Parathormone as a mediator of inorganic phosphate diuresis during saline infusion in the rat

Summary Normal rats were infused with isotonic saline at 0.50 ml/min for 2 hours in order to expand their extracellular fluid volume. Under these conditions fractional excretion of inorganic phosphate was found to be as high as 38.8±3.0% of filtered phosphate, while fractional sodium excretion was 12.9±0.7% of filtered sodium. The combined addition of calcium and magnesium to the infusion solution decreased inorganic phosphate excretion significantly (P<0.001) to 11.2±3.6% (presumably by inhibiting parathyroid gland activity), while sodium excretion was unchanged (13.5±1.1%).Parathyroidectomized rats were infused with isotonic saline at 0.50 ml/min to achieve a similar extent of extracellular fluid volume expansion as in the normal rats. In these animals inorganic phosphate excretion was as low as 0.9±0.9% of filtered phosphate, while sodium excretion was 11.8±2.2% of filtered sodium. Administration of parathormone to volume expanded parathyroidectomized rats resulted in marked increases or inorganic phosphate excretion to 41.5±3.1% of filtered phosphate (P<0.001), while sodium excretion remained unaltered (12.0±2.8% of filtered sodium), thus resembling very closely the results in normal volume expanded rats.From these results it is concluded, that saline induced phosphaturia in normal rats is mediated primarily by parathormone. Furthermore, sodium excretion during volume expansion of extracellular fluid appears to be independent of inorganic phosphate excretion and independent of changes in parathyroid activity.This work was supported in part by a grant from the Deutsche Forschungsgemeinschaft.