Evidence for physiological importance of calcitonin in the regulation of plasma calcium in rats.

To determine the physiological importance of calcitonin in the regulation of plasma calcium, studies were carried out in fasting animals to (a) assess the acute effects of thyroparathyroidectomy (TPTX) and thyroidectomy (TX) on plasma and urinary calcium; (b) investigate whether the changes in plasma calcium produced by removal of the glands were dependent on the presence of the kidney; and (c) determine if the effect of TPTX on plasma calcium is affected by age. Except where otherwise indicated, all studies were carried out on fasting male Wistar rats weighing over 300 g. The following observations were made. (a) TPTX and TX caused an increase in plasma calcium in nephrectomized animals. (b) This increase was not dependent on nephrectomy since in intact animals bearing autoparathyroid transplants TX also caused a significant rise in the mean plasma calcium level (0.37 mg/100 ml at 1 1/2 h). (c) Urinary calcium increased twofold in the 3-h period immediately after TX. (d) In unnephrectomized immature (50-g) rats, TPTX caused a progressive decrease in plasma calcium in contrast to old (360-g) rats, where a significant fall observed at 6 h was preceded by an increase in plasma calcium (0.5 mg/100 ml at 1 1/2 h). From these observations we conclude that: (a) calcitonin must play an important physiological role in the regulation of plasma calcium since the termination of its basal secretion caused an immediate but transient increase in plasma calcium in old unfed rats; (b) the relative importance of calcitonin and parathyroid hormone in the acute regulation of plasma calcium is age-related; and (c) the action of parathyroid hormone on bone may be modified by changes in ambient calcitonin concentration.