Maximal PTH secretory rate and set point for calcium in normal subjects and patients with primary hyperparathyroidism. In vivo studies
Gardin, J.P.; Patron, P.; Fouqueray, B.; Prigent, A.; Paillard, M.
Mineral and Electrolyte Metabolism 14(4): 221-228
1988
ISSN/ISBN: 0378-0392 PMID: 2850460 Document Number: 306209
The characteristics of PTH secretion, which have been extensively studied in vitro with dispersed cells of normal and abnormal parathyroid glands, remain poorly studied in vivo. We performed ethylenediamine tetraacetate (EDTA) intravenous infusions in 12 normal subjects and 5 patients with hypercalcemia (serum ionized calcium between 2.72 and 2.89 mEq/l) and surgically proven primary hyperparathyroidism (PHPT) to establish the relationship between nephrogenous cyclic AMP (NcAMP) used as an index of PTH secretion and serum ionized calcium. We determined the maximal NcAMP taken as an index of maximal secretory rate for PTH, the set point and sensitivity of parathyroid cells for calcium. In normal subjects, mean values (.+-. SD) were 4.04 .+-. 0.47 nmol/dl glomerular filtrate (GF) for maximal NcAMP, 2.23 .+-. 0.04 mEq/l for set point, and -250 .+-. 58 for sensitivity when NcAMP was expressed in percent of maximal value and serum calcium in mEq/l. In patients with PHPT, the differences between maximal and basal values of NcAMP represented 50% or more of maximal NcAMP, indicating that PTH secretion was suppressible. Sensitivity values were within normal limits in all patients. Calculated set point values were abnormally elevated (between 2.64 and 2.83 mEq/l) in all patients. Maximal NcAMP values ranged from low to normal (2.77 nmol/dl GF) to abnormally high (6.64 nmol/dl GF), and a positive linear correlation was observed with parathyroid cell mass. Basal serum calcium concentrations were not correlated with either parathyroid cell mass or maximal NcAMP values, and were close to calculated set point values for each patient. We concluded that (1) the vivo relationship between NcAMP and plasma ionized calcium established from EDTA infusions is a reliable tool to characterize PTH secretion in normal subjects and patients with PHPT, and (2) an abnormally, elevated set point of parathyroid cells for calcium, in addition to the increase in tissue cell mass, may be responsible for chronic hypercalcemia and inappropriate PTH secretion.