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L. D. Haarsma, D. S. Will, J. L. Ubels; Elevated Extracellular K+ Reduces UV-induced K+ Efflux From Corneal Epithelial Cells. Invest. Ophthalmol. Vis. Sci. 2008;49(13):5304. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
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.
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.
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.
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.
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