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

1. The inwardly rectifying K+ channel current (I-K(IR)) recorded from isola ted retinal pigmented epithelial (RPE) cells showed poor dependence on exte rnal K+ ([K+](o)) and low sensitivity to block by Ba2+. We examined the mol ecular identity and specific subcellular localization of the K-IR channel i n RPE cells. 2. The Kir7.1 channel current heterologously expressed :in HEK293T cells (h uman embryonic kidney cell line) showed identical properties to those of th e RPE I-K(IR), i.e. poor dependence on [K+](o) and low sensitivity to Ba2block. 3. Expression of Kir7.1 mRNA and protein was detected in RPE cells by RT-PC R 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 als o detected. 5. The middle-distal portions of apical processes of RFE cells in the intac t tissue exhibited immunoreactivity of Kir4.1, a common K-IR channel. Tn th e isolated RPE cells, however, Kir4.1 immunoreactivity was largely lost, wh ile 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 api cal processes. Go-localization with Na+,K+-ATPase suggests that the Kir7.1 channel may provide the pathway for recycling of K+ to maintain pump activi ty and thus is essential for K+ handling in RPE cells.