Mechanisms of ion and acid‐base regulation at the gills of freshwater fish

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Abstract
This review examines the branchial mechanisms utilized by freshwater fish to regulate internal acid‐base status and presents a model to explain the underlying basis of the compensatory processes. Rainbow trout, Oncorhynchus mykiss, and brown bullhead, Ictalurus nebulosus, were examined under a variety of experimental treatments which induced respiratory and metabolic acid‐base disturbances. Acid‐base regulation was achieved by appropriate adjustments of Na + and Cl− net fluxes across the gills which, in turn, were accomplished by variable contributions of three different branchial mechanisms: 1) differential changes in Na+ and Cl− diffusive effluxes, 2) changes in internal substrate (H+, HCO3−) availability, and 3) morphological adjustments to the gill epithelium. Differential diffusive efflux of Na+ over Cl− was involved only during periods of metabolic alkalosis. The importance of internal substrate availability was demonstrated using a two‐substrate model. According to the model, ionic flux rates (JinCl−, JinNa +) are determined not only by the concentration of the external ion (Na+, Cl−) but also by the concentration of the internal counterion (H+, HCO3−). This system provides for an “automatic negative feedback” to aid in the compensation of metabolic acid‐base disturbances. Morphological alteration of the gill epithelia and the associated regulation of chloride cell (CC) fractional area is an essential third mechanism which is especially important during respiratory acid‐base disturbances. Specifically, fish vary the availability of the CC associated Cl−/HCO3− exchange mechanism by physical covering/uncovering of CCs by adjacent pavement cells.