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 studied in an Ussing chamber modified so that a strain gauge transducer could be attached longitudinally on the serosal side of the intestine. Ileal segments from 22 rats were mounted as flat sheets and voltage clamped at zero transmural potential (PD). Changes in short-circuit current (delta ISC) in the absence of carbachol and longitudinal muscle relaxations in the presence of carbachol in response to transmural field stimulation (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 after treatment with L-NNA. In the control period, the delta ISC and muscle relaxation were frequency dependent with maximal responses generated at a frequency of 10 Hz. Tetrodotoxin (5 x 10(-6) M) blocked muscle relaxation and decreased delta ISC by 94% during TMS at 10 Hz. L-NNA blocked the muscle relaxation induced by TMS but failed to alter delta ISC. 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 ISC either in the presence or absence of carbachol. These results support a role for NO as a neurotransmitter mediating relaxation of ileal smooth muscle but not mediating changes in epithelial ISC.