Renal adaptation to changes in dietary phosphate during development.

The present study tested the hypothesis that the influence of dietary phosphate (Pi) on the renal handling of Pi changes during development. We evaluated whether variations in the dietary Pi content would alter the tubular capacity of Pi reabsorption [Max RPi/glomerular filtration rate (GFR)] in immature rats, which have a relatively greater Max RPi/GFR compared with adult rats. Then we examined the response of immature and adult Pi-deprived rats to dietary Pi replenishment. Studies were performed in acutely thyroparathyroidectomized Wistar rats at three different stages of development: immature (3- to 4-wk old), young (6- to 7-wk old), and adult (12- to 13-wk old). Animals were fed either low (0.07%)-, normal (0.7%), or high (1.8%)-phosphate diet (LPD, NPD, and HPD, respectively) for 4 days and were then prepared for renal clearance experiments to determine the Max RPi/GFR. On all dietary regimens, the Max RPi/GFR was highest in immature rats and decreased progressively with age. When fed LPD, immature rats, with an already elevated rate of phosphate transport, displayed a remarkable 68 +/- 13% increase in the Max RPi/GFR (from 5.58 +/- 0.29 to 9.47 +/- 0.76 mumol/ml, P less than 0.01). This was significantly greater than the 38 +/- 3% increase observed in adult rats (from 3.50 +/- 0.18 to 4.81 +/- 0.09 mumol/ml). Conversely, in response to HPD, the decrease in the Max RPi/GFR was smallest in immature rats (-42 +/- 2%) compared with the decrement in either young (-54 +/- 3%) or adult (-61 +/- 6%) rats.(ABSTRACT TRUNCATED AT 250 WORDS)