Characterization of Ca2+ transport in Euglena gracilis mitochondria

The present study was designed to establish the characteristics of the Ca2+ fluxes in isolated mitochondria of the protist Euglena gracilis. Uptake of Ca2+ and Sr2+ was supported by succinate and lactate oxidation. Ca2+ influx was slightly inhibited by 5 microM Ruthenium red and completely blocked by La3+ with a half-maximal inhibition attained at 50 microM. The addition of inorganic phosphate induced a 3-fold stimulation of Ca2+ uptake. Ca2+ uptake was inhibited by Mg2+ only in the absence of phosphate. Ca2+ efflux was induced by Na+, Li+ and K+ through a diltiazem-insensitive reaction. Ca2+ release, collapse of membrane potential and swelling were induced by Hg2+ and Cd2+ but not by carboxyatractyloside; cyclosporin A did not prevent the Ca2+ release induced by the heavy metal ions. Ca2+ uptake was achieved in the presence of 3 microM antimycin or 0.1 mM cyanide; this finding indicates that the alternative respiratory chain present in Euglena mitochondria can support this energy-dependent reaction. The data obtained suggest similar pathways, but different regulatory mechanisms, for Ca2+ transport between protist and mammalian mitochondria.