Acid-base maneuvers and phosphate transport in the isolated rat kidney

The influence of bicarbonate (HCO3-), the carbon dioxide tension (PCO2), and pH on phosphate (Pi) excretion were assessed in isolated rat kidneys, perfused in vitro with recirculating synthetic solutions. After establishing control values, the perfusate HCO3- or PCO2 was altered separately. When the perfusate pH was decreased, either by increasing PCO2 or decreasing HCO3-, the absolute and fractional Pi excretions increased. Perfusate alkalinization by slightly decreasing the PCO2 did not affect Pi excretion, but increasing the perfusate pH with addition of HCO3- elicited phosphaturia. Other kidneys were perfused with a solution from which the CO2/HCO3- buffer system was nominally absent. Alkalinization of HCO3--free perfusate had no effect upon Pi excretion, but acidification resulted in marked phosphaturia. Perfusate acidification, whether achieved by decreasing HCO3-, increasing PCO2, or by adding hydrogen ions, uniformly elicited phosphaturia. The data indicate that either decreasing the extracellular pH or increasing the extracellular HCO3- inhibits renal Pi reabsorption.