Elevated Extracellular K+ Reduces UV-induced K+ Efflux From Corneal Epithelial Cells

Abstract

Purpose: : Previous studies showed that UV-C exposure increases K⁺ efflux through ion channels in corneal epithelial cells (IOVS 44:5095, 2003). One purpose of this study was to determine if UV-B has a similar effect. The previous study also showed that reducing K⁺ efflux with K⁺ channel blockers can prevent UV-induced apoptosis. Tears have a higher K⁺ concentration (about 20 mM) compared to other extracellular fluid (about 4.2 mM), but the function of this elevated extracellular [K⁺] is unknown. A second goal of this study was to measure whether increased extracellular [K⁺] reduces UV-induced K⁺ efflux and, if so, the magnitude of that response.

Methods: : Perforated patch-clamp recordings were made on cultured human corneal limbal epithelial (HCLE) cells. Prior to UV exposure, the resting potential was measured and whole-cell K⁺ currents as a function of voltage (–80 to +160 mV) were recorded. Each cell was then exposed to either UV-B (80-160 mJ/cm²) or UV-C (0.8 - 1.6 mJ/cm²), and the current measurements were repeated. K⁺ currents were recorded during superfusion of cells with external solutions in which Na⁺ was isosmotically replaced with K⁺, to concentrations of 5, 20, 50, and 155 mM [K⁺]_o.

Results: : UV exposure resulted in a rise in resting membrane potential, typically from about –20 mV prior to UV to near 0 mV after UV. Large increases in outward K+ currents were observed in most cells after UV exposure. Voltage-activated currents increased within 5 minutes of exposure and continued to fluctuate for 30 minutes. UV-B (84 mJ/cm²) caused an average 11-fold increase in current at +100 mV, while an average 23-fold increase in current at +100 mV resulted from UV-C (0.8 mJ/cm²) exposure. In cells exposed to elevated [K⁺]_o, increasing [K⁺]_o systematically reduced UV-induced K⁺ efflux at all voltages.

Conclusions: : UV-B radiation at levels relevant to ambient daylight activates K⁺ channels, leading to increased K⁺ efflux from corneal epithelial cells. Increased [K⁺]_o reduces this efflux. This suggests that the higher [K⁺] in tears relative to other extracellular fluids helps to protect the ocular surface from UV damage that can cause apoptosis by preventing loss of intracellular K⁺ when channels are activated by UV radiation.

Keywords: cornea: epithelium • radiation damage: light/UV • apoptosis/cell death