A number of studies have implied a linkage between acid­base and ion exchanges in both freshwater and seawater fish, although little is known about the branchial and renal acid­base transfers involved as the animals move between different salinities. To investigate the role of these transfers in a marine teleost fish as it is exposed to a dilute environment, we measured plasma acid­base values and net movements from fish to water of NH4+, HCO3- and H+ in long-horned sculpin (Myoxocephalus octodecimspinosus) placed in 100 %, 20 %, 8 % or 4 % sea water for 24­48 h. Renal excretion of H+ was also monitored in fish exposed to 4 % sea water. Sculpin proved to be somewhat euryhaline for they were able to maintain plasma ion and acid­base transfers in hypo-osmotic (20 %) sea water, but could not tolerate greater dilutions for more than several days. Plasma pH and carbon dioxide concentration (CCO2) increased in the 20 % and 8 % dilution groups, with CCO2 nearly doubling (control, 4.56 mmol l-1; 8 % group, 8.56 mmol l-1) as a result of a combined increase in the partial pressure of plasma CO2 (PCO2) and [HCO3-]. During a 44­46 h exposure, HCO3- transfers increased progressively in the most dilute water, with animals in the 8 % and 4 % groups exhibiting a net H+ loss that was smaller than that of seawater fish (control, 5.1 mmol kg-1; 8 %, 0.9 mmol kg-1; 4 %, -2.9 mmol kg-1). Animals exposed to 4 % sea water for 24 h and then returned to normal sea water had a variable plasma pH, an elevated CCO2 and a net efflux of H+ that effectively stopped (control, 0.10 mmol kg-1 h-1; 4 %, 0.02 mmol kg-1 h-1; seawater recovery, 0.20 mmol kg-1 h-1) during the low-salinity period. Renal acid excretion remained relatively constant throughout the experiment but only made up a significant portion (approximately 40 %) of the total acid transfers during the 4 % dilution period (control rate approximately 3 µmol kg-1 h-1: 3 % of branchial rate). We postulate that the increase in plasma CCO2 during exposure to low salinity may be due to mobilization of base from the intracellular bone compartment. The decrease in external salinity could induce base loss by alteration of gill ion exchanges (Na+/H+, Cl-/HCO3-) and/or changes in branchial HCO3- permeability. For the first time, we have shown that the effects of a dilute environment on acid­base transfers may be an important limitation to the survival of a euryhaline species in brackish or fresh water.

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