A DIRECT EFFECT IN-VITRO OF PHOSPHATE ON PTH RELEASE FROM BOVINE PARATHYROID TISSUE-SLICES BUT NOT FROM DISPERSED PARATHYROID CELLS

Background. Phosphate retention has long been considered to be of impo rtance for the pathogenesis of secondary hyperparathyroidism in chroni c renal failure. Hyperphosphatemia in vivo is associated with alterati ons of calcium and vitamin D levels, both of which are known to alter the parathyroid hormone (PTH) release independently. Materials and met hods. We have investigated the direct effect of phosphate on PTH relea se in vitro using two different preparations of bovine parathyroid tis sue: 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 cont aining 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 disper sed 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 f ixed calcium concentration of either 0.8, 1.2 or 1.8 mM responded with a dose dependent stimulation of PTH release. The degree of stimulatio n of PTH release by high phosphate (3.5 mM), was significantly (P<0.05 ) higher at low calcium levels (0.8 mM), 172% above baseline values (1 .0 mM phosphate) as compared to high calcium levels (1.8 mM), 139% abo ve baseline values. Conclusions. This study shows that phosphate direc tly 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 indicates that mainte nance of near normal architecture of the parathyroid glands is essenti al in order to elicit the effect of high phosphate on the PTH release.