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 (Fundulus heteroclitus) and Atlantic salmon (Salmo salar) transferred from low salinity (0.1 ppt) to high salinity (25-35 ppt). In parr and presmolt, Salmo salar gill Na(+),K(+)-ATPase activity started to increase 3 days after transfer. Exposure of Fundulus heteroclitus to 35 ppt seawater (SW) induced a rise in gill Na(+), K(+)-ATPase activity 3 hr after transfer. After 12 hr, the values dropped to initial levels but showed a second significant increase 3 days after transfer. The absence of detergent in the enzyme assay resulted in 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 resulted 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(+)-ATPase. 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 results show a very rapid and transitory increase in gill Na(+),K(+)-ATPase activity in the first hours after the transfer of Fundulus heteroclitus to SW that is dependent on translational and transcriptional processes. J. Exp. Zool. 287:263-274, 2000.