A survey was made of the capacity of mitochondria isolated from a number of different tissues and species to accumulate Ca2+ from the suspending medium during electron transport. The species examined included the rat, mouse, rabbit, hamster, guinea pig, cow, chicken, turtle, blowfly, yeast and Neurospora crassa. The tissues examined included vertebrate liver, kidney, brain, heart, spleen, thyroid and adrenal cortex, and the flight muscle of the blowfly. The mitochondria from all vertebrate tissues examined showed: (a) stimulation of State 4 respiration by added Ca2+ (Ca2+/~ activation ratio about 2.0), accompanied by accumulation of Ca2+ and ejection of H+, with a H+/Ca2+ ratio about 1.0; (b) a requirement of phosphate for accumulation of large amounts of Ca2+; (c) respiration-independent high-affinity binding sites for Ca2+; (d) endogenous Ca2+, which is largely released by uncoupling agents. However, mitochondria from yeast and blowfly flight muscle are unable to accumulate Ca2+ in a respiration-dependent process and possess no high-affinity Ca2+-binding sites. These findings support the view that the high-affinity sites represent the ligand-binding sites of a specific Ca2+ ‘permease’ or transport system in the membrane. The relatively high affinity for Ca2+, which equals or exceeds the affinity for ADP, and the generally uniform characteristics of Ca2+ transport in all the vertebrate mitochondria tested strongly suggest that respiration-linked Ca2+ accumulation plays a general and fundamental role in vertebrate cell physiology.

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