NITRIC-OXIDE SYNTHESIS BY TRACHEAL SMOOTH-MUSCLE CELLS BY A NITRIC-OXIDE SYNTHASE-INDEPENDENT PATHWAY

Nitric oxide (NO)is known to be synthesized from L-arginine in a react ion catalyzed by NO synthase. Liver cytochrome P-450 enzymes also cata lyze the oxidative cleavage of C=N bonds of compounds containing a -C( NH2)=NOH function, producing NO in vitro. The present study was design ed to investigate whether there was evidence of a similar pathway for the production of NO in tracheal smooth muscle cells. Formamidoxime (1 0(-2) to 10(-4) M), a compound containing -C(NH2)=NOH, relaxed carbach ol-contracted tracheal rings and increased intracellular cGMP in cultu red tracheal smooth muscle cells, whereas L-arginine had no such effec t. NO was detectable in the medium containing cultured tracheal smooth muscle cells when incubated with formamidoxime. Ethoxyresorufin (10(- 7) to 10-4 M), an alternate cytochrome P-450 substrate, inhibited form amidoxime-induced cGMP accumulation as well as tracheal ring relaxatio n in cultured tracheal smooth muscle cells. The NO synthase inhibitors N-omega-nitro-L-arginine (10(-3) M) and N-G-monomethyl-L-arginine (10 -3 M) had no effect on formamidoxime-induced cGMP accumulation. These results suggest that NO can be synthesized from formamidoxime in trach eal smooth muscle cells, presumably by a reaction catalyzed by cytochr ome P-450.