Ca2+-Dependent Bk Channels in Human Rpe Cells Are Coupled to Voltage-Dependent Ca2+ Channels and Not to Ryanodine Receptors

Abstract

Purpose: : In different tissues the activation of BK channels has been shown to be coupled to voltage-gated Ca²⁺ channels and/or ryanodine receptors. As activation of BK channels leads to hyperpolarization of the cell they provide a negative feedback mechanism for Ca²⁺-induced functions. Many cellular functions of the RPE are coupled to changes in [Ca²⁺]_i. The aim of this study was to identify to which Ca²⁺ entry pathway the activation of BK channels is coupled in the RPE.

Methods: : We used freshly isolated human RPE cells and the ARPE-19 cell line for molecular biological detection of BK channel subunits. Patch-clamp measurements were used to characterize BK channels and Fura-2 to monitor changes in [Ca²⁺]_i in ARPE-19 cells.

Results: : Freshly isolated human RPE cells and ARPE-19 cells have been shown to express BK channels. In ARPE-19 cells these channels have been 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 [Ca²⁺]_i by 34.16%. Nevertheless, caffeine-induced Ca²⁺ signals were not sufficient to activate BK channels. Instead, the activation of L-type Ca²⁺ 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 expresses BK channels. These channels are activated in RPE cells by increases in [Ca²⁺]_i that are mediated by the opening of voltage gated L-type Ca²⁺ channels. As Ca²⁺ entering the RPE cells through these Ca²⁺ channels are known to be important for growth factor secretion and light-induced transepithelial transport, we speculate that BK channels coupled directly to these Ca²⁺ channels may provide a good tool for negative feedback control of the L-type Ca²⁺ channels.

Keywords: ion channels • retinal pigment epithelium • signal transduction