Rapid activation of gill Na+,K+-ATPase in the euryhaline teleost Fundulus heteroclitus

The rapid activation of gill Na+,K+-ATPase was analyzed in the mummichog (F undulus heteroclitus) and Atlantic salmon (Salmo salar) transferred from lo w salinity (0.1 ppt) to high salinity (25-35 ppt). In parr and presmolt, Sa lmo salar gill Na+,K+-ATPase activity started to increase 3 days after tran sfer. Exposure of Fundulus heteroclitus to 35 ppt seawater (SW) induced a r ise in gill Na+,K+-ATPase activity 3 hr after transfer. After 12 hr, the va lues dropped to initial levels but showed a second significant increase 3 d ays after transfer. The absence of detergent in the enzyme assay resulted i n lower values of gill Na+,K+-ATPase, and the rapid increase after transfer to SW was not observed. Na+,K+-ATPase activity of gill filaments in vitro for 3 hr increased proportionally to the osmolality of the culture medium ( 600 mosm/kg > 500 mosm/kg > 300 mosm/kg). Osmolality of 800 mosm/kg resulte d in lower gill Na+,K+-ATPase activity relative to 600 mosm/kg. Increasing medium osmolality to 600 mosm/kg with mannitol also increased gill Na+,K+-A TPase. Cycloheximide inhibited the increase in gill Na+,K+-ATPase activity observed in hyperosmotic medium in a dose-dependent manner (10(-4) M > 10(- 5) M > 10(-6) M). Actinomycin D or bumetanide in the culture (doses of 10(- 4) M, 10(-5) M, and 10(-6) M) did not affect gill Na+,K+-ATPase. Injection of fish with actinomycin D prior to gill organ culture, however, prevented the increase in gill Na+,K+-ATPase activity in hyperosmotic media. The resu lts show a very rapid and transitory increase in gill Na+,K+-ATPase activit y in the first hours after the transfer of Fundulus heteroclitus to SW that is dependent on translational and transcriptional processes. (C) 2000 Wile y-Liss, Inc.