Effect of argon laser irradiation on rabbit aortic smooth muscle: evidence for endothelium independent contraction and relaxation.

To study the effect of argon laser irradiation on vascular smooth muscle reactivity, ring segments of rabbit thoracic aorta were mounted isometrically in Krebs bicarbonate buffer. Laser irradiation was performed via a 200-micron core optical fibre interfaced with an argon ion gas laser. Laser irradiation at powers greater than 1.0 watt (W) (n = 34) produced contraction in each case, regardless of the duration of exposure. Conversely, irradiation at powers less than 0.1 W (n = 41) consistently produced relaxation. When the power employed ranged from 0.1 to 1.0 W (n = 100), the typical response consisted of a combination of low amplitude contraction and relaxation. At each power level, the responses observed were independent of the presence or absence of intact endothelium, assessed functionally by the response to acetylcholine and anatomically by scanning electron microscopy. Recordings of tissue temperature profiles performed during continuous-wave laser irradiation suggest that the findings observed in this study may be explained by a combination of heat induced contraction and light induced relaxation. At powers greater than 1.0 W, heat generated by laser irradiation is predominant, causing contraction. At powers less than 0.1 W, light predominates over heat, causing relaxation. When the power range is intermediate between these values, the heterogeneous response recorded reflects the combination of light and heat. In all cases, contraction and relaxation are endothelium independent.