Effects of thyroid hormone on Na-K-adenosine triphosphatase activity along the rat nephron

Na-K-adenosine triphosphatase (ATPase) activity was determined in individual nephron segments obtained from the kidneys of euthyroid and hypothyroid rats. One group of animals was made hypothyroid by feeding 0.05% aminotriazole (ATZ) in the diet for 2 weeks. A second group received the same amount of ATZ plus 500 micrograms/kg body weight of L-thyroxine (T4) given subcutaneously each day for 2 weeks. A third group received the same diet without ATZ or T4. There was a 57% (P less than 0.01) decrease in Na-K-ATPase activity in the proximal convoluted tubule (PCT) in ATZ-treated rats that was corrected by the simultaneous administration of T4 with ATZ. A smaller (15% to 25%) and statistically nonsignificant decrease in Na-K-ATPase activity was observed in the cortical portion of the proximal straight tubule and in both the cortical and the medullary portions of the thick ascending limb in ATZ-treated rats. These changes in the enzyme activity were also corrected by simultaneous administration of T4 with ATZ. The results suggest that under the conditions of these experiments the PCT is a major site of action of thyroid hormone in the rat kidney.