Ouabain resistance of a human trophoblast cell line is not related to its reactivity to ouabain

Ouabain is a specific inhibitor of sodium, potassium-dependent adenosine triphosphatase (Na,K-ATPase), a P-type ion-transporting ATPase which is essential for the maintenance of adequate concentrations of intracellular Na+ and K+ ions. The present study describes the establishment of a ouabain-resistant mutant, TLouaR, from a human trophoblast cell line TL. Morphologically TL and TLouaR are indistinguishable, but, TLouaR is about 1000 times more resistant to the cytotoxic effect of ouabain and > 2000 times to that of bufalin and yet ouabain can retard the growth of the TLouaR cells and in parallel reduce its cloning efficiency in a time- and dose-dependent manner. Furthermore, Na,K-ATPase activity from TLouaR cells is inhibitable by ouabain albeit with lower efficiency. [3H]ouabain binding studies reveal that TLouaR cells have less (P < 0.05) ouabain binding sites (1.7 +/- 0.15 x 10(4)/cell vs. 2.3 +/- 0.115 x 10(4)/cell in the control). However, affinities (dissociation constants Kd) to ouabain for TL and TLouaR cells are not significantly different. Lastly, Na,K-ATPase activity (1.375 +/- 0.25 micromole ATP/min mg protein) of TLouaR cells is significantly higher (P < 0.05) than that of the TL cells (0.895 +/- 0.12 micromole ATP/min x mg protein). These studies show that the interactions between ouabain and Na,K-ATPase can be mediated through different pathways resulting in diverse phenotypic characteristics. In addition, ouabain resistance does not necessarily reflect the lack of response to the digitalis drug. The exact mechanisms of ouabain resistance observed in the present study remain to be determined but the TLouaR cells may be the best tool to uncover the many functional characteristics of Na,K-ATPase.

Different methods for measuring endogenous digitalis-like factor(s)

It is well recognized that one of the difficulties in the search for endogenous ligand(s) with digitalis-like properties (endogenous digitalis-like factor(s), EDLF) in mammals has been the lack of a unique, specific method for the accurate measurement of EDLF. Using C18 solid-phase extracts of plasma from normal adults and various patient groups, and purified extracts from umbilical cord plasma by affinity resin chromatography and HPLC, different methods to measure EDLF were evaluated. These were: (a) a human placenta radioreceptor assay (RRA) developed on the premise that competition for cardiac glycoside receptors was an absolute requirement for EDLF; (b) the inhibition of 86Rb uptake in human erythrocytes to estimate the potassium transport by the sodium pump; (c) an enzyme immunoassay specific for ouabain recently introduced in the market (DuPont Ouabain EIA Reagent Pack). The human placenta RRA was found to have the same ease of application as immunoassay, but could have major advantages in detecting active molecules, being "biologically more meaningful". Ouabain immunoreactivity correlated with EDLF values obtained by RRA, but in some instances the two assays were completely unrelated. Moreover, the high specificity of the DuPont antibody for ouabain (< 3% cross reactivity with digoxin) could be disadvantageous to detect EDLF not strictly resembling ouabain. The 86Rb uptake inhibition method correlated with RRA for EDLF purified by HPLC. It tested the complete enzymatic cycle and could therefore better reflect the in-vivo inhibitory activity of EDLF. However, it appeared not suitable for the routine EDLF evaluation in clinical studies since it was susceptible to sample osmolarity and required daily isolation of human erythrocytes possibly from the same donor. Results of the present study demonstrate that every assay has its limitations, and would suggest the use of multiple assays for EDLF detection.