Changes in NMR-visible kidney cell phosphate with age and diet: relationship to phosphate transport

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