EFFECT OF SR-142801 ON NITRIC OXIDE-DEPENDENT AND INDEPENDENT RESPONSES TO TACHYKININ NK3, RECEPTOR AGONISTS IN ISOLATED GUINEA-PIG COLON

We have determined the ability of the novel nonpeptide tachykinin (TK) NK3 receptor antagonist, SR 142801, propyl)-4-phenylpiperidin-4-yl)-N -methylacetamide] in inhibiting the nitric oxide (NO)-independent prej unctional inhibition of cholinergic twitches and the NO-dependent rela xation produced by the NK3 receptor selective agonist, senktide, in th e circular muscle of the guinea-pig proximal colon. Under moderate loa d (10 mN) and isometric recording of mechanical activity, single pulse electrical field stimulation (EFS) produced atropine- and tetrodotoxi n-sensitive twitch contractions of mucosa-free circular muscle strips from the guinea-pig proximal colon. In the presence of NK1 and NK2 rec eptor antagonists (SR 140333 0.01 mu M and GR 94800 0.1 mu M, respecti vely) the NK3 receptor selective agonist, senktide (EC(50) 33 pM) and the NK3 receptor preferring natural TK, neurokinin B (NKB, EC(50) 13 P M) produced a concentration-dependent slowly developing inhibition of cholinergic twitches. Senktide (1 nM) did not affect the contractile r esponse to acetylcholine (1 mu M) indicating that depression of evoked twitches occurs prejunctionally. The inhibitory effect of senktide wa s unaffected when evoked in the presence of the cyclooxygenase inhibit or (S)-ketoprofen (10 mu M), guanethidine (10 mu M), naloxone (0.3 mu M), the GABA(B) receptor antagonist 2-hydroxysaclofen (10 mu M) or the combined application of the adenosine A(1) and A(2) receptor antagoni sts, 8-cyclopentyl-1,3-dipropylxanthine (10 mu M) and 3,7-dimethyl-1-p ropargylxanthine (30 mu M) respectively. In the presence of NK1 and NK 2 receptor antagonists, the NO-synthase inhibitor L-nitroarginine (L-N OARG 30-100 mu M) did not affect twitch inhibition induced by senktide (EC(50) 33 pM). The response to NKB (EC(50) 95 pM) was slightly reduc ed by L-NOARG, yet the bulk of the inhibitory effect of both agonists on cholinergic twitches was substantially independent of NO generation . SR 142801 (0.1-0.3 mu M) produced a moderate rightward shift of the concentration-response curve to senktide without depression of the Ema x to the agonist, yielding an apparent pK(B) value of 7.65. Under low resting tone (3 mN) and isotonic recording of mechanical activity, muc osa-free circular muscle strips from the guinea-pig proximal colon gai ned a high intrinsic tone suitable for testing the response to relaxan t agents. In the presence of atropine (1 mu M), guanethidine (3 mu M), SR 140333 (0.01 mu M) and GR 94800 (0.1 mu M), senktide (EC(50) 50 pM ) produced a concentration-dependent relaxation of the strips, which w as blocked by L-NOARG. SR 142801 (0.01-0.1 mu M) produced a large righ tward shift of the L-NOARG-sensitive concentration-response curve to s enktide yielding an apparent pK(B) value of 8.62. Under isometric reco rding condition, SR 143801 (0.1 mu M) did not affect twitch inhibition produced by 3 nM clonidine. Under isotonic recording condition, SR 14 2801 did not affect the L-NOARG-sensitive relaxation produced by EFS. The present results indicate that NK3 receptor stimulation produces a NO-dependent relaxation of the guinea-pig colon and a substantially NO -independent prejunctional inhibition of cholinergic twitches. The var iable affinities of SR 142801 in antagonizing various senktide-induced neuromodulatory effects in the guinea-pig intestine suggest a possibl e intraspecies heterogeneity of NK3 receptors in the enteric nervous s ystem.