Electron-opaque granules are deposited in isolated rat-liver mitochondria concomitant with the energy-linked accumulation of Sr++ by these organelles. High temperature microincineration (600°C) of thin sections of mitochondria containing different amounts of Sr++ shows that a clear qualitative correlation exists between the number of inorganic residues remaining after incineration and the amount of Sr++ translocated into the mitochondria. By loading the mitochondria with consecutive pulses of small amounts of Sr++ ("multiple-pulse" loading), very early stages of granule formation can be detected; the first detectable deposits are seen closely associated with the cristae. The evidence presented supports the hypothesis that mineral deposition following or during the in vitro accumulation of ions by mitochondria occurs, at least initially, at sites on these membranes and not as nonspecific precipitates in the mitochondrial matrix. The large number of electron-opaque deposits (100 to 200) seen in single thin sections of individual mitochondria having accumulated intermediate levels of Sr++ clearly exceeds the number of normal dense granules in rat-liver mitochondria, indicating that the normal matrix granules per se do not constitute sites essential for deposition. At the highest levels of Sr++ uptake studied in the multiple-pulse loading experiments, needlelike deposits are seen, a result which suggests that the structural form ("crystallinity") of the mineral deposits may be determined by the rate of accumulation.

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