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Functional Kir7.1 channels localized at the root of apical processes in rat retinal pigment epithelium

Shunji Kusaka*†, Atsushi Inanobe*, Akikazu Fujita*, Yasunaka Makino*, Masayuki Tanemoto*, Kenji Matsushita*†, Yasuo Tano† and Yoshihisa Kurachi*

1. The inwardly rectifying K⁺ channel current (I_K(IR)) recorded from isolated retinal pigmented epithelial (RPE) cells showed poor dependence on external K⁺ ([K⁺]_o) and low sensitivity to block by Ba²⁺. We examined the molecular identity and specific subcellular localization of the K_IR channel in RPE cells.

2. The Kir7.1 channel current heterologously expressed in HEK293T cells (human embryonic kidney cell line) showed identical properties to those of the RPE I_K(IR), i.e. poor dependence on [K⁺]_o and low sensitivity to Ba²⁺ block.

3. Expression of Kir7.1 mRNA and protein was detected in RPE cells by RT-PCR and immunoblot techniques, respectively.

4. Immunohistochemical studies including electron microscopy revealed that the Kir7.1 channel was localized specifically at the proximal roots of the apical processes of RPE cells, where Na⁺,K⁺-ATPase immunoreactivity was also detected.

5. The middle-distal portions of apical processes of RPE cells in the intact tissue exhibited immunoreactivity of Kir4.1, a common K_IR channel. In the isolated RPE cells, however, Kir4.1 immunoreactivity was largely lost, while Kir7.1 immunoreactivity remained.

6. These data indicate that the only I_K(IR) recorded in isolated RPE cells is derived from the functional Kir7.1 channel localized at the root of apical processes. Co-localization with Na⁺,K⁺-ATPase suggests that the Kir7.1 channel may provide the pathway for recycling of K⁺ to maintain pump activity and thus is essential for K⁺ handling in RPE cells.

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