The role of calcium ions in the permeability changes produced by external ATP in transformed 3T3 cells

External ATP causes a rapid increase in passive permeability to nucleotides and phosphate esters in transformed cell lines, such as 3T6 mouse fibroblasts. However, untransformed lines, such as 3T3, do not show a similar sensitivity to external ATP. Ca²⁺ inhibits permeabilization, but only at concentrations approaching those of external ATP. In contrast, La³⁺ and Tb³⁺ inhibit ATP-dependent permeabilization at one-fifth the concentration of external ATP. Considering reports that lanthanides can substitute for calcium ion in many enzymatic reactions, often with a higher affinity, it would appear that Ca²⁺ plays a specific role in the maintenance of a passive membrane permeability barrier and in opposing the effects of external ATP.Other data suggest a regulatory role for the Ca²⁺-calmodulin complex in the permeabilization process. Trifluoperazine, chlorpromazine and W-7, compounds which inhibit cellular functions dependent on the Ca²⁺-calmodulin complex, are able to enhance the effect of external ATP. Thus, a dramatic stimulation of nucleotide permeability occurs with concentrations of external ATP and inhibitor that are ineffective when added alone. Calmodulin antagonists and low concentrations of external ATP increased membrane permeability to Na⁺ and K⁺ as was previously shown for permeabilization with ATP alone. Earlier studies have shown that energy inhibitors which reduce intracellular ATP levels greatly increase the sensitivity of transformed cells to external ATP. However, the Ca²⁺-calmodulin antagonists used in the present study exert their effects at concentrations which do not alter intracellular ATP levels.