FIG. 1.

Fluorescence excitation spectra of fura-2 in the presence of metal cations. Fluorescence excitation spectra of fura-2 in the presence of saturating concentrations of Ca²⁺, Ba²⁺, Sr²⁺, and La³⁺ (A) or Ni²⁺, Co²⁺, and Mn²⁺ (B) are shown. Fluorescence emission was recorded at 510 nm. Under all conditions, the cuvette contained 1 mM cation in a solution of 10 μM fura-2 free acid, 10 mM HEPES (pH 7.2), and 100 mM KCl. The spectrum of fura-2 in the absence of any cation was obtained in the presence of 10 mM K₂H₂EGTA in the same medium to chelate traces of contaminant divalent cations. The y-axis scale in panel B has been increased by a factor of two.