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

Abstract

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.

Figure 1.

Water nitrite and nitrate (NO_x; mean ± SE) for the housing environments (Table 2) of all species tested. Light gray bars, water sampled at the same time that plasma samples were obtained; open bars, repeat sampling of natural water environments (plasma not obtained with this set of samples); USDI, United States Department of the Interior. The 3-y Missouri (MO) River scour pit did not exist at the time of the second sampling.

Figure 2.

Nitrite and nitrate (NO_x) measured in 2 species of jawless fish: Vertebrate Class 1, Agnatha. Data are presented as individual points along with the plasma average. Water NO_x data are from Figure 1. Note break in abscissa. *, P < 0.0001

Figure 3.

Nitrite and nitrate (NO_x) measured in 2 species of cartilaginous fish: Vertebrate Class 2, Chondrichthye. Data are presented as individual points along with the plasma average. Water NO_x data are from Figure 1. Note break in abscissa. *, P < 0.0001.

Figure 4.

Nitrite and nitrate (NO_x) measured in the plasma of 3 species of Chondrostei bony fish: Vertebrate Class 3, Osteichthye. Data are presented as individual points along with the plasma average. Water NO_x data are from Figure 1. Location for paddlefish was Blind Pony Fish Hatchery (Table 2). Note break in abscissa. *, P < 0.0001.

Figure 5.

Nitrite and nitrate (NO_x) measured in the plasma of 11 species of Teleostei bony fish: Vertebrate Class 3, Osteichthye. Data are presented as individual points along with the plasma average. Water NO_x data are from Figure 1. Data for brown bullhead, black bullhead, and channel catfish were pooled as catfish. Location for channel catfish was a private pond. Location for largemouth bass and bluegill was a 3-y Missouri River scour pit. One outlier for catfish (20.6 nmol/ml) and one outlier for carp (32.0 nmol/ml) were not included in the analysis. *, P < 0.0001.

Figure 6.

Nitrite and nitrate (NO_x) measured in plasma for fish species sampled in 2 geographic locations. Data are presented as individual points along with the plasma average. Water NO_x data are from Figure 1. Black solid markers are data provided for comparison from Figure 4 for paddlefish (Blind Pony Fish Hatchery) and Figure 5 for channel catfish, largemouth bass, and bluegill. Two outlier data points for largemouth bass (12.3 and 15.8 nmol/ml) and 1 outlier for bluegill (245.6 nmol/ml) were not included in the analysis. #, P = 0.01; *, P < 0.0001; &, P = 0.02.