the e.j.p. and, at higher concentrations (IC50 22 microM and 16 microM 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 microM) 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 microM and 45 microM, respectively) induced by the tachykinin NK2 receptor agonist [betaAla8]neurokinin A (4-10). When tested in the presence of nifedipine (1 microM), 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 [betaAla8]neurokinin A (4-10) (IC50 41 microM). 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 [betaAla8]neurokinin A (4-10) (IC50 60 microM and 54 microM, respectively). In radioligand binding experiments otilonium bromide produced a concentration-dependent inhibition of the binding of both an agonist ([125I]neurokinin A, Ki 7.2 microM) and an antagonist ([3H]SR 48968, Ki 2.2 microM) to membranes of Chinese hamster ovary cells transfected with the human tachykinin NK2 receptor. In conclusion, the present findings demonstrate that, in the microM 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.

No abstract

Substance P (SP) and neurokinin A (NKA) are synthesized by enteric cholinergic motorneurons that project to the longitudinal and circular muscle of the mammalian intestine. Thus, acetylcholine, SP, and NKA are the excitatory neuromuscular transmitters in the intestine. Tachykinin NK1 and NK2 receptors are expressed by smooth muscle cells in most regions of the intestine: the corelease of SP and NKA from nerves thus realizes paradigms of tachykininergic cotransmission. Examples have been found in which a cooperative model can be applied to account for the action of SP-NKA acting at NK1 and NK2 receptors (e.g., circular muscle of guinea-pig duodenum), as well as examples in which the message produced by activation of the two receptors diverges sharply in producing responses that have a markedly different time course and use different effector systems (e.g., circular muscle of guinea-pig colon). NK3 receptors are expressed on both excitatory and inhibitory motor neurons: indirect contractions (via release of acetylcholine and tachykinins) and relaxations (via release of nitric oxide) can be evoked in the gut by selective stimulation of NK3 receptors. Although a role of NK3 receptors in certain enteric reflexes has been evidenced, the importance of this system in mediating hexamethonium-resistant enteric transmission appears less important than previously speculated.