TIPSMARK, CK*; MADSEN, SS; BORSKI, RJ; North Carolina State Univ., Raleigh; Univ. of Southern Denmark, Odense; North Carolina State Univ., Raleigh: Expression of gill ion transporters in striped bass (Morone saxatilis) during salinity acclimation

The time course of osmoregulatory adjustments and changes in expression of three key ion transporters in the gill were investigated in striped bass (Morone saxatilis) exposed to shifts in salinity. Transfer from fresh water (FW) to seawater (SW; 35 ppt) induced an increase in serum [Na⁺] after 6 h being corrected after 24 h. Gill Na,K-ATPase (NKA) activity and Na,K,2Cl cotransporter (NKCC) protein expression increased after 3 and 7 d. Abundance of NKA α-subunit mRNA and protein was unchanged. Changes in NKCC protein were preceded by an increase in NKCC1 mRNA after 24 h. Decreased expression of B-subunit v-type H-ATPase mRNA was seen after 3 d. Transfer from SW to FW caused a decrease in serum [Na⁺] after 6 h that was partly corrected after 3 d. No change in gill NKA expression was seen. However, NKCC mRNA and protein expression was decreased after 24 h and 7 d, respectively. Expression of H-ATPase mRNA increased in response to FW transfer after 7 d. In brackish water (15 ppt) fish transferred to FW or SW serum [Na⁺] was altered by 6 h and restored after 24 h. Gill NKA activity increased in both the SW and FW group at 7 d, with higher activity in the SW group. NKCC expression only increased in the SW group. Striped bass show more rapid ion-compensatory adjustments than most euryhaline teleosts. This seems to rely on the higher degree of preparedness of the osmoregulatory apparatus. We have found that SW-induced increase in gill NKCC and NKA is less prominent in bass than brown trout. Gill H-ATPase activity was negligible in bass and was 10-fold higher in FW trout. In bass Na⁺-uptake in the FW gill may depend more on high abundance of NKA compared to trout where H-ATPase is essential to establish a thermodynamically favorable gradient for Na⁺-uptake.