Abstract
The time course of adaptation of the serum calcium level to the gradual deprivation of parathyroid hormone was studied in adult male and female rats. In addition, the role of thyrocalcitonin and thyroid hormones in adaptive changes in serum calcium was assessed in the surgically parathyroidectomized animals. Simultaneous bilateral parathyroidectomy, thyroparathyroidectomy or the second unilateral parathyroidectomy performed 72hr after the first unilateral parathyroidectomy produced a prompt fall in serum calcium level leading to the lowest value at 10hr and the significantly lower calcium level retained for one week-observation. Unilateral parathyroidectomy resulted in a transient fall in the serum calcium level and to a rapid return to the subnormal range in 24hr in both sexes. When the second unilateral parathyroidectomy was carried out 1 or 6 weeks after the first unilateral parathyroidectomy, the serum calcium in female rats showed a slight decrease until 10hr and returned to the normal level in one week, but in male rats no further decline or a gradual decrease was seen. Bilateral thyroidectomy performed 10weeks after the second unilateral parathyroidectomy resulted in a gradual increase in the serum calcium above the normal range in female rats. In male rats, the serum calcium level tended to drop until 10hr and returned to the initial level or to the normal range. Bilateral thyroidectomy performed one week after simultaneous bilateral parathyroidectomy led to a further decline in serum calcium in both male and female rats. The results indicate that adaptation of the serum calcium level toward the normal range occurs in the absence of parathyroid hormone in adult male and female rats if the parathyroid hormone is gradually deprived. A greater ability of female rats to restore the calcium level, especially seen when thyroidectomized after adaptation to the lack of parathyroid hormone, suggests that estrogens play a significant role in the homeostatic control of serum calcium in chronic disorders in calcium regulation.
