CHARACTERIZATION OF LETHAL ACTION OF NEAR-ULTRAVIOLET ON RETINAL-PIGMENT EPITHELIAL-CELLS IN-VITRO

The lethal effect of near ultraviolet (NUV) with low intensity on cult ured RPE cells has been investigated. RPE cultures with various cell d ensities were exposed to NUV (peaking at 365 nm) with or without ambie nt oxygen in phenol-red-free Dulbecco's PBS containing Ca2+, Mg2+ and glucose (PBS+). The cell viability was determined by dye exclusion and was expressed as cell death ratio (CDR, dead cells/total cells). When RPE cells at 5X10(3) cells/cm(2), a non-contact low density, were irr adiated either at a fixed irradiance (900 mu W/cm(2)) with different e xposure times (4 to 8h) or vice versa (8 h with irradiance from 430 to 900 mu W/cm(2)), the change of CDR represented a similar linear funct ion. The replotted data from both the time- and the irradiance-depende nt curves indicated that the killing of RPE cells is dependent on the total energy dose of NUV. When a single NUV energy (19.44 J/cm(2)) was used, CDR was RPE cell density dependent. At confluence, NW at the hi ghest dosage tested (26 J/cm(2)) did not show any lethality An oxygen- free condition abolished the NW lethality on RPE cells even though the RPE cells were at a non-contact state. The presence of an antioxidant enzyme, catalase, in oxygen-saturated PBS' protected RPE cells agains t NUV killing, but superoxide dismutase did not protect the RPE cells against NUV killing. These findings demonstrate that NUV possesses a l ethal effect on RPE cells in vitro. Two key factors determine the magn itude and nature of this lethal effect: first, total NUV energy dose d etermines the nature of NUV's lethal effect; second, RPE growth condit ions suggest the importance of cell-cell interaction in protecting the se cells from NUV injury. Because an oxygen-free condition abolishes N UV lethality, it suggests that the oxidative stress is directly relate d to NW lethal action. The selective inhibition by catalase of NUV kil ling of RPE cells suggests that the killing is oxidative species speci fic. NUV radiation might be highly risky to RPE viability in vivo, esp ecially when the integrity of the RPE layer has been lost.