Further studies on the hypercalcemic effect of acute calcitonin deficiency in rats.

Studies were carried out to determine the origin of the immediate increase in plasma calcium following acute calcitonin deficiency in mature rats. Animals were pre-labelled with 45calcium 24 hours and 4 weeks before thyroparathyroidectomy (TPTX) and 1 hour, 24 hours, 2 and 4 weeks before nephrectomy and TPTX and bled serially over the following 3 hours. In each study the final average weight of the rats was over 300 g. Plasma calcium increased after TPTX. In rats labelled with 45calcium 1 and 24 hours previously, the rise was to small to alter the specific activity of calcium although radiocalcium was unchanged. In contrast, in animals pre-labelled with 45calcium 2 and 4 weeks before TPTX, the increase in stable calcium was associated with a parallel increase in radiocalcium. Consequently, the specific activity of plasma calcium did not differ appreciably from that of controls. These findings confirm the theory that in mature unfed rats acute calcitonin deficiency results in an immediate rise in plasma calcium. Since this increase is due mainly to enhanced transport of calcium from deep bone, our observations are in accord with the view that calcitonin decreases plasma calcium primarily by inhibiting calcium transport from "stable" bone.