Evidence for induction of a phosphate appetite in juvenile rats

This study examined whether dietary phosphate (P(i)) restriction stimulates an appetite for P(i) in the juvenile rat, which normally has a high metabolic P(i) demand for growth. Juvenile Wistar rats were placed in individual cages with unrestricted access to tap water and a low (LPD, 0.02% P(i)) or normal P(i) diet (NPD, 0.6% P(i)) for 7 days. On day 8, both groups of rats were given unlimited access to a solution of 0.3 M potassium phosphate water (P(i)H₂O) for 8 additional days. Rats fed LPD consumed 70- 100% more P(i)H₂O then those rats fed NPD (P < 0.001). The increase in P(i)H₂O intake resulted in a marked rise in the growth rate of rats fed LPD during days 8-15. A similar P(i) intake was inducible after only 2 days of LPD and was associated with significant reductions in both plasma and cerebrospinal fluid (CSF) P(i) levels; these levels remained low throughout P(i) restriction, despite a significant P(i)H₂O intake. Furthermore, the renal adaptation to enhance P(i) reabsorption (T(m)P(i)) during P(i) deprivation remained elevated despite enhanced P(i)H₂O intake. Replenishment with a high-P(i) diet rapidly quenched the P(i)H₂O appetite and was associated with restoration of both plasma and CSF P(i) levels. These findings suggest that an appetite for P(i) can be induced in juvenile rats, perhaps through lowered plasma and CSF P(i) levels. This behavioral response may serve as an additional mechanism to maintain an adequate supply of P(i) necessary for growth and development of the animal.