Conditions modulating the ionic selectivity of transport by monensin examined on Enterococcus hirae (Streptococcus...

Factors likely to modulate the ionic selectivity of monensin were examined on Enterococcus hirae (Streptococcus faecalis) in two states previously characterized: the resting (de-energized) cell and the active (energized) cell. Internal and external Na+ were followed by corresponding 23Na-NMR resonances K+ concentrations were measured by atomic absorption. For a given cellular population of de-energized cells, the apparent transport rates and the final cationic concentrations reached at the steady state were decreasing with the ionophore dose. Monensin was selective for sodium only at low concentrations, in the range 1 mM-10(-4) mM the transport was depending on the effective cationic gradients. Comparison of the activity curves for two cell populations (7.10(9) and 7.10(10) cells/ml) showed the importance of the ratios of monensin/mg phospholipid and also of the ratios of external/internal volumes. On energized cells, except for low monensin concentrations, the main effect was a K(+)-induced efflux and not a Na+ influx. Two factors were modulating the resulting selectivity of this ionophore: the response of the intrinsic bacterial carriers and the generation of the gradients (mainly the external pH) which were favourable to a K+/Na+ transport. Once again the results obtained for two cell populations could be compared, the determining factors were the ratio external/internal volume and the generation of the pH gradient.