Abstract
The present studies were performed in an effort to examine the characteristics of the control system governing phosphate excretion in uremic man. In a group of patients with glomerular filtration rates (GFR) ranging from normal to 2 ml/min, it was found that the lower the GFR the lower the fraction of filtered phosphate reabsorbed (TRP). On a fixed phosphate intake, phosphate excretion rate was the same in patients with GFRs ranging from 60 to 3 ml/min. When plasma phosphate concentrations were diminished to subnormal levels in hyperphosphatemic, hypocalcemic uremic patients, TRP values increased but did not return to normal. TRP failed to rise substantially when GFR, as well as plasma phosphate concentrations, were diminished. In patients with unilateral renal disease, TRP values were equal bilaterally, and values were substantially higher in the diseased kidneys than in patients with bilateral involvement. When plasma calcium concentrations were raised to normal for 2-3 wk in uremic patients in whom plasma phosphate concentrations had previously been lowered to subnormal levels, TRP values rose to an average value of 86%. Values remained in the normal range when phosphate concentrations were allowed to increase while normocalcemia was maintained. The data are interpreted to indicate that in advancing renal disease, the changing patterns of phosphate excretion are mediated by a control system in which parathyroid hormone serves as a major effector element. An increase in GFR per nephron, hyperphosphatemia, and intrinsic inability of the surviving nephrons to transport phosphate do not appear to be of primary importance in the progressive reduction in TRP.