CELLULAR MECHANISM OF RENAL PHOSPHATE (Pi) TRANSPORT BY THE NEWBORN

Abstract

We have provided evidence that the enhanced renal reabsorption of Pi in growing animals is due to a large extent to an increase in the fractional reabsorption of Pi by the proximal tubule. This prompted us to explore the cellular mechanism responsible for this phenomenon. Vesicles of proximal tubule brush border membranes obtained from guinea pigs of either 1-7 or > 30 days of age were used to assess the kinetics of the Na⁺ -Pi cotransporter. No significant differences were found between these two age groups either in the Km (0.61 vs 0.75 mM, p > .4) or in the Vmax (1282 vs 1960 pmole/mg protein at 12 sec, p >.2). The Ki, measured in the presence of arsenate in the incubation media, was also found to be similar (3.0 vs. 2.3 mM, p > .2). To assess the Pi concentration gradient across the luminal membrane, we measured intracellular concentration of Pi by spectrophotometry (SP) and by nuclear magnetic resonance (NMR) and found it to be ≃ 3-fold lower in the newborn than in the adult. The exchangeable Pi, measured by NMR, was found to represent only ≃ 25% of the total intracellular Pi, measured by SP (.25 ±.6 vs 1.0±0.10 mM, p <.001 in the newborn and .60 ± .07 vs 2.06 ± .09 mM in the adult, p<.005). In addition, the Na⁺-independent uptake of Pi by brush border microvesicles was significantly higher in the newborn than in the adult (p<0.05) at concentrations of Pi in the media varying between 0.1 and 4.0 mM. We surmise that, contrary to previous assumptions, a steep downhill Pi concentration gradient exists across the luminal membrane of the proximal tubule and that passive diffusion along this concentration gradient may contribute to the enhanced renal reabsorption of Pi observed in growing animals.

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