EFFECTS OF THYROID-HORMONE ON THE NEONATAL RENAL CORTICAL NA+ H+ ANTIPORTER/

The neonatal proximal tubule has a lower rate of bicarbonate absorptio n than that of adults. This is due, in part, to a lower rate of apical membrane NA(+)/H+ antiporter activity. The purpose of these studies w as to examine if thyroid hormone could be a factor in the maturational increase in Na+/H+ antiporter activity. Hypothyroid (0.01% propylthio uracil in drinking water starting at day 14 gestation and throughout t he postnatal period), euthyroid, and hyperthyroid (intraperitoneal tri iodothyronine, 10 mu g/100 g body wt, once daily on days 17 to 20 of p ostnatal life) rats were all studied at 21 days of life. Renal cortica l brush border Na+/H+ antiporter activity was 453 +/- 24, 527 +/- 30 a nd 608 +/- 25 pmol/mg protein in the hypothyroid, euthyroid and hypert hyroid groups, respectively (P < 0.001). Hyperthyroid neonates had sim ilar to twofold greater renal cortical NHE-3 mRNA abundance than euthy roid and hypothyroid neonates (P < 0.05). Brush border membrane NHE-3 protein abundance in hypothyroid and hyperthyroid neonates was one-thi rd and twofold that of euthyroid 21-day-old rats, respectively (P < 0. 001). These data are consistent with a potential role of thyroid hormo ne in the postnatal increase in Na+/H+ antiporter activity.