Na⁺ and Cl⁻ Uptake Kinetics, Diffusive Effluxes and Acidic Equivalent Fluxes Across the Gills Of Rainbow Trout: I. Responses To Environmental Hyperoxia Available to Purchase

Endogenous respiratory acidosis and metabolic alkalosis were induced in bladder-catheterized freshwater rainbow trout by exposure to environmental hyperoxia (72 h) and its subsequent removal. Unidirectional and net fluxes of Na⁺, Cl⁻ and acidic equivalents across the gills were examined over 0.5 h intervals. Hyperoxia resulted in a positive Na⁺ balance, negative Cl⁻ balance and net acidic equivalent excretion. Return to normoxia caused a negative Na⁺ balance, a positive Cl⁻ balance and net basic equivalent excretion (=acidic equivalent uptake). Cl⁻/basic equivalent exchange was more important than Na⁺/acidic equivalent exchange in the homeostatic responses, and alkalosis was a more potent stimulus than acidosis for change in branchial ion fluxes. Kinetic analysis demonstrated that alterations in ion fluxes were achieved by complex changes in both the K_m (inverse of affinity) and the J_max (maximal transport rate) of the branchial C1⁻/HCO₃⁻ (OH⁻) and Na⁺/H⁺(NH₄⁺) transporters. (control=165μequivl⁻¹) and 114μequivl⁻¹) were increased during hyperoxic acidosis to 250 and 44μ μequivl⁻¹, respectively. (291μequiv kg⁻¹h⁻¹) and (456 μequiv kg⁻¹ h⁻¹) did not change significantly. During post-hyperoxic alkalosis, was further increased to 559μequivl⁻¹, increased to 445μequivkg⁻¹h⁻¹, while and decreased to 137μequivl⁻¹ and 309μequiv kg⁻¹ h⁻¹, respectively. Diffusive efflux was examined using a novel method. There was no significant differential diffusive efflux of Na⁺ and Cl⁻ during hyperoxia but diffusive Na⁺ efflux exceeded Cl⁻ efflux during posthyperoxic alkalosis, thereby serving as an additional mechanism for basic equivalent excretion.