TP5 Mechanisms of Toxic Action
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() Mechanistic basis for large differences in resistance to nitrite among freshwater fishes.

Tomasso, J1, Grosell, M2, 1 Clemson University, Clemson, South Carolina, USA2 University of Miami, Miami, Florida, USA

ABSTRACT- Uptake of environmental nitrite by most freshwater fishes occurs at the gills where nitrite is actively transported into the blood by the chloride-uptake pathway. Some freshwater fishes do not concentrate nitrite in their plasma, regardless of environmental-nitrite exposure and exhibit a high degree of resistance to nitrite. Recent studies indicate that freshwater-adapted killifish (Fundulidae: Fundulus heteroclitus) and European eel (Anguillidae: Anguilla Anguilla), unlike most freshwater fishes, do not actively transport chloride into the blood by way of the gills; rather, chloride requirements are met in other ways (probably dietary). We hypothesized that the presence or absence of chloride-uptake activity by the gill may explain the observed differences in nitrite uptake and consequent toxicity among freshwater fishes. Chloride influx rates of channel catfish (Ictaluridae: Ictalurus punctatus), a species that concentrates nitrite in the plasma and is sensitive to nitrite, and bluegill (Centrarchidae: Lepomis macrochirus), a species that does not concentrate nitrite in the plasma and is resistant to nitrite, were determined over a range of environmental chloride concentrations. Channel catfish actively transported chloride into the plasma (Km = 98.9 + 34.7 umol/L Cl-; Jmax = 230.4 + 18.5 nmol/g/h; + SE). In contrast, bluegill exhibited negligible chloride uptake. Differences in chloride-uptake mechanisms among groups of freshwater fishes may explain, in large part, the wide range of sensitivity (>100 fold, based on median-lethal concentrations) to environmental nitrite. Nitrite uptake determinations may also prove to be an easy screening method when studying chloride-uptake mechanisms in fishes. For example, several species of the families Centrarchidae and Moronidae have been examined and do not concentrate environmental nitrite, suggesting that chloride uptake does not occur at the gill. The observations within these families also suggest that the absence of chloride uptake at the gills may be more widespread among freshwater fishes than previously thought.

Key words: chloride flux, freshwater fishes, nitrite, gill

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