Adaptation of deep and superficial nephrons to changes in dietary phosphate intake

Dietary phosphate intake is now recognized to be a primary regulator of renal phosphate excretion. However, the nephron sites involved in the adaptation to changes in dietary phosphate are unclear. We tested the hypothesis that deep and superficial nephrons respond differently to changes in dietary phosphate by comparing fractional phosphate delivery (FDP%) from proximal tubules of both nephron populations. Because phosphate reabsorption is not detectable in the ascending loop of Henle, FDP% to the superficial early distal tubule and papillary loop of Henle reflect delivery from superficial and deep nephron proximal tubules, respectively. Micropuncture experiments were performed in 17 acutely TPTX rats fed either a low (0.07%) or a high (1.8%) phosphate diet for 4 days prior to the experiment. In low phosphate diet, fractional phosphate excretion was 0.93 +/- 0.26%. FDP% was 7.5 +/- 0.5 and 9.1 +/- 2.2% from superficial and deep nephron proximal tubules, respectively (P greater than 0.05). In high phosphate diet, fractional phosphate excretion was 29.6 +/- 5.0%. FDP% was significantly greater from superficial proximal tubules, 33.9 +/- 4.6%, compared with that from deep nephron proximal tubules, 14.0 +/- 2.7% (P less than 0.05). We conclude that significant adaptation of phosphate reabsorption in response to changes in dietary phosphate intake occurs in the superficial but not in the deep nephron proximal tubule in acutely TPTX volume-expanded rats. In addition, the presence of distal phosphate reabsorption was not evident in high phosphate diet but must occur in low phosphate diet.