Abstract: The transmembrane behavior of La³⁺ and Eu³⁺ was examined by using Fura-2 technique. The result showing a low concentration La³⁺ or Eu³⁺ (5×10^-6mol/L) could not enter fresh and ATPdepleted human red cells. But in the presence of ionomycin, obvious transmembrane phenomenon was observed when La³⁺ (or Eu³⁺)^of 5×10^-6mol/Lwas added. KCl and Bay K8644 could not promote the influx of La³⁺ (or Eu³⁺). Moreover, a low concentration La³⁺ (or Eu³⁺) had no effect on the influx of Ca ²⁺ in ATPdepleted human red cells. When a higher concentration La³⁺ or Eu³⁺ (5×10^-4mol/L) was added, red cells became hemolysis. Acharacterization of La³⁺ (or Eu³⁺)-Fura-2 interactions showed that they formed a 1∶1 La³⁺ (or Eu³⁺)-Fura-2 complex. In vitro studies confirmed that Fura-2 fluorescence was sensitive to La³⁺ (or Eu³⁺) and the apparent dissociation constants of La³⁺ and Eu³⁺ for Fura-2 (K_d) were 1.7×10^-12 and 4.95×10^-14mol/L, respectively (pH=7.05).

Key words: ATP depleted, Human red cells, Fura-2, Rare earth ions