In this study effects of Rho kinase inhibitors have been examined on the mouse gastric fundal smooth muscle reactivity and neurotransmitter (acetylcholine) release. Two Rho-kinase inhibitors, Y-27632 and fasudil (HA-1077), conspicuously suppressed the contractile responses to carbachol (CCh) and KCl as well as electrical field stimulation (EFS, 40 V, 0.5 ms, and 20 s). pEC(50) value for CCh and EC50 value for KCl were 6.68 +/- 0.15 M and 10.4 +/- 2.8 mM, respectively. EFS induced reproducible contraction (38.3 +/- 4.75 mN/g tissue) which was almost abolished and potentiated in the presence of atropine (10(-6) M) and eserine (10-6 M), respectively. The Rho-kinase inhibitors relaxed the fundic strips preconstricted by submaximal concentration of CCh or KCl in a concentration dependent manner. With CCh-elicited contraction, the pEC(50) values of Y-27632 and fasudil were 5.45 +/- 0.14 and 5.11 +/-0.14 M, respectively (p > 0.05). However, the pEC(50) values for Y-27632 and fasudil on KCl-induced tone were 6.09 +/- 0.1 and 5.35 +/-0.06 M (p < 0.001), respectively. Moreover, [H-1] acetylcholine ([H-3]ACh) release upon EFS from the gastric fundus was measured and it was found that Y-27632 (10(-4) M) significantly impaired the release. At 3 Hz the radioactivity ratio obtained after and before EFS (S-2/S-1, ratio) was 0.88 +/- 0.03 in control but 0.63 +/- 0.08 in the presence of 10(-4) M Y-27632 (p < 0.05). These results suggest that Rho kinase inhibitors can not only relax the gastric fundus but also modulate CCh, cholinergic nerve stimulation, and KCl-induced contraction. Furthermore, Rho/Rho kinase signalling may play a role in the neurotransmitter (ACh) release in the mouse gastric fundus. (C) 2003 Elsevier Science (USA). All rights reserved.