Strategies for maintaining Na⁺ balance in zebrafish (Danio rerio) during prolonged exposure to acidic water.

The objective of the present study was to characterize the capacity of zebrafish (Danio rerio) to regulate whole body Na⁺ levels during exposure to acidic (pH 3.8-4.0) water. Exposure to acidic water significantly affected the mRNA levels of 14 claudin and two occludin isoforms, tight junction proteins thought to be involved in regulating paracellular efflux. Despite these changes, Na⁺ efflux as well as uptake of polyethylene glycol (PEG), a marker for paracellular pathway, was persistently elevated during the 2-week period of acid exposure, although there was a transient recovery between 12- and 72-h. Pre-exposing fish to acidic water for 2 weeks failed to attenuate the increase in Na⁺ efflux associated with acute exposure to acidic water of low [Ca²⁺]. However, during recovery in water of circumneutral pH following exposure to acidic water, normal rates of Na⁺ efflux were restored within 5h. The rate of Na⁺ uptake was significantly elevated between 4 and 7 days of exposure to acidic water; the increase was associated with significant increases in maximal Na⁺ uptake capacity (J(MAX)Na⁺) and affinity constant (K(M)). These results demonstrate that in acidic water, zebrafish maintain their whole body Na⁺ balance primarily by regulating Na⁺ uptake, rather than Na⁺ efflux.