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

Inhibition of Na + /K + -ATPase from gill plasma membranes of the shore crab Carcinus maenas by cadmium was investigated and compared with inhibitory eects by known antagonists (ouabain and Ca 2+ ). For comparative considerations the Cd 2+ -inhibition of the enzyme from dog kidney was also tested. Na + /K + -ATPase from dog kidney and from crab gill diered greatly in sensitivity against ouabain. The inhibition constant K i of the dog enzyme amounted to 9.1 ´10 A7 mol l A1 , i.e. more than 300-fold smaller than the K i of 2.9 ´10 A4 mol l A1 determined for the crab enzyme. Ca 2+ inhibited the activity of Na + /K + -ATPase from crab gill plasma membranes with a K i of 4.3 ´10 A4 mol l A1 . The Na + /K + -ATPase from crab gill was inhibited by Cd 2+ with a K i of 9.1 ´10 A5 mol l A1 . Cd 2+ inhibited the Na + /K + -ATPase from dog kidney with a K i (6.4 ´10 A5 mol l A1 ) comparable to that observed in the crab gill enzyme. Under experimental conditions Cd 2+ -inhibition of Na + /K + -ATPase was irreversible. Repeated washing, centrifugation and homogenization of the plasma membranes (four times) with Cd 2+ -free buer did not restore any activity lost in the presence of 1 ´10 A3 mol l A1 Cd 2+ . Since ouabain-insensitive (non-speci®c) ATPases in the plasma membrane fraction of crab gills were inhibited by Cd 2+ in the same way as Na + /K + -ATPase, the heavy metal is considered as an unspeci®c ATPase inhibitor. Comparing these results with literature data on Cd 2+ -binding to electrophoretically separated proteins suggests that Na + /K + -ATPase is a Cd 2+ -binding enzyme. The results obtained on Na + /K + -ATPase were re¯ected by Cd 2+ -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-speci®c 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 ®ndings are ecologically and biomedically signi®cant: 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 ®rst inhibitory eects occur.