Effects of transmural field stimulation in isolated muscle strips from rabbit sphincter of Oddi and duodenum

The role of nitric oxide in the electrical field stimulation-induced contractions of sphincter of oddi and gallbladder strips in Guinea pigs

The aim of this study was to investigate the modulatory role of nitric oxide (NO) in the electrical field stimulation (EFS)-induced contractions of isolated sphincter of Oddi (SO) and gallbladder strips from guinea pigs. EFS was used to activate the intrinsic nerves in SO and gallbladder strips. EFS produced frequency-dependent biphasic contractile responses in the SO strips. A smaller contraction, "on response", occurred during EFS, which was followed by a bigger contraction, "off response". Both responses were completely and irreversibly abolished by tetrodotoxin (TTX) (10(-6) M). Atropine (10(-6) M) inhibited the "on response", but not the "off response". EFS produced frequency-dependent monophasic contractile responses in gallbladder strips, which were completely and irreversibly abolished by TTX (10(-6) M) and atropine (10(-6) M). A nitric oxide synthase (NOS) inhibitor, NG-nitro-L-arginine (10(-4) M and 3 x 10(-4) M, in SO and gallbladder strips, respectively), significantly increased all EFS-induced contractions of SO and gallbladder strips. L-Arginine, but not D-arginine reversed the effect induced by the NOS inhibitor, at all frequencies, in both strips. These results suggested that NO released from nitrergic nerve endings might play a regulatory role in the cholinergic neurotransmission of guinea pig SO and gallbladder strips. The "off response" in he SO preparations might be a rebound increase that was modulated by the nonadrenergic, noncholinergic inhibitory mediator NO.

Pattern of nonadrenergic, noncholinergic responses during short- or long-lasting electrical stimulation in guinea-pig ileum

1. The pattern of responses of longitudinally oriented guinea pig ileum organ bath preparations was studied during short- (1-5 sec) or long-lasting (20 sec) electrical field stimulation (EFS, 0.8 msec, 40 V, 1-20 Hz). 2. In the presence of phentolamine (5 microM), propranolol (5 microM), and atropine (3 microM), the EFS elicited nonadrenergic, noncholinergic (NANC), tetrodotoxin (0.3 microM)-sensitive responses. 3. The 1-sec EFS evoked relaxation. The response to 5-sec EFS consisted of relaxation followed by twitch, whereas relaxation, twitch and tonic contraction characterized the NANC response to 20-sec EFS. The maximum relaxation was observed at 10-Hz short- or long-lasting EFS. 4. Both N-G-nitro-L-arginine (L-NNA, 0.1-0.5 mM) and apamin (1-5 microM) concentration dependently inhibited the relaxation of the NANC response to 10-Hz 20-sec EFS. During L-NNA treatment, the twitch and the tonic contractions were increased. The inhibitory effect of L-NNA was reversed by L-arginine (0.1-0.5 mM) but not by D-arginine. Sodium nitroprusside (1-10 microM) was without effect. 5. AP 13.2 ACOH (0.1 microM), a blocker of Substance P receptors, inhibited the twitch and the tonic contractions. The contractions were decreased after desensitization of purinoceptors by ATP and in the presence of the GABA(A) receptor antagonist bicuculline (30 microM). 6. Depending on the EFS duration, a subsequent occurrence of relaxation and contractions characterized the NANC responses. It seems that relaxation is mediated by nitric oxide whereas Substance P and ATP are involved in the maintenance of the twitch and the tonic contractions. Nitric oxide appears to exert an inhibitory effect on the excitatory transmitters, whereas purinergic mechanism(s) could modulate the nitric oxide-dependent relaxation.