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M. Valtink, A. Farah, S. Duesing, V. Zubaty, O. Strauss, K. Engelmann; Expression and Localisation of Voltage-gated L-type Ca2+ ion Channels in Adult Human Retinal Pigment Epithelium . Invest. Ophthalmol. Vis. Sci. 2003;44(13):399.
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Purpose: L-type channel currents have been described in freshly isolated and cultured retinal pigment epithelial (RPE) cells from various species. Recent data point out a role of L-type channels in growth factor secretion in RPE cells. To gain further information about the role of these channels for RPE function, we determined expression and localisation of L-type Ca2+-channel subunits α1C and α1D in adult human RPE. Methods: Human RPE cells in situ (frozen sections) and in vitro (cultured serum-supplemented or serum-free for up to 3 passages) were immunostained with polyclonal antibodies against α1C and α1D subunits. Specific staining was confirmed using corresponding blocking peptides. Antibody-labelling in retina sections was performed by LSAB-2, in cultured cells by immunofluorescence. Results: In vivo, human RPE cells mainly express α1D subunits, which are localised in basolateral membranes. Proliferating RPE cells (cultured under serum supplementation) expressed both α1D and α1C subunits during subculture at least until passage 3. α1D subunits were mainly localised in the cell membrane, whereas α1C was mainly found in the endoplasmic reticulum. α1D subunit expression decreased during subculture, while α1C subunit expression increased. Quiescent, repigmented RPE cells (cultured serum-free) preferentially expressed Ca2+-channel subunit α1D. In these cells α1D subunits were localised in the basolateral membrane. Conclusions: In vivo and in vitro, human RPE cells express mainly the neuroendocrine subtype of L-type Ca2+-channels (corresponding with the α1D subunit). In both, in vivo and in vitro, the Ca2+-channels are localised in the basolateral membrane. The expression and localisation of the correct L-type channel subtype is dependent on the cell cycle. The expression of the neuroendocrine subtype, which is localized in the basolateral membrane, implies that L-type channels in the RPE regulate secretion of growth factors to the basolateral side of the RPE.
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