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Journal of Experimental Biology 152,521-547 (1990)
Published by Company of Biologists 1990

Na⁺ and Cl^- Uptake Kinetics, Diffusive Effluxes and Acidic Equivalent Fluxes Across the Gills of Rainbow Trout : I. Responses to Environmental Hyperoxia

GREG G. GOSS ¹ and CHRIS M. WOOD ²

¹ Department of Biology, McMaster University, 1280 Main Street West, Hamilton, Ontario, Canada L8S 4K1; Department of Biology, University of Ottawa, 30 George Glinski Pr., Ottawa, Ontario, Canada K1N 6N5
² Department of Biology, McMaster University, 1280 Main Street West, Hamilton, Ontario, Canada L8S 4K1

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 Cl^-/HCO₃^-(OH^-) and Na⁺/H⁺(NH₄⁺) transporters. K_m^Cl (control=165 µequiv l^-1) and K_m^Na (114 µequiv l^-1) were increased during hyperoxic acidosis to 250 and 445 µequiv l^-1, respectively. J_max^Cl, (291 µequiv kg^-1 h^-1) and J_max^Na (456 µequiv kg^-1 h^-1) did not change significantly. During post-hyperoxic alkalosis, K_m^Na was further increased to 559 µequiv l^-1, J_max^Cl increased to 445 µequiv kg^-1 h^-1, while K_m^Cl and J_max^Na decreased to 137 µequiv l^-1 and 309 µequiv 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.

Key words: hyperoxia, ion fluxes, uptake kinetics, K_m, J_max, efflux

Accepted on May 15, 1990

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