Electron microscopic evidence is presented, from mitochondria in whole cells of toad urinary bladder and from isolated rat kidney mitochondria, indicating that the divalent cations calcium, strontium, and barium are accumulated in granules localized in the mitochondrial matrix. This accumulation occurs under conditions in which divalent ions are present in the medium bathing either whole cells or isolated mitochondria. The evidence indicates that the divalent ions are deposited on, or in a pre-existing granule, possibly in exchange for other ions. It suggests a possible role of the intramitochondrial granules in the regulation of the internal ionic environment of the mitochondrion. Certain biochemical and physiological implications of this phenomenon are discussed.

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