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 un ique not only in its pore sequence but also in its dense localization in th e follicular cells of the thyroid gland. Northern blot analysis revealed th at the channel message was abundantly expressed in the thyroid gland and sm all intestine, and moderately in the kidney, stomach, spinal cord and brain . Immunohistochemistry of the rat thyroid, intestine and choroid plexus dem onstrated the expression of the channel protein in the follicular cells and epithelial cells, suggesting a role in the regulation of the ion-transport ing functions of these specialized cells. The unique pore properties of Kir 7.1 make it a strong candidate for the hypothetical low-conductance K+ chan nel that is functionally coupled with Na+,K+-ATPase by recycling K+. We the refore 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 f ollicular cell; in the choroid plexus, which is known to be unique in havin g Na+,K+-ATPase in the apical side of the epithelial cells, Kir7.1 was foun d in the apical membrane, implying a close functional coupling between the channel and Na+,K+-ATPase.