Comparison of the effects of prostaglandin E2 and parathyroid hormone on plasma calcium concentration and osteoclast function

Prostaglandin Es (PGEs) are known to be stimulators of bone resorption, but there are conflicting reports as to the effects of PGE2 and synthetic 16,16-dimethyl-PGE2 on plasma calcium concentration. This study involves iv infusion of PGE2 or its dimethyl analog into male thyroparathyroidectomized, parathyroidectomized, or thyroidectomized rats. Rats were given an initial dose of 5 μg PGE2 or its analog, followed immediately by infusion at a rate of 10 μg/ml-h for up to 12 h. An additional group of rats was injected with 0.01 U parathyroid hormone. (PTH)/g BW-h. 45Ca was injected ip at either 10 h or 6 days before commencing the experiment. In contrast to PTH, PGE2 and its analog affected bone lining cells only by increasing their quantity of filipodia. The PGs and PTH influenced osteoclast function by increasing the size of the population and the activity of individual osteoclasts. However, only PTH raised the plasma calcium level. PTH produced the expected results in plasma 45Ca and 45Ca specific activity values. PGE2 infusion did not affect 45Ca values when 45Ca was injected 10 h before start of infusion, but it did increase 45Ca values when the isotope was injected 6 days earlier in spite of having no effect on plasma calcium concentrations. The data permit the conclusion that PGs appear to duplicate the action of PTH on osteoclasts but not on plasma calcium control. This is in support of the concept of a separation of changes in the rate of bone resorption from the control of plasma calcium concentration