CICA CLAMP EVALUATION OF PARATHYROID RESPONSIVENESS IN CHRONIC HYPOPARATHYROIDISM - A SEQUENTIAL CITRATE AND CALCIUM CLAMP STUDY

The objective of this study was to examine if the Cica clamp technique , sequential citrate and calcium administration sufficient to promote steady-state-blood-ionized calcium concentrations (B-Ca2+) of about 0. 20 mmol/l below and above the individual baseline concentrations, was able to produce reciprocal. changes in serum intact parathyroid hormon e [S-PTH(1-84)] in chronic surgical hypoparathyroidism (HP; n = 10) an d chronic idiopathic HP (n = 2). The calcium set point according to Br own [J Clin Endocrinol Metab 1993;56:572-581] was calculated when poss ible. Data from 22 controls were included for comparison. Within 5-10 min B-Ca2+ lowering in responding patients with surgical HP (n = 7) an d controls demonstrated transient S-PTH(1-84) peaks from 1.3 +/- 0.7 t o 3.5 +/- 3.2 pmol/l (p < 0.05) and from 3.4 +/- 1.2 to 19.1 +/- 6.7 p mol/l (p < 0.001), respectively. Subsequently S-PTH(1-84) declined to steady-state hypersecretion levels of about 1.9 +/- 1.2 and 8.6 +/- 2. 6 pmol/l, respectively. An increase of B-Ca2+ made S-PTH(1-84) unmeasu rable in all HP responders except one, while S-PTH(1-84) remained meas urable, 0.9 +/- 0.4 pmol/l, in all controls. In responding patients wi th surgical HP and controls the respective calcium set points averaged 1.05 +/- 0.06 and 1.13 +/- 0.04 mmol/l, respectively (p < 0.001). The remaining nonresponders with surgical and idiopathic HP did not respo nd at all. To summarize, 7 out of 10 patients with surgical HP demonst rated a normal pattern of parathyroid response to sequential B-Ca2+ de creases and increases. In these patients the dependency of S-PTH(1-84) on B-Ca2+ was characterized by inverse sigmoidal curves with generall y reduced spans, shifted downwards and to the left, reflecting reduced calcium set points.