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The potential for salt toxicity: Can the Trans-Epithelial Potential (TEP) across the gills serve as a metric for major ion toxicity in fish?
Citation:
Wood, C., M. McDonald, M. Grosell, Dave Mount, W. Adams, B. Po, AND K. Brix. The potential for salt toxicity: Can the Trans-Epithelial Potential (TEP) across the gills serve as a metric for major ion toxicity in fish? AQUATIC TOXICOLOGY. Elsevier Science Ltd, New York, NY, 226:1-12, (2020). https://doi.org/10.1016/j.aquatox.2020.105568
Impact/Purpose:
This manuscript describes a series of experiments to determine the change in trans-epithelial potential (TEP) in fish exposed to major ion salts across a range of concentrations. TEP is the difference in electrical potential between the interior of the fish and the surrounding water, and is influenced by the organism’s regulation of the internal concentrations of various ions (e.g., Na+, K+). TEP is a theorized basis for a proposed toxicity model to predict the aquatic toxicity of major ions (Na, K, Ca, Mg, Cl, SO4, HCO3) to fish and other aquatic organisms. The present work explores whether direct measures of TEP are consistent with the theory underpinning that toxicity model. Generally speaking, TEP in fathead minnows responded similarly to exposure to all the major ion salts tested, and the concentration causing changes in TEP was proportional to the acute toxicity of those salts to fathead minnows. For the two other species tested, responses of channel catfish were generally similar to those of fathead minnows, while bluegill sunfish showed different responses for some salts; the different responses noted for bluegill sunfish may related to differences in how that species obtains and regulations internal ions. Overall, the data provide support to the potential applicability of TEP as the basis for predicting the toxicity of major ions to fish and perhaps other aquatic organisms.
Description:
Contamination of freshwater by multiple major ions is an issue of growing concern. An emerging Multi-Ion Toxicity (MIT) model for assessment of environmental salt pollution is based on the premise that major ion toxicity to aquatic organisms is related to a critical disturbance of the transepithelial potential across the gills (ΔTEP), which in turn can be predicted by electrochemical theory. However, the model is based on many unproven assumptions and has never been evaluated physiologically. We directly tested key assumptions of the model by examining the individual effects of eight different salts (NaCl, Na2SO4, MgCl2, MgSO4, KCl, K2SO4, CaCl2, and CaSO4) on measured TEP in three different fish species (fathead minnow (Pimephales promelas) = FHM; channel catfish (Ictalurus punctatus) = CC; bluegill (Lepomis macrochirus) = BG) using a geometric concentration series based on previously reported 96-h LC50s for FHM. In general, all salts caused concentration-dependent changes in TEP (TEP) to less negative/more positive values in a pattern well-described by the Michaelis-Menten equation. In each species, the ΔTEP responses for different salts coincided well when concentrations were expressed as a percentage of the LC50 and a plateau was reached at 100% where the ΔTEP values were remarkably consistent (FHM = 5.9 to 8.9 mV; CC = 4.9 to 6.9 mV; BG = 1.9 to 4.2 mV). This low 1.4 - 2.2-fold variation in ΔTEP contrasts with 28-fold variation in salt concentration, 9.6-fold in total dissolved solids, and 7.9-fold in conductivity at the LC50. Further analyses showed that ΔTEP responses were not a direct effect of osmolarity (all species), were related to specific cation rather than specific anion concentrations (FHM), and were stable for up to 24 h (CC). A few deviations in the BG from the general patterns may be related to its unusual ionoregulatory physiology. Overall, the results provide physiological support for the MIT model, are coherent with electrochemical theory, and point to areas of uncertainty for future research.
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DOI: The potential for salt toxicity: Can the Trans-Epithelial Potential (TEP) across the gills serve as a metric for major ion toxicity in fish?