Modulation of M-Type K+ Current in Monnkey Retinal Pigment Epithelium (RPE) by the KCNQ Channel Opener Retigabine

Abstract

Purpose:: Native human and monkey RPE cells exhibit a sustained, outwardly rectifying K⁺ current that has electrophysiological and pharmacological properties resembling those of M-type K⁺ currents in excitable cells, which are mediated by channels composed of KCNQ and KCNE subunits. At ARVO last year, we presented results showing that KCNQ1, KCNQ4, and KCNQ5 transcripts are expressed in human and monkey RPE. Studies by other groups have shown that the anti-epileptic drug retigabine is an opener of KCNQ2-5 channels but not KCNQ1 channels. The purpose of this study was to determine the effect of retigabine on the M-type K⁺ current in native monkey RPE.

Methods:: Whole-cell currents were recorded in enzymatically dissociated monkey RPE cells at room temperature using the standard patch-clamp technique. M-type K⁺ current was identified by its characteristic activation and deactivation kinetics. The pipette solution contained (in mM) 30 KCl, 83 K-gluconate, 10 HEPES, 5.5 EGTA, 0.5 CaCl₂, 4 MgCl_2, and 4 K₂ATP, and 0.5 Na-GTP, pH 7.2. The control bath solution was a HEPES-buffered Ringer’s solution containing (in mM) 135 NaCl, 5 KCl, 10 HEPES, 10 glucose, 1.8 CaCl₂, and 1.0 MgCl₂, pH 7.4. Retigabine was dissolved in DMSO and added to Ringer’s solution to achieve the desired concentration.

Results:: Exposure of monkey RPE cells to retigabine augmented M-type K⁺ current in a concentration dependent and reversible manner. At a concentration of 10 µM, retigabine increased the M-type K⁺ conductance at +30 mV from 3.4 ± 1.6 nS to 4.8 ± 1.7 nS (n = 6), shifted its voltage of half-maximum activation from -33 ± 3.5 mV to -49 ± 3.7 mV, and hyperpolarized the membrane potential by approximately 10 mV. Retigabine also slowed the rate of current deactivation.

Conclusions:: Our findings identify M-type K⁺ channels in native monkey RPE cells as a target of retigabine and are consistent with the idea that these channels are composed of KCNQ4 or KCNQ5 subunits.

Keywords: ion channels • electrophysiology: non-clinical • retinal pigment epithelium