Pulsed ultraviolet laser irradiation produces endothelium-independent relaxation of vascular smooth muscle

Recent studies have shown that continuous wave laser irradiation induces contraction of vascular smooth muscle, except at powers far below the threshold for tissue ablation. To determine the corresponding effects of pulsed laser irradiation on vascular smooth muscle tone, vascular rings of rabbit thoracic aorta were mounted isometrically with 1 g tension in Krebs-bicarbonate buffer and irradiated with 308 or 351 nm from an excimer laser through a 400-microns optical fiber. A total of 250 exposures were performed with 1-6.5 mJ/pulse (fluence = 0.8-5.5 J/cm2), 10-50 Hz, and cumulative exposures of 10-120 seconds. Excimer laser irradiation in combinations of pulse energy (PE), repetition rate (RR), and cumulative exposure below, at, or above threshold for tissue ablation consistently produced relaxation unassociated with contraction in each of the 250 exposures. For the total 250 exposures, the magnitude of relaxation (reduction in recorded tension, Rmax) was 55 +/- 4% (mean +/- SEM) of maximum vasomotor reactivity recorded in the specimen in response to administration of serotonin. Rmax varied directly with both PE and RR. When PE was increased from 1 to 5 mJ/pulse (n = 13), Rmax increased from 57 +/- 19% to 80 +/- 19% (p less than 0.0001); when RR was increased from 10 to 50 Hz (n = 10), Rmax increased from 27 +/- 8 to 46 +/- 8 (p less than 0.0001). Rmax varied independently of endothelial integrity (assessed anatomically and pharmacologically) and wavelength (308 vs. 351 nm). Simultaneously recorded tissue-temperature profiles disclosed that during pulsed laser irradiation, tissue temperature rise did not exceed 5 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)