Bicarbonate-induced phosphaturia: Dependence upon the magnitude of phosphate reabsorption

The acute effect of bicarbonate infusion on inorganic phsophate (Pi) reabsorption was determined in thyroparathyroidectomized (TPTX) rats stabilized on a high phsophorus diet. In the normophosphatemic state, bicarbonate did not affect Pi reabsorption significantly. On the other hand, after hyperphosphatemia was induced by phosphate loading, bicarbonate inhibited Pi reabsorption. In order to determin if eht aboslute level of plasma or filtered Pi was an improtant determinant of this phosphaturic action of bicarbonate during hyperphosphatemia, equivalent rates of Pi reabsorption were achieved after phosphate infusion in TPTX rats previously stabilized on a low dietary phosphorus intake, but at significantly lower plasma Pi concentrations and filtered Pi loads than in their high dietary phosphorus counterparts. Bicarbonate inhibited Pi reabsorption equivalently in both hyperphosphatemic groups, indicating that the magnitude of Pi transport itself was of importance in determining the inhibitory response to bicarbonate. In addition, the results suggest that transport mechanisms for bicarbonate and Pi may interact competitively.