Lack of influence of 24,25-dihydroxyvitamin D3 on parathyroid hormone secretion from normal or hyperplastic glands

The role of 24,25(OH)2D3 on parathyroid gland function remains controversial. The present studies were performed in vitro using (a) dispersed normal bovine parathyroid cells (bPTC) and (b) dispersed canine PTC (cPTC) prepared from glands of normal dogs, dogs with chronic renal failure (CRF), and dogs with CRF treated with 24,25(OH)2D3, 2.5 micrograms orally every day for more than 6 months. Bovine parathyroid cells were incubated for up to 180 min at 0.5, 1.0, and 3.0 mM external calcium in the presence or absence of 24,25(OH)2D3 (100 or 1000 nM). Similar experiments were conducted with cells incubated for 24 h in the presence of either the ethanol vehicle or 24,25(OH)2D3 (1000 nM). Parathyroid hormone secretion, measured in the supernatant by both C-terminal and N-terminal assays, did not show any differences between control and experimental groups at any time interval. Canine parathyroid cells obtained from uremic animals showed an average threefold increase in the total amount of PTH secreted, on a per cell basis over 180 min at 0.5 mM Ca2+, when compared with normal controls. However, there was no significant difference in PTH secretion at any level of calcium concentration between the cells obtained from parathyroid glands of CRF dogs and 24,25(OH)2D3-treated CRF dogs. Acute exposure to 24,25(OH)2D3 (1000 nM) in vitro of the cells obtained from the glands of CRF dogs also had no effect on PTH secretion. We conclude that 24,25(OH)2D3 has no direct effect on PTH secretion from dispersed parathyroid cells of either normal or uremic animals.