Inwardly rectifying K+ channel Kir7.1 is highly expressed in thyroid follicular cells, intestinal epithelial cells and choroid plexus epithelial cells: implication for a functional coupling with Na+,K+-ATPase.

A novel inwardly rectifying K+ channel, Kir7.1, with unique pore properties, was cloned recently. Working in the field of osmoregulation, we have also identified the same human and rat channel and found that the channel is unique not only in its pore sequence but also in its dense localization in the follicular cells of the thyroid gland. Northern blot analysis revealed that the channel message was abundantly expressed in the thyroid gland and small intestine, and moderately in the kidney, stomach, spinal cord and brain. Immunohistochemistry of the rat thyroid, intestine and choroid plexus demonstrated the expression of the channel protein in the follicular cells and epithelial cells, suggesting a role in the regulation of the ion-transporting functions of these specialized cells. The unique pore properties of Kir7.1 make it a strong candidate for the hypothetical low-conductance K+ channel that is functionally coupled with Na+,K(+)-ATPase by recycling K+. We therefore further examined the co-localization of Kir7.1 and Na+,K(+)-ATPase and found that both are localized in the basolateral membrane of the thyroid follicular cell; in the choroid plexus, which is known to be unique in having Na+,K(+)-ATPase in the apical side of the epithelial cells, Kir7.1 was found in the apical membrane, implying a close functional coupling between the channel and Na+,K(+)-ATPase.