The treatment of rats with cortisone diminishes the ATP or substrate-dependent binding of calcium to subsequently isolated liver mitochondria in vitro. Treatment with 17/3-estradiol or deoxycorticosterone (DOCA) enhances calcium binding under the same conditions. These effects of steroid hormone treatment are demonstrable in intact, parathyroidectomized or vitamin D-depleted animals, and appear to be independent of a mitochondria! interaction with parathyroid hormone or vitamin D. Treatment with glucocorticoids probably interferes with the utilization of both the substrate and/or the ATP necessary to support calcium binding. Parathyroidectomy or vitamin D -repletion of depleted animals causes an increase in mitochondrial calcium binding which is independent of the steroid hormone effects. These results are discussed in relation to previous work in this area. (Endocrinology 80: 89,1967)

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