THE ROLE OF NITRIC-OXIDE IN CHOLINERGIC NEUROTRANSMISSION IN RAT TRACHEA

1 We have investigated the role of nitric oxide (NO) in cholinergic contraction in rat trachea. 2 Methylene blue (10 nM to 30 muM) potentiated cholinergic contraction induced by electrical field stimulation (EFS) at 5 Hz in a concentration-dependent fashion. At a concentration of 30 muM, methylene blue decreased responses to log EFS frequency, producing 50% of maximum contraction from a control value of 0.74 +/- 0.09 Hz to 0.30 +/- 0.05 Hz without a significant effect on concentration-response curves to acetylcholine (ACh). 3 N(G)-monomethyl-L-arginine (L-NMMA; 100 muM) also potentiated cholinergic contraction induced by EFS at 5 Hz (131.5 +/- 4.6% of control) without having any effect against ACh (3 muM)-induced contractions. Likewise, L-NMMA (100 muM) significantly increased EFS (5 Hz)-evoked release of ACh from tracheal segments into the bath solution (51.4 +/- 4.0 pmol ml-1 in the presence of L-NMMA and 35.0 +/- 1.8 pmol ml-1 in the absence of L-NMMA, respectively). 4 Administration of NO (present in acidified soluton of NaNO2) (1 nM to 10 muM) and sodium nitroprusside (100 nm to 10 muM) concentration-dependently reduced EFS (5 Hz)-induced cholinergic contractions without having a significant effect on ACh (3 muM)-induced contractions. These results were unaffected by prior exposure of the tissues to L-NMMA (100 muM). 5 Dibutyryl cyclic GMP (3 mM) also reduced cholinergic contractions induced by EFS at 5 Hz (70.1 +/- 3.6% of control) without any significant effect on ACh (3 muM)-induced contractions. 6 Pretreatment of tissues with capsaicin (30 muM) or alpha-chymotrypsin (1 u ml-1) failed to inhibit methylene blue (30 muM)-induced potentiation of responses to EFS at 5 Hz. 7 These results suggest that an endogenous NO-like factor may mediate prejunctional inhibition of cholinergic contraction through a cyclic GMP-dependent mechanism in rat trachea.