Inhibition of Na+/K+-ATPase and of active ion-transport functions in the gills of the shore crab Carcinus maenas induced by cadmium

Inhibition of Na+/K+-ATPase from gill plasma membranes of the shore crab Carcinus maenas by cadmium was investigated and compared with inhibitory effects by known antagonists (ouabain and Ca2+). For comparative considerations the Cd2+-inhibition of the enzyme from doe kidney was also tested. Na+/K+-ATPase from dog kidney and from crab gill differed greatly in sensitivity against ouabain. The inhibition constant K-i of the dog enzyme amounted to 9.1 x 10(-7) mol l(-1) i.e. more than 300-fold smaller than the K-i of 2.9 x 10(-4) mol l(-1) determined for the crab enzyme. Ca2+ inhibited the activity of Na+/K+-ATPase from crab gill plasma membranes with a K-i of 4.3 x 10(-4) mol l(-1). The Na+/K+-ATPase from crab gill was inhibited by Cd2+ with a K-i of 9.1 x 10(-5) mol l(-1). Cd2+ inhibited the Na+/K+-ATPase from dog kidney with a K-i (6.4 x 10(-5) mol l(-1)) comparable to that observed in the crab gill enzyme. Under experimental conditions Cd2+-inhibition of Na+/K+-ATPase was irreversible. Repeated washing, centrifugation and homogenization of the plasma membranes (four times) with Cd2+-free buffer did not restore any activity lost in the presence of 1 x 10(-3) mol l(-1) Cd2+. Since ouabain-insensitive (nonspecific) ATPases in the plasma membrane fraction of crab gills were inhibited by Cd2+ in the same way as Na+/K+-ATPase, the heavy metal is considered as an unspecific ATPase inhibitor. Comparing these results with literature data on Cd2+-binding to electrophoretically separated proteins suggests that Na+/K+-ATPase is a Cd2+-binding enzyme. The results obtained on Na+/K+-ATPase were reflected by Cd2+-inhibition of the branchial ion-transport functions depending on this enzyme. The transepithelial short-circuit current of isolated gill half lamellae, a direct measure of area-specific active ion uptake, and the transepithelial potential difference of isolated, perfused whole gills, also indicative of active ion uptake, were inhibited by the heavy metal in a time- and dose-dependent mode. Remarkably these inhibitions were also irreversible. These findings are ecologically and biomedically significant: even when the actual environmental or tissue concentrations measured are low, biological microstructures such as Na+/K+-ATPase may accumulate the heavy metal by tight binding over prolonged periods until the first inhibitory effects occur.