Stable intracellular acidification upon polyamine depletion induced by α-difluoromethylornithine or N¹, N¹²-bis(ethyl)spermine in L1210 leukaemia cells

Polyamines play major roles in ionic and osmotic regulation, but their exact involvement in specific ion transport processes is poorly defined. Treatment of L1210 mouse leukaemia cells with either 5mM α-difluoromethylornithine (DEMO), a suicide substrate of ornithine decarboxylase, or 25 μM N¹, N¹²-bis(ethyl)spermine (BE-3-4-3), a dysfunctional polyamine analogue, caused a stable decrease in intracellular pH (pH,) by 0.1-0.4 unit from steady-state control values between 7.4 and 7.6, as measured either by partition of a weak acid or with a fluorescent pH-sensitive probe. This effect was not related to cell growth status or differences in metabolic acid generation, and was observed in either the presence or absence of HCO₃¹. Exogenous spermidine (10-25 μM) or putrescine (25-50 μM) fully reversed DFMO- or BE-3-4-3-induced acidification within 2 and 8 h respectively. Recovery of pH(i) in L1210 cells after a nigericin- or NH₄⁺-mediated acid load in HCO₃^- -free buffers was mediated by Na⁺/H⁺ antiporter activity, in addition to a minor Na⁺-independent and amiloride-insensitive pathway. Decreased steady-state pH(i) was maintained in polyamine-depleted L1210 cells after recovery from acid stress. Moreover, the pH(i)-dependence of the rate of Na⁺-dependent H⁺- extrusion after an acid stress was altered by DFMO and BE-3-4-3, resulting in a set-point which was lower by 0.25-0.30 pH unit in polyamine depleted cells. On the other hand, neither the rate nor the magnitude of Na⁺/H⁺-exchanger-mediated alkalinization induced by hypertonic shock was decreased by polyamine depletion. Thus polyamine depletion induces a persistent defect in pH, homeostasis which is due, at least in part, to a stable decrease in the pH(i) set-point of the Na⁺/H⁺ exchanger.