THE EFFECT OF A HIGH PHOSPHORUS DIET ON THE PARATHYROID CELL-CYCLE

High extracellular phosphorus directly increases parathyroid hormone ( PTH) secretion and gene transcription. The present study evaluates the effect of high phosphorus diet on the parathyroid cell cycle in rats with normal renal function. Rats were divided into two groups, receivi ng either a high phosphorus diet (HPD, P=1.2%) or a normal phosphorus diet (NPD, P=0.6%). The dietary calcium content was 0.6% in both diets . Rats were pair fed and sacrified on days 0, 1, 5, 10 and 15 after in itiation of the diet. The parathyroid glands were removed and parathyr oid cells dispersed for evaluation of cell cycle and apoptosis by flow cytometry. Serum calcium, phosphorus, PTH and calcitriol were measure d. As compared with NPD, the ingestion of a HPD resulted in an increas ed number of cells in the S phase of the cell cycle from day 1 to 10 ( 1.2+/-0.09% vs 0.6+/-0.04% for day 1, 1.2+/-0.11% vs 0.6+/-0.06% for d ay 5 and 1.0+/-0.09% vs 0.5+/-0.04% for day 10, P<0.01). By day 15, th e percentage of cells in the S phase in NPD and HPD were not different . In the rats fed the HPD, serum PTH increased significantly from day 5 through 15 (P<0.01). Parathyroid cell apoptosis was minimal and unaf fected by the diet. At day 15, the parathyroid gland size in HPD was i ncreased by 27% as compared with NPD (P<0.05). This increase should be attributed to cell proliferation since parathyroid cell size remained unchanged. Serum calcitriol and calcium were not significantly differ ent in the two groups. In HPD, an increase in serum phosphorus was obs erved only on day 1. The results show that an HPD results in the stimu lation of the parathyroid cell cycle independently of changes in calci um and calcitriol.