Failure to up-regulate gill Na+,K+-ATPase α-subunit isoform α1b may limit seawater tolerance of land-locked Arctic char (Salvelinus alpinus)

Many populations of Arctic char (Salvelinus alpinus) are land-locked, physically separated from the ocean by natural barriers and unable to migrate to sea like anadromous populations. Previous studies which experimentally transferred land-locked Arctic char to seawater report high mortality rates due to osmoregulatory failure and an inability to up-regulate gill Na+,K+-ATPase activity. This study examined the mRNA expression of two recently discovered α-subunit isoforms of gill Na+K+-ATPase (α1a and α1b) during seawater exposure of land-locked Arctic char. mRNA levels of these gill Na+,K+-ATPase α-subunit isoforms were compared to Na+,K+-ATPase activity and protein levels and related to osmoregulatory performance. Land-locked Arctic char were unable to regulate plasma osmolality following seawater exposure. Seawater exposure did not induce an increase in gill Na+,K+-ATPase activity or protein levels. Na+,K+-ATPase isoform α1a mRNA quickly decreased upon exposure to seawater, while isoform α1b levels were unchanged. These results suggest the inability of land-locked Arctic char to acclimate to seawater is due a failure to up-regulate gill Na+,K+-ATPase activity which may be due to their inability to increase Na+,K+-ATPase α1b mRNA expression.