No abstract

The activity of Na-K-ATPase was determined in the posterior gills of the shore crab Carcinus maenas during a period following transfer from 35 to 10 %o salinity and vice versa at 15 °C. After transfer from high to low salinity, Na-K-ATPase activity increased from 3.2 to 7.0 ~tmoles Pimg protein -1 h-1 within a period of 2 to 3 weeks. Transfer of crabs from low to high salinity resulted in reduction of activity from 7.4 to 4.5 ~moles Pimg protein -1 h -1 within about the same period. The relatively slow response following salinity change indicates that the amounts of Na-K-ATPase in the gills may play a role in hyperionic Na regulation in relatively constant brackish-water environments. Instant responses to salinity result from activation and inhibition of Na-K-ATPase activity by Na. Gill Na-K-ATPase is activated by the Na concentration of the incubation medium to attain a steep maximum at about 75 mM Na, which corresponds to the lowest environmental Na levels tolerated by C. maenas equivalent to a salinity of ca 6 %°. Activity greatly decreased towards higher Na levels, equivalent to the salinity of normal sea water, at which hyperregulation no longer occurs. Selective addition of either Na or C1 to brackish water of 9 %0 S resulted in effective hyperregulation of the non-increased ion, and passive distribution between medium and blood of the increased ion. These data indicate that under appropriate conditions the normally coupled transport of Na and Ct may be uncoupled and take place independently of each other.