Contact-mediated communication of ouabain resistance in mammalian cells in culture

THE active transport of Na⁺ and K⁺ across the plasma membrane, mediated by Na⁺-K⁺-ATPase, can be inhibited specifically by the cardiac glycoside ouabain¹. Ouabain is toxic to mammalian cells in culture, the degree of sensitivity varying with species ²; however, cells resistant to its cytotoxic effects have been obtained in rodent^2-4 and human^5,6 cell strains. Ouabain-resistant (oua^r) cells are thought to have a mutation affecting the Na⁺-K⁺-ATPase so that their enzyme has a lower affinity for ouabain than the enzyme of ouabain-sensitive (oua ^s) cells. Therefore, oua^r cells can actively transport Na⁺ and K⁺ at ouabain concentrations that would inhibit oua^s cells^2,4,5. We have observed that oua^s human cells, when cocultured densely with oua^r human cells, acquire a functional resistance to ouabain as evidenced by their ability to proliferate in medium containing the drug (unpublished observations). We suspected that ouabain resistance was communicated from oua^r to oua^s cells and that this transfer was a manifestation of ionic transport through a pathway independent of the Na⁺-K⁺-ATPase. There is substantial evidence that a specialised membrane structure, the gap junction, is the ultrastructural pathway for the transfer of ions, metabolites and other small molecules between cells in contact^7-9. To determine whether the acquisition of ouabain resistance we observed was mediated through gap junctions, we have compared-for the ability to communicate ouabain resistance-cells which are known to form gap junctions with those that cannot. Our results indicate that ouabain resistance can be communicated from oua ^r to oua^s cells, that the process is contact dependent and that it does not occur in cells unable to form gap junctions.