Background: Phosphate retention has long been considered to be of importance for the pathogenesis of secondary hyperparathyroidism in chronic renal failure. Hyperphosphatemia in vivo is associated with alterations of calcium and vitamin D levels, both of which are known to alter the parathyroid hormone (PTH) release independently.
Materials and methods: We have investigated the direct effect of phosphate on PTH release in vitro using two different preparations of bovine parathyroid tissue: Acutely dispersed bovine parathyroid cells and tissue slices of 0.5 x 0.5 mm were prepared from bovine parathyroid glands. Parathyroid dispersed cells and tissue slices were incubated for 4 h in media containing normal phosphate (1.0 mM) or high phosphate (3.5 mM).
Results: High phosphate induced a significant (P < 0.01) increase in PTH release in the preparation of tissue slices, but not in preparations of dispersed cells. The 4 h incubation in high phosphate medium did not change the set-point for calcium. Bovine parathyroid tissue slices incubated in increasing phosphate concentrations from 1.0 to 3.5 mM and with a fixed calcium concentration of either 0.8, 1.2 or 1.8 mM responded with a dose dependent stimulation of PTH release. The degree of stimulation of release by high phosphate (3.5 mM), was significantly (P < 0.05) higher at low calcium levels (0.8 mM), 172% above baseline value (1.0 mM phosphate) as compared to high calcium levels (1.8 mM), 139% above baseline values.
Conclusions: This study shows that phosphate directly stimulates the PTH release in bovine parathyroid glands, and that this effect is only seen in preparations of parathyroid tissue slices and not in preparations of dispersed cells. This indicated that maintenance of near normal architecture of the parathyroid glands is essential in order to elicit the effect of high phosphate on the PTH release.