Volume-induced increase of K+ and Cl− permeabilities in Ehrlich ascites tumor cells. Role of internal Ca2+

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Abstract
Ehrlich ascites tumor cells resuspended in hypotonic medium initially swell as nearly perfect osmometers, but subsequently recover their volume within 5 to 10 min with an associated KCl loss. 1. The regulatory volume decrease was unaffected when nitrate was substituted for Cl−, and was insensitive to bumetanide and DIDS. 2. Quinine, an inhibitor of the Ca2+-activated K+ pathway, blocked the volume recovery. 3. The hypotonic response was augmented by addition of the Ca2+ ionophore A23187 in the presence of external Ca2+, and also by a sudden increase in external Ca2+. The volume response was accelerated at alkaline pH. 4. The anti-calmodulin drugs trifluoperazine, pimozide, flupentixol, and chlorpromazine blocked the volume response. 5. Depletion of intracellular Ca2+ stores inhibited the regulatory volume decrease. 6. Consistent with the low conductive Cl− permeability of the cell membrane there was no change in cell volume or Cl− content when the K+ permeability was increased with valinomycin in isotonic medium. In contrast, addition of the Ca2+ ionophore A23187 in isotonic medium promoted Cl− loss and cell shrinkage. During regulatory volume decrease valinomycin accelerated the net loss of KCl, indicating that the conductive Cl− permeability was increased in parallel with and even more than the K+ permeability. It is proposed that separate conductive K+ and Cl− channels are activated during regulatory volume decrease by release of Ca2+ from internal stores, and that the effect is mediated by calmodulin.