Transient analysis of macromolecular transport across microvascular wall and into interstitium

We studied the dynamics of macromolecular transport across the microvascular wall in the hamster cheek pouch using intravital microscopy and digital video-image analysis. We used fluorescein isothiocyanate-dextrans of 70,000 and 150,000 Da (FITC-Dextran 70 and 150, respectively) as tracers. We applied our mathematical model and our in vivo calibration to determine the diffusion coefficient (D) and the average fluid velocity (V) in the microvascular wall and in the interstitium from the experimental data. The value of D for FITC-Dextran 70 was 0.90 ± 0.04 x 10^-11 cm²/s in the wall and 1.29 ± 0.05 x 10^-8 cm²/s in the interstitium. In both regions, V was 2.05 ± 0.05 x 10^-8 cm/s. The transport parameters for FITC-Dextran 150 were 0.27 ± 0.02 x 10^-11 cm²/s, 0.55 ± 0.05 x 10^-8 cm²/s, and 1.71 ± 0.48 x 10^-8 cm/s for D in the wall and interstitium and V, respectively. The topical application of either calcium ionophore A23187 (7 x 10^-7 M) or bradykinin (5 x 10^-7 M) increased D for FITC-Dextran 70 and 150 2-fold and V 10-fold relative to their control values. We used these values to quantify the relative importance of the diffusive and convective mechanisms in the total solute flux. Molecular diffusion dominates convective transport in both the microvascular wall and the interstitial space.