Na+/K+-ATPase isoform regulation in three-spine stickleback (Gasterosteus aculeatus) during salinity acclimation
Most marine and all freshwater fishes regulate the ionic and osmotic composition of their extracellular fluids different from the outside environment. The gills play a prominent osmoregulatory role in maintaining whole body ion and water balance in fish living in both freshwater and saltwater environments. The ability of the gill to regulate salts is dependent on the action of the Na+/K+-ATPase. When euryhaline fishes, fishes able to tolerate changes in external salinity, experience changes in environmental salinity, they alter their gill physiology and Na+/K+-ATPase activity to handle changing osmotic and ionic stresses. The activity and relative expression of Individual Na+/K+-ATPase isoforms is known to change in some euryhaline species during acclimation to changing salinity, however the regulation of this important enzyme is poorly understood. The three-spine stickleback, Gasterosteus aculeatus, is a euryhaline fish species that inhabits a wide variety of environments ranging from freshwater to seawater with some freshwater populations being landlocked. The genome of G. aculeatus has been sequenced and contains four Na+/K+-ATPase isoforms, of which three are expressed in the gills. This study examined the expression of these three Na+/K+-ATPase isoforms in wild lake and marine populations of stickleback following transfer to either freshwater or seawater. Plasma chloride levels, muscle water content, gill Na+/K+-ATPase activity and gill Na+/K+-ATPase subunit isoform mRNA expression levels were monitored. Marine and freshwater stickleback were able to regulate both plasma chloride levels and muscle water content, suggesting they can successfully acclimate to changing salinity; which is consistent with other studies. Marine fish transferred to freshwater showed a significant increase in gill Na+/K+-ATPase activity while freshwater fish transferred to seawater did not. No significant change in mRNA expression was seen in either ATP1A3 isoforms following freshwater or seawater acclimation. However, a significant increase in the ATP1A1 isoform was observed when acclimating to seawater and a significant decrease was seen in the ATP1A1 isoform when acclimating to freshwater, suggesting ATP1A1 plays a role in ion secretion in marine habitats. These results confirm that three-spine stickleback independently regulate individual Na+/K+-ATPase subunit isoforms in their gills and that landlocked populations of the species have retained the ability to acclimate to seawater.