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It is well known that serum calcitonin (CT) levels in infantile rats are higher than those in adult rats. In the present experiment, the effects of parathyroidectomy or calcium administration on serum calcitonin levels were observed in suckling rats. Bilateral parathyroidectomy (Px) on 3-, 5-, 7-, 8- or 14-day-old Wistar-Imamichi female rats was performed surgically under a dissecting microscope. Ether anesthesia was used. In the first experiment, the Px rats of various age groups were killed 1 week after the operation. In the second experiment, time course effects of Px on one-week-old rats were observed 1, 2 or 3 weeks after the operation. CT in the serum and thyroid extracts (0.001N HCl) was measured using radioimmunoassay kits for human CT. Serum Ca was measured by the OCPC method and serum inorganic phosphates was measured by the Takahashi method. Serum calcium levels showed a significant decrease 1 week after Px in all age groups. In these rats, serum CT levels showed a slight increase or no changes, and a significant increase in CT concentrations in the thyroid gland was observed in all age groups. Moreover, the increase in CT contents in the thyroid gland became larger 3 weeks after Px in rats operated on at one week of age. Thyroid extract containing 70 pg immunoreactive CT was effective in decreasing 1.22 mg/100 ml serum calcium at 15 min after i.v. injection in a bioassay system in which a dose response curve of human calcitonin was used as standards. Acute s.c. injection of CaCl2 solution resulted in a significant increase in serum CT levels 15 and 30 min later in intact 2- and 3-week-old rats, whereas no significant change in serum CT levels was observed in one-week-old intact rats. Three and 15 min after s.c. injection of Na2EDTA, serum Ca levels decreased markedly, but thyroidal CT contents tended to increase 3 and 5 min after the injection. The results indicated that parathyroidectomy in infantile rats induces a marked increase in biologically active calcitonin in the thyroid gland, inspite of the significant decrease of serum Ca levels.

An Increase in Calcitonin Levels in the Thyroid Gland of Infant Rats Following Parathyroidectomy