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Parathyroid hormone effects on skeletal exchangeable calcium and bone blood flow.

The effects of alterations in serum calcium levels on the volume of distribution of a calcium tracer in bone and on bone blood flow were investigated in dogs by manipulation of parathyroid status. The volume of distribution increases with increasing serum calcium. This implies that there is a concentration-dependent binding mechanism in the extravascular space of bone, which is not saturated at the levels of serum calcium achieved in these experiments. The skeletal exchangeable calcium was increased in hypercalcemic dogs and decreased in hypocalcemic dogs. Cancellous bone contains proportionately more exchangeable calcium than cortical bone. Bone blood flow was decreased by 50% 15 min after injection of a bolus of parathyroid hormone (PTH) but returned to control values by 240 min. Fifty-two hours after the induction of hypercalcemia with repeated doses of PTH, there was no difference in blood flow over control animals.