Chlortetracyclin‐mediated continuous Ca 2+ oscillations in mitochondria of digitonin‐treated Tetrahymena pyriformis

Ca 2+ transport in mitochondria was studied in situ using digitonin‐permeabilized cells of the ciliate protozoan Tetrahymena pyriformis GL. In the presence of oxidizable substrates and inorganic phosphate, mitochondria were able to accumulate a large amount of the added Ca 2+ without subsequent uncoupling and mitochondrial damage. However, the maximal Ca 2+ uptake dramatically decreased in the presence of micromolar concentrations of the fluorescent calcium indicator, chlortetracyclin, which in aerobic conditions caused an uncoupling of the respiration in Ca 2+ ‐loaded mitochondria. Moreover, on reaching hypoxia, when the rate oxygen diffusion from the air to the stirred incubation medium became a limiting factor, continuous Ca 2+ oscillations were observed. Ca 2+ fluxes were synchronous with the cyclic changes of the membrane potential and were followed with a significant delay by the changes of the membrane‐associated fluorescence of Ca‐chlortetracyclin complexes. Both the chlortetracyclin‐induced uncoupling of the respiration and the oscillations were prevented by either EGTA or ruthenium red. It is suggested that in conditions of the limited rate of respiration the oscillations are generated as a result of the functioning of the two Ca 2+ ‐transport pathways: a Ca 2+ uniport and a chlortetracyclin‐mediated electroneutral Ca 2+ efflux.