Macromolecular transport in the cat mesentery

Abstract

Movement of macromolecules from the bloodstream across microvascular walls, into and through the surrounding tissue was studied in the cat mesentery with intravital fluorescence microscopy. Rapid passage of small molecules, such as unconjugated fluoroscein isothiocyanate (FITC), MW 389, and FITC-Dextran, MW = 3400, was found along the entire length of most microvessels, although more prominently on venous capillaries and venules. For species with MW 19,000 or higher, a distinctly spotty pattern of movement out of the microvessels was seen, mostly in localized areas on venous capillaries or venules. These localized regions may represent the “large pore” regions frequently postulated. Diffusion coefficients for movement within the interstitial space were calculated for dextrans as well as for bovine serum albumin (BSA), using a one-dimensional diffusion equation with a time dependent boundary condition at the origin. The calculated diffusion coefficients for dextrans of MW 41,200 and below, and for BSA (MW 69,000) are, within experimental error, the same as the free diffusion coefficients for dextrans of the same molecular weight in water. Dextrans with MW = 152,700 and MW = 393,900 showed smaller diffusion coefficients in the tissue than in water, consistent with increased volume exclusion for larger molecules. The calculated diffusion coefficient for BSA is only two-thirds of that in water. This may possibly be explained by the negative charge on the BSA.