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Renal tubular sites of increased phosphate transport and NaPi-2 expression in the juvenile rat
C Woda1, S E Mulroney, N Halaihel
1Department of Physiology and Biophysics, Georgetown University School of Medicine, Washington, District of Columbia 20007, USA.
Insights
Juvenile rats exhibit enhanced renal phosphate reabsorption due to increased proximal tubule sodium-dependent phosphate transporter 2 (NaPi-2) expression. Dietary phosphate restriction further upregulates NaPi-2, boosting reabsorption in juvenile animals.
Area of Science:
- Nephrology
- Renal Physiology
- Mineral Metabolism
Background:
- Renal phosphate handling is crucial for mineral homeostasis.
- Phosphate reabsorption occurs primarily in the proximal tubules.
- Developmental differences in renal phosphate handling are not fully understood.
Purpose of the Study:
- To investigate the tubular sites and mechanisms of enhanced renal phosphate reabsorption in juvenile rats.
- To compare phosphate reabsorption in juvenile versus adult rats under varying dietary phosphate conditions and parathyroid hormone (PTH) levels.
- To elucidate the role of NaPi-2 transporter expression in these age-related differences.
Main Methods:
- Renal micropuncture experiments in acutely thyroparathyroidectomized juvenile and adult Wistar rats.
- Dietary manipulation with normal and low phosphate diets.
- Brush-border membrane (BBM) vesicle uptake studies.
- Western blot analysis for NaPi-2 protein expression.
- Immunofluorescence microscopy for NaPi-2 localization.
Main Results:
- Juvenile rats showed greater phosphate reabsorption in proximal convoluted (PCT) and straight tubules (PST) compared to adults, irrespective of PTH presence.
- Higher NaPi-2 protein abundance was observed in BBM vesicles from superficial and outer juxtamedullary cortices of juvenile rats.
- Dietary phosphate restriction in juveniles significantly increased phosphate reabsorption and NaPi-2 expression in the proximal tubule BBM.
Conclusions:
- Renal phosphate reabsorption in PCT and PST segments contributes to the attenuated PTH response in juvenile animals.
- Dietary phosphate restriction upregulates BBM NaPi-2 expression in juvenile rats, enhancing proximal tubular phosphate reabsorption.
- Age-dependent differences in NaPi-2 expression play a significant role in renal phosphate handling.
Abstract:
To determine the tubular sites and mechanisms involved in enhanced renal phosphate (P(i)) reabsorption seen in the juvenile animal, renal micropuncture experiments were performed in acutely thyroparathyroidectomized adult (>14 wk old) and juvenile (4 wk old) male Wistar rats fed either a normal P(i) diet (NPD, 0.6% P(i)) or low P(i) diet (0.07% P(i)) for 2 days, in the presence and absence of parathyroid hormone (PTH). P(i) reabsorption was greater in proximal convoluted (PCT) and straight tubules (PST) of the juvenile compared with adult rats fed NPD, whether or not PTH was present. These findings were consistent with a greater P(i) uptake in brush-border membrane (BBM) vesicles from both superficial (SC) and outer juxtamedullary (JMC) cortices of juvenile animals. Western blot analysis revealed a 2- and 1.8-fold higher amount of NaPi-2 protein in the SC and JMC, respectively, in juvenile rats. Immunofluorescence microscopy also indicated that NaPi-2 protein expression was present in the proximal tubule (PT) BBM to a greater extent in juvenile rats. Dietary P(i) restriction in juvenile rats resulted in a significant increase in P(i) reabsorption in the PCT and PST segments. NaPi-2 expression in the PT BBM was also increased, as was the expression of intracellular NaPi-2 protein. These studies indicate that P(i) reabsorption in both the PCT and PST segments of the renal tubule contributes to the attenuated response to PTH in the normal juvenile animal. In addition, dietary P(i) restriction in the juvenile rat upregulates BBM NaPi-2 expression, which is associated with a further increase in proximal tubular P(i) reabsorption.
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