Physiological Basis for Large Differences in Resistance to Nitrite among Freshwater and Freshwater-Acclimated Euryhaline Fishes

Uptake of environmental NO2 - by most freshwater fishes occurs at the gills where NO2 - is actively transported into the blood by the Cl- uptake pathway. Some freshwater fishes do not concentrate NO2 - in their plasma, regardless of environmental NO2 - exposure and exhibit a high degree of resistance to NO2 -. Recent studies indicate that freshwater-adapted killifish (Fundulidae: Fundulus heteroclitus) and European eel (Anguillidae: Anguilla anguilla) have no or minimal Cl- uptake activity at the gills relative to most freshwater fishes; rather, Cl- requirements are met in other ways (probably dietary). We hypothesized that different rates of Cl- uptake by the gill may explain the observed differences in NO2 - uptake and consequent toxicity among freshwater fishes. Cl- influx rates of channel catfish (Ictaluridae: Ictalurus punctatus), a species that concentrates NO2 - in the plasma and is sensitive to NO2 -, and bluegill (Centrarchidae: Lepomis macrochirus), a species that does not concentrate NO2 - in the plasma and is resistant to NO2 -, were determined over a range of environmental Cl- concentrations. Channel catfish actively transported chloride into the plasma (K m = 155.6 ± 101.2 μmol/L Cl-; J max = 414.9 ± 51.4 nmol/g/h; ± SEM). In contrast, bluegill exhibited no observable Cl- uptake. We placed our results and previously reported results in a phylogenetic context and concluded that differences in Cl- uptake mechanisms among groups of freshwater fishes may explain, in large part, the wide range of sensitivity to environmental NO2 -. NO2 - uptake determinations may also prove to be an easy screening method when studying the phylogenetic distribution and nature of Cl- uptake mechanisms in the gills of fishes.