Apoptosis of Corneal Epithelial Cells Caused by Ultraviolet B-induced Loss of K+ is Inhibited by Ba2+

UVB exposure at ambient outdoor levels triggers rapid K+ loss and apoptosis in human corneal limbal epithelial (HCLE) cells cultured in medium containing 5.5 mM K+, but considerably less apoptosis occurs when the medium contains the high K+ concentration that is present in tears (25 mM). Since Ba2+ blocks several K+ channels, we tested whether Ba2+-sensitive K+ channels are responsible for some or all of the UVB-activated K+ loss and subsequent activation of the caspase cascade and apoptosis. Corneal epithelial cells in culture were exposed to UVB at 80 or 150 mJ/cm2. Patch-clamp recording was used to measure UVB-induced K+ currents. Caspase-activity and TUNEL assays were performed on HCLE cells exposed to UVB followed by incubation in the presence or absence of Ba2+. K+ currents were activated in HCLE cells following UVB-exposure. These currents were reversibly blocked by 5 mM Ba2+. When HCLE cells were incubated with 5 mM Ba2+ after exposure to UVB, activation of caspases-9, -8, and -3 and DNA fragmentation were significantly decreased. The data confirm that UVB-induced K+ current activation and loss of intracellular K+ leads to activation of the caspase cascade and apoptosis. Extracellular Ba2+ inhibits UVB-induced apoptosis by preventing loss of intracellular K+ when K+ channels are activated. Ba2+ therefore has effects similar to elevated extracellular K+ in protecting HCLE cells from UVB-induced apoptosis. This supports our overall hypothesis that elevated K+ in tears contributes to protection of the corneal epithelium from adverse effects of ambient outdoor UVB.