Dynamic tests of parathyroid hormone secretion using hemodialysis and calcium infusion cannot be compared

Background. Extracellular Ca2+ concentration [Ca++] and parathormone (PTH) are related by a sigmoidal function. The set point of the control system is the [Ca++] that produces a half-maximal inhibition of PTH secretion. Wheth er or not this set point is abnormal in patients with chronic renal failure (CRF) and secondary hyperparathyroidism (SHP) is controversial. Methods. We investigated whether the way [Ca++] is varied [hemodialysis (HD ) or calcium gluconate/sodium citrate infusions (INF)] and the way the curv e is constructed (four-parameter model or adapted four-parameter, created b y Felsenfeld) could influence this set point. We performed dynamic tests of PTH secretion in 12 patients with CRF and SHP during either HD or INF. Bot h the four-parameter model or adapted four-parameter methods were used, cre ating four combinations: (a) hypocalcemia and hypercalcemia induced during HD, calculated by Brown's formula (HDB); (b) hypocalcemia and hypercalcemia induced during HD, calculated by Felsenfeld's formula (HDF); (c) hypocalce mia and hypercalcemia induced during infusion: calculated by Brown's formul a (INFB); and (d) hypocalcemia and hypercalcemia induced during infusion, c alculated by Felsenfeld's formula (INFF). Results. The set points obtained with HDB correlated perfectly with those o btained with HDF (R-2 = 0.999). A similar relationship was found between IN FB and INFF (R-2 = 0.9997). In contrast, there was no correlation between e ither HDB and INFB (R-2 = 0.0157) or HDF and INFF (R-2 = 0.0204). Conclusions. These findings indicate that the calculated [Ca++] set point i n patients with CRF and SHP is determined by the way [Ca++] is varied, rath er than by the mathematical model used to generate the curves. Further stud ies are needed to determine the differing physiological mechanisms triggere d by HD and INF and the way they influence [Ca++] homeostasis in this setti ng.