Effect of Internally Loaded Iodide, Thiocyanate, and Perchlorate on Sodium-Dependent Iodide Uptake by Phospholipid Vesicles Reconstituted with Thyroid Plasma Membranes: Iodide Counterflow Mediated by the Iodide Transport Carrier

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Abstract
Na+-dependent I− transport and I− counterflow were studied using phospholipid vesicles (P-vesicles) made of porcine thyroid plasma membranes and soybean phospholipid by sonication. 1) I− uptake by P-vesicles incubated in the presence of external Na+ was higher than that by P-vesicles incubated in choline+ instead of Na+. The vesicles exhibited Na+-dependent I− uptake. When P-vesicles were internally loaded with I− prior to incubation in Na+, a further increase in Na+-dependent I- uptake was observed, although the concentration of internal I− was very much higher than that outside. In the absence of external Na+, I− uptake by P-vesicles preloaded with I− was comparable to baseline uptake. 2) Na+-dependent I− uptake by P-vesicles not loaded with I− and enhanced Na+-dependent I− uptake by P-vesicles preloaded with I− were both inhibited by either of SCN− and CIO4− added outside the vesicles. 3) When P-vesicles were loaded with SCN− instead of I− and incubated in Na+, I− uptake by these vesicles was also higher than baseline Na+-dependent I− uptake. However, a CIO4− load did not result in an increase in I− uptake. These results indicate that Na+-dependent I− transport including Na+-dependent I− counterflow is specifically mediated by the thyroid I− carrier. SCN− -I− counterflow in addition to I−-I−counterflow occurs dependently on Na+, but CIO4− -I− counterflow does not.

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