Evidence that tachykinin NK1 and NK2 receptors mediate non‐adrenergic noncholinergic excitation and contraction in the circular muscle of guinea‐pig duodenum

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Abstract
1 In the presence of atropine (1 μm), guanethidine (3 μm), indomethacin (3 μm), apamin (0.1 μm) and l-nmtroarginine (l-NOARG, 30 μm), electrical field simulation (EFS) produced a nonadrenergic, non-cholinergic (NANG) excitatory junctional potential (ej.p.), action potentials and contraction of the circular muscle of the guinea-pig proximal duodenum, recorded by the single sucrose gap technique. 2 The selective tachykinin (TK) NK, receptor antagonist, GR 82,334 (30 nM-3 μm) produced a concentration-dependent inhibition of the EFS-evoked NANC e.j.p. and contraction. Similarly, the selective NK2 receptor antagonists, MEN 10,627 (30 nM-3 μm) and GR 94,800 (100 nM-10 μm), both produced a concentration-dependent inhibition of the EFS-evoked NANC e.j.p. and contraction. GR 82,334 inhibited the electrical and mechanical NANC responses to EFS in an almost parallel manner, while MEN 10,627 and GR 94,800 were more effective in inhibiting the mechanical than the electrical response to EFS. 3 Activation of the NK1 or NK2 receptor by the selective agonists, [Sar9]substance P (SP) sulphone and [βAla8]neurokinin A (NKA) (4–10), respectively (0.3 μm each), produced depolarization, action potentials and contractions. GR 82,334 selectively inhibited the responses to [Sar9]SP sulphone, without affecting the responses to [βAla8]NKA (4–10). MEN 10,627 and GR 94,800 inhibited or abolished the responses to [βAla8]NKA (4–10), without affecting the responses to [Sar9]SP sulphone. 4 Nifedipine (1 μm) abolished the action potentials and contraction produced either by EFS or by the TK receptor agonists [Sar9]SP sulphone or [βAla8]NKA (4–10). 5 In the presence of nifedipine, the NANC ej.p. produced by EFS was biphasic: in the majority of strips tested (21 out of 29) an early fast phase of depolarization was followed by a second slow component. The combined administration of GR 82,334 and GR 94,800 (3 μm each) reduced both components, the slow phase being inhibited to a greater extent than the fast phase. 6 The P2 purinoreceptor antagonist, suramin (100 μm) reduced the fast phase of the e.j.p. produced by EFS in the presence of nifedipine, without affecting the slow phase. The combined administration of suramin, GR 82,334 and GR 94,800 produced a nearly complete blockade of the e.j.p. produced by EFS in the presence of nifedipine. 7 When tested in the absence of apamin and l-NOARG, EFS induced a NANC inhibitory junction potential (i.j.p.) followed by an e.j.p., and the selective P2Y receptor agonist, adenosine-5′-O-(2-thiodiphosphate) (ADPβS, 10 μm), produced membrane hyperpolarization. After addition of apamin and l-NOARG, the i.j.p. was blocked, and EFS produced a pure NANC e.j.p.; ADPPS produced depolarization, action potentials and contraction. 8 Suramin (100 μm) blocked the depolarization, action potentials and contractions produced by ADPPS in the presence of apamin and l-NOARG, without affecting the responses produced by the NK, receptor agonist, [Sar9]SP sulphone. 9 We conclude that NK1 and NK2 receptors cooperate in producing NANC excitation and contraction of the circular muscle in the guinea-pig proximal duodenum. Activation of either TK receptor produces membrane depolarization and both receptors contribute to generate action potentials which are essential for producing muscle contraction, via nifedipine-sensitive calcium channels. It appears that endogenous ATP chiefly acts as an inhibitory transmitter but, after blockade of NANC inhibitory mechanism(s), ATP may act as a fast signalling excitatory transmitter.