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  • Title: Changes in renal phosphate reabsorption in the aged rat.
    Author: Mulroney SE, Woda C, Haramati A.
    Journal: Proc Soc Exp Biol Med; 1998 May; 218(1):62-7. PubMed ID: 9572153.
    Abstract:
    Depletion of inorganic phosphate (Pi) reserves occurs frequently in aged animals and can result in diminished bone mineralization and osteoporosis. This altered Pi balance results from a reduction in intestinal Pi absorption and an elevation in renal Pi excretion. Since the kidney plays a central role in maintaining Pi homeostasis, we tested whether the increased phosphaturia seen with aging is a consequence of changes in the intrinsic tubular capacity to reabsorb Pi (TmPi). Male Wistar rats (12-, 18-, and 24-months-old) were acutely thyroparathyroidectomized (TPTX) and prepared for renal clearance studies in the presence and absence of fixed levels of parathyroid hormone (synthetic PTH-(1-34), 1 U/kg/min). The maximum capacity for Pi transport (TmPi) was assessed by infusion of Pi at progressively higher rates (0-6 micromol/min) to increase the filtered load of Pi and facilitate the determination of the TmPi. TmPi declined significantly with age (3.51 +/- 0.12 vs 3.04 +/- 0.19 vs 2.30 +/- 0.18 micromol/ml, for 12-, 18-, and 24-month-old rats, respectively, P < 0.05) in TPTX rats. Administration of PTH markedly reduced the TmPi in all age groups. Although the TmPi attained was similar among the age groups (1.15 +/- 0.13 vs 1.15 +/- 0.06 vs 1.03 +/- 0.09 micromol/ml, for 12-, 18-, and 24-month-old rats, respectively), the magnitude of the reduction in the presence of PTH declined from 67% in 12-month-old rats to 62% and 55% in 18- and 24-month-old rats, respectively. These results demonstrate that aging is associated with a PTH-independent decrease in the intrinsic capacity of the kidney to reabsorb phosphate. Further, the kidney of the aged rat can respond to a pharmacological dose of PTH with appropriate reductions in the TmPi although the magnitude of the response declines with age.
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