Summary

The contribution of the secondary circulatory system to acid-base regulation and epithelial ion transport was evaluated qualitatively in freshwater-acclimated rainbow trout. The dorsal aorta (DA) and the lateral cutaneous vessel (LCV) (which is considered to be the venous drainage of the secondary system) were chronically cannulated and the fish were exposed to environmental hypercapnia (2 % CO₂) after establishment of normocapnic control values. Fluid sampled from the LCV contained much less haemoglobin (0.14 g 100 ml⁻¹) and fewer blood cells (packed cell volume, PCV, 1.2-1.7%) than DA blood ([Hb] 8.2-8.9 g 100 ml⁻¹, PCV 27.2-32.5%) regardless of ambient CO₂ levels, indicating highly limited access of red blood cells to the secondary circulatory system through anastomoses connecting it to the primary system. There was no significant difference between the two sampling sites for any of the acid-base variables (pH, P_CO2 [HCO₃⁻]) and most plasma ion concentrations ([Na⁺], [Ca²⁺], [Mg²⁺] and [Cl⁻]) during normocapnia, although plasma [K⁺] and osmolarity were significantly lower in LCV fluid. Upon exposure to hypercapnia, the pH of LCV fluid became significantly higher than that of DA plasma because of the higher [HCO₃⁻] in the LCV plasma than in the DA plasma. The LCV plasma concentration of Cl⁻ was significantly lower than that of DA plasma during hypercapnia. These data suggest that the secondary circulatory system draining the body surface is involved in ionic acid-base regulation in fish, probably via Cl⁻/HCO₃⁻ exchange processes across the extrabranchial epithelium of the body surface.