Ca2+-channels in the RPE.

The retinal pigment epithelium closely interacts with photoreceptors and helps to maintain the activity of photoreceptors. Investigations using patch-clamp techniques on cultured or freshly isolated retinal pigment epithelial cells from various species demonstrated the expression of voltage-dependent Ca2+ channels with characteristics of L-type channels. Since retinal pigment epithelial cells rarely display changes of the membrane potential which lead to the activation of these Ca2+ channels, their function seemed to be unclear. Recent findings shed light onto the possible role of these Ca2+ channels. First of all, the subtype of these ion channels could be identified as neuroendocrine subtype of L-type channels. Recent studies demonstrated that the neuroendocrine subtype of L-type channels is regulated by serine/ threonine kinases and protein tyrosine kinases. These phosphorylation-dependent regulatory mechanisms lead to Ca2+ fluxes into the cell which are independent of changes in the membrane potential and induced by a shift in the voltage-dependence ofthese ion channels. The regulation modality implied that L-type Ca2+ channels play an important role in signal transduction pathways which are important for a communication between retinal pigment epithelium and photoreceptors. L-type Ca2+ channels in the retinal pigment epithelium seem to be involved in the regulation of secretion of various factors, in growth factor-dependent intracellular signalling and in the regulation of the phagocytosis of photoreceptor outer membranes. Thus, voltage-dependent Ca2+ channels in the retinal pigment epithelium are of importance for the function of photoreceptors.