Nitric oxide synthase activity and non‐adrenergic non‐cholinergic relaxation in the rat gastric fundus

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Abstract
1 In the presence of atropine (1 μm) and guanethidine (5 μm), electrical field stimulation (EFS, 120 mA, 1 ms, 0.5–16.0 Hz, trains of 2 min) induced frequency-dependent relaxations of 5-hydroxytryptamine (3 μm)-precontracted longitudinal muscle strips from the rat gastric fundus. 2 L-Citrulline concentrations were measured in the incubation medium of precontracted strips before and after EFS to investigate nitric-oxide (NO) synthase activity and its possible relation to non-adrenergic non-cholinergic (NANC) relaxation. 3 Basal NO synthase activity was reflected by the finding of prestimulation levels of L-citrulline of  30 nM. These levels were unaffected by tetrodotoxin (3 μm) and NG-nitro-D-arginine methyl ester (D-NAME, 100 μm), slightly reduced by a calcium-free medium and halved by NG-nitro-L-arginine methyl ester (L-NAME, 100 μm). 4 EFS evoked significant, frequency-dependent increases in bath levels of L-citrulline at all frequencies tested. The increases evoked by 16-Hz EFS were abolished by tetrodotoxin (3 μm), a calcium-free medium and L-NAME (100 μm) but not by D-NAME (100 μm). 5 L-NAME (0.1 μm − 1.0 mM) produced significant reduction of 4-Hz EFS-induced L-citrulline production (100% inhibition at 10 μm), but had less marked effects on basal production (50% reduction at 100 μm) and 4-Hz EFS-induced NANC relaxation (50% reduction at 1 mM). 6 L-Arginine (1 mM), but not D-arginine (1 mM), increased basal L-citrulline levels and reversed the inhibitory effect of L-NAME (10 μm). 7 These findings represent clear biochemical evidence of both basal and EFS-stimulated NO synthase activity in the rat gastric fundus.

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