Since in some studies in hemodialysis patients calcitriol treatment has resulted in a reduction of both parathyroid hormone (PTH) levels and the set point of calcium, it has been suggested that that the set point of calcium reflects a reduction in the magnitude of hyperparathyroidism. However, others have maintained that the set point of calcium is primarily an indicator of the serum calcium at which PTH is secreted and may be dissociated form the magnitude of hyperparathyroidism. The present study was designed to evaluate how a reduction in PTH levels associated with an increase in the predialysis (basal) serum calcium would affect the set point of calcium. Two different treatments were used to produce a reduction in PTH that was associated with an increase in predialysis serum calcium. In the first group, hemodialysis patients received 2 micrograms of intravenous calcitriol and were dialyzed with a 3.5 mEq/liter calcium dialysate for six weeks; in the second group, hemodialysis patients were dialyzed with a 4 mEq/liter calcium dialysate and had oral calcium supplementation increased for six weeks. In both groups, low and high calcium studies were performed to determine the PTH-calcium relationship before treatment, at the end of six weeks of treatment, and six weeks after the discontinuation of treatment. In the calcitriol group the predialysis calcium increased form 9.62 +/- 0.34 to 10.56 +/- 0.31 mg/dl, P < 0.05 and the set point of calcium increased from 9.34 +/- 0.23 to 9.79 +/- 0.25 mg/dl, P < 0.05 at the same time as maximally stimulated PTH decreased from 2637 +/- 687 to 1555 +/- 617 pg/ml, P < 0.05. In the high calcium dialysate group, the predialysis serum calcium increased from 9.19 +/- 0.31 to 9.84 +/- 0.28 mg/dl, P < 0.05, and set point of calcium increased form 9.01 +/- 0.28 to 9.39 +/- 0.22 mg/dl, P < 0.05 at the same time as maximally stimulated PTH decreased from 1642 +/- 450 to 1349 +/- 513 pg/ml, P < 0.05. Discontinuation of treatment for six weeks resulted in a return to pretreatment values. In conclusion, our results would suggest that (1) the set point of calcium may not be a reliable indicator of the magnitude of hyperparathyroidism during calcitriol treatment, and (2) PTH secretion may adapt to the ambient serum calcium concentration.

Calcitriol putatively suppresses bone activity by decreasing parathyroid hormone (PTH) levels. Results of studies in a 52-year-old female maintenance hemodialysis patient suggest that calcitriol may also have a direct suppressive effect on bone. The PTH-calcium relationship was evaluated through the use of low (1 mEq/1) and high (4 mEq/1) calcium hemodialyses that were performed before the initiation of calcitriol treatment, at the end of 6 weeks of thrice-weekly intravenous calcitriol administration, and 6 weeks after the discontinuation of calcitriol. During the low-calcium dialysis, serum calcium decreased more rapidly and to a greater magnitude after calcitriol treatment despite no appreciable difference in basal and maximally stimulated PTH levels; during the high-calcium dialysis, calcitriol treatment resulted in a more rapid increase in serum calcium despite no appreciable difference in basal and maximally suppressed PTH levels. Discontinuation of calcitriol resulted in responses to the low and high calcium dialyses that were similar to those observed before calcitriol treatment. In conclusion, the results suggest that calcitriol may have a direct suppressive effect on bone that is independent of PTH.