In vitro characterization of tachykinin NK 2 ‐receptors modulating motor responses of human colonic muscle strips

Human in vitro preparations of transverse or distal colonic circular smooth muscle were potently and dose‐dependently contracted by neurokinin A (EC 50 , 4.9 n m ), the tachykinin NK 2 ‐receptor selective agonist [β‐Ala 8 ]neurokinin A (4–10) ([β‐Ala 8 ]NKA (4–10)) (EC 50 , 5.0 n m ), neurokinin B (EC 50 , 5.3 n m ) and substance P (EC 50 , 160 n m ), but not by the tachykinin NK 1 ‐receptor selective agonist [Sar 9 Met(O 2 ) 11 ] substance P, or the NK 3 ‐receptor selective agonists, senktide and [MePhe 7 ] neurokinin B. No regional differences between transverse and distal colon were observed in response to [β‐Ala 8 ]NKA (4–10). Atropine (1 μ m ) and tetrodotoxin (1 μ m ) did not significantly inhibit responses to [β‐Ala 8 ]NKA (4–10), neurokinin A, substance P or neurokinin B. The newly developed non‐peptide antagonists for tachykinin NK 2 ‐receptors SR 48968, SR 144190 and its N‐demethyl (SR 144743) and N,N‐demethyl (SR 144782) metabolites, were used to challenge agonist responses, as appropriate. SR 144190 and the metabolites all potently and competitively antagonized the response to [β‐Ala 8 ]NKA (4–10), with similar potency (Schild plot pA 2 values 9.4, 9.4 and 9.3, slope = 1). SR 48968 antagonism was not competitive: the Schild plot slope was biphasic with a high (X intercept∼9.3) and a low (X intercept 8.4, slope 1.6) affinity site. Co‐incubation of SR 48968 (10, 100 n m ) and SR 144782 (10 n m ) produced additive effects; in this experimental condition, SR 48968 apparent affinity (pK B ) was 8.2. In addition, SR 144782 (0.1 μ m ) antagonized responses to neurokinin A, substance P and neurokinin B, with pK B consistent with its affinity for tachykinin NK 2 ‐receptors. The potent and selective NK 1 and NK 3 ‐receptor antagonists, SR 140333 and SR 142801 (both 0.1 μ m ), failed to inhibit contractions induced by SP or NKB. In conclusion, the in vitro mechanical responses of circular smooth muscle preparations from human colon are strongly consistent with the presence of non‐neuronal tachykinin NK 2 ‐receptors, but not tachykinin NK 1 ‐ or NK 3 ‐receptors. Our findings with SR 48968 suggest the existence of two tachykinin NK 2 ‐receptor subtypes, that it seems to distinguish, unlike SR 144190 and its metabolites. However, the precise nature of SR 48968 allotopic antagonism remains to be elucidated, since allosteric effects at the tachykinin NK 2 ‐receptor might well account for the complexity of the observed interaction. British Journal of Pharmacolo