The iodide channel of the thyroid. II. Selective iodide conductance inserted into liposomes.
An iodide channel has been previously identified in the plasma membrane of bovine throcytes [Golstein et al., Am. J. Physiol. 263 (Cell Physiol. 32): C590-C597, 1992]. The plasma membrane proteins were solubilized and ultrafiltered, and the protein fraction collected above 100 kDa was inserted in liposomes. Voltage-sensitive uptake of radiolabeled I- by these proteoliposomes was studied. To this end, an outward I- gradient was set up by loading the proteoliposomes with KI and removing extraliposomal I-. I- exit from the proteoliposome induces an inside positive membrane potential, which leads to the uptake of 125I- added to the incubation medium. This uptake was abolished by valinomycin, which in the presence of K+ short circuits the liposomal membrane potential, demonstrating the conductive nature of this uptake. A double reciprocal plot of I- influx over I- concentration suggests the existence of a single population of channels in these proteoliposomes with a Michaelis-Menten constant for I- of approximately 9 microM. When the proteoliposomes were loaded with KCl or KSCN instead of I-, no conductive uptake occurred anymore, suggesting that these anions are unable to diffuse through the I- conductance, hence do not generate a diffusion potential. I- uptake by KI-loaded proteoliposomes was not inhibited in the presence of a 1,000-fold excess of extraliposomal Cl- but was completely inhibited by a 1,000-fold excess of extraliposomal SCN-, indicating that Cl- does not permeate the I- channel, whereas SCN- inhibits it. SCN- and flufenamate were both shown to be competitive inhibitors of the I- channel with an inhibitor constant of approximately 10 and 750 microM, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)
Golstein
P E
PE
Laboratory of Physiology and Pathophysiology, School of Medicine, Université Libre de Bruxelles, Brussels, Belgium.
Sener
A
A
Beauwens
R
R
D000818:Animals; D000838:Anions / Q000494:pharmacology; D002417:Cattle; D004553:Electric Conductivity; D005439:Flufenamic Acid / Q000494:pharmacology; D007454:Iodides / Q000378:metabolism*; D007473:Ion Channels / Q000037:antagonists & inhibitors / Q000502:physiology*; D008081:Liposomes / Q000378:metabolism; D010539:Permeability; D011510:Proteolipids / Q000378:metabolism; D013861:Thiocyanates / Q000494:pharmacology; D013961:Thyroid Gland / Q000378:metabolism*; D014634:Valinomycin / Q000494:pharmacology
D000838:Anions; D007454:Iodides; D007473:Ion Channels; D008081:Liposomes; D011510:Proteolipids; D013861:Thiocyanates; C020485:proteoliposomes; D014634:Valinomycin; D005439:Flufenamic Acid; C031760:thiocyanate