PHYSIOLOGICAL-RESPONSES OF RAINBOW-TROUT (ONCORHYNCHUS-MYKISS) TO PROLONGED EXPOSURE TO SOFT-WATER

Adult rainbow trout (Oncorhynchus mykiss) were acclimated to artificia l soft water to evaluate potential physiological responses compensatin g for the thickening of the blood-to-water diffusion barrier that is k nown to accompany branchial lamellar chloride cell proliferation. Indi rect estimates of gill ventilation volumes using the Fick principle de monstrated pronounced hyperventilation in the soft-water-acclimated fi sh (2,280 +/- 434 mL min(-1) kg(-1)) when compared to the control fish (1,217 +/- 166 mL min(-1) kg(-1)). Despite the hyperventilation, the soft-water-acclimated fish displayed significantly lowered arterial Po -2 values (93.7 +/- 2.4 Torr [1 Torr = 133.32 Pa] vs. 102.5 +/- 1.6 To rr); arterial Pco(2) was unaffected by acclimation to soft water An in vitro radioZsotopic assay was used to demonstrate that an;intrinsic a cceleration in the rate of HCO3- flux through the red blood cell and h ence accelerated H2CO3 dehydration was not contributing to the mainten ance of arterial Pco(2) in the soft-water-acclimated fish. The constru ction of blood O-2 equilibrium curves in vitro revealed an increased a ffinity of haemoglobin-O-2 binding in the soft-water-acclimated fish ( P-50 = 12.3 +/- 1.3 Torr) when compared to the controls (17.6 +/- 0.9 Torr). In response to hypoxia (water Po-2 = 30 - 90 Torr), both groups of fish released catecholamines into the circulation. However, catech olamine release occurred at higher levels of water Po-2 (80 Torr compa red to 50 Torr) in the soft-water-acclimated fish. The results demonst rate that prolonged exposure of rainbow trout to soft water promotes s everal physiological responses that may serve to alleviate or reduce t he detrimental consequences of the thickened diffusion barrier elicite d by chloride cell proliferation.