AJP - Cell Physiology, Vol 263, Issue 3 C590-C597, Copyright © 1992 by American Physiological Society
The iodide channel of the thyroid: a plasma membrane vesicle study
P. Golstein, M. Abramow, J. E. Dumont and R. Beauwens
Department of Physiology and Pathophysiology, School of Medicine, Universite Libre de Bruxelles, Belgium.
The uptake of radioactive iodide or chloride by plasma membrane vesicles of
bovine thyroid was studied by a rapid filtration technique. A Na(+)-I-
cotransport was demonstrated. When this Na(+)-I- cotransport is inactive
(i.e., at 4 degrees C and in the absence of Na+), an uptake of iodide above
chemical equilibrium could be induced, driven by the membrane potential.
The latter was set up by allowing potassium to diffuse into the membrane
vesicles in the presence of valinomycin and of an inward K+ gradient. This
potential difference (positive inside) induced the uptake of iodide (or
other anion present). The data support the existence of two anionic
channels. The first one, observed at low near-physiological iodide
concentration (micromolar range), which exhibits a high permeability and
specificity for iodide (hence called the iodide channel), has a Km of 70
microM. The other one appears similar to the epithelial anion channel as
described by Landry et al. (J. Gen. Physiol. 90: 779-798, 1987); it is
still about fourfold more permeable to iodide than to chloride and presents
a Km of 33 mM. Under physiological conditions the latter channel would
mediate chloride transport, and the iodide channel, which is proposed to be
restricted to the apical plasma membrane domain of the thyrocyte,
transports iodide from the cytosol to the colloid space.
