Effect of differing concentrations of parathyroid hormone on rat renal electrolyte excretion

Urinary clearance studies were performed on acutely thyroparathyroidectomized rats to study the effect of a range of parathyroid hormone (PTH) concentrations on renal electrolyte transport. PTH 0.1 U, prime and per hour, significantly increased the plasma calcium concentration, yet fractional calcium excretion was increased by 47.5% and inorganic phosphate excretion remained unaltered. PTH 1 U produced a similar increase in the plasma calcium concentration. However, urinary calcium excretion was unchanged when compared with control animals and a small phosphaturia occurred (from 0.1 +/- 0.1 to 0.8 +/- 0.2%; P less than 0.05). PTH 10 U decreased fractional calcium excretion by 50% and increased fractional phosphate excretion from 0.4 +/- 02. to 19.4 +/- 1.7% (P less than 0.01). Changes in urinary magnesium excretion were similar to those of calcium, being increased with the lowest PTH concentration yet decreased with a 100-fold increase in PTH concentration. These data suggest that PTH, at a low physiological concentration that acts on bone, does not directly alter renal electrolyte handling. High physiological concentrations of PTH, however, produce a brisk phosphaturia and reduce calcium and magnesium excretion. A reappraisal of the accepted role of PTH on renal calcium conservation is therefore required.