EFFECT OF 3,5,3'-TRIIODOTHYRONINE ON MATURATION OF RAT RENAL PHOSPHATE-TRANSPORT - KINETIC CHARACTERISTICS AND PHOSPHATE TRANSPORTER MESSENGER-RIBONUCLEIC-ACID AND PROTEIN ABUNDANCE

The plasma concentration of T-3 increases during the third postnatal w eek in rats when the young are weaned. Renal phosphate (Pi) transport matures during this period, with increases in the carrier affinity for Na-cotransported Pi (1/K-m) and in the maximal transport rate (V-max. As prevention of weaning prevents the normal increase in the plasma T -3 concentration and blocks the maturation of Pi trans port, this stud y examines the influence of T-3 on Pi transport maturation. The brush border membranes from 21-day-old rats rendered hypothyroid with 6-n-pr opyl-2-thiouracil (PTU) or prevented from weaning (PW) were examined. These two treatments effectively prevented normal maturation, as the a pparent K-m was significantly higher in the PTU (0.243 +/- 0.04) and P W (0.159 +/- 0.008) rats than in controls [0.111 +/- 0.032 (n = 6; P < 0.05) and 0.104 +/- 0.005 (n = 6; P < 0.05)], and the V-max was signi ficantly lower [3192 +/- 170 us. 4269 +/- 343 (n = 6; P < 0.01) and 34 79 +/- 478 us. 4184 +/- 368 (n = 6; P < 0.05)]. Administration of T-3 to PTU and PW rats restored the apparent K-m and V-max, to central val ues. The amounts of Na-Pi cotransporter messenger RNA and protein were also determined. The amounts of NaPi-2 messenger RNA and protein were decreased about 5-fold in PTU and PW rats, and T-3 restored them to c ontrol levels. The effect of T-3 on the maturation of Pi transport was independent of PTH, GH, or glucocorticoids. Thus, thyroid hormone hel ps control the maturation of Pi transport in the rat during weaning, a nd thyroid hormone magi be a dominant triggering factor in this proces s.