Different Mechanisms of Acid-Base Regulation in Rainbow Trout (Oncorhynchus mykiss) and American Eel (Anguilla rostrata) during NaHCO3Infusion

Rainbow trout (Oncorhynchus mykiss) and American eels (Anguilla rostrata) were infused intra-arterially with NaHCO₃ (∼800 μmol/kg/h) to assess their acid-base regulatory mechanisms during metabolic alkalosis. In both species, plasma pH and [HCO₃ ⁻] increased during infusion, although the changes were more pronounced in the eel. In trout, pH and [HCO₃⁻] were returned to preinfusion values by 12 h after infusion was stopped, while in the eel there was only incomplete clearance of accumulated [HCO₃⁻] during the 24-h postinfusion period. In each case, approximately 20 mmol/L of HCO₃⁻ was removed from the plasma during postinfusion. The predominant mechanism employed by both trout and eel to regulate blood acid-base status during alkalosis was manipulation of branchial net Cl⁻ flux (\documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage{wasysym} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document}$$J_{{\mathrm{net}}}^{{\mathrm{Cl-}}}$$\end{document}), although this was accomplished with markedly different strategies. Trout relied primarily on adjustments of branchial Cl⁻ uptake (\documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage{wasysym} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document}$$J_{{\mathrm{in}}}^{{\mathrm{Cl-}}}$$\end{document}) to control \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage{wasysym} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document}$$J_{{\mathrm{net}}}^{{\mathrm{Cl-}}}$$\end{document}. In the eel, however, owing to the absence of an appreciable influx component to \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage{wasysym} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document}$$J_{{\mathrm{net}}}^{{\mathrm{Cl-}}}$$\end{document}, regulation of Cl⁻ efflux \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage{wasysym} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document}$$J_{{\mathrm{out}}}^{{\mathrm{Cl-}}}$$\end{document}was the dominant strategy. The surface area of gill filament chloride cells (CCs) was increased during alkalosis in trout and appeared to be the mechanism underlying the stimulation of \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage{wasysym} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document}$$J_{{\mathrm{in}}}^{{\mathrm{Cl-}}}$$\end{document}and clearance of the infused base load. In the eel, CC surface area was low and not significantly altered by NaHCO₃ infusion. In summary, different strategies were used by trout and eel to regulate blood acid-base status during NaHCO₃ infusion. Trout relied on (i) elevation of \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage{wasysym} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document}$$J_{{\mathrm{in}}}^{{\mathrm{Cl-}}}$$\end{document}(ii) reduction of \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage{wasysym} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document}$$J_{{\mathrm{out}}}^{{\mathrm{Cl-}}}$$\end{document}, and (iii) intracellular buffering. In contrast, the eel relied almost exclusively on reduction of \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage{wasysym} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document}$$J_{{\mathrm{out}}}^{{\mathrm{Cl-}}}$$\end{document}. Manipulation of CC surface area to increase \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage{wasysym} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document}$$J_{{\mathrm{in}}}^{{\mathrm{Cl-}}}$$\end{document}or the use of intracellular buffering appeared to be unimportant for regulating metabolic alkalosis in eel.