Amino terminal cleavage of PTH(1-84) to PTH(7-84) is regulated by serum calcium concentration via calcium-sensing receptor in hemodialysis patients.

BACKGROUND: Secondary hyperparathyroidism is one of the critical complications of end-stage renal disease patients. Conventionally intact parathyroid hormone (iPTH) was used to assess secondary hyperparathyroidism, but this assay measures both PTH(1-84) (full-length parathyroid hormone) and PTH(7-84) (amino (N)-terminal-cleaved parathyroid hormone). PTH(7-84) is biologically inactive or antagonistic for PTH. In this study, we examined the relationship between serum calcium concentration and PTH(7-84)/PTH(1-84) ratio and the effect of calcimimetics on the ratio in hemodialysis (HD) patients.
METHODS: Ionized-calcium (iCa), iPTH, and whole PTH (wPTH) were measured at the start of HD sessions on HD patients. Patients were divided into four groups by presence (+) or absence (-) of vitamin D (VD) and calcimimetics (CM). RESULT: PTH(7-84)/PTH(1-84) ratios of the four groups [VD(-)CM(-), VD(+)CM(-), VD(-)CM(+) and VD(+)CM(+)] were 0.735, 0.799, 0.844, and 1.156, respectively. In VD(-)CM(-) and VD(+)CM(-) groups, iCa and PTH(7-84)/PTH(1-84) ratio showed equilateral correlation (r = 0.634, p < 0.001 and r = 0.360, p < 0.01, respectively). In calcimimetics-treated group, iCa and PTH(7-84)/PTH(1-84) ratio did not show correlation.
CONCLUSION: Whereas in the absence of calcimimetics cleavage of N-terminal PTH was regulated by serum calcium concentration, this regulation was abolished in the presence of calcimimetics. This suggests that cleavage of N-terminal PTH is regulated by calcium concentration via a calcium-sensing receptor and that calcimimetics may have a novel effect to reduce PTH level.