Direct upregulation of parathyroid calcium-sensing receptor and vitamin D receptor by calcimimetics in uremic rats.

To investigate whether the effect of the calcimimetic AMG 641 and calcitriol on CaSR and VDR expression could be separated from their ability to reduce parathyroid cell proliferation, five-sixth nephrectomized (5/6 Nx) rats received vehicle, AMG 641, calcitriol, or AMG 641+calcitriol either daily for 13 days (long-term protocol) or in a single dose (short-term protocol). In the long-term protocol, AMG 641, calcitriol, and their combination significantly reduced the percentage of proliferating parathyroid cells. Proliferation was uncontrolled in the short-term protocol. A significant increase in CaSR mRNA (% vs. beta-actin) was detected in rats treated with both calcitriol (1.60 +/- 0.30) and AMG 641 (1.66 +/- 0.25) for 13 days (P = 0.01 vs. 5/6 Nx+vehicle, 0.89 +/- 0.09); and there was a further increase when both drugs were administered simultaneously (2.46 +/- 0.33). In the short-term protocol, only rats receiving AMG 641 alone (2.01 +/- 0.33, P < 0.001) showed increased expression of CaSR mRNA, whereas the combination (1.81 +/- 0.20) produced no additional benefit. AMG 641 also increased CaSR mRNA expression in vitro. Changes in VDR mRNA paralleled those of CaSR mRNA. In the long-term treatment, both AMG 641 (0.87 +/- 0.14) and calcitriol (0.99 +/- 0.12) increased VDR mRNA (P < 0.05 vs. 5/6 Nx+vehicle, 0.49 +/- 0.10), and the increase was more accentuated when the drugs were combined (1.49 +/- 0.45). In the short-term protocol, only treatment with AMG 641, alone (1.52 +/- 0.41) or combined with calcitriol (1.86 +/- 0.24), increased VDR mRNA. In conclusion, our results demonstrate an acute increase in CaSR mRNA and VDR mRNA in the parathyroid glands of uremic rats treated with AMG 641, in which cell proliferation was uncontrolled, thus supporting a direct effect of calcimimetics on CaSR and VDR expression by hyperplastic parathyroid cells.