Low intensity laser irradiation inhibits tritiated thymidine incorporation in the hemopoietic 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 incorporation in two hemopoietic cell lines, HL-60 and U937. Cells were suspended at a concentration of 1 x 10(6)/ml in their respective serum-free media and irradiated at energy densities from 1.0 to 11.5 J/cm2. Twenty-four hours after irradiation the cells were assayed for their ability to incorporate tritiated thymidine (3H-TdR) in comparison with nonirradiated cells. Analysis by two-way analysis of variance (ANOVA) for unrelated groups showed that laser irradiation at all energy densities > or = 5.8 J/cm2 produced a significant decrease in 3H-TdR incorporation (P < 0.05) into HL-60 cells. In U937 cells, irradiation at energy densities of 5.8, 7.2, and 11.5 J/cm2 caused a similar reduction in 3H-TdR incorporation (P < 0.01), although not at 8.6 and 9.6 J/cm2. The temperature of each cell suspension was recorded both during and immediately postirradiation, and no significant thermal changes were observed. These findings demonstrate a direct photobiological effect of laser irradiation on these two cell lines. The precise mechanism for this effect is unknown but may have significance in understanding the biological action of laser's known therapeutic effectiveness in promoting wound repair.