Effects of weaning on phosphate transport maturation in the rat kidney. Clearance and brush border membrane studies

Proximal tubular phosphate reabsorption: molecular mechanisms

Renal proximal tubular reabsorption of P(i) is a key element in overall P(i) homeostasis, and it involves a secondary active P(i) transport mechanism. Among the molecularly identified sodium-phosphate (Na/P(i)) cotransport systems a brush-border membrane type IIa Na-P(i) cotransporter is the key player in proximal tubular P(i) reabsorption. Physiological and pathophysiological alterations in renal P(i) reabsorption are related to altered brush-border membrane expression/content of the type IIa Na-P(i) cotransporter. Complex membrane retrieval/insertion mechanisms are involved in modulating transporter content in the brush-border membrane. In a tissue culture model (OK cells) expressing intrinsically the type IIa Na-P(i) cotransporter, the cellular cascades involved in "physiological/pathophysiological" control of P(i) reabsorption have been explored. As this cell model offers a "proximal tubular" environment, it is useful for characterization (in heterologous expression studies) of the cellular/molecular requirements for transport regulation. Finally, the oocyte expression system has permitted a thorough characterization of the transport characteristics and of structure/function relationships. Thus the cloning of the type IIa Na-P(i )cotransporter (in 1993) provided the tools to study renal brush-border membrane Na-P(i) cotransport function/regulation at the cellular/molecular level as well as at the organ level and led to an understanding of cellular mechanisms involved in control of proximal tubular P(i) handling and, thus, of overall P(i) homeostasis.

Distribution of the sodium/phosphate transporter during postnatal ontogeny of the rat kidney

Renal phosphate reabsorption via the type II sodium/ phosphate cotransporter (NaPi-2) in the brush border membrane (BBM) of proximal tubules underlies alterations during aging. The ontogeny of NaPi-2 in kidneys from newborn to 6-wk-old rats was investigated. NaPi-2 protein distribution in the kidneys of neonatal, 13-d-old, 22-d-old, and 6-wk-old rats was immunohistochemically analyzed, and NaPi-2 mRNA distribution in neonatal and 6-wk-old rats was analyzed by in situ hybridization. In kidneys of newborn rats, the appearance of NaPi-2 protein and mRNA coincided with the development of the brush border (assessed by actin staining) on proximal tubular cells. NaPi-2 was not detectable in the nephrogenic zone or in the outgrowing straight sections of proximal tubules, which lack a brush border. In 13-d-old suckling rats, strong NaPi-2 staining was seen in the BBM of convoluted proximal tubules of all nephron generations. In contrast, in 22-d-old weaned rats, NaPi-2 staining in the BBM of superficial nephrons was weaker than that in the BBM of juxtamedullary nephrons. Western blotting demonstrated that the overall abundance of NaPi-2 protein in the BBM of 22-d-old rats was decreased to approximately 70% of that in 13-d-old rats. In kidneys of 6-wk-old rats, the internephron gradient for NaPi-2 abundance in the BBM corresponded to that in adult rats. The data suggest that the NaPi-2 system in the kidney is fully functional and possesses the capacity for regulation as soon as nephrogenesis is completed. The manifestation of NaPi-2 internephron heterogeneity immediately after weaning might be related to the change in dietary inorganic phosphate content.

Maturation of rat renal phosphate transport: effect of triiodothyronine

1. Prevention of weaning prevents the increase in the carrier affinity for Na(+)-cotransported phosphate (Pi) and the normal elevation of plasma 3,5,3'-triiodothyronine (T3) which occur between postnatal days 14 and 21. 2. This study examines the possible role of T3 in the control of the maturation process of Pi transport. Clearance experiments and brush-border membrane studies were performed on 14-day-old rats given T3. 3. The fractional excretion of Pi of T3-treated rats was 33% lower compared with controls (P < 0.01). After Pi perfusion, it remained at a lower level, and the amount of Pi reabsorbed per minute, corrected for the glomerular filtration rate (RPi/GFR), was higher. 4. The membrane vesicles from 14-day-old rats given T3 showed a 30% increase in carrier affinity for Na(+)-cotransported Pi. In addition to this maturational effect of T3, a 46% increase in Vmax was also observed. 5. The amount of immunoreactive Pi transporter, detected using anti-(NaPi-2) antiserum, was increased in T3-treated rats. 6. Glucose transport, another Na(+)-dependent transport system, was not altered by T3. 7. It is concluded that exogenous T3 given before the third postnatal week specifically induced precocious maturation of renal Pi transport in 14-day-old rats, suggesting that thyroid hormone is normally involved in this maturation.