Characterization of receptors mediating contraction induced by tachykinins in the guinea‐pig isolated common bile duct
Authors: Patacchini Riccardo , Barthó Lorand , Maggi Carlo Alberto 
Published: January 2005

We studied the effect of the natural tachykinins and of synthetic agonists selective for the tachykinin NK1, NK2 and NK3 receptors, on the motility of guinea‐pig isolated common bile duct longitudinally‐oriented smooth muscle.
All the tachykinins tested (both natural and synthetic) produced a concentration‐dependent contractile response of the guinea‐pig isolated common bile duct: these effects underwent a marked tachyphylaxis, especially the responses elicited by NK1 and NK3 receptor‐selective agonists.
Among the natural tachykinins neurokinin B (EC50=3.2 nm; 95% c.l.=2.0–5.1; n=4) was the most potent, being about 40 and 25 fold more potent than substance P (EC50=121.6 nm; 95% c.l.=94–157; P<0.01; n=4) and neurokinin A (EC50=83.4 nm; 95% c.l.=62–112; P<0.01; n=4), respectively. Among the synthetic analogues the NK3 receptor‐selective agonist senktide (EC50=1.1 nm; 95% c.l.=0.7–1.8; n=8) was the most potent, being about 120, 110 and 20 fold more potent than [Sar9]substance P sulfone (NK1 receptor‐selective) (EC50=130.4 nm; 95% c.l.=99–172; P<0.01; n=8), [βAla8]NKA (4–10) (NK2 receptor‐selective) (EC50=120.1 nm; 95% c.l.=95–151; P<0.01; n=8) and septide (NK1 receptor‐selective) (EC50=22.6 nm; 95% c.l.=18–28; P<0.01; n=8), respectively. All tachykinins (natural or synthetic receptor agonists) produced a similar Emax, averaging about 50% of that produced by KCl (80 mm).
Atropine (1 μm) did not affect the responses to either NK1 or NK2 receptor‐selective agonists, whereas it reduced the Emax of senktide by about 50%, without affecting its potency (EC50). Tetrodotoxin (1 μm) totally blocked senktide‐induced contractions, as did the combined pretreatment with atropine plus the tachykinin NK1 and NK2 receptor‐selective antagonists GR 82334 and MEN 11420 (1 μm each), respectively.
GR 82334 (1 μm) blocked with apparent competitive kinetics septide‐ (apparent pKB=7.46±0.10; n=5) and [Sar9]substance P sulfone‐ (apparent pKB=6.80±0.04; n=4) induced contractions. MEN 11420 (30–300 nm), a novel potent NK2 receptor antagonist, potently antagonized [βAla8]NKA (4–10), with competitive kinetics (pKB=8.25±0.08; n=12: Schild plot slope=−0.90; 95% c.l.=−1.4; −0.35). The NK3 receptor‐selective antagonist SR 142801 (30 nm) produced insurmountable antagonism of the senktide‐induced contractions (Emax inhibited by 64%). None of the above antagonists, tested at the highest concentrations employed against tachykinins, affected the concentration–response curve to methacholine (0.1–300 μm).
We conclude that tachykinins produce contraction of the guinea‐pig isolated common bile duct by stimulating NK1, NK2 and NK3 receptors. The responses obtained by activating NK1 and NK2 receptors are atropine‐resistant. The contraction obtained by stimulating NK3 receptors is totally neurogenic, being mediated by the release of endogenous acetylcholine and tachykinins; the latter act, in turn, on postjunctional tachykinin NK1/NK2 receptors. The role of the NK3 receptor as prejunctional mediator of the excitatory transmission operated by tachykinins is discussed.