High blood CO2 levels in rainbow trout exposed to hypercapnia in bicarbonate-rich hard fresh water--a methodological verification.

Heisler (1984, 1986) suggested that fish have a maximum extracellular bicarbonate concentration of about 30mmoll~, which can be attained and used for the compensation of blood acidosis induced during environmental hypercapnia. In contrast, Borjeson (1977) and Dimberg & Hoglund (1987) reported HCO3~ concentrations, analysed using Astrup's tonometric method (Astrup, 1956), of 40—SOmmoll" in venous blood drawn by a syringe from captured fish. However, blood pH in rainbow trout is greatly affected by sampling stress induced by venous puncture (Railo, Nikinmaa & Soivio, 1985). This may introduce error into the determination of blood [HCO3~] when using a tonometric technique based on pH measurements. Consequently, the present study was performed to determine whether the maximal blood HCC>3~ level in hypercapnic fish is limited to 30 mmol l~\ and if the higher reported blood bicarbonate values could be the result of sampling and/or analytical errors. Rainbow trout (Salmo gairdneri) were exposed to different levels of hypercapnia and acid-base variables were analysed either in venous blood taken with a syringe in the ductus Cuvieri from captured fish or in blood sampled via a dorsal aortic catheter from undisturbed fish. In addition, tonometry and gasometry were carried out separately to determine total blood CO2 concentration. Rainbow trout, weighing 250—350 g, were acclimated to flowing Uppsala tap water (8-10°C, pH7-88, PCO2 = 0-36kPa, [02] = l l m g r 1 , [Na] = 0-6 mmol 1"', [CP] = 0-SSmmoir, [Ca] =2-60mmoir, [HCO3"] =4mmoir 1 ) for at least 3 months. Three weeks before the experiments, fish were anaesthetized with MS-222, and the dorsal aorta was cannulated as described by Soivio, Nyholm & Westman (1975). Six fish were then transferred to 100-1 test aquaria with water of the same quality as that used during acclimation, except that the pH was 7-65 and the PCO2 was 0-57 kPa.