Expression profile of voltage-dependent Ca2+ channel subunits in the human retinal pigment epithelium.

BACKGROUND

The secretion of a variety of factors by the retinal pigment epithelium (RPE) is essential for the structural integrity of the neuronal retina and choroid, but also plays a pivotal role in the etiology of diseases such as choroidal neovascularisation. A recent study showed that the secretory activity of the RPE is regulated by the activity of a certain type of voltage-dependent Ca(2+) channels, the L-type channel. In order to provide a better base for the understanding of the underlying Ca(2+) signalling in these cells, we investigated the expression profile of voltage-dependent Ca(2+) channel subunits in RPE cells.

METHODS

Using RT-PCR techniques with cDNA isolated from RPE cells, we investigated the expression pattern of Ca(2+) channel subunits. Furthermore, we analysed Ba(2+) currents through voltage-dependent Ca(2+) channels in RPE cells by the patch-clamp technique.

RESULTS

We detected the expression of two L-type channel subtypes and the expression of two different T-type channel subtypes. As accessory subunits, they expressed beta2 and beta4 and all known alpha(2)delta subunits. In general, we were able to confirm these data with cDNA from the ARPE-19 cell line. They only showed some differences in their expression pattern of accessory subunits. Since the expression of T-type channels was so far unknown in RPE cells, we confirmed their expression in the RPE using cDNA isolated from freshly isolated human RPE cells. Furthermore, the patch-clamp analysis of Ba(2+) currents showed a heterogeneous pattern of voltage-dependent inward currents in RPE cells. In some cells, typical slowly inactivating L-type currents were detected, whereas in other cells fast inactivating T-type currents could be detected.

CONCLUSIONS

These data indicate the expression of a so far not detected subtype of voltage-dependent Ca(2+) channels, the T-type channels. Together with the expression of L-type channels, RPE cells show a comparable expression pattern to that of other secretory cells, such as beta-islets of the pancreas.