PTH secretion in patients with chronic renal failure assessed by a modified CiCa clamp method: effects of 1-year calcitriol therapy.

Background: Secondary hyperparathyroidism (2 °HPT) in patients with chronic renal failure (CRF) is characterized by parathyroid gland hyperplasia and an intrinsic defect in the recognition of parathyroid hormone (PTH) secretion. Conflicting results have been reported regarding the set point for calcium-regulated PTH release and its modification by calcitriol therapy in hemodialysis patients. Additionally, the effect of calcitriol on the calcium/PTH relationship in predialysis CRF patients with early 2 °HPT has not been investigated. Our objective in this controlled study was to investigate the calcium/PTH relationship and to determine the calcium set point in patients with early stages of CRF before and after a 1-year treatment with calcitriol and in normal volunteers. Methods: Nine patients with an early stage of CRF (GFR between 20 and 50 ml/min × 1.73 m 2 b.s.) aged 35 - 77 years and 13 healthy volunteers (HV) aged 26 - 60, years were included in the study. All participants were investigated by sequential lowering and raising of serum calcium levels comprising the following phases: blood-ionized calcium (Ca 2+ ) was lowered by about 0.2 mmol/l (3 steps), steady-state hypocalcemia of Ca 2+ 0.2 mmol/l below the baseline (step 4), stop of the infusion for 5 minutes (step 5), Ca 2+ was raised to about 0.2 mmol/l above baseline (steps 6 and 7), and a steady state hypercalcemia of Ca 2+ 0.2 mmol/l above baseline (step 8). Ionized calcium and intact PTH (iPTH) were measured at 30 time points during 240 minutes. The calcium set point was determined using the classical 4-parameter model. The CiCa clamp test was performed before and after a 1-year treatment with 0.5 μg of calcitriol thrice weekly. Results: No differences in the set point were observed between HV and CRF patients with early 2 °HPT. Four of 9 patients responded to calcitriol treatment with a decrease in basal serum iPTH levels (responders). There was no difference between renal function (GFR 18 ± 6 vs. 17 ± 8 ml/min x 1.73 m 2 b.s.), set point (Ca 2 1.07 ± 0.13 vs. 1.07 ± 0.06 mmol/l) and suppressibility of PTH secretion (PTH min% 7.3 ± 1.6 vs. 8.2 ± 2.9) in responders vs non-responders, nor did these values change after treatment with calcitriol. PTH min% decreased significantly in the whole group after treatment (10.4 ± 8.5 vs. 7.8 ± 2.4). Conclusions: Although the calcium set point was not different in predialysis CRF patients with early 2 °HPT compared to HV, calcitriol treatment improved the calcium-related suppression of PTH secretion (PTH min% ).