Question: 

The retinal pigment epithelium (RPE) is part of the blood-retinal barrier. Insufficient suppression of the complement system at the RPE is associated with age-related macular degeneration. Sublytic complement activation at the cell surface of RPE cells leads to VEGF secretion which is thought to be controlled partly by L-type calcium channel activation. Since complement activation results in calcium influx, we investigated the ion channel-dependent cellular response to complement exposure.

Methods: 

Electrophysiological recordings: human ARPE-19 cells, Ba²⁺ as a charge carrier to dissect L-type calcium currents.

Results: 

Complement activation by treatment with NHS for 10 minutes produced a shift in the voltage dependence of L-type calcium channels towards more positive values (heat-inactivated control: -14.77 mV, NHS: -10.77 mV; p=0.028). Current clamp experiments (physiological conditions) revealed an expeditious hyperpolarization in contrast to control from -38.31 ± 2.74 mV (mean ± SEM) to -62.95 ± 1.45 mV after 70.74 ± 5.13 s of onset of treatment, followed by a depolarization to -3.15 ± 0.64 mV (after 10 min). Voltage clamp experiments showed a linear membrane conductance during the hyperpolarizing phase; hyperpolarizing currents were carried by potassium ions, insensitive to paxilline (50 nM) and iberiotoxin (100 nM); followed by an outwardly rectifying membrane conductance at the late depolarization.

Conclusion: 

Our experiments demonstrate that the application of NHS leads to complex modulations of membrane conductance in human RPE: activation of non-rectifying ion channels, initial hyperpolarization and final depolarization. Additionally, L-type calcium channels show a strong shift in voltage-dependence.