NITRIC-OXIDE RESPONSES OF AIR-BREATHING AND WATER-BREATHING FISH

Nitric oxide (NO), exogenously administered or endogenously produced b y NO synthase (NOS), is an important regulator of lung ventilation and perfusion in mammals. This study attempts to investigate the evolutio nary history of this system in fish and its possible relationship to a ir breathing. The gas bladder of Hoplerythrinus unitaeniatus (air-brea thing teleost) and Oncorhynchus mykiss (non-air-breathing teleost) and the lung of Lepidosiren paradoxa (air-breathing dipnoan) all exhibite d elevated guanosine 3',5'-cyclic monophosphate (cGMP) levels in respo nse to 1 mu M sodium nitroprusside. Only the H. unitaeniatus gas bladd er responded to 10 mu M acetylcholine chloride (ACh) with increased cG MP levels. The ACh response was inhibited by N-omega-nitro-L-arginine methyl ester, which inhibits NOS. These data suggest that although tis sues from each species may respond to exogenous NO, only the gas bladd er of H. unitaeniatus appears to synthesize NO through NOS, This is th e first report of constitutive NOS outside of the central nervous syst em in a teleost. These results also imply that NOS did not necessarily coevolve with air breathing in fish.