Retinal Vessel Oxygen Saturation during 100% Oxygen Breathing in Healthy Individuals.

PURPOSE: To detect how systemic hyperoxia affects oxygen saturation in retinal arterioles and venules in healthy individuals. METHODS: Retinal vessel oxygen saturation was measured in 30 healthy individuals with a spectrophotometric retinal oximeter (Oxymap T1). Oximetry was performed during breathing of room air, 100% oxygen (10 minutes, 6L/min) and then again room air (10 minutes recovery). RESULTS: Mean oxygen saturation rises modestly in retinal arterioles during 100% oxygen breathing (94.5%±3.8 vs. 92.0%±3.7% at baseline, p<0.0001) and dramatically in retinal venules (76.2%±8.0% vs. 51.3%±5.6%, p<0.0001). The arteriovenous difference decreased during 100% oxygen breathing (18.3%±9.0% vs. 40.7%±5.7%, p<0.0001). The mean diameter of arterioles decreased during 100% oxygen breathing compared to baseline (9.7±1.4 pixels vs. 10.3±1.3 pixels, p<0.0001) and the same applies to the mean venular diameter (11.4±1.2 pixels vs. 13.3±1.5 pixels, p<0.0001). CONCLUSIONS: Breathing 100% oxygen increases oxygen saturation in retinal arterioles and more so in venules and constricts them compared to baseline levels. The dramatic increase in oxygen saturation in venules reflects oxygen flow from the choroid and the unusual vascular anatomy and oxygen physiology of the eye.

Flicker-induced retinal arteriole dilation is reduced by ambient lighting.

PURPOSE: To investigate the impact of ambient room lighting on the magnitude of flicker light-induced retinal vasodilations in healthy individuals. METHODS: Twenty healthy nonsmokers participated in a balanced 2 × 2 crossover study. Retinal vascular imaging was performed with the dynamic vessel analyzer under reduced or normal ambient lighting, then again after 20 minutes under the alternate condition. Baseline calibers of selected arteriole and venule segments were recorded in measurement units. Maximum percentage dilations from baseline during 20 seconds of luminance flicker were calculated from the mean of three measurement cycles. Within-subject differences were assessed by repeated measures analysis of variance with the assumption of no carryover effects and pairwise comparisons from the fitted model. RESULTS: Mean (SD) maximum arteriole dilations during flicker stimulation under reduced and normal ambient lighting were 4.8% (2.3%) and 4.1% (1.9%), respectively (P = 0.019). Maximum arteriole dilations were (mean ± 95% confidence interval) 0.7% ± 0.6% lower under normal ambient lighting compared with reduced lighting. Ambient lighting had no significant effect on maximum venular dilations during flicker stimulation or on the baseline calibers of arterioles or venules. CONCLUSIONS: Retinal arteriole dilation in response to luminance flicker stimulation is reduced under higher ambient lighting conditions. Reduced responses with higher ambient lighting may reflect reduced contrast between the ON and OFF flicker phases. Although it may not always be feasible to conduct studies under reduced lighting conditions, ambient lighting levels should be consistent to ensure that comparisons are valid.

Dynamic responses in retinal vessel caliber with flicker light stimulation in eyes with diabetic retinopathy.

PURPOSE: This study investigated the responses of retinal vessels to flickering light in diabetic patients with various stages of diabetic retinopathy (DR). METHODS: This cross-sectional observational study evaluated adult subjects with diabetes mellitus. The Dynamic Vessel Analyzer (DVA) was used to measure retinal vascular dilatation in response to diffuse illuminance flicker. Diabetic retinopathy was graded from retinal photography. RESULTS: There were 279 subjects in total, with a mean age of 59.9 ± 9.2 years. The majority were male (73%) and the mean HbA1c level and mean duration of diabetes were 7.7% ± 1.4% and 13.9 ± 10.4 years, respectively. After adjustments for age, sex, smoking, duration of diabetes, HbA1c, hypertension, and hyperlipidemia, the responses of both retinal arterioles and venules to flicker stimulation decreased continuously with increasing stages of diabetic retinopathy (P = 0.008 and <0.001, respectively). Subjects with reduced arteriolar dilation responses were more likely to have any DR (odds ratio, OR, 1.20, [95% confidence interval (CI), 1.01-1.45], P = 0.045, per SD decrease). Subjects with reduced venular dilation responses were more likely to have any DR, moderate DR, or vision-threatening DR (OR: 1.27 [1.04-1.53], P = 0.02; OR: 1.27 (1.06-1.49), P = 0.007; and OR: 1.51 (1.14-1.50), P = 0.002; per SD decrease, respectively). CONCLUSIONS: The responses of retinal arterioles and venules to flickering light are reduced in subjects with DR, and decrease progressively with more severe stages of DR.

External beam radiotherapy for head and neck cancers is associated with increased variability in retinal vascular oxygenation.

BACKGROUND: Radiation retinopathy is a possible post-treatment complication of radiation therapy. The pathophysiologic mechanism is hypothesized to be microvascular in origin, but evidence is limited. In an effort to study retinal oxygenation in these patients, we herein evaluate the repeatability and variability of retinal oximetry measurements in subjects who had previously received radiation and make comparisons to a cohort of unirradiated subjects. METHODS: Using retinal oximetry, a non-invasive imaging modality, we performed in vivo measurements of arteriole (SaO2) and venule SO2 (SvO2) in subjects (n = 9, 18 retinas) who had received incidental radiation to their retinas (≥ 45 Gy to one retina) and in healthy subjects (n = 20, 40 retinas). A total of 1367 SO2 observations on 593 vessels in 29 persons were analyzed to assess three sources of variance in vessel SO2: 1) variance in repeated measurements of the same vessel ("repeatability"), 2) variance in different vessels within the same subject ("within-subject variability"), and 3) variance between subjects ("between-subject variability"). RESULTS: Retinal oximetry measurements were highly repeatable in both irradiated patients and unirradiated subjects. The within-subject variability of SvO2 and SaO2 measurements constituted the highest component of variance in both groups and was significantly higher in venules vs. arterioles (relative effect size 1.8, p<0.001) and in irradiated subjects vs. unirradiated subjects (relative effect size 1.6, p<0.001). CONCLUSIONS: Retinal oximetry is a highly repeatable technology and can be reliably used to study vascular oxygenation in irradiated subjects. Different vessels within the same subject exhibit a high degree of variability, suggesting that pooled analyses of multiple vessels are most likely to be informative of regional retinal oxygenation. Finally, irradiated subjects exhibited significantly higher within-subject variability in SO2 measurements, suggesting that radiation may cause regional alterations in retinal oxygen delivery and/or metabolism.

Microbeam radiation-induced tissue damage depends on the stage of vascular maturation.

PURPOSE: To explore the effects of microbeam radiation (MR) on vascular biology, we used the chick chorioallantoic membrane (CAM) model of an almost pure vascular system with immature vessels (lacking periendothelial coverage) at Day 8 and mature vessels (with coverage) at Day 12 of development. METHODS AND MATERIALS: CAMs were irradiated with microplanar beams (width, ∼25 µm; interbeam spacing, ∼200 µm) at entrance doses of 200 or 300 Gy and, for comparison, with a broad beam (seamless radiation [SLR]), with entrance doses of 5 to 40 Gy. RESULTS: In vivo monitoring of Day-8 CAM vasculature 6 h after 200 Gy MR revealed a near total destruction of the immature capillary plexus. Conversely, 200 Gy MR barely affected Day-12 CAM mature microvasculature. Morphological evaluation of Day-12 CAMs after the dose was increased to 300 Gy revealed opened interendothelial junctions, which could explain the transient mesenchymal edema immediately after irradiation. Electron micrographs revealed cytoplasmic vacuolization of endothelial cells in the beam path, with disrupted luminal surfaces; often the lumen was engorged with erythrocytes and leukocytes. After 30 min, the capillary plexus adopted a striated metronomic pattern, with alternating destroyed and intact zones, corresponding to the beam and the interbeam paths within the array. SLR at a dose of 10 Gy caused growth retardation, resulting in a remarkable reduction in the vascular endpoint density 24 h postirradiation. A dose of 40 Gy damaged the entire CAM vasculature. CONCLUSIONS: The effects of MR are mediated by capillary damage, with tissue injury caused by insufficient blood supply. Vascular toxicity and physiological effects of MR depend on the stage of capillary maturation and appear in the first 15 to 60 min after irradiation. Conversely, the effects of SLR, due to the arrest of cell proliferation, persist for a longer time.

Dynamic retinal vessel response to flicker in obesity: A methodological approach.

Obesity and related metabolic disorders affect vascular endothelial function. The use of the Dynamic Vessel Analyzer (DVA) represents a modern methodological approach to analyze vascular function in the retinal microcirculation. Whether the dynamic reaction to flicker stimulation in retinal vessels is altered in obese subjects is investigated. Retinal vessel reactions to flicker stimulation were examined by DVA in 46 obese individuals (49.6±10.0years) and 46 age- and gender-matched healthy controls. The clinical examination included anthropometry, blood pressure measurements and blood sampling. Mean maximal arteriolar dilation in response to flicker was reduced in the obese group (3.2±1.8%) compared to controls (4.1±2.0%, p<0.05) and the time to maximal arteriolar dilation was prolonged (18.0±9.4s vs. 14.6±3.8s, p=0.03). In addition, mean maximal venular dilation was reduced in obese subjects (3.9±1.7% vs. 4.7±1.8%, p<0.05). Among the microvascular parameters, the most significant correlation with waist circumference was found for the "area under the reaction curve 50-80s after stimulation" in arterioles (r=-0.40; p<0.001). Functional retinal arteriolar reactivity to flicker stimulation differs between obese and healthy lean subjects. Time course analysis of retinal vessel response and its quantitative parameters can comprehensively characterize alterations of retinal vessel reactivity in metabolic disease.

Flicker light-induced retinal vasodilation in diabetes and diabetic retinopathy.

OBJECTIVE: Flicker light-induced retinal vasodilation may reflect endothelial function in the retinal circulation. We investigated flicker light-induced vasodilation in individuals with diabetes and diabetic retinopathy. RESEARCH DESIGN AND METHODS: Participants consisted of 224 individuals with diabetes and 103 nondiabetic control subjects. Flicker light-induced retinal vasodilation (percentage increase over baseline diameter) was measured using the Dynamic Vessel Analyzer. Diabetic retinopathy was graded from retinal photographs. RESULTS: Mean +/- SD age was 56.5 +/- 11.8 years for those with diabetes and 48.0 +/- 16.3 years for control subjects. Mean arteriolar and venular dilation after flicker light stimulation were reduced in participants with diabetes compared with those in control subjects (1.43 +/- 2.10 vs. 3.46 +/- 2.36%, P < 0.001 for arteriolar and 2.83 +/- 2.10 vs. 3.98 +/- 1.84%, P < 0.001 for venular dilation). After adjustment for age, sex, diabetes duration, fasting glucose, cholesterol and triglyceride levels, current smoking status, systolic blood pressure, and use of antihypertensive and lipid-lowering medications, participants with reduced flicker light-induced vasodilation were more likely to have diabetes (odds ratio 19.7 [95% CI 6.5-59.1], P < 0.001 and 8.14 [3.1-21.4], P < 0.001, comparing lowest vs. highest tertile of arteriolar and venular dilation, respectively). Diabetic participants with reduced flicker light-induced vasodilation were more likely to have diabetic retinopathy (2.2 [1.2-4.0], P = 0.01 for arteriolar dilation and 2.5 [1.3-4.5], P = 0.004 for venular dilation). CONCLUSIONS: Reduced retinal vasodilation after flicker light stimulation is independently associated with diabetes status and, in individuals with diabetes, with diabetic retinopathy. Our findings may therefore support endothelial dysfunction as a pathophysiological mechanism underlying diabetes and its microvascular manifestations.

Radiation-induced astrogliosis and blood-brain barrier damage can be abrogated using anti-TNF treatment.

PURPOSE: In this article, we investigate the role of tumor necrosis factor-alpha (TNF) in the initiation of acute damage to the blood-brain barrier (BBB) and brain tissue following radiotherapy (RT) for CNS tumors. METHODS AND MATERIALS: Intravital microscopy and a closed cranial window technique were used to measure quantitatively BBB permeability to FITC-dextran 4.4-kDa molecules, leukocyte adhesion (Rhodamine-6G) and vessel diameters before and after 20-Gy cranial radiation with and without treatment with anti-TNF. Immunohistochemistry was used to quantify astrogliosis post-RT and immunofluorescence was used to visualize protein expression of TNF and ICAM-1 post-RT. Recombinant TNF (rTNF) was used to elucidate the role of TNF in leukocyte adhesion and vessel diameter. RESULTS: Mice treated with anti-TNF showed significantly lower permeability and leukocyte adhesion at 24 and 48 h post-RT vs. RT-only animals. We observed a significant decrease in arteriole diameters at 48 h post-RT that was inhibited in TNF-treated animals. We also saw a significant increase in activated astrocytes following RT that was significantly lower in the anti-TNF-treated group. In addition, immunofluorescence showed protein expression of TNF and ICAM-1 in the cerebral cortex that was inhibited with anti-TNF treatment. Finally, administration of rTNF induced a decrease in arteriole diameter and a significant increase in leukocyte adhesion in venules and arterioles. CONCLUSIONS: TNF plays a significant role in acute changes in BBB permeability, leukocyte adhesion, arteriole diameter, and astrocyte activation following cranial radiation. Treatment with anti-TNF protects the brain's microvascular network from the acute damage following RT.

Evaluation of the treatment of venous lakes with the 595-nm pulsed-dye laser: a case series.

Venous lakes (VLs) are common benign venous ectasias in the upper dermis. They are treated to improve cosmesis and occasionally to prevent bleeding. Numerous methods have been used, such as cryotherapy, infrared coagulation and various types of lasers. They are variable in their success and all can be complicated by scarring. We report our experiences of using the 595 nm pulsed-dye laser (PDL), which has not been previously described. Eight patients were treated but sufficient resolution was achieved in only three patients. The limited success with this laser could be attributed to insufficient thermal energy being generated to close all the blood vessels permanently. A large prospective study would provide further data regarding the efficacy of the PDL. The use of compression and longer pulse durations may improve the efficacy of the 595 nm PDL to treat VLs.

Protective effect of superoxide dismutase in radiation-induced intestinal inflammation.

PURPOSE: To analyze the therapeutic value of Cu/Zn-superoxide dismutase (SOD1) supplementation in an experimental model of radiation-induced intestinal inflammation and explore its mechanistic effects. METHODS AND MATERIALS: Mice were subjected to abdominal irradiation with 10 Gy or sham irradiation and studied 24 or 72 hours after radiation. Groups of mice were treated with 0.1, 4, or 6 mg/kg/day of SOD1 or vehicle. Leukocyte-endothelial cell interactions in intestinal venules were assessed by intravital microscopy. Endothelial intercellular adhesion molecule-1 (ICAM-1) expression was determined with radiolabeled antibodies. Effects of SOD1 on histologic damage and levels of lipid hydroperoxides were also measured. RESULTS: A significant increase in the flux of rolling leukocytes and number of firmly adherent leukocytes in intestinal venules was observed at 24 and 72 hours after irradiation. Treatment with SOD1 had no effect on leukocyte rolling but significantly and dose-dependently decreased firm leukocyte adhesion to intestinal venules. Treatment with SOD1 at doses that reduced leukocyte recruitment abrogated the increase in hydroperoxides in intestinal tissue and ICAM-1 upregulation in intestinal endothelial cells. The inflammatory score, but not a combined histology damage score, was also significantly reduced by SOD1. CONCLUSIONS: Treatment with SOD1 decreases oxidative stress and adhesion molecule upregulation in response to abdominal irradiation. This is associated with an attenuation of the radiation-induced intestinal inflammatory response.

Irradiation increases the interactions of platelets with the endothelium in vivo: analysis by intravital microscopy.

Adhesion of platelets to the endothelium is believed to be a major factor contributing to thrombosis and vascular occlusion after radiotherapy or endovascular irradiation. In the present study, platelet-endothelium interactions were analyzed in vivo by intravital microscopy in mesenteric venules of mice according to three parameters: (1) platelet rolling, (2) platelet adhesion, and (3) the presence of platelet clusters. A 10-Gy total-body irradiation of mice resulted in an increase in the frequency of appearance of these three types of platelet-endothelium interactions in postcapillary venules 6 and 24 h after exposure, whereas only minor alterations were seen in large venules. In addition, the duration of platelet adhesion was increased 24 h after irradiation in both postcapillary and large venules. However, P-selectin was not up-regulated on the platelet membrane and platelet-leukocytes were not seen rolling together, suggesting that changes in platelet-endothelial cell interaction result from endothelial cell activation rather than platelet activation. Our data suggest that irradiation transforms resting endothelial cells to a pro-adhesive surface for platelets, which could ultimately lead to thrombosis.

Near-wall micro-PIV reveals a hydrodynamically relevant endothelial surface layer in venules in vivo.

High-resolution near-wall fluorescent microparticle image velocimetry (micro-PIV) was used in mouse cremaster muscle venules in vivo to measure velocity profiles in the red cell-depleted plasma layer near the endothelial lining. micro-PIV data of the instantaneous translational speeds and radial positions of fluorescently labeled microspheres (0.47 microm) in an optical section through the midsagittal plane of each vessel were used to determine fluid particle translational speeds. Regression of a linear velocity distribution based on near-wall fluid-particle speeds consistently revealed a negative intercept when extrapolated to the vessel wall. Based on a detailed three-dimensional analysis of the local fluid dynamics, we estimate a mean effective thickness of approximately 0.33 micro m for an impermeable endothelial surface layer or approximately 0.44 micro m assuming the lowest hydraulic resistivity of the layer that is consistent with the observed particle motions. The extent of plasma flow retardation through the layer required to be consistent with our micro-PIV data results in near complete attenuation of fluid shear stress on the endothelial-cell surface. These findings confirm the presence of a hydrodynamically effective endothelial surface layer, and emphasize the need to revise previous concepts of leukocyte adhesion, stress transmission to vascular endothelium, permeability, and mechanotransduction mechanisms.

Lack of in vivo function of CD31 in vascular thrombosis.

A murine model of endothelial cell injury-based vascular thrombosis was used to test the role of platelet-endothelial cell adhesion molecule-1 (PECAM-1, CD31) in blood cell aggregate formation and vessel occlusion in vivo. Photochemically-induced thrombus formation was analyzed in detail using intravital fluorescence microscopy of individual microvessels in cremaster muscle preparations of CD31-deficient and wildtype mice. In venules, epi-illumination induced rapid thrombus formation with first platelet deposition after 0.56 +/- 0.11 min and complete vessel occlusion within 5.05 +/- 0.45 min. In arterioles, thrombus formation was markedly delayed with first platelet deposition after 3.03 +/- 0.47 min and complete vessel occlusion within 10.04 +/- 1.26 min. Kinetics of thrombus formation in both venules (first platelet deposition: 0.52 +/- 0.1 min; vessel occlusion: 5.03 +/- 0.52 min) and arterioles (first platelet deposition: 3.06 +/- 0.68 min; vessel occlusion: 10.02 +/- 1.38 min) of CD31-deficient mice was found almost identical compared with that in wildtype animals. Tail bleeding time was 233 +/- 24 s in wildtype and 243 +/- 32 s in CD31-deficient mice. Moreover, CD31-deficient and wildtype mice revealed comparable interaction of leukocytes to endothelium. This study shows for the first time in vivo that CD31 is not critically involved in blood cell thrombus formation upon endothelial cell injury.

Laser fluence for permanent damage of cutaneous blood vessels.

Treatment of vascular disorders may be improved by a more thorough understanding of laser-blood vessel interaction. In this study, the probability of permanent damage to a given type and size of blood vessel was determined as a function of fluence at the top (superficial edge) of the vessel lumen. A 532 nm wavelength, 10 ms pulse duration, 3 mm spot size laser was used to perform approximately 250 irradiations of subdermal blood vessels in the hamster dorsal skin flap preparation. The radiant exposure required for a 50% probability of permanent damage was calculated using a probit analysis of experimental results. Threshold radiant exposure increased with larger blood vessel diameters and was greater for arterioles than venules. Monte Carlo modeling of a typical blood vessel geometry revealed that fluence at the top of the blood vessel lumen was amplified by a factor of approximately 2.4 over tissue surface radiant exposure, due to light scattering in the tissue and internal reflection at the skin-air interfaces.

Role of leukocyte-endothelial cell adhesion in radiation-induced microvascular dysfunction in rats.

BACKGROUND/AIMS: Recent evidence suggests an active role of endothelial cells and inflammatory cells in radiation-induced vascular dysfunction and organ damage. The aim of this study was to characterize the endothelial cell-leukocyte interactions, their molecular mechanisms, and the associated microvascular dysfunction in postcapillary venules exposed to ionizing radiation. METHODS: Leukocyte rolling, adherence, and emigration and leakage of fluorescein isothiocyanate albumin in rat mesenteric venules were measured in control conditions and at 2, 4, and 6 hours after abdominal irradiation. Some animals were treated with monoclonal antibodies against leukocyte (CD18) or endothelial cell (intercellular adhesion molecule 1, P-selectin) adhesion molecules before radiation and 5 hours thereafter. RESULTS: In comparison with controls, irradiated animals had a marked increase in the number of rolling leukocytes at 2 hours after radiation. In animals studied 6 hours after radiation, a significant increment in the number of adherent and emigrated leukocytes was observed. This was associated with an increased permeability to fluorescein isothiocyanate albumin. Treatment with antibodies against either CD18 or intercellular adhesion molecule 1, but not P-selectin, significantly attenuated leukocyte adherence, emigration, and the increase in permeability induced by radiation. CONCLUSIONS: Radiation-induced leukocyte adherence and emigration involves an interaction between CD11/CD18 on leukocytes and intercellular adhesion molecule 1 on vascular endothelium. These interactions are implicated in the early increase in vascular permeability after irradiation.

The effects of photodynamic therapy using differently substituted zinc phthalocyanines on vessel constriction, vessel leakage and tumor response.

The effects of four different zinc phthalocyanines were studied during and after photodynamic therapy (PDT). Measurements of vessel constriction, vessel leakage, tumor interstitial pressure, eicosanoid release, and tumor response of chondrosarcoma were made in Sprague-Dawley rats. Animals were injected intravenously with 1 mumol/kg of mono-, di-, or tetrasulfonated zinc phthalocyanine, or 1 mumol/kg of a zinc phthalocyanine substituted with four tertiary butyl groups. Tissues were exposed to 400 J/cm2 670 nm light 24 h after photosensitizer injection. An additional group of animals was given indomethacin before treatment. The use of the monosulfonated and tertiary butyl substituted zinc phthalocyanines in PDT caused the release of specific eicosanoids, caused vessel constriction, and induced venule leakage and increases in tumor interstitial pressure. Tumor cures of 27% and 7% were observed. Photodynamic therapy using the disulfonated zinc phthalocyanine did not induce vessel constriction or the release of eicosanoids, however, tumor cure was 43%. The use of the tetrasulfonated zinc phthalocyanine caused intermediate effects between the mono- and disulfonated compounds. The administration of indomethacin to animals completely inhibited the effects of PDT using the monosulfonated compound but had minimal effects on treatment using the disulfonated compound. This suggests that the monosulfonated and disulfonated compounds act by different mechanisms of destruction.

Microscopic instrumentation and analysis of laser-tissue interaction in a skin flap model.

A dorsal skin flap model for microcirculatory studies has been modified for "in vivo" studies of laser-tissue interaction with microcirculation. An experimental apparatus has been built implementing a laser delivery system, video microscopy during irradiation, and thermal recordings. This model has been used to study irradiation effects on microcirculation using the argon laser (488 and 514.5 nm) and the argon pumped dye laser at 577 nm. The results include: measurements of the optical properties of the model; dosimetry measurements for the production of embolized and stationary coaguli in arterioles and venules; and focal vessel disappearance of venules irradiated with the argon or the argon pumped dye laser at 577 nm; a method to determine light attenuation in the model; a unique method for measurements of blood flow velocity in arterioles and venules and measurements obtained with this method; measurements of transient and steady state temperatures during irradiation and a study of laser induced photorelaxation phenomena in venules.

[The link between the parameters of the thrombus formation process and blood flow velocity in the microvessels of the rat mesentery]. / Sviaz' parametrov protsessa tromboobrazovaniia i skorosti krovotoka v mikrososudakh bryzheiki krysy.

The role of shear rate in the processes of thrombocytes adhesion and aggregation was studied in vivo and in vitro. Certain parameters were obtained describing the thrombo-formation process in the rat mesenteric venules in different blood flow velocities. The experimental model took into consideration the dependence of the thrombocytes activation period on the distance from lesioned portion of vessel's endothelium.

Laser-irradiation-induced relaxation of blood vessels in vivo.

The response of blood vessels to laser irradiation in vivo was studied in the dorsal skin flap glass window chamber model of hamsters. The vasodilatory response of venules was critically dependent on the wavelength of irradiating laser. Relaxation was not produced in arterioles, although it was tried repeatedly. Vessels were irradiated with the 514.5 nm single line argon laser with irradiances from 1 to 10 W/cm2 on a 1.2 mm-diameter spot. Irradiation of venules with 2.2 W/cm2 and 4.25 W/cm2 produced reversible relaxation. Venules relaxed initially and after the interruption of irradiation returned to their original diameter. At higher irradiances (8.5 W/cm2) an irreversible relaxation was observed. At irradiances of 10 W/cm2 and above initial relaxation was accompanied with constriction, focal coaguli, and hemostasis. Irradiation with the argon-pumped dye laser at 595 nm did not produce any significant relaxation.