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 T3 increases during the third postnatal week 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 (Vmax). As prevention of weaning prevents the normal increase in the plasma T3 concentration and blocks the maturation of Pi transport, this study examines the influence of T3 on Pi transport maturation. The brush border membranes from 21-day-old rats rendered hypothyroid with 6-n-propyl-2-thiouracil (PTU) or prevented from weaning (PW) were examined. These two treatments effectively prevented normal maturation, as the apparent K(m) was significantly higher in the PTU (0.243 +/- 0.04) and PW (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 Vmax was significantly lower [3192 +/- 170 vs. 4269 +/- 343 (n = 6; P < 0.01) and 3479 +/- 478 vs. 4184 +/- 368 (n = 6; P < 0.05)]. Administration of T3 to PTU and PW rats restored the apparent K(m) and V(max) to control values. The amounts of Na-Pi co-transporter 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 T3 restored them to control levels. The effect of T3 on the maturation of Pi transport was independent of PTH, GH, or glucocorticoids. Thus, thyroid hormone helps control the maturation of Pi transport in the rat during weaning, and thyroid hormone may be a dominant triggering factor in this process.