Voltage-dependent Ca2+ channels, not ryanodine receptors, activate Ca2+-dependent BK potassium channels in human retinal pigment epithelial cells.
OBJECTIVE
In different tissues the activation of large conductance Ca2+-activated (BK) potassium channels has been shown to be coupled to voltage-gated Ca2+ 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 Ca2+-induced functions. Many cellular functions of the retinal pigment epithelium (RPE) are coupled to changes in [Ca2+]i. The aim of this study was to identify which Ca2+-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 alpha subunits. Patch-Clamp measurements were used to characterize BK channels in ARPE-19 cells electrophysiologically. To monitor changes in [Ca2+]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 [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 cells express 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.
Wimmers
Sönke
S
Experimentelle Ophthalmologie, Klinik und Poliklinik für Augenheilkunde, Universitätsklinikum Hamburg-Eppendorf, Hamburg, Germany. wimmers.soenke@mh-hannover.de
Halsband
Claire
C
Seyler
Sebastian
S
Milenkovic
Vladimir
V
Strauss
Olaf
O
D001498:3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester / Q000494:pharmacology; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000494:pharmacology*; D020746:Calcium Channels, L-Type / Q000378:metabolism; D002460:Cell Line; D005786:Gene Expression Regulation / Q000187:drug effects; D006801:Humans; D015640:Ion Channel Gating / Q000187:drug effects*; D051036:Large-Conductance Calcium-Activated Potassium Channels / Q000235:genetics / Q000378:metabolism*; D021122:Protein Subunits / Q000235:genetics / Q000378:metabolism; D055213:Retinal Pigment Epithelium / Q000166:cytology* / Q000187:drug effects / Q000378:metabolism*; D019837:Ryanodine Receptor Calcium Release Channel / Q000378:metabolism*
D020746:Calcium Channels, L-Type; D051036:Large-Conductance Calcium-Activated Potassium Channels; D021122:Protein Subunits; D019837:Ryanodine Receptor Calcium Release Channel; D002110:Caffeine; D001498:3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester; D002118:Calcium