Warm acclimation changes the expression of the Na+,K+-ATPase α subunit isoforms in Antarctic fish gills

Acclimation of the Antarctic teleost Trematomus bernacchii at + 4 °C results in a 25% decrease in serum osmolality and a doubling of gill Na+,K+-ATPase activity relative to − 1.5 °C. Protein and mRNA of the α1, α2, and α3 isoforms of Na+,K+-ATPase were previously reported in T. bernacchii gill and studies in other systems have shown these isoforms display different kinetic and physiologic properties. We propose that the increase in Na+,K+-ATPase activity resulting from + 4 °C acclimation is due to a change in Na+,K+-ATPase α isoform contribution and accounts for the decrease in serum osmolality. In response to acclimation at + 4 °C, the apparent affinity of the enzyme for sodium increased significantly (measured by K′Na and K0.5Na), while the apparent affinity for potassium and ATP showed no change. Analysis of Na+,K+-ATPase phosphoenzyme intermediates indicated the α3 contribution to total Na+,K+-ATPase decreased from 33.72 ± 13.03% to 0 ± 6.67% while α1 and α2 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. Immunoblotting of α isoform protein showed significant increases in α1 and α2 and a significant decrease in α3 with + 4 °C acclimation. However, there was not a significant change in the Na+,K+-ATPase α subunit steady state transcription levels as measured by real-time RT-PCR. These observations (through the application of a functional biochemical characterization to an environmental physiological change) suggest that the T. bernacchii reduce serum osmolality in response to increased environmental temperature by altering the α subunit composition of Na+,K+-ATPase.