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A Pan, D Yang, A Swaminathan, BA Hughes; Expression of the Inwardly Rectifying K+ Channel Kir7.1 in the Apical Processes of Bovine Retinal Pigment Epithelium . Invest. Ophthalmol. Vis. Sci. 2002;43(13):2616.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose:Our previous patch-clamp studies indicated that a high density of Kir7.1 channels are expressed at the apical surface bovine RPE cells. Immunohistochemical studies on rat RPE suggested that Kir7.1 channels are localized to the root of apical processes, whereas Kir4.1 channels are present on middle and distal regions (Kusaka et al. J. Physiol .2001; 531:27-36). The purpose of this study was to determine the distribution of Kir7.1 and Kir4.1 channels in bovine RPE. Methods:Expression of Kir7.1 and Kir4.1 channel proteins in native bovine RPE and neural retina was assayed by Western blot analysis. Distribution of Kir channel proteins was determined by indirect immunofluorescence microscopy using bovine retina-RPE choroid fixed in 4% paraformaldehyde and cut as frozen sections ∼ 10 microns thick. Results:Western blot analysis of whole-cell lysates prepared from native bovine RPE and neural retina revealed that our poly-clonal anti-Kir7.1 antibody recognized ∼50 kDa band in the RPE, while a poly-clonal anti-Kir4.1 antibody (gift of Dr. Paulo Kofuji) recognized a ∼60 kDa band in the neural retina but not in the RPE. The specificity of both anti-Kir7.1 and anti-Kir4.1 antibodies was confirmed by control peptide blocking experiments. In retinal sections, Kir4.1 immunoreactivity was detected in Muller cells, but not in the RPE. Intense Kir7.1 immunoreactivity was found at the apical surface of all RPE cells and extended into the distal half of the subretinal space. Kir7.1 co-localized with ezrin, a marker of RPE apical processes. Not all RPE cells exhibiting intense Na+/K+-ATPase immunoreactivity, but in those that did, Na+/K+-ATPase colocalized with Kir7.1 in the apical microvilli. Conclusion:In bovine RPE, Kir4.1 channels are absent and Kir7.1 channels are distributed along the entire length of apical processes. Although Kir7.1 appears to be expressed at high levels in all RPE cells, Na+/K+-ATPase expression varies among cells (Burke et al., Invest Ophthalmol Vis Sci. 2000 ; 41: 1945-1952). This suggests that in addition to maintaining pump activity by providing a pathway for the recycling of K+, Kir7.1 channels likely serve other functions necessary for retinal function.
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