<i>In Vivo</i>Inhibition of Bone Resorption by Thyrocalcitonin

Abstract

The effect of thyrocalcitonin on the relative excretion of 85Sr was studied in young Holtzman rats whose skeletons were labeled with 85Sr 21 days before study. Under these conditions, thyrocalcitonin decreased the ratio of urinary to tibial 85Sr by 60 to 70% whether or not parathyroid hormone was co-administered. When the skeletons of Holtzman rats were labeled 1 day before thyrocalcitonin challenge, a 33% decrease in the ratio of urinary to tibial 85Sr 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 85Sr was present at skeletal sites undergoing resorption 21 days after 85Sr administration than 1 day after 85Sr administration, these data support in vitro observations that the hypocalcemic action of thyrocalcitonin is mediated by its ability to inhibit bone resorption. (Endocrinology80: 61, 1967)