Branchial Ion and Acid-Base Transfer in Freshwater Teleost Fish: Environmental Hyperoxia as a Probe

In freshwater fish, linked ion and acid-base fluxes across the gills are dynamically manipulated to regulate internal acid-base status in the face of environmental challenges. Relevant theory, methodology, and a synthesis of current ideas are presented, based on experiments with hypercapnia, low pH, acid and base infusion, temperature change, enforced exercise, toxicants, and other challenges. Hyperoxia may be the most useful of these treatments, for it is devoid of confounding factors caused by other probes-high water [H⁺], catecholamine mobilization, and metabolic rate changes. In rainbow trout, hyperoxia (inspired Po2[Pio2] > 500 Torr) induces a pure respiratory acidosis that is fully compensated over 72 h by net acid excretion, more than 90% of which occurs via the gills. Return to normoxia induces a pure metabolic alkalosis that is more rapidly compensated by branchial acid uptake (= base excretion). Use of this protocol has demonstrated that (1) respiratory acidosis is largely compensated by adjustments of Cl⁻ versus base exchange, while both Cl⁻ versus base and Na⁺ versus acid exchanges contribute during metabolic alkalosis; (2) these dynamic adjustments in Na⁺ and Cl⁻ influx are achieved by complex changes in both substrate afinity and maximum transport capacity (Jmax); Na⁺ and Cl⁻ afinities appear to be maximal under control conditions and can only be decreased, but Jmax values can be either increased or decreased depending on the internal acid-base status; (3) differential diffusive efflux of Na⁺ relative to Cl⁻ provides an additional mechanism of net acid flux during alkalosis; (4) large decreases in branchial intracellular fluid volume, Na⁺, and Cl⁻ levels occur during respiratory acidosis and are corrected during normoxic recovery; (5) mean gill intracellular pH remains extremely stable. The importance of the internal acid-base status of the transport cells, and a possible regulatory role for extracellular pH, are discussed.