Nitrogen mustard interference with potassium transport systems in Ehrlich ascites tumor cells

Nitrogen mustard (N-mustard) inhibits the ouabain-sensitive and the furosemide-sensitive Rb uptake of Ehrlich ascites tumor cells, whereas the transport, which is resistant to both inhibitors, is not affected by the alkylating agent. At N-mustard concentrations below 10 microM, the reduction in Rb uptake is predominantly due to an interference with the furosemide-sensitive system. The dose response curve for the inhibition by N-mustard of the furosemide-sensitive Rb uptake closely parallels the dose response curve for the anti-tumor activity of the alkylating drug. This is in contrast to the behaviour of the ouabain-sensitive Rb transport. The inhibition of the furosemide-sensitive Rb uptake is expressed much less in cells which are resistant to N-mustard. The recovery of the furosemide-sensitive transport system after a single exposure to N-mustard is relatively slow and characterized by an initial 4 h lag period, whereas the repair of DNA-interstrand cross-links starts immediately after removal of the drug. At mM concentrations furosemide blocks the multiplication of Ehrlich ascites tumor cells. However, lower concentrations of furosemide which cause a 50% reduction in the furosemide-sensitive Rb uptake do not interfere with cell proliferation. This is in contrast to the behaviour of N-mustard which exerts a clear-cut depression of cell growth at concentrations leading to a 50% inhibition of the furosemide-sensitive Rb transport. It is concluded, therefore, that the inhibition of the furosemide-sensitive system alone is not sufficient to explain the anti-tumor activity of the alkylating agent. The effect is discussed as part of a more extended N-mustard-induced membrane alteration which may be important for the growth inhibitory effect of the alkylating agent.