Influence of ionic composition on acid-base regulation in rainbow trout (Oncorhynchus mykiss) exposed to environmental hypercapnia

The influence of ambient calcium, bicarbonate and chloride levels on acid-base regulation was investigated in rainbow trout acclimated and exposed to hypercapnia in five different water types. In soft water (low [Ca++] and [HCO3-]), compensation of the respiratory acidosis was slow and incomplete within 72h. High ambient [HCO3-] clearly improved extracellular HCO3- accumulation, and pH recovery was accomplished within 24h. This may result from stimulation of branchial HCO3- (influx)/Cl- (outflux) exchange. Elevation of ambient [Cl-] had a small, positive effect on pH compensation. High ambient [Ca++] improved the degree of pH compensation. Plasma [HCO3-] and [Cl-] showed an inverse 1:1 relationship in all acclimation groups, revealing an ubiquitous chloride-mediated acid-base regulation. Ventilation activity was increased by hypercapnia and only returned to control values in hard water (high [HCO3-] and [Ca++]). During progressive hypercapnia (up to 3% CO2), hard water acclimated fish obtained significantly higher plasma [HCO3-] (51.2 mM) than fish acclimated to low [Ca++]/high [HCO3-] (44.7 mM). This suggests an additive effect of ambient Ca++ on plasma HCO3- accumulation. At levels of CO2 above 1 %, some mortality was induced in low [Ca++]/high [HCO3-] water. Dying fish could be distinguished from surviving fish by an excessive Cl- loss and increasing extracellular anion gap.