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J. Adijanto, N. J. Philp, T. Banzon, S. Jalickee, S. S. Miller; Lactate Transport in Human Retinal Pigment Epithelium. Invest. Ophthalmol. Vis. Sci. 2009;50(13):1857.
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The retina is highly glycolytic--95% of glucose consumed by the retina is metabolized via glycolysis into lactic acid that is deposited into the subretinal space (SRS). In this study, we examined lactate transport in the RPE by co-transporters, exchangers, and channels. We also studied their interactions with carbonic anhydrase and HCO3-transporters.
All experiments were performed with confluent monolayers of cultured human fetal RPE (hfRPE) grown on transwells (Maminishkis et al., IOVS, 2006). We used pH-sensitive fluorescence dye (BCECF) to monitor intracellular pH (pHi) while simultaneously recording transepithelial potential (TEP) and total epithelial resistance (RT).
The RPE transports lactate out of the SRS (to the choroid) with MCT1 (apical) and MCT3 (basolateral) to maintain pH homestasis in the SRS. In hfRPE, H/Lac co-transport by MCT1 was inhibited by niflumic acid and pCMBS. Proton-entry via MCT1 activated the Na/H exchanger at the apical membrane, causing a slow alkalinization that was absent in the presence of apical amiloride. Apical lactate entry caused a TEP-increase that can be inhibited by basal DIDS, indicating that a DIDS-sensitive electrogenic pathway is activated. Functional interaction between MCT1, carbonic anhydrase II (CA II) and NBC1 (Na/HCO3 co-transporter) has been described (Becker et al., jbc, 2005). However, blocking NBC1 with DIDS at the apical membrane did not affect H/Lac entry via MCT1. Dorzolamide, a membrane-permeant CA-inhibitor, increased apical lactate induced pHi and TEP responses, suggesting that CA-inhibition increased lactate transport across the RPE.
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