EFFECTS OF TRANSMURAL FIELD STIMULATION IN ISOLATED MUSCLE STRIPS FROM RABBIT SPHINCTER OF ODDI AND DUODENUM

The purpose of the study was to compare the effect of transmural field stimulation (TMS) on isolated smooth muscle strips from rabbit sphinc ter of Oddi (SO), duodenal circular layer (D-c) and duodenal longitudi nal layer (D-l). The strips were suspended in thermostatically control led 5-ml organ baths containing Krebs solution constantly bubbled with 5% CO2 in O-2. TMS was delivered through platinum electrodes (140 V, 0.4 ms, 5 s trains, 40 Hz). The TMS responses could be divided in two main responses: (1) contraction initiated after cessation of the stimu lus train, preceded by an inhibitory phase during TMS ('off'); and (2) contraction initiated during TMS ('duration'). The 'duration' respons e was observed in one out of 20 strips in the SO and D-c compartments, whereas 11 D-l strips (55%) showed 'duration' responses (P < 0.001). Atropine (10(-6)) converted all 'duration' responses to an 'off' respo nse preceded by an inhibitory phase during TMS and reduced the contrac tile amplitudes with 40-65%. L-NNA significantly increased the number of 'duration' responses in all types of muscle, and caused a 40% incre ase in D-l contractile amplitude. Inhibitory responses could not be re moved by atropine, propranolol and phentolamine. The results suggest t hat the intrinsic innervation of SO and duodenal muscle consists of a mixture of excitatory, cholinergic and inhibitory NANC pathways. The l atter may utilize, wholly or partly, NO or a related compound as trans mitter. A relative dominance of excitatory, cholinergic responses was present in the D-l strips, whereas inhibitory responses were dominatin g in the SO and D-c strips.