EVIDENCE THAT TACHYKININ NK2 RECEPTORS MODULATE RESTING TONE IN THE RAT ISOLATED SMALL-INTESTINE

1 In the progress of experiments aimed at evaluating the role of tachy kinins as enteric nonadrenergic noncholinergic (NANC) transmitters, we noted that certain tachykinin receptor antagonists produce a relaxati on of circular muscle strips in the rat small intestine. This study ai med to assess the nature of this response and to determine the recepto r type involved. The majority of the experiments were performed in cap saicin- (10 mu M for 15 min) pretreated mucosa-free circular muscle st rips from the rat small intestine, in the presence of atropine (1 mu M ), guanethidine (3 mu M) and indomethacin (10 mu M). 2 Under isometric recording of mechanical activity, the tachykinin NK1 receptor antagon ist SR 140,333 (0.1 mu M) had no effect on resting tone or spontaneous activity in duodenal or ileal circular muscle strips. The NK2 recepto r antagonists, MEN 10,627 (0.1 mu M) and GR 94,800 (0.1 mu M) produced , after a delay of 10-15 min, a relaxation which averaged 61 +/- 3 and 57 +/- 6% (n = 6 and 4, respectively) of the maximal response (E(max) ) to isoprenaline (1 mu M). The effect of maximal concentrations of ME N 10,627 and GR 94,800 when applied together was non-additive. The rel axant effect of MEN 10,627 (0.1 mu M) was similar in the absence and p resence of apamin (0.3 mu M) and L-nitroarginine (100 mu M). 3 Under i sotonic recording of mechanical activity, MEN 10,627 (10 nM - 1 mu M) produced a concentration- and time-related relaxation of duodenal stri ps. The maximal relaxation averaged 72 +/- 4 and 69 +/- 4% (n = 5 each ) of E(max) to isoprenaline (1 mu M) and was achieved 15-20 or 20-30 m in after application of 1.0 or 0.1 mu M MEN 10,627, respectively. 4 Du odenal strips were relaxed by other NK2 receptor selective antagonists (values in parentheses are % of E(max) to isoprenaline at the given c oncentration of antagonist): GR 94,800 (69 +/- 3% at 1 mu M, n = 4), S R 48,968 (60 +/- 3% at 1 mu M, n = 4) and MDL 29,913 (66 +/- 4% at 1 m u M, n = 4). SR 48,965 (1 mu M), the inactive enantiomer of SR 48,968, was without effect. The NK1 receptor selective antagonists, SR 140,33 3 (0.1 mu M), FK 888 (10 mu M) RP 67,580 (1 mu M) and GR 82,334 (10 mu M) were also without effect (n = 4-5). 5 A cocktail of peptidase inhi bitors, thiorphan, bestatin and captopril (1 mu M each) had no signifi cant effect on tone or spontaneous activity of duodenal strips. In the presence of peptidase inhibitors, MEN 10,627 (1 mu M) produced a rela xation of duodenal strips (72 +/- 6% of E(max) to isoprenaline, n = 5) , whilst GR 82,334 (10 mu M, n = 6) had no significant effect. 6 The r elaxant response to MEN 10,627 was preserved in mucosa-free strips not pre-exposed to capsaicin. Tetrodotoxin (1 mu M), saxitoxin (1 mu M), hexamethonium (100 mu M) and omega-conotoxin (0.1 mu M) had no signifi cant effect on the resting tone of duodenal strips nor did they affect the relaxation to MEN 10,627. L-Nitroarginine (100 mu M) increased th e tone of the strips but did not affect the response to MEN 10,627. Ni fedipine (1 mu M) relaxed the strips by 62 +/- 4% (n = 4), but in its presence a small relaxant effect to MEN 10,627 (26 +/- 5%, n = 4) was still evident. 7 Under isotonic recording of mechanical activity along the longitudinal axis, MEN 10,627 (1 mu M) produced a slowly developi ng relaxation (39 +/- 3% of E(max) to isoprenaline; n = 6) of whole se gments of rat duodenum. When similar experiments were performed on who le segments of rat proximal colon MEN 10,627 had no effect. 8 The pres ent findings document the observation that tachykinin NK2 receptors co ntribute to the maintenance of resting tone of the rat isolated small intestine. We found no evidence to suggest that this effect follows th e blockade of the contractile effect of spontaneously released endogen ous tachykinins. The present findings raise the possibility that const itutively active NK2 receptors account for the relaxant effect produce d by NK2 receptor antagonists in rat small intestine.