Physiological impact of salinity increase at organism and red blood cell levels in the European flounder (Platichthys flesus)

Blood respiratory, acid-base, and ionic changes in response to hyperosmotic shock were studied in vivo and in vitro in the European flounder. One primary aim was to evaluate regulatory changes in red blood cell (RBC) volume and its interrelationship with blood O₂ transporting properties. An acute increase in the ambient salinity from 10 to 30 ppt caused small but significant increases in extracellular osmolality (<20 mosM kg^-1), [Na⁺], and [Cl^-], which were corrected within 48 h. RBC volume was not significantly changed 3 h after the in vivo exposure to elevated salinity. A small metabolic acidosis was fully developed within 3 h, and this acidosis seemed responsible for a modest decrease in blood O₂ affinity (i.e., increased P₅₀-O₂ tension at 50% O₂ saturation). RBC organic phosphates were unchanged. In vitro elevation of whole blood extracellular osmolality by 60 mosM kg^-1 caused immediate RBC shrinkage. The subsequent regulatory volume increase (RVI) showed a graded dependency on blood O₂ saturation (S_O2). At S_O2 values of 0% and 20%, there were full RBC volume recoveries within 120 min, RVI was partial at S_O2 values of 45% and 55%, and RVI was absent at a S_O2 of 100%. S_O2 and P₅₀ did not change significantly during RBC shrinkage and RVI. Thus, the up-concentration of cellular haemoglobin and organic phosphates in hyperosmotically shrunken RBCs had minimal influence on blood O₂ transporting properties. The degree of cell shrinkage and time needed for RVI were positively correlated with the magnitude of the rise in extracellular osmolality. The RVI proceeded via elevation of cellular [Na⁺], [Cl^-], and to some extent also [K⁺]. Cell volume regulatory mechanisms are only needed to correct minor volume disturbances in vivo, because changes in extracellular osmolality were limited by an efficient osmotic regulation at the epithelial interface between extracellular compartment and environment.