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 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 utilized by an acid sensitive species to maintain ion balance in low pH and appeared to be a result of reduced gill paracellular permeability, though we did not observe any significant differences in the transcript abundance of 6 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 that, like the rainbow trout, possess acid sensitive mechanisms of ion absorption.