IN VIVO INHIBITION OF BONE RESORPTION BY THYROCALCITONIN

The effect of thyrocalcitonin on the relative excretion of Sr85 was studied in young Holtzman rats whose skeletons were labeled with Sr85 21 days before study. Under these conditions, thyrocalcitonin decreased the ratio of urinary to tibial Sr85 by 60 to 70% whether or not parathyroid hormone was coadministered. When the skeletons of Holtzman rats were labeled 1 day before thyrocalcitonin challenge, a 33% decrease in the ratio of urinary to tibial Sr85 occurred, a value comparable to the expected decrease in the plasma concentration and glomerular filtration of calcium during thyrocalcitonin administration. Since more of the retained Sr85 was present at skeletal sites undergoing resorption 21 days after Sr85 administration than 1 day after Sr85 administration, these data support in vitro observations that the hypocalcemic action of thyrocalcitonin is mediated by its ability to inhibit bone resorption.