Plasma levels of nitrite and nitrate in early and recent classes of fish

The stable metabolite of nitric oxide in plasma is NO_x, the sum of nitrite plus nitrate. Measures of plasma NO_x may provide information about the nitric oxide tonus of the entire endothelium including capillary microvessels. Although data are available for mammalian species, plasma NO_x measurements in early vertebrate species are scarce. The purpose of this study was to test the hypothesis that plasma NO_x would be similar to the NO_x in the water environment for fish in early classes (Agnatha and Chondrichthye) and would exceed water NO_x levels in the known nitrite-sensitive fish (Osteichthye). Plasma samples were obtained from 18 species of adult fish (n = 167) and from their housing or natural water environment. NO_x was measured by using chemiluminescence. Plasma NO_x was detected in all species and ranged from 0.5 nmol/ml (skate) to 453.9 nmol/ml (shortnose gar). Average plasma NO_x was significantly higher in sea lamprey than in Atlantic hagfish whereas that of little skate was 3-fold lower than in spiny dogfish shark. Plasma NO_x differed significantly among early bony fish (paddlefish, pallid sturgeon, gar) yet was similar among modern bony fish, with the exception of rainbow trout. Plasma NO_x reflected water NO_x in only 2 species (hagfish and shark), and levels did not coincide with nitrite sensitivity. This study provides an expanded comparative view of plasma NO _x levels across 3 groups of early fish. The data obtained suggest a nitric oxide system in early and modern fish.