Near-Infrared Irradiation Nonthermally Induces Long-lasting Vasodilation by Causing Apoptosis of Vascular Smooth Muscle Cells

Background: Moderate sunburn after prolonged sun exposure is thought to cause long-lasting inflammatory vasodilation due to thermal and ultraviolet radiation from the sun. We previously reported that near-infrared irradiation that simulates solar near-infrared can penetrate the skin and nonthermally affect the dermis, superficial muscles, and other tissues. To clarify the possible effect of near-infrared on long-lasting vasodilation, we evaluated how near-infrared affects subcutaneous vascular smooth muscle cells in rats. Methods: The central back tissues of rats were irradiated with a near-infrared device that simulates solar radiation, which has specialized contact cooling to avoid thermal effects. The total energy emitted was equivalent to approximately 8.75 hours of sunbathing in North America. Histological evaluation was performed on the subdermal plexus over the panniculus carnosus muscle of the near-infrared–irradiated rats at postirradiation days 7, 30, 60, and 90, and compared with nonirradiated controls. The vascular smooth muscle cells were evaluated by the transferase-mediated dUTP nick-end labeling assay and staining with an anti-smooth muscle actin antibody. Results: There was no evidence of inflammation by the increased movement of leucocytes around the dilated vessels in irradiated samples. Near-infrared irradiation induced apoptosis of the vascular smooth muscle cells and significantly induced intense, long-lasting vasodilation of the subdermal plexus at postirradiation day 7. Conclusions: Near-infrared irradiation nonthermally induces long-lasting vasodilation by causing apoptosis of vascular smooth muscle cells. Since solar near-infrared radiation nonthermally induces damage of the subcutaneous tissues, exposed skin should be protected with sunscreens that block not only ultraviolet but also near-infrared radiation.