A nanosecond fluorescence study of the simultaneous influx of Ca2+ and Cd2+ into liposomes

Nanosecond fluorescence decay characteristics of the calcium-binding probe Quin2 and two of its cation complexes were examined by time-resolved fluorescence spectroscopy. Binding of Ca2+ and Cd2+ resulted in fluorescence lifetime enhancements as compared to that of free Quin2 ('tau' = 0.9 ns). The Quin2-Ca2+ complex displays a monoexponential decay of tau = 7.4 ns, while the cadmium complex gives an average decay time of ca. 4 ns. Lifetime measurements made on heterogeneous cationic solutions demonstrate that decay times for individual complexes can be retrieved. Time-resolved measurements were used to monitor the kinetics of ionomycin-mediated calcium and cadmium transport across artificial membranes. Fluorescence decays, collected on the time-scale of second, were sufficient to measure individual ion fluxes or those of mixtures into liposomes. The combination of steady-state and time-resolved fluorescence techniques offers the unique advantage of simultaneously detecting other cations in the presence of calcium.