ACUTE EFFECT OF CALCITONIN-DEFICIENCY ON PLASMA CALCIUM IN FASTED YOUNG-RATS

Studies were carried out to assess the acute effect of calcitonin deficiency on plasma calcium in fasted young rats varying in age from 3 to 9 weeks. In the first experiment, animals were thyroparathyroidectomized; in the second, animals were nephrectomized and immediately thyroparathyroidectomized; in the third, animals bearing autoparathyroid transplants were thyroidectomized, and in the final study 1 year-old rats were fasted and then thyroparathyroidectomized. In all studies, control rats were sham operated and animals were bled just before surgery and at timed intervals subsequently. The following observations were made. Thyroparathyroidectomy (TPTX) caused a fall in plasma calcium level. In 3, 6, and 8 week-old animals the decrease was progressive, and in 1 year-old animals the fall was preceded by a rise in calcium of 0.80 mg/100 ml of plasma 3 hours after TPTX. The acute increase in plasma calcium, which is usually observed within 6 hours after nephrectomy, was augmented by concomitant TPTX in 4, 6 and 8 week-old rats. Twenty-four hours after surgery, plasma calcium had fallen below the pre-surgery level in all rats, but the decrease was more in thyroparathyroidectomized animals than in nephrectomized controls. In 6 and 9 week-old rats bearing autoparathyroid transplants, thyroidectomy raised plasma calcium level. A maximum increase of 0.60 mg/100 ml of plasma occurred 1.5 hours after surgery but 6 hours later plasma calcium had returned to the presurgery level. These observations are consistent with the view that during fasting the removal of the source of calcitonin causes an immediate but unsustained increase in plasma calcium in young rats. Since in fasted old rats thyroidectomy also causes an immediate rise in plasma calcium, it is suggested that endogenous calcitonin is involved in the maintenance of normocalcemia during fasting in both young and mature rats.