[Dynamics of calcium-regulated PTH secretion in secondary hyperparathyroidism: comparison between "in vivo" vs. "in vitro" responses].

AIM

To compare the dynamics in vivo and in vitro calcium-PTH release of uremic patients with secondary hyperparathyroidism and their hyperplasic parathyroid glands after parathyroidectomy.

MATERIALS AND METHODS

Seven patients with secondary HPT and their 23 hyperplasic glands obtained after surgical parathyroidectomy were evaluated. In vivo studies of the PTH secretion curve were obtained by induction of hypocalcemia and hypercalcemia with a continuous endovenous infusion of sodium EDTA and Ca gluconate, respectively. For the in vitro studies, small parathyroid pieces of 1 mm were sequentially transferred to wells with varying Ca concentrations: 0.4, 0.6, 0.8, 1, 1.25 and 1.5 mM. iPTH concentrations were determined in the medium.

RESULTS

The in vivo set point did not correlate with the basal, maximal or minimal PTH concentrations, although it correlated significantly with the basal serum Ca concentration (r = 0.62, p < 0.02). Both in vivo and in vitro PTH secretion curves were sigmoidal, although the in vivo set point was higher than the in vitro (1.57 +/- 0.05 vs. 1.27 +/- 0.07 mM, p < 0.001). The degree of maximal PTH inhibition were similar in both circumstances (30.5 +/- 8.1 vs. 33.6 +/- 5.4 %; p = NS) with a significant direct correlation (r = 0.901; p < 0.01).

CONCLUSIONS

The in vivo set point of calcium is more closely related to serum calcium concentration than to basal iPTH concentration. Although there are differences between the in vivo and in vitro calcium set point the maximal degree of PTH inhibition was similar in both circumstances.