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J. Adijanto, Q. Wan, N. J. Philp, R. Li, S. S. Miller; Lactic Acid Activates Kir7.1 and ClC-2 in Human Retinal Pigment Epithelium. Invest. Ophthalmol. Vis. Sci. 2010;51(13):479.
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In the dark, the photoreceptors produce lactic acid that is released to the subretinal space (SRS) and crosses the RPE apical membrane via monocarboxylate transporters (MCT1). In this study, we describe the ensuing physiological changes that take place within the RPE, which could in turn affect the chemical composition of the SRS.
All experiments were performed with confluent monolayers of cultured human fetal RPE (hfRPE) grown on transwells (Maminishkis et al., IOVS, 2006). A pH-sensitive fluorescence dye (BCECF) was used to monitor intracellular pH (pHi) while simultaneously recording transepithelial potential (TEP) and total epithelial resistance (RT). Intracellular microelectrodes measured apical and basolateral membrane voltages and resistances (VA, VB & RA/RB), TEP and RT.
Microelectrode experiments showed that adding lactate to the apical bath acidified the cells & produced a 2-phased electrical response: VB depolarization followed by VA hyperpolarization. VB depolarization is consistent with activation of Cl-channels at the basolateral membrane, possibly ClC-2. Immunofluorescence show that ClC-2 is localized at the basolateral membrane, and physiological experiments with ClC-2 inhibitor (Zn2+) indicate that ClC-2 was activated by apical lactate. In addition, microelectrode experiments show that apical lactate activated a Ba2+-sensitive K-channel at the apical membrane, probably Kir7.1. Intracellular acidification also activated ClC-2 and Kir7.1.
Our data show that apical H/Lac entry via MCT1 caused intracellular acidification that subsequently activated Kir7.1 at the apical membrane and ClC-2 at the basolateral membrane. Lactic acid transport has been shown to cause cell swelling, and by stimulating KCl efflux via Kir7.1 and ClC-2, the RPE prevents swell-induced osmotic stress. In addition, lactate induced activation of Kir7.1 allows the RPE to regulate K-level in the subretinal space following transitions between light and dark.
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