The distribution of biologically active and inactive radioiodinated human calcitonin in the rat

The anatomical distribution of a biologically active and biologically inactive preparation of radioiodinated human calcitonin (hCT) was studied in rats. The former was prepared by electrolysis and the latter by chloramine-T oxidation. When administered iv, the distribution of the two species of [125I]hCT was nearly identical. The kidney was the principle organ of distribution for the hormone while less was present in liver, intestine, or bone. In vitro, kidney homogenates rapidly degraded [125I]hCT. This degradation was partially blocked by trypsin inhibitor and fully blocked by ACTH at both 4 and 37 C. These studies demonstrate the importance of kidney in the metabolism of calcitonin and suggest that the in vivo distribution of biologically active and inert species of the hormone may be indistinguishable.