Interaction of palytoxin and cardiac glycosides on erythrocyte membrane and (Na+ + K+) ATPase

Palytoxin (PTX), at extremely low concentrations (0.01-1 nM), caused K+ release from rabbit erythrocytes. Among the various chemical compounds tested, cardiac glycosides potently inhibited the PTX-induced K+ release. The order of inhibitory potency (IC50) was cymarin (0.42 microM) greater than convallatoxin (0.9 microM) greater than ouabain (2.3 microM) greater than digitoxin (88 microM) greater than digoxin (90 microM). Their corresponding aglycones, even at 10 microM, did not inhibit the K+ release, but competitively antagonized the inhibitory effect of the glycosides. All these cardiotonic Steroids inhibited the activity of (Na+ + K+)-ATPase prepared from hog cerebral cortex in narrow concentration ranges (IC50 = 0.15-2.4 microM), suggesting that the inhibition of K+ release is not related to their inhibitory potency on the (Na+ + K+)-ATPase activity, and the sugar moiety of cardiac glycosides is involved in the inhibition. On the other hand PTX, at higher concentrations (greater than 0.1 microM), inhibited the (Na+ + K+)-ATPase activity. However, this inhibitory effect of PTX was not antagonized by ouabain. It is suggested that, compared with ouabain, PTX has additional binding site(s) on the (Na+ + K+)-ATPase.