Ionic dependence of the extracellular ATP‐induced permeabilization of transformed mouse fibroblasts: Role of plasma membrane activities that regulate cell volume

Extracellular ATP rendered the plasma membrane of transformed mouse fibro‐blasts permeable to normally impermeant molecules. This permeability change was prevented by increasing the ionic strength of the isotonic medium with NaCl. Conversely, the cells exhibited increased sensitivity to ATP when the NaCl concentration was decreased below isotonicity, when the KCl concentration was increased above 5 mM while maintaining isotonicity, and when the pH of the medium was raised above 7.0. These conditions as well as the addition of ATP itself caused cell swelling. However, the effect of ATP was independent of cell volume and dependent upon the ionic strength and not the osmolarity of the medium since (1) addition of sucrose to isotonic medium did not prevent permeabilization although media made hypertonic with either sucrose or NaCl caused a decrease in cell volume; and (2) addition of sucrose or NaCl to hypotonic media caused a decrease in cell volume, but only NaCl addition decreased the response to ATP. Conditions that have been shown to inhibit plasma membrane proteins that play a reciprocal role in cell volume regulation had reciprocal effects on the permeabilization process, even though the effect of ATP was independent of cell volume. For example, inhibition of the Na⁺, K⁺‐ATPase by ouabain increased sensitivity of cells to ATP while conditions which inhibit Na⁺, K⁺, Cl⁻‐cotransporter activity, such as treatment of the cells with the diuretics furosemide or bumetanide or replacement of sodium chloride in the medium with sodium nitrate or thiocyanate, inhibited permeabilization. The furosemide concentration that inhibited permeabilization was greater than the concentration that inhibited Na⁺, K⁺, Cl⁻‐cotransporter‐mediated ⁸⁶Rb⁺ (K⁺) uptake, suggesting that the effect of furosemide on the permeabilization process may not be specific for the Na⁺, K⁺, Cl⁻‐cotransporter.