title: The use of micropuncture, isolated tubule, and vesicle technique in the study of the action of thyroid hormones on the proximal tubule function.
abstract: 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)
mesh_terms: D000042:Absorption; D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D017301:Ca(2+) Mg(2+)-ATPase / Q000378:metabolism; D002463:Cell Membrane Permeability; D007037:Hypothyroidism / Q000503:physiopathology*; D007687:Kidney Tubules, Proximal / Q000503:physiopathology*; D008297:Male; D008871:Microvilli / Q000378:metabolism; D011188:Potassium / Q000378:metabolism; D051381:Rats; D011919:Rats, Inbred Strains; D012964:Sodium / Q000378:metabolism; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism; D013963:Thyroid Hormones / Q000502:physiology*; D013965:Thyroidectomy; D014284:Triiodothyronine / Q000494:pharmacology
chemical_list: D013963:Thyroid Hormones; D014284:Triiodothyronine; D012964:Sodium; D017301:Ca(2+) Mg(2+)-ATPase; D000254:Sodium-Potassium-Exchanging ATPase; D011188:Potassium