EFFECT OF SERUM PHOSPHATE ON PARATHYROID-HORMONE SECRETION DURING HEMODIALYSIS

Background. Recent studies have demonstrated that a high concentration of phosphate directly stimulates parathyroid hormone (PTH) secretion. High serum levels of phosphate are usually observed in patients with end-stage renal disease. The aim of the present study was to evaluate whether serum phosphate concentration had an acute effect on PTH secre tion in hemodialysis patients. The levels of serum phosphate were mani pulated during the hemodialysis session by using a phosphate free dial ysate or a dialysate with a high content of phosphate. Methods. Ten st able hemodialysis patients with PTH values above 300 pg/ml were includ ed in the study. A PTH-calcium curve was obtained during both high pho sphate and phosphate free hemodialysis. Results. The serum phosphate c oncentration remained high (2.17 +/- 0.18 mM) throughout the high phos phate hemodialysis and decreased progressively to normal levels (1.02 +/- 0.06 mM) during the phosphate free hemodialysis. The serum PTH lev els at maximal inhibition by hypercalcemia (minimal PTH) were greater during the high phosphate than the phosphate free hemodialysis (413 +/ - 79 vs. 318 +/- 76 pg/ml, P < 0.003). In all patients the values of m inimum PTH were greater during the high phosphorus than the phosphorus free hemodialysis. The Values of maximally stimulated PTH during hypo calcemia and the set point of the PTH-calcium curve were similar durin g the high phosphate and the phosphate free hemodialysis. Conclusion. The maintenance of high serum phosphorus levels during hemodialysis pr evented, in part, the inhibition of PTH patients high serum phosphate contributes directly to the elevation of PTH levels despite normal or high serum calcium concentration.