LOW-INTENSITY LASER IRRADIATION INHIBITS TRITIATED-THYMIDINE INCORPORATION IN THE HEMATOPOIETIC-CELL LINES HL-60 AND U937

The purpose of this study was to determine the effect of low intensity laser irradiation (660 nm, 12 mW, 5 kHz) on tritiated thymidine incor poration in two hemopoietic cell lines, HL-60 and U937. Cells were sus pended at a concentration of 1 x 10(6)/ml in their respective serum-fr ee media and irradiated at energy densities from 1.0 to 11.5 J/cm(2). Twenty-four hours after irradiation the cells were assayed for their a bility to incorporate tritiated thymidine (H-3-TdR) in comparison with nonirradiated cells. Analysis by two-way analysis of variance (ANOVA) for unrelated groups showed that laser irradiation at all energy dens ities greater than or equal to 5.8 J/cm(2) produced a significant decr ease in H-3-TdR incorporation (P < 0.05) into HL-60 cells. In U937 cel ls, irradiation at energy densities of 5.8, 7.2, and 11.5 J/cm(2) caus ed a similar reduction in H-3-TdR incorporation (P < 0.01), although n ot at 8.6 and 9.6 J/cm(2) The temperature of each cell suspension was recorded both during and immediately postirradiation, and no significa nt thermal changes were observed. These findings demonstrate a direct photobiological effect of laser irradiation on these two cell lines. T he precise mechanism for this effect is unknown but may have significa nce in understanding the biological action of laser's known therapeuti c effectiveness in promoting wound repair. (C) 1994 Wiley-Liss, Inc.