NMR-STUDIES OF PHOSPHATE-METABOLISM IN THE ISOLATED PERFUSED KIDNEY OF DEVELOPING RATS

During growth, the capacity for renal phosphate (Pi) reabsorption varies as a function of Pi demand. These changes occur in the absence of changes in extracellular concentration of Pi and are also observed in renal cells cultured in defined media. These findings suggest a direct regulatory effect of intracellular Pi on its transport systems. We postulate that a low intracellular Pi concentration ([Pi]i) occurs in the developing kidney as a consequence of differences in Pi metabolism between growing and mature cells and that a low [Pi]i, in turn, leads to adaptive changes in renal Pi transport. In order to assess this hypothesis, we used31P-nuclear magnetic resonance (NMR) to measure the intracellular concentrations of NMR-visible Pi and phospho-metabolites and the rates of Pi turnover due to adenosine triphosphate (ATP) synthesis, in isolated perfused kidneys of 3- to 4-week-old and 12- to 13-week-old rats. The [Pi]i was lower (1.7±0.1 vs 2.7±0.1 mM, P<0.05) in kidneys of growing than of adult rats, while the ATP (2.9±0.3 vs 2.8±0.5 mM) and adenosine diphosphate (ADP)(-0.2 mM) concentrations were similar at the two ages, consistent with a high phosphorylation potential in the kidneys of the younger animals. Radiofrequency irradiation of the γ-P of ATP resulted in reduction in the intensity of the Pi resonance of 62±5% in the newborn and 38±3% in the adult (P<0.05). The corresponding 1.6-fold higher fractional turnover rate of the Pi pool in the younger than in the older rats accounts for the similar rates of ATP synthesis at the two ages (30±7 vs 35±4 μmol/min per g, P>0.3), despite the smaller intracellular Pi pool present in the younger than in the older animals. The low [Pi]i may stimulate the synthesis of 1,25 hydroxivitamin D3 and the expression of Pi transport related proteins. The high phosphorilation potential drives the ATP flux necessary for growth related transport and biosynthetic processes. © 1990 IPNA.