The effect of intravenous calcitriol on parathyroid function was evaluated in nine chronic hemodialysis patients with secondary hyperparathyroidism. Two micrograms of calcitriol were administered intravenously after dialysis thrice weekly for ten weeks. Parathyroid function was assessed by inducing hypo- and hypercalcemia with low calcium (1.0 mEq/liter) and high calcium (4.0 mEq/liter) dialyses before and after ten weeks of intravenous calcitriol therapy. To avoid hypercalcemia during calcitriol administration, the dialysate calcium was reduced to 2.5 mEq/liter. Parathyroid hormone (PTH) values (pg/ml) from dialysis-induced hypo- and hypercalcemia were plotted against serum ionized calcium, and the sigmoidal relationship between PTH and calcium was evaluated. Basal PTH levels fell from 902 ± 126 pg/ml to 466 ± 152 pg/ml (P <0.01) after therapy without a significant change in the serum total calcium concentration. The ionized calcium-PTH sigmoidal curve shifted to the left and downward after calcitriol therapy. The maximal PTH response during hypocalcemia decreased after calcitriol from 1661 ± 485 pg/ml before calcitriol to 1031 ± 280 pg/ml afterward (P <0.05). The PTH level at maximal inhibition due to hypercalcemia decreased from 281 ± 76 pg/ml before calcitriol to 192 ± 48 pg/ml afterward (P <0.05). The slope of the sigmoidal curve changed from -2125 ± 487 to -1563 ± 385 (P <0.05). The set point of ionized calcium (4.60 ± .11 mg/dl before vs. 4.44 ± .07 mg/dl after) did not change significantly with calcitriol therapy. In summary, ten weeks of intravenous calcitriol therapy decreased PTH secretion across a wide range of serum ionized calcium concentrations, shifting the ionized calcium-PTH sigmoidal curve toward normal (left and downward). There were no significant changes in basal serum total calcium concentration throughout the study. These results demonstrate a direct inhibitory effect of intravenous calcitriol on parathyroid function in dialysis patients with secondary hyperparathyroidism.
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