Transport of phosphate, D-glucose, and L-valine in newborn rat kidney brush border

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.

Characterization of the fetal glucose transporter in rabbit kidney. Comparison with the adult brush border electrogenic Na+-glucose symporter

Glucose transport was characterized in rabbit renal brush border membrane vesicles (BBMV) of the fetus late in gestation. Highly purified, osmotically reactive fetal BBMV contained a glucose transporter that was qualitatively indistinguishable from that in the adult: both are concentrative, Na+ dependent, electrogenic, stereospecific, and sensitive to phlorizin. Although the apparent Km for glucose is similar in the fetus and adult, the Vmax is significantly higher in the adult. When the membrane potential was clamped with a protonophore, this difference diminished; however, Vmax remained significantly higher in adult BBMV. This postnatal increase in Vmax was paralleled by a similar increase in the number of phlorizin binding sites. These findings indicate that the maturational increase in glucose transport is, in part, consequent to a more favorable electrical potential for Na+-dependent glucose transport and, in part, the result of the insertion of new transporters. The homogenate activity of several brush border enzymes also demonstrated significant maturational increases. The magnitude of these changes was variable and enzyme dependent. These combined observations suggest that mature expression of membrane proteins (transporters and enzymes) occurs at different stages of development of renal proximal tubule cells.