Role of sodium in thyroid hormone uptake by rat skeletal muscle.
Whether Na+ movement through the plasma membrane plays a role in thyroid hormone uptake was investigated in intact rat soleus muscles. After preincubation for 120 min at 37 degrees C in modified Krebs-Ringer bicarbonate containing 140 or 5 mM Na+ plus choline or lithium to maintain osmolarity, muscles were incubated with 50 pM [125I]triiodo-L-thyronine (T3) or [125I]L-thyroxine (T4) for 60 min. T3 uptake was decreased when extracellular Na+ was replaced by either choline or lithium, the amount of decrease corresponding to the specific (or saturable) uptake component. Monensin, an ionophore that stimulates Na+ entry, increased T3 uptake at 140 mM Na+ but not at 5 mM Na+. Amiloride, a Na+/H+ exchange inhibitor, had no effect on T3 uptake under basal conditions or when Na+ was replaced by choline, but reversed the action of lithium. Ouabain, an inhibitor of Na+/K+ ATPase, reduced specific T3 uptake. T4 uptake was unaffected by low extracellular Na+. These results are consistent with a major role of Na+ movement in T3 uptake by skeletal muscle, but not in T4 uptake, and suggest an involvement of membrane pumps in this process.
Centanni
M
M
Clinical Endocrinology Branch, National Institute of Diabetes and Digestive and Kidney Diseases, Bethesda, Maryland 20892.
Robbins
J
J
D000584:Amiloride / Q000494:pharmacology; D000818:Animals; D001693:Biological Transport, Active; D002352:Carrier Proteins / Q000378:metabolism; D002462:Cell Membrane / Q000378:metabolism; D002794:Choline / Q000378:metabolism; D008094:Lithium / Q000378:metabolism; D008297:Male; D008985:Monensin / Q000494:pharmacology; D009132:Muscles / Q000378:metabolism*; D010042:Ouabain / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D012964:Sodium / Q000378:metabolism*; D017923:Sodium-Hydrogen Exchangers; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism; D013963:Thyroid Hormones / Q000493:pharmacokinetics*; D013974:Thyroxine / Q000493:pharmacokinetics; D014284:Triiodothyronine / Q000493:pharmacokinetics
D002352:Carrier Proteins; D017923:Sodium-Hydrogen Exchangers; D013963:Thyroid Hormones; D014284:Triiodothyronine; D010042:Ouabain; D000584:Amiloride; D008985:Monensin; D008094:Lithium; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; D002794:Choline; D013974:Thyroxine