NITRIC-OXIDE IS INVOLVED IN MUSCLE-RELAXATION BUT NOT IN CHANGES IN SHORT-CIRCUIT CURRENT IN RAT ILEUM

L-Arginine (L-Arg)-nitric oxide (NO) pathways in rat ileum were studie d in an Ussing chamber modified so that a strain gauge transducer coul d be attached longitudinally on the serosal side of the intestine. Ile al segments from 22 rats were mounted as flat sheets and voltage clamp ed at zero transmural potential (PD). Changes in short-circuit current (Delta I-SC) in the absence of carbachol and longitudinal muscle rela xations in the presence of carbachol in response to transmural field s timulation (TMS; 5-s trains of impulses, 0.4-ms impulse duration, 1-10 Hz) were recorded during a control period, in the presence of N-omega -nitro-L-arginine (L-NNA; 10(-4) M), and in the presence of L-Arg afte r treatment with L-NNA. In the control period, the Delta I-SC and musc le relaxation were frequency dependent with maximal responses generate d at a frequency of 10 Hz. Tetrodotoxin (5 x 10(-6) M) blocked muscle relaxation and decreased Delta I-SC by 94% during TMS at 10 Hz. L-NNA blocked the muscle relaxation induced by TMS but failed to alter Delta I-SC. Muscle relaxation to TMS was restored dose dependently by L-Arg . In segments from another group of eight rats, saturated NO solutions relaxed the muscle but failed to change I-SC either in the presence o r absence of carbachol. These results support a role for NO as a neuro transmitter mediating relaxation of ileal smooth muscle but not mediat ing changes in epithelial I-SC.