Adaptive respiratory responses of trout to acute hypoxia. I. Effects of water ionic composition on blood acid-base status response and gill morphology.

The effects of various levels of hypoxia (PWO2 ranging from 10 to 60 Torr) on arterial blood gases (PaO2 and PaCO2) and acid-base status were investigated in trout at 15 degrees C. The hypoxic responses of two stocks of trout living in natural waters having very different levels of NaCl (1.0 mmol.L-1 and 0.1 mmol.L-1) and carbonate alkalinity (0.4 mmol.L-1 and 2.4 mmol.L-1) were compared. The use of an extracorporeal circulation method made it possible to continuously monitor the pH changes. The different patterns of the acid-base status observed in response to hypoxia depend on the evolution of PaO2. Two critical PaO2 thresholds were defined. Crossing the upper (about 15 Torr) induces metabolic acidosis which is normally followed by pH recovery, while crossing the lower (about 10 Torr) promotes loss of capacity to compensate acidosis. The NaCl concentration of the water drastically modifies the fish sensitivity to hypoxia: fish living in water with a low NaCl concentration have less resistance to hypoxic exposure. This may be explained by the fact that in fish living in low NaCl concentrations, the secondary gill lamellae are surrounded by chloride cells, which considerably reduce the surface area available for gas exchange. Consequently a modest fall in PWO2 induces a drastic reduction of the arterial oxygen tension which crosses the lower critical PaO2 threshold.