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 pla sma 3,5,3'-triiodothyronine (T-3) which occur between postnatal days 1 4 and 21. 2. This study examines the possible role of T-3 in the contr ol of the maturation process of P-i transport. Clearance experiments a nd brush-border membrane studies were performed on 14-day-old rats giv en 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 rema ined at a lower level, and the amount of P-i reabsorbed per minute, co rrected 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% inc rease in carrier affinity for Na+-cotransported P-i. In addition to th is maturational effect of T-3, a 46% increase in V-max was also observ ed. 5. The amount of immunoreactive P-i transporter, detected using an ti-(NaPi-2) antiserum, was increased in T-3-treated rats. 6. Glucose t ransport, another Na+-dependent transport system, was not altered by T -3. 7. It is concluded that exogenous T-3 given before the third postn atal week specifically induced precocious maturation of renal P-i tran sport in 14-day-old rats, suggesting that thyroid hormone is normally involved in this maturation.