NITRERGIC MODULATION OF CHOLINERGIC RESPONSES IN THE OPOSSUM LOWER ESOPHAGEAL SPHINCTER

1 Electrical field stimulation (EFS) of the superfused lower oesophage al sphincter from opossum (Monodelphis domestica) elicited biphasic re sponses. The first phase (relaxation) was strictly dependent on the du ration of the EFS. The second phase (contraction) started following te rmination of the EFS (less than or equal to 15 Hz). EFS at frequencies above 15 Hz led only to contraction, which started immediately upon i nitiation of the stimulation. 2 In the presence of N-G-nitro-L-arginin e (L-NOARG; 0.1-300 mu M), the relaxation phase was abolished and the contractile response started with the initiation of EFS (at all freque ncies) and was greater in magnitude. The contractile response to EFS w as completely blocked with scopolamine (10 mu M). 3 Exogenous acetylch oline (1-100 mu M) elicited concentration-dependent contractions of th e sphincter in the presence of botulinum toxin. These contractions wer e abolished when EFS was applied during administration of acetylcholin e. This inhibitory effect of EFS was completely reversed when the tiss ue was treated with L-NOARG (100 mu M). 4 These results suggest that t he cholinergic response in the opossum lower oesophageal sphincter is under nitrergic control.