Gill Na super(+)-K super(+)-ATPase activity correlates with basolateral membrane lipid composition in seawater- but not freshwater-acclimated Arctic char (Salvelinus alpinus)

The successful migration of euryhaline teleost fish from freshwater to seawater requires the upregulation of gill Na super(+)-K super(+)-ATPase, an ion transport enzyme located in the basolateral membrane (BLM) of gill chloride cells. Following 39 days of seawater exposure, Arctic char had similar plasma sodium and chloride levels as individuals maintained in freshwater, indicating they had successfully acclimated to seawater. This acclimation was associated with an eightfold increase in gill Na super(+)-K super(+)-ATPase activity but only a threefold increase in gill Na super(+)-K super(+)-ATPase protein number, suggesting that other mechanisms may also modulate gill Na super(+)-K super(+)-ATPase activity. We therefore investigated the influence of membrane composition on Na super(+)-K super(+)-ATPase activity by examining the phospholipid, fatty acid, and cholesterol composition of the gill BLM from freshwater- and seawater-acclimated Arctic char. Mean gill BLM cholesterol content was significantly lower (622%) in seawater-acclimated char. Gill Na super(+)-K super(+)-ATPase activity in individual seawater Arctic char was negatively correlated with BLM cholesterol content and positively correlated with %phosphatidylethanolamine and overall %18:2n6 (linoleic acid) content of the BLM, suggesting gill Na super(+)-K super(+)-ATPase activity of seawater-acclimated char may be modulated by the lipid composition of the BLM and may be especially sensitive to those parameters known to influence membrane fluidity. Na super(+)-K super(+)-ATPase activity of individual freshwater Arctic char was not correlated to any membrane lipid parameter measured, suggesting that different lipid-protein interactions may exist for char living in each environment.