Basal calcium entry in retinal pigment epithelial cells is mediated by TRPC channels.

PURPOSE

Ca(2+) is a major regulator of cell function. In the retinal pigment epithelium (RPE), intracellular free Ca(2+) concentration (Ca(2+)) is essential for the maintenance of normal retinal function. Therefore, accurate control of Ca(2+) is vital in these cells. Because Ca(2+) is permanently extruded from the cytosol, RPE cells need a basal Ca(2+) entry pathway that counteracts this Ca(2+) efflux. The purpose of this study was to identify the molecular basis of basal Ca(2+) entry into the RPE.

METHODS

Ca(2+) was measured using Fura-2-loaded ARPE-19 cells. The expression pattern of TRPC channels was investigated by RT-PCR with RNA extracted from ARPE-19 cells and freshly isolated RPE cells from human donor eyes.

RESULTS

In most cells, basal Ca(2+) is highly controlled by cell membranes that are only slightly permeable to Ca(2+) and by the activity of Ca(2+) pumps and transporters. The authors show here that RPE cells have a basal Ca(2+) conductance that is dose dependently blocked by La(3+). Basal Ca(2+) was also strongly reduced by the TRP channel blockers Gd(3+), Ni(2+), 2-APB, and SKF96365 and was insensitive to blockers of other Ca(2+) channels. In confirmation of this pharmacologic profile, RPE cells expressed TRPC1 and TRPC4 channels, as shown by RT-PCR experiments.

CONCLUSIONS

Ca(2+) is needed for several permanently occurring regulatory processes in RPE cells. The Ca(2+) influx pathway identified in this study is essential to define a resting basal Ca(2+). This resting Ca(2+) may contribute, for example, to basal cytokine secretion essential for the maintenance of normal retinal function.