CHANGES IN NMR-VISIBLE KIDNEY-CELL PHOSPHATE WITH AGE AND DIET - RELATIONSHIP TO PHOSPHATE-TRANSPORT

To test the hypothesis that growth and dietary P(i) affect the intracellular concentration of P(i) ([P(i)]i), as well as its renal reabsorption, we measured nuclear magnetic resonance (NMR)-visible [P(i)]i in isolated perfused kidneys of < 1- and > 4-wk-old guinea pigs fed various amounts of P(i) reabsorption (FRP(i)) in vivo and in capacity (V(max)) for Na+-P(i) cotransport in microvilli derived from animals of similar age and fed the same diets. In animals fed normal (0.76% P(i)) diet, [P(i)]i was lower (0.91 +/- 0.14 vs. 1.85 +/- 0.23 mM, P < 0.05), whereas FRP(i) was higher (0.90 +/- 0.02 vs. 0.70 +/- 0.03, P < 0.01) in < 1- than in > 4-wk-old guinea pigs. P(i) deprivation decreased [P(i)]i in mature animals to 0.74 +/- 0.29 mM, P < 0.05, and increased FRP(i) to 0.99 +/- 0.01. Excess dietary P(i) increased [P(i)]i in immature animals to 1.67 +/- 0.56 mM, P < 0.05, and decreased FRP(i) to 0.55 +/- 0.03. Diet-induced changes in [P(i)]i were associated with reciprocal changes in V(max) of similar absolute magnitude in immature and mature animals. However, diets that resulted in comparable [P(i)]i at the two ages were associated with higher (P < 0.05) V(max) in < 1- than in > 4-wk-old animals. The reciprocal nature of the relationship between [P(i)]i and renal P(i) transport indicates that [P(i)]i is primarily determined by P(i) efflux from the cells or P(i) organification rather than P(i) influx through Na+-P(i) cotransport. Findings indicate that changes in [P(i)]i with growth or diet may be a cause but cannot be the consequence of changes in abundance or maximal mobility of Na+-P(i) cotransporters. Data also indicate that factors in addition to low [P(i)]i contribute to the high Na+-P(i) cotransport capacity observed in renal microvilli of growing animals.