Analysis of microvascular permeability to macromolecules by video-image digital processing

The dynamics of macromolecular transport across microvascular walls were studied in the hamster cheek pouch by intravital fluorescence microscopy. A graded series of fluorescein isothiocyanate-labeled dextrans (FITC-Dx) of 20,000-70,000 MW was used as macromolecular tracers. The time-dependent extravasation of FITC-Dxs was videotaped for about 2.5 hr to allow for tracer equilibration in the interstitial space. Permeation of macromolecules from individual microvessels was quantified by digital video-image processing. Histograms of the light intensity distributions for selected fields at various times were measured and used to construct integral optical density-time profiles of the extravasated fluorochromes for particular leaky sites. A nonlinear regression algorithm was employed to determine the effective microvascular permeability (P) for the macromolecules studied using a one-dimensional two-compartmental diffusion model and a step change in macromolecular concentration at the boundary. The calculated P's (X 10(-8) cm/sec) were 47.8 +/- 8.7 for FITC-Dx 20; 31.7 +/- 5.9 for FITC-Dx 40, and 17.5 +/- 4.1 for FITC-Dx 70. Our values are comparable to those obtained by whole organ techniques. The observed differences can be explained by explicit consideration of interstitial resistance in the calculations.