Renal response to alterations in dietary phosphate in the young beagle

Hyperphosphatemia is present in the young of many species compared to adult animals. We studied the renal response to alterations in dietary phosphate in 4- to 7-day-old beagle puppies. Puppies were phosphate restricted with oral aluminum hydroxide gel (Group I) or phosphate loaded with oral sodium phosphate (Group II). Puppies in both groups had hyperphosphatemia (Group I 8.6, Group II 12.2 mg/dl) relative to dogs (3.4--4.8 mg/dl). Plasma phosphate concentration (P) and rate of excretion of phosphate (Ep) were independent of glomerular filtration rate (GFR). At any filtered load of phosphate Ep was greater and rate of reabsorption less in Group II independent of P. The maximal rate of phosphate reabsorption (TmP) was higher in Group I (100 micrograms/ml GFR) than Group II (26 micrograms/ml GFR). Negative values for phosphate reabsorption in 4 animals support the presence of a secretory mechanism for phosphate. Relative hyperphosphatemia in the puppy is not secondary to a renal limitation in phosphate excretion. The 1- to 2-week-old puppy adjusts renal reabsorption of phosphate to maintain a concentration of phosphate in plasma higher than in the adult animal. The renal alterations in response to variations in dietary phosphate occur independently of plasma phosphate concentration, GFR, or filtered load of phosphate.