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

The stable metabolite of nitric oxide in plasma is NOx, the sum of nitrite plus nitrate. Measures of plasma NOx may provide information about the nitric oxide tonus of the entire endothelium including capillary microvessels. Although data are available for mammalian species, plasma NOx measurements in early vertebrate species are scarce. The purpose of this study was to test the hypothesis that plasma NOx would be similar to the NO in the water environment for fish in early classes (Agnatha and Chondrichthye) and would exceed water NOx 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. NOx was measured by using chemiluminescence. Plasma NO was detected in all species and ranged from 0.5 nmol/ml (skate) to 453.9 nmol/ml (shortnose gar). Average plasma NOx 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 differed significantly among early bony fish (paddlefish, pallid sturgeon, gar) yet was similar among modern bony fish, with the exception of rainbow trout. Plasma NOx reflected water NO 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, levels across 3 groups of early fish. The data obtained suggest a nitric oxide system in early and modern fish.