EVIDENCE FOR THE PARTICIPATION OF THE L-ARGININE-NITRIC OXIDE PATHWAYIN NEURALLY INDUCED RELAXATION OF THE ISOLATED RAT DUODENUM

1. Electrical field stimulation (EFS) of intrinsic nerves in the rat p roximal duodenum induces a frequency-dependent non-adrenergic-non-chol inergic (NANC) relaxation response. 2. The inhibitors of L-arginine-NO synthase L-N-G-nitro arginine methy-ester (L-NAME) and L-NOARG (L-N-G -nitro arginine) reduced the NANC relaxations elicited by EFS in a dos e- and time-dependent manner; L-NOARG was two times more potent than L -NAME (IC50 = 14.3 vs 25.2 mu M) and these effects were partially reve rted by the addition of 300-1000 mu M L-arginine but not of 300-1000 m u M D-arginine. Relaxation caused by vasoactive intestinal peptide (VI P;0.1 mu M) or ATP (20 mu M) was not blocked by L-NAME or L-NOARG. 3. The magnitude of the blockade caused by L-NAME and L-NOARG was depende nt on the frequency of stimulation. At low frequencies (below 1 Hz) bo th L-NAME and L-NOARG abolished the relaxations, while at 2 to 8 Hz on ly partial inhibition was observed (maximal inhibition: 44.6% +/- 5.2 and 63.4% +/- 3.4, respectively) 4. The basal tonus of the duodenum wa s increased by 10-300 mu M L-NAME and 10-300 mu M L-NOARG and this eff ect was blocked by 1 mM L-arginine. 5. Nitric oxide generated from aci dified NaNO2 caused a dose-dependent (EC(50)=2.75 mu M) relaxation of the duodenum which was not affected by 100 mu M L-NAME, 100 mu M L-NOA RG or 1 mu M tetrodotoxin (TTX). 6. NADPH-diaphorase positive neurons and fibers identified by histochemistry were present in the myenteric plexus and along both circular and longitudinal muscle fibers indicati ng that nitric oxide could be synthetized by these neural structures.