Gill O2 uptake, CO2 excretion, ventilation and blood respiratory/acid&shy;base variables were evaluated in control and softwater-acclimated trout (Oncorhynchus mykiss) to test the hypothesis that gill chloride cell (CC) proliferation, elicited by 2 weeks of softwater exposure, impairs the diffusion of respiratory gases across the gill. The proliferation of CCs in softwater fish was verified using light microscopy, and its impact on respiratory gas transfer was assessed in vivo by continuous monitoring of arterial blood PO2 (PaO2), PCO2 (PaCO2) and pH (pHa) using an extracorporeal blood circulation under conditions of normoxia and graded hypoxia [water PO2 (PwO2) was lowered from 20.0 kPa to 5.3 kPa within 20 min]. During normoxia, ventilation frequency was significantly higher in the softwater trout (78&plusmn;4 versus 57&plusmn;4 breaths min-1; mean &plusmn; s.e.m.), while ventilation amplitude was similar in both groups (1.0&shy;1.1 cm opercular displacement). PaCO2 and plasma HCO3- concentration were significantly lower in the softwater fish and the blood acid&shy;base status was characterized by a mixed respiratory alkalosis and metabolic acidosis such that blood pH was not statistically different between the two groups. CO2 excretion (2.5&shy;2.8 mmol kg-1 h-1) and O2 uptake rates (2.3&shy;5.1 mmol kg-1 h-1), as measured during normoxia, were unaffected by acclimation to soft water. During hypoxia, ventilation frequency and amplitude increased in the control trout, whereas only ventilation amplitude increased in the softwater-acclimated fish. The rate of PaO2 reduction during hypoxia was significantly greater in the softwater fish (0.84&plusmn;0.06 versus 0.65&plusmn;0.06 kPa PaO2 kPa-1 PwO2) and, at the most severe level of hypoxia (PwO2=5.3 kPa), PaO2 was significantly lower in the softwater fish. The rate of PaCO2 reduction (caused by hyperventilation) was significantly lower in the softwater-acclimated fish (0.002&plusmn;0.001 versus 0.005&plusmn;0.001 kPa PaCO2 kPa-1 PwO2; mean &plusmn; s.e.m.; P<0.06) and, indeed, was not statistically different from zero. Blood pH did not change significantly during hypoxia in either group but, through much of the hypoxic period (7&shy;15 kPa PwO2), pHa was statistically lower in the softwater-acclimated fish. These results demonstrate that exposure of trout to soft water for 2 weeks is associated with proliferation of lamellar CCs and impaired branchial gas transfer. Hyperventilation was identified as a compensatory physiological adjustment.

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