MATURATION OF RAT RENAL PHOSPHATE-TRANSPORT - EFFECT OF TRIIODOTHYRONINE

1. Prevention of weaning prevents the increase in the carrier affinity for Na+-cotransported phosphate (P-i) and the normal elevation of plasma 3,5,3'-triiodothyronine (T-3) which occur between postnatal days 14 and 21. 2. This study examines the possible role of T-3 in the control of the maturation process of P-i transport. Clearance experiments and brush-border membrane studies were performed on 14-day-old rats given T-3. 3. The fractional excretion of P-i of T-3-treated rats was 33% lower compared with controls (P < 0.01). After P-i perfusion, it remained at a lower level, and the amount of P-i reabsorbed per minute, corrected for the glomerular filtration rate (R(P1)/GFR), was higher. 4. The membrane vesicles from 14-day-old rats given T-3 showed a 30% increase in carrier affinity for Na+-cotransported P-i. In addition to this maturational effect of T-3, a 46% increase in V-max was also observed. 5. The amount of immunoreactive P-i transporter, detected using anti-(NaPi-2) antiserum, was increased in T-3-treated rats. 6. Glucose transport, another Na+-dependent transport system, was not altered by T-3. 7. It is concluded that exogenous T-3 given before the third postnatal week specifically induced precocious maturation of renal P-i transport in 14-day-old rats, suggesting that thyroid hormone is normally involved in this maturation.