Molecular Pharmacology, Vol 18, 259-266, Copyright © 1980 by the American Society for Pharmacology and Experimental Therapeutics

Effects of Sodium Iodide, Lithium Bromide, and Deoxycholate on Dissociation of [³H]Ouabain-Na,K-ATPase Complex during Enzyme Isolation

¹ Department of Pharmacology, Northwestern University, Chicago, Illinois 60611

In an attempt to resolve the current controversy regarding whether or not myocardial Na,K-ATPase remains inhibited following washout of digitalis inotropy in isolated hearts, the effects of various protein extracting agents used in enzyme isolation procedures were investigated on dissociation of the drug-enzyme complex. The drug-enzyme complex, [³H]ouabain-Na,K-ATPase, was isolated both after in vitro incorporation of [³H]ouabain to crude rabbit heart Na,K-ATPase fraction and after ex vivo binding of [³H]ouabain in isolated Lagendorff rabbit hearts. The drug-enzyme complex was then treated at 0-4°C with 2 M NaI, 0.1% deoxycholate (DOC), or 0.1% DOC plus 2 M NaI, the protein extracting agents used in many laboratories, and compared against treatment with LiBr, the extracting agent used in our laboratory. It was found that a sequential treatment of 0.1% DOC followed by 2 M NaI resulted in over 95% of the [³H]ouabain dissociating from both ex vivo and in vitro bound drug-enzyme complexes in comparison to control. For ex vivo bound drug-enzyme complex, 2 M NaI treatment alone removed approximately 90% of the [³H]ouabain activity and 0.1% DOC removed approximately 45% of the drug. In contrast to these results, treatment of the ex vivo and in vitro labeled drug-enzyme complexes with 1 M LiBr caused no detectable difference in drug dissociation in comparison to control complex treated with 1 mM Tris-HCl. Therefore, these findings may account for the inability of some investigators to demonstrate inhibition of Na,K-ATPase following enzyme isolation from inotropically stimulated Lagendorff rabbit hearts as well as from noninotropic hearts of various species following drug washout due to dissociation of the drug-enzyme complex by NaI and DOC. Since LiBr does not appreciably dissociate the drug-enzyme complex, in comparison to 1 mM Tris-HCl control, the present findings support our original observation that inhibition of Na,K-ATPase is not responsible for the inotropic action of digitalis, because enzyme inhibition is still present in noninotropic rabbit hearts following drug washout.