Tachykinin NK1 and NK2 receptor antagonists and atropine‐resistant ascending excitatory reflex to the circular muscle of the guinea‐pig ileum

1 The aim of this study was to investigate the effect of various antagonists, selective for the tachykinin NK1 or NK2 receptor, on the atropine‐resistant ascending excitatory reflex (AER) to the circular muscle of the guinea‐pig ileum elicited by radial stretch (balloon distension) or electrical field stimulation. 2 Submaximal and maximal atropine‐ (1 μm) resistant AER elicited by balloon distension averaged about 40–50% and 70–90% of maximal circular spasm to 80 mm KCl, respectively. The NK1 receptor antagonist, (±)‐CP 96,345 (1 μm) inhibited both maximal and submaximal AER. FK 888 (1–3 μm) inhibited submaximal AER only. RP 67,580 (1 μm) was ineffective. The NK2 receptor antagonist, GR 94,800, inhibited both maximal and submaximal AER at all concentrations tested (0.1–3.0 μm), while SR 48,968 was effective only at 1.0 μm. The NK2 receptor antagonists, MEN 10,376 and MEN 10,573 inhibited both submaximal and maximal AER at 10 and 1.0 μm, respectively. 3 In other experiments, an NK1 receptor antagonist, (±)‐CP 96,345 or FK 888 (1.0 μm in each case) was administered first and the effect of GR 94,800 (1.0 μm) on the residual AER response was determined; or GR 94,800 was administered first and the effect of (±)‐CP 96,345 or FK 888 was determined. The results of these experiments indicated an additive effect produced by the combined treatment with NK1 and NK2 receptor antagonists. 4 Electrical field stimulation (10 Hz for 0.5 s, 10–20 V, 0.15–0.3 ms pulse width) with electrodes placed at 1.4–1.8 cm anal to the recording site, produced ascending contractions which were almost abolished by 10 μm hexamethonium (electrically‐evoked AER). In the presence of apamin (0.1 μm) and NG‐nitro‐l‐arginine (30 μm) these contractions were reproducible over 10 consecutive stimulation cycles. GR 94,800 (1 μm) and FK 888 (1 μm) both produced a partial inhibition of the electrically‐evoked AER and their combined administration produced an inhibitory effect which was larger than that induced by each antagonist alone. 5 FK 888 (1–3 μm), GR 94,800 (1–3 μm), MEN 10,573 (1 μm) and MEN 10,376 (10 μm) did not significantly affect the atropine‐sensitive twitch contractions produced by electrical field stimulation of the guinea‐pig ileum longitudinal muscle‐myenteric plexus preparation, which were abolished by 10–30 μm procaine, 1 μm tetrodotoxin or 1 μm atropine. (±)‐CP 96,345 (1 μm) and SR 48,968 (1 μm) produced 12% and 27% inhibition of cholinergic twitches in the longitudinal muscle of the ileum, respectively. 6 We conclude that both NK1 and NK2 receptors mediate the atropine‐resistant AER to the circular muscle of the ileum. NK2 receptor activation plays a more important role than NK1 receptor activation in the AER evoked by radial stretch. Since a consistent fraction of the distension‐ and electrically‐evoked atropine‐resistant AER persists in the presence of combined NK1 and NK2 receptor blockade, the existence of a third excitatory transmitter to the circular muscle of the ileum, in addition to acetylcholine and tachykinins, is suggested.