Phosphate deprivation and acute respiratory alkalosis both stimulate phosphate conservation by the kidney. To evaluate the response of the superficial proximal tubule to increases in the filtered load of phosphate (Pi), micropuncture studies were performed in these two models of Pi conservation, rats fed a low-phosphate diet (0.07% Pi), and rats with acute respiratory alkalosis (RA). All animals were acutely thyroparathyroidectomized. After basal collections, the animals were infused with Pi at a rate of 4 microM/min and then the samples recollected 30 min after the initiation of the Pi infusion. Infusion of phosphate increased the filtered load of phosphate (FLPi) of single nephrons from 112 +/- 13 to 266 +/- 44 pmol/min in phosphate-derived rats and from 123 +/- 21 to 217 +/- 44 pmol/min in respiratory alkalotic rats. However, absolute reabsorption of phosphate (ARPi) increased only in phosphate deprivation (94 +/- 16 to 126 +/- 25 pmol/min). These results indicate that the superficial proximal convoluted tubule responds to phosphate infusion by increasing reabsorption in the phosphate-deprived model, but not in the respiratory alkalotic model, of phosphate conservation.