{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,9,24]],"date-time":"2026-09-24T15:54:39Z","timestamp":1790265279980,"version":"4.1.0"},"reference-count":28,"publisher":"Wiley","issue":"2","license":[{"start":{"date-parts":[[2005,7,19]],"date-time":"2005-07-19T00:00:00Z","timestamp":1121731200000},"content-version":"vor","delay-in-days":18,"URL":"http:\/\/onlinelibrary.wiley.com\/termsAndConditions#vor"}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["The Journal of Physiology"],"published-print":{"date-parts":[[2005,7]]},"abstract":"<jats:p>In mammalian cells (including those of the ocular system), the water\u2010soluble vitamin B<jats:sub>2<\/jats:sub> (riboflavin, RF) assumes an essential role in a variety of metabolic reactions and is critical for normal cellular functions, growth and development. Cells of the human retinal pigment epithelium (hRPE) play an important role in providing a sufficient supply of RF to the retina, but nothing is known about the mechanism of the vitamin uptake by these cells and its regulation. Our aim in the present study was to address this issue using the hRPE ARPE\u201019 cells as the retinal epithelial model. Our results show RF uptake in the hRPE to be: (1) energy and temperature dependent and occurring without metabolic alteration in the transported substrate, (2) pH but not Na<jats:sup>+<\/jats:sup> dependent, (3) saturable as a function of concentration with an apparent <jats:italic>K<\/jats:italic><jats:sub>m<\/jats:sub> of 80 \u00b1 14 n<jats:sc>m<\/jats:sc>, (4) <jats:italic>trans<\/jats:italic>\u2010stimulated by unlabelled RF and its structural analogue lumiflavine, (5) <jats:italic>cis<\/jats:italic>\u2010inhibited by the RF structural analogues lumiflavine and lumichrome but not by unrelated compounds, and (6) inhibited by the anion transport inhibitors 4,4\u2032\u2010diisothiocyanatostilbene\u20102,2\u2032\u2010disulphonic acid (DIDS) and 4\u2010acetamido\u20104\u2032\u2010isothiocyanatostilbene\u20102,2\u2032\u2010disulphonic acid (SITS) as well as by the Na<jats:sup>+<\/jats:sup>\u2013H<jats:sup>+<\/jats:sup> exchange inhibitor amiloride and the sulfhydryl group inhibitor <jats:italic>p<\/jats:italic>\u2010chloromercuriphenylsulphonate (<jats:italic>p<\/jats:italic>\u2010CMPS). Maintaining the hRPE cells in a RF\u2010deficient medium led to a specific and significant up\u2010regulation in RF uptake which was mediated via changes in the number and affinity of the RF uptake carriers. While modulating the activities of intracellular protein kinase A (PKA)\u2010, protein kinase C (PKC)\u2010, protein tyrosine kinase (PTK)\u2010, and nitric oxide (NO)\u2010mediated pathways were found to have no role in regulating RF uptake, a role for the Ca<jats:sup>2+<\/jats:sup>\u2013calmodulin\u2010mediated pathway was observed. These studies demonstrate for the first time the involvement of a specialized carrier\u2010mediated mechanism for RF uptake by hRPE cells and show that the process is adaptively regulated in RF deficiency, and also appears to be under the regulation of an intracellular Ca<jats:sup>2+<\/jats:sup>\u2013calmodulin\u2010mediated pathway.<\/jats:p>","DOI":"10.1113\/jphysiol.2005.085811","type":"journal-article","created":{"date-parts":[[2005,5,7]],"date-time":"2005-05-07T00:19:30Z","timestamp":1115425170000},"page":"369-377","source":"Crossref","is-referenced-by-count":24,"title":["Mechanism of riboflavin uptake by cultured human retinal pigment epithelial ARPE\u201019 cells: possible regulation by an intracellular Ca<sup>2+<\/sup>\u2013calmodulin\u2010mediated pathway"],"prefix":"10.1113","volume":"566","author":[{"given":"Hamid 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