Effects of thyroid hormone on the neonatal renal cortical Na⁺/H⁺ antiporter

The neonatal proximal tubule has a lower rate of bicarbonate absorption 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 was to examine if thyroid hormone could be a factor in the maturational increase in Na⁺/H⁺ antiporter activity. Hypothyroid (0.01% propylthiouracil in drinking water starting at day 14 gestation and throughout the postnatal period), euthyroid, and hyperthyroid (intraperitoneal triiodothyronine, 10 μg/100 g body wt, once daily on days 17 to 20 of postnatal life) rats were all studied at 21 days of life. Renal cortical brush border Na⁺/H⁺ antiporter activity was 453 ± 24, 527 ± 30 and 608 ± 25 pmol/mg protein in the hypothyroid, euthyroid and hyperthyroid groups, respectively (P < 0.001). Hyperthyroid neonates had ~twofold greater renal cortical NHE-3 mRNA abundance than euthyroid and hypothyroid neonates (P < 0.05). Brush border membrane NHE-3 protein abundance in hypothyroid and hyperthyroid neonates was one-third and twofold that of euthyroid 21-day-old rats, respectively (P < 0.001). These data are consistent with a potential role of thyroid hormone in the postnatal increase in Na⁺/H⁺ antiporter activity.