Voltage-dependent Ca(2+) channels, not ryanodine receptors, activate Ca(2+)-dependent BK potassium channels in human retinal pigment epithelial cells

PURPOSE: In different tissues the activation of large conductance Ca(2+)-activated (BK) potassium channels has been shown to be coupled to voltage-gated Ca(2+) channels as well as ryanodine receptors. As activation of BK channels leads to hyperpolarization of the cell, these channels provide a negative feedback mechanism for Ca(2+)-induced functions. Many cellular functions of the retinal pigment epithelium (RPE) are coupled to changes in [Ca(2+)](i). The aim of this study was to identify which Ca(2+)-entry pathway leads to the activation of BK channels in the RPE. METHODS: We used freshly isolated human RPE cells and the ARPE-19 cell line for the detection of transcripts of BK channel α subunits. Patch-Clamp measurements were used to characterize BK channels in ARPE-19 cells electrophysiologically. To monitor changes in [Ca(2+)](i) ARPE-19 cells were loaded with Fura-2. RESULTS: Freshly isolated human RPE cells and ARPE-19 cells were shown to express BK channels. In ARPE-19 cells these channels were shown to be functionally active. Application of iberiotoxin led to a block of outward currents by 28.15%. At +50 mV ARPE-19 cells had a BK channel-mediated current density of 2.42 pA/pF. Activation of ryanodine receptors by caffeine led to a significant increase in [Ca(2+)](i) by 34.16%. Nevertheless, caffeine-induced Ca(2+) signals were not sufficient to activate BK channels. Instead, the activation of L-type Ca(2+) channels by BayK 8644 caused a dramatic increase in BK channel activity and a shift of the reversal potential of the ARPE-19 cells by −22.6 mV. CONCLUSIONS: We have shown here for the first time that human RPE cells express BK channels. These channels are activated in RPE cells by increases in [Ca(2+)](i) that are mediated by the opening of voltage gated L-type Ca(2+) channels. As Ca(2+) entering the RPE cells through these Ca(2+) channels are known to be important for growth factor secretion and light-induced transepithelial transport, we speculate that BK channels coupled directly to these Ca(2+) channels may provide a good tool for negative feedback control of the L-type Ca(2+) channels.