Paracellular ion transport in trout opercular epithelium models osmoregulatory effects of acid precipitation

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Abstract
The opercular epithelium of brook trout, isolated in vitro in Ussing-style membrane chambers, is used to model the effects of acid precipitation on ion balance in fish. The epithelium normally has a low permeability to sodium, chloride, and mannitol, consistent with the barrier function of the skin. Treatment of the outside surface of the epithelium with low pH (threshold pH = 3.9) reversibly reduces transepithelial resistance (Rt). Reduction of outer pH to 3.61 ± 0.05 decreases Rt and increases the efflux of ions (Na+ and Cl−) and uncharged solutes, indicating that the epithelium rapidly and reversibly becomes leaky to solutes. Simultaneous mannitol and ion fluxes indicated that a common pathway, in which the solutes moved in relation to their free solution mobilities, is opened by low pH. The results are best explained by the acid-induced opening of a simple water-filled paracellular pathway. Low pH also altered the shape of nonlinear current-voltage relations, reducing the current density required to effect a reduction in Rt. The results are consistent with a model in which the titration by low pH of Ca2+ away from structural sites in the apical "tight" junctions weakens the intercellular junctions, thus leading to increased paracellular ion permeation. The effect may account in part for increased ion loss that leads to the morbidity of fish in soft-water acidic environments.