Proliferation in hyperplastic human and normal rat parathyroid glands: role of phosphate, calcitriol, and gender.

Background: Parathyroid gland hyperplasia develops in azotemic patients. A phosphate excess and calcitriol deficiency play critical roles in its development. Our goals were to determine whether differences in serum phosphate values at parathyroidectomy (PTX) in hemodialysis patients with refractory hyperparathyroidism: (1) correlated with parathyroid cell proliferation; and (2) affected the antiproliferative response to in vitro calcitriol. Studies were also performed to determine whether the phosphate concentration in the medium affected the antiproliferative response to calcitriol, and whether a high phosphate diet and calcitriol treatment affected parathyroid cell proliferation and parathyroid hormone (PTH) levels in normal rats.

Methods: Forty-seven parathyroid glands from 19 hemodialysis patients were obtained at PTX. Flow cytometry was used to determine cell proliferation (percent cells in S phase) in excised parathyroid glands. Similarly, cell proliferation was determined in parathyroid tissue incubated for 24 hours in medium with or without 10(-7) mol/L calcitriol and with 1 or 4 mmol/L phosphate. In normal rats, the effect of 3 days of a high phosphate diet (1.2% P) and calcitriol treatment (100 pmol/kg) on PTH values and cell proliferation was evaluated.

Results: In cells from freshly removed parathyroid glands obtained at PTX from hemodialysis patients, there were no significant correlations between the percent cells in S phase and age, gender, and serum phosphate, calcium, and PTH. While incubation of parathyroid tissue with 10(-7) mol/L calcitriol did reduce cell proliferation (P < 0.001), both the pre-PTX serum phosphate value (P= 0.003) and female gender (P=0.003) were associated with a decreased response to calcitriol. Incubation of parathyroid tissue in medium containing 4 mmol/L phosphate did not increase cell proliferation. In normal rats, a high phosphate diet for 3 days increased cell proliferation (P < 0.05) and PTH levels (P < 0.05), and calcitriol treatment was without effect.

Conclusion: Our findings suggest a high phosphate burden, as well as female gender, favor parathyroid cell proliferation and both may reduce the inhibition of parathyroid function by calcitriol.