Interaction of Ca2+ with blowfly flight muscle mitochondria

Blowfly flight muscle mitochondria differ markedly from mammalian mitochondria in their interaction with Ca 2+ . State 4 respiration of blowfly mitochondria is not stimulated by Ca 2+ , nor does Ca 2+ induce a reversible shift in the oxidation-reduction state of cytochrome b . Nevertheless, Ca 2+ is accumulated by respiring blowfly mitochondria, but with a low efficiency; the Ca 2+ : ∼ accumulation ratio is about 0.6 with pyruvate plus proline, and about 0.2 with glycerol 1-phosphate, compared with values of about 2.0 observed in mammalian mitochondria. Ca 2+ uptake is blocked by respiratory chain inhibitors and by uncouplers, but not by oligomycin. The presence of permeant anions in the medium is essential for Ca 2+ uptake by blowfly mitochondria. At high levels of Ca 2+ , phosphate supports Ca 2+ uptake better than acetate, but at low Ca 2+ levels, phosphate and acetate are equally effective. Since acetate penetrates blowfly mitochondria passively it is probable that respiration-coupled uptake of Ca 2+ in blowfly mitochondria must be accompanied by passive entry of matching anions. The accumulation of Ca 2+ by blowfly mitochondria is very slow and does not exhibit saturation kinetics; moreover, no high affinity Ca 2+ sites characteristic of a specific Ca 2+ carrier could be detected. For these reasons it is concluded that the respiration-dependent accumulation of Ca 2+ in blowfly mitochondria is not mediated by a specific carrier, but occurs through slow, concentration-dependent, physical diffusion through the membrane, in response to an electrochemical gradient generated by electron transport. The number of low affinity Ca 2+ -binding sites is surprisingly low, about one-tenth of that found in mammalian mitochondrial, and has been related to the peculiar phospholipid composition of these mitochondria. The findings described strongly suggest that the mitochondria of blowfly flight muscle do not actively segregate Ca 2+ at the low concentrations at which it is presumed to occur in the sarcoplasm in vivo .