DIRECT EFFECT OF PHOSPHORUS ON PTH SECRETION FROM WHOLE RAT PARATHYROID-GLANDS IN-VITRO

Phosphorus retention is an important factor in the development of hype rparathyroidism secondary to renal failure. In vivo manipulation of ph osphorus is associated with changes in serum calcium and calcitriol le vels which in turn can modify parathyroid hormone synthesis and secret ion. The present in vitro study evaluates whether high extracellular p hosphorus has a direct effect on parathyroid hormone secretion, Fresh rat parathyroid glands were incubated in a media with phosphorus conce ntrations of 1, 2, 3, and 4 mM and subsequently exposed to calcium lev els ranging from 0.4 to 1.35 mM. In 1.25 mM calcium, the parathyroid h ormone secretion rate was similar in 1 and 2 mM phosphorus; however, a phosphorus concentration of 3 and 4 mM produced a 3- and 4-fold incre ase in the parathyroid hormone secretion, respectively, as compared wi th 1 mM phosphorus, While in 1 or 2 mM phosphorus an increase in calci um from 0.6 to 1.35 mM reduced parathyroid hormone secretion to 37%, i n 4 mM phosphorus the same increase in calcium only inhibited parathyr oid hormone secretion to 75%. Furthermore, the addition of arachidonic acid 20 mu M, a substrate for inhibitory intracellular signal pathway , to the 4 mM phosphorus-1.35 mM calcium incubation media reduced the parathyroid hormone secretion to 34.5% (p < 0.05), In conclusion, our results indicate that in vitro, high phosphorus directly increases par athyroid hormone secretion.