Determining The Temperature Sensitivity Of Gill Na+/K+ Atpase Function: An Alternative Method and Interpretation

Both freshwater and marine fishes depend on Na⁺/K⁺ ATPase for osmoregulation.  Due to its high temperature sensitivity, this metabolically-dependent pump may fail at extreme temperatures.  The function of juvenile red drum gill Na⁺/K⁺ ATPase at various temperatures, including lower lethal limits, was investigated.  This marine sport fish, located in the southeastern US, experiences extreme cold during its first year of life at the northern extent of its range.  Kinetic assays of juvenile red drum gill tissue, comparing activity with and without the Na⁺/K⁺ ATPase-inhibiting ouabain, were run in a temperature-controlled water bath at 1, 5, 10, 15, or 25°C.  By complete solubilization of protein, the reaction was stopped at set time points in order to manually determine in situ Na⁺/K⁺ ATPase activity.  The break in the Arrhenius plot created for the temperature range of 1 – 25°C indicates that the activation energy needed for the Na⁺/K⁺ ATPase reaction increases at temperatures below 6.3°C.  However, it does not appear that the enzyme completely inactivated within the tested range.  By applying this new alternative method to measure Na⁺/K⁺ ATPase activity, we were able to conclude that this important osmoregulatory enzyme retains its function in juvenile red drum, even at a minimum of 1°C.