Elementary processes in cells after light absorption do not depend on the degree of polarization: implications for the mechanisms of laser phototherapy.

OBJECTIVE The objective of this work was to evaluate the importance of the degree of light polarization in stimulation of cellular metabolism. BACKGROUND DATA Although the possible role of polarization's effects on the mechanisms of laser phototherapy is sometimes discussed in the literature, there are still no clear answers. MATERIAL AND METHODS A model system (HeLa cell suspension) was used in which the lengths of light scattering (l sc) and absorption (l a) were much larger than the thickness of the irradiated layer (L = 3 mm). The cell suspension (1 x 10(6) cells/cm3) was irradiated with a diode laser (lambda = 637 nm, D = 65.7 J/m2, tau = 10 sec, I = 6.57 W/m2). The polarization degree (99.4%, 60.9%, and 34.2%) of the beam was changed by means of optical fibers of different lengths. The irradiated suspension was incubated at 37 degrees C for 30 min, and the attached cells were counted afterwards. RESULTS The cell fraction stimulated to adhere by red light at 637 nm was nearly the same in all three experimental groups (58.1% +/- 2.5%, 57.6% +/- 3.5%, and 62.5% +/- 3.2% for degree of beam polarization of 99.4%, 66.9%, and 34.2%, respectively). There was no statistically significant difference in these results (p <0.8, <0.6, and <0.7, respectively). At the same time, all three groups had statistically significant differences (p < 0.01) in adherence from the sham-irradiated control group (39.1% +/- 2.2%). CONCLUSION The biological effect (stimulation of cell attachment) of light with lambda = 637 nm on cells in our model system was pronounced, but did not depend on the degree of light polarization. Elementary processes in cells (light absorption and photochemistry) do not appear to depend on the degree of light polarization.