Fluorescein-labeled dextran MW 150K (FITC-Dx 150) is commonly utilized as a macromolecular fluorochrome in inflammatory studies. We examined the influence of FITC-Dx 150 on leukocyte activity as assessed by adherence to cheek pouch microvessels. Transcapillary exchange was evaluated as fluorochrome clearance (Cl). Few leukocytes adhered to the microvascular wall in venous segments prior to and immediately following administration of the dextran fluorochrome. Spontaneous leukocyte adhesion to venous endothelium ensued within 30 min in epi-illuminated, FITC-Dx 150-loaded vessels. However, the Cl of FITC-Dx 150 (1.75 +/- 0.68 nl/min) remained unaltered (2.21 +/- 1.83 nl/min). Hamsters pretreated with FITC-Dx 150 (a minimum of 7 hr prior to experimentation) and subsequently exposed to epi-illumination showed marked leukocyte adhesion in small venules and an impressive pool of marginating leukocytes in large venules. Leukocyte aggregation, thrombus formation, lymphatic distension, and leukocyte adherence to arteriolar endothelium also ensued as a consequence of epi-illumination. Uptake of FITC-Dx 150 by extravascular phagocytes was evident. Some hamsters pretreated with FITC-Dx 150 developed spontaneous leaky sites upon epifluorescent stimulation. Focal accumulation of interstitial phagocytes was suggestive of fluorochrome uptake and elicitation by FITC-Dx 150. These effects were not observed in control animals treated with FITC-albumin. The results of this study suggest caution in employing FITC-Dx 150 as a permeability probe for studies involving inflammatory processes, especially those dependent on neutrophil/endothelial interactions and prolonged circulation time of fluorochrome.
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