Effects of inorganic sea water constituents on branchial Na-K-ATPase activity in the shore crab Carcinus maenas

Abstract

Influences of salinity, Na, K, Ca and Mg on Na−K-ATPase activity in the posterior gills of Carcinus maenas (L.) have been investigated with respect to the role of the enzyme in hyperosmotic regulation. K and Mg ions were obligatory for enzyme activity. The dependence on the K concentration can be seen in a saturation curve of the Michaelis-Menten type. Low concentrations of Ca (0.2–3 mM) in the incubation medium strongly inhibited Na−K-ATPase activity. Activities inhibited by Ca could be reactivated to non-inhibited values by the addition of higher amounts of Mg (25 mM). Activity increased along with the salinity of the sea water used as incubation medium up to about 10‰ S. Here, maximum activity was observed. Further salinity increases of the incubating sea water were inhibitory. Salinity dependence is assumed to be based on Na dependence of the Na-pump. Comparative investigations of the Na−K-ATPase activity and its affinity to sodium in five species of decapod crustaceans indicated that levels of Na−K-ATPase differed in the posterior gills of stenohaline and euryhaline species. The results obtained confirm previous assumptions of a central role of the branchial Na−K-ATPase in hyperosmotic regulation. Properties of the Na−K-ATPase, such as affinity for substrates or dependence on ionic sea water constituents, are kept constant with respect to salinity changes. Modifications due to salinity only concern enzyme amounts especially in the posterior gills. The finding that the Na-pump is localized in basolateral parts of ion-transporting epithelial cells confirms the aforementioned results.