The effects of chloroquine and other weak bases on the accumulation and efflux of digoxin and ouabain in HeLa cells

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Abstract
We have studied the effects of the weak bases chloroquine, NH4Cl and amantadine on the handling of certain cardiac glycosides by HeLa cells. When these weak bases are applied acutely to HeLa cells they have only minor effects on the binding of cardiac glycosides to the sodium pumps and on the recovery of pump function following block. When cells are grown in these weak bases there is a variable (10–30%) reduction in pump numbers. This effect is additive to that of chronic treatment with cardiac glycosides. If all sodium pumps are blocked with ouabain, digoxin or digitoxin then recovery of function recovers with a T1/2 of about 7 h (10% h−1); digoxin and digitoxin molecules are excreted at a similar rate but ouabain excretion occurs at a much slower rate (3% h−1). These weak bases greatly slow (× 3) the rate of excretion of digoxin and digitoxin but do not alter that of ouabain. The process affected by chloroquine was estimated to have a T1/2 of 8 h. Cells grown in the presence of cardiac glycosides accumulate large numbers of glycoside molecules; chloroquine, NH4Cl and amantadine increase the accumulation of digoxin and digitoxin and may decrease that of ouabain. Quantitatively these results fit a model whereby cardiac glycosides are accumulated by HeLa cells bound to the sodium pumps, are processed by the lysosomes and then excreted. The results are consistent with a process of internalisation and renewal of sodium pumps by HeLa cells.