Metabolism of radioiodinated salmon calcitonin in rats

Synthetic salmon calcitonin (CT) was iodinated by an electrolytic method. The iodinated hormone retained full biological activity as determined in both an in vivo and in vitro bioassay. The metabolism of this hormone was studied in rats of two different ages, 24 days and 8 months old. Two types of uptake were observed: Rapid accumulation followed by rapid loss or degradation, as represented by the liver, and sustained accumulation with slow degradation, as represented by the kidney and bone. The liver accumulated a large fraction of the dose in 1 minute. Most of this was degraded or gone by 10 min and nearly all by 30 min. The kidney was a primary target organ in terms of concentrating the hormone. Nearly 50% of the initially accumulated CT, however, remained undegraded 60 min post-injection. The bone, to a lesser extent, concentrated the hormone where 70% remained undegraded 60 min post-injection. The bone accumulated significantly less CT while the kidney and liver accumulated more in 8-month-old rats compared to 24-day-old rats. This suggests that the relative ineffectiveness of CT in older animals as a hypocalcemic agent may be due in part to the inability of the bone to accumulate available hormone.