Abstract
The Antarctic teleost Trematomus bemacchii lives in the waters surrounding Antarctica which are a constant -1.8°C. As an adaptive property they possess a uniquely high serum osmolality of -600 mOsm kg"1 which is nearly double that of temperate teleosts. Acclimation of the T. bemacchii to +4°C results in a 25% decrease in serum osmolality and a doubling of activity of the ion pump in the gill, the Na+,K+-ATPase, that is energetically responsible for osmoregulation. The Na+,K+-ATPase consists of two obligate subunits (a catalytic a and membrane trafficking ß), however in some systems an accessory (y) subunit has also been shown to colocalize with the obligate subunits.|The a subunit is the active enzymatic portion and consists of 4 isoforms with differing biochemical properties. The affinity of the a3 for Na+ is less than that of the al and a2. However, all isoforms have similar affinities for K+ and ATP. Protein and mRNA of the al, a2, and a3 isoforms of Na+,K+-ATPase were previously reported in T. bemacchii gill. The effects of +4°C acclimation were investigated by measuring serum osmolality, oxygen consumption, substrate concentration dependent activation of the Na+,K+- ATPase, and ouabain inhibition of phosphoenzyme intermediate formation. Acclimation to +4°C caused the T. bemacchii to significantly decrease its serum osmolality from 581 + 11 mOsm kg"1 at -1.5°C to 477 + 4 mOsm kg'1 at +4°C. The apparent affinity of the enzyme for sodium increased significantly (K'Na of 2.3 + 0.2 mM to 1.5 + 0.3 mM) with +4°C acclimation but the apparent affinity for potassium and ATP showed no change. Concentration dependent inhibition of the formation of phosphoenzyme intermediate of Na+,K+-ATPase by ouabain indicated the contribution of a3 to total Na+,K"l"-ATPase decreased from 33.72 + 13.03% to 0 + 6.67% while al and a2 increased from 33.12 + 12.72% and 33.15 + 0.30% to 44.87 + 6.67% and 55.12 + 6.67%, respectively upon +4°C acclimation. This is the first report of the application of a functional biochemical assay to characterize an environmental physiological change in this organism. The a3 isoform of Na+,K+-ATPase was of particular interest. Gene specific primers for the a3 were designed and the sequence of the a3 determined. The T. bernacchii a3 isoform amino acid sequence showed 86% identity to the a3 isoform sequence from mammals and 87% to the sequence of the a3 isoform from other fishes (individual identity was as high as 91% when compared to Tilcipia mossambica). These results suggest that the T. bernacchii reduce serum osmolality in response to increased environmental temperature by altering the cell surface Na+,K+-ATPase a subunit composition.