Maintenance of sodium ion balance in rainbow trout exposed to moderately acidic water is achieved through reduction of sodium efflux

In most freshwater fishes, exposure to acidic water decreases rates of active ion uptake and increases rates of passive ion efflux. To survive in low pH conditions, fish must possess mechanisms to maintain net ion flux rates close to zero. In this study, rainbow trout (Oncorhynchus mykiss) exposed to moderate acidity (pH = 4.5 or 5.5) in hard water (∼180 mg/L CaCO₃) demonstrated Na⁺ influx inhibition relative to fish held in control conditions (pH = 8.0). However, Na⁺ efflux rates were significantly lower in fish exposed to low pH, which minimized the net loss of Na⁺. This represents a relatively novel mechanism to maintain ion balance in low pH for an acid sensitive species, which appeared to be a result of reduced gill paracellular permeability. However, there were no significant differences in the transcript abundance of six different tight junction genes in response to low pH exposure. Overall, we conclude that acid tolerance may be dictated by differences in efflux rates in fishes which, like the rainbow trout, possess acid sensitive mechanisms of ion absorption.