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

Abstract

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.