Ion channels and transporters of the retinal pigment epithelium.

Ion channels and ion transporters play essential roles in the function of the retinal pigment epithelium (RPE). The use of cell cultures has been exploited as a key method for successfully identifying and studying ion channels and transporters of the RPE. Cultured RPE cells enable robust and long-lasting patch-clamp recordings, Ussing chamber investigations of the transepithelial transport within the isolated RPE, and analyses of the intracellular Ca(2+) or pH with fluorescent probes. Furthermore, cultured RPE can be transfected at high success rates, permitting the easy use of siRNA to study the involvement of ion channels on the molecular level. However, the expression patterns of the ion channels in the RPE appear to be a very sensitive marker reflecting the extent of RPE differentiation in vitro. Having originated from the neuroectoderm, cultured RPE cells seem to retain some capacity to change into a more neuronal phenotype expressing TTX-blockable Na(+) channels or synaptic Ca(2+) channels. Therefore, the identification of ion channels and transporters in cultured cells should be verified in freshly isolated RPE cells and in situ preparations of the RPE, via immunohistochemistry and the analysis of RPE-specific signals in the electroretinogram from transgenic animals.