The use of micropuncture, isolated tubule, and vesicle technique in the study of the action of thyroid hormones on the proximal tubule function.

In hypothyroid rats (TX), the isotonic fluid reabsorption (Jv), that is closely linked to the transepithelial sodium transport (JNa), is impaired. The administration of physiological doses (10 micrograms/kg body weight per day) of tri-iodothyronine (T3) doubles Jv in three days (TX+T3). This phenomenon could be explained by several mechanisms: a direct stimulation of Na-K-ATPase, an increase in the Na+ entry step, changes in the permeability properties of the luminal and/or basal lateral membranes. Using a kinetic microassay, Na-K-ATPase activity was measured in early (S1) and late (S2) proximal tubules segments isolated from control, TX, and TX+3T3 animals. In TX rats the enzyme activity was lower (70%) in both segments versus control rats, it remained unchanged after 3 days, and it increased after 7 days of T3 substitution. The Na+ permeability of brush border membrane (BBM) vesicles isolated from TX and TX+T3 rats was identical. However the valuation of the K+ membrane permeability by in vivo perfusion of the lumen and peritubular space of proximal tubules of TX rats, with perfusate containing the K+ ionophore valinomycin (1 microgram/ml), induced a significant increase in Jv that accounted for 40% of that elicited by T3. Taken together, the in vivo and in vitro experiments suggest that the early effect on Jv of physiological doses of T3 cannot be explained by a direct action of T3 either on the Na+ entry step across the BBM or on the Na+ exit step (i.e., the Na-K-ATPase), but rather by an increase in K+ permeability of proximal tubular cell membranes.(ABSTRACT TRUNCATED AT 250 WORDS)