Thyroid Hormones Increase Na+-H+Exchange Activity in Renal Brush Border Membranes

Na+-H+exchange activity, i.e., amiloride-sensitive Na+and H+flux, in renal proximal tubule brush border (luminal) membrane vesicles was increased in the hyperthyroid rat and decreased in the hypothyroid rat, relative to the euthyroid animal. A positive correlation was found between Na+-H+exchange activity and serum concentrations of thyroxine (T4) and triiodothyronine (T3). The thyroid status of the animal did not alter amiloride-insensitive Na+uptake. The rate of passive pH gradient dissipation was higher in membrane vesicles from hyperthyroid rats compared to the rate in vesicles from hypothyroid animals, a result which would tend to limit the increase in Na+uptake in vesicles from hyperthyroid animals. Na+-dependent phosphate uptake was increased in membrane vesicles from hyperthyroid rats; Na+-dependent D-glucose and L-proline uptakes were not changed by the thyroid status of the animal. The effect of thyroid hormones in increasing the uptake of Na+in the brush border membrane vesicle is consistent with the action of the hormones in enhancing renal Na+reabsorption. Further, the regulation of transtubular Na+flux has now been shown to be concomitant with modulation of the entry of Na+into the tubular cell across its luminal membrane, mediated by the exchange reaction, and with the previously reported control of the pumping of Na+out of the cell across its basolateral membrane, mediated by the Na+,K+-ATPase.