Time course of extracellular acid-base adjustments under hypo- or hyperosmotic conditions in the euryhaline fish Platichthys flesus

In order to better understand the basis for the euryhalinity of the flounder, Platichthys flesus , which tolerates large variations in water salinity, experiments have been designed to characterize the time course of extracellular ionic and acid-base adjustments under hypo- or hyperosmotic conditions. Abrupt transfer from sea water (SW) to fresh water (FW) provokes a transient decrease in the plasma osmolality (Posm) and a concomitant transient metabolic alkalosis (whole blood pH 7.78 in SW and 8.04 five days after FW transfer) associated with a marked, persistent hypercapnia. After 33 days in FW, Posm and whole blood pH are not significantly different from those in SW, but whole blood Pco₂ and plasma bicarbonate concentration are always higher than SW values. Opposite transitory fluctuations, i.e. a metabolic acidosis associated with a respiratory alkalosis, occur when flounder long-acclimated to FW are again exposed to SW. The mechanisms involved in these salinity-dependent acid-base disturbances are rather complex and remain to be elucidated. These observations attest to the importance of the extracellular acid-base changes that may be (i) linked to extracellular anisosmotic regulation and/or to cellular metabolic adjustments, and (ii) compensated partially by ventilatory adjustments.