Mutants of HeLa cells resistant to ouabain and cassaine: Genetic evidence for the common site of action of cardiac glycosides and erythrophleum alkaloids

Stable mutants highly resistant to the cardiac glycoside ouabain (OuaR) and the erythrophleum alkaloid cassaine (CasR) are obtained at similar frequencies in different experiments in mutagen-treated cultures of HeLa cells. All of the OuaR and CasR mutants examined (twelve of each kind from two independent experiments) exhibited identical cross resistance patterns to ouabain and cassaine and to a number of other cardiac glycosides and aglycones such as oleandrin, strophanthidin, digitoxigenin and digitoxin, which were examined. The apparent identity of OuaR and CasR mutants as suggested by these studies indicates that resistance to these two groups of compounds results from the same primary lesion, which from earlier studies on the OuaR mutants is known to involve the plasma membrane Na+, K+-ATPase. Results of genetic studies support the notion that these two groups of compounds, i.e. cardiac glycosides and erythrophleum alkaloids, possess common structural determinants which are responsible for their biological activities, and mutants are selected in both cases against these common determinants. The OuaR and CasR mutants, however, did not show any cross resistance to other types of inhibitors of Na+,K+-ATPase function such as ethacrynic acid, sanguinarine, quindonium bromide, suramin and oligomycin, indicating that the site of action of these inhibitors is different from that of cardiac glycosides and erythrophleum alkaloids.