Macromolecular Permeability and Hydraulic Conductivity Through Large Pores Across a Single Venular Capillary

Abstract

To determine the hydraulic conductivity and permeability of macromolecules across capillary and venular walls in the rabbit tenuissimus muscle, we developed a new fluorescence vital microscope in which a pair of well-defined slit laser beams (10–50 μm in thickness) were focused, so as to cross in a chosen depth of the tissue region and to excite the fluorescent tracer intensively at the crossing portion adjusted to the focal plane of the objective. After injecting a fluorescent tracer (FITC-Dextran: FD40, 70, 150; M.W. = 40 000, 70 000, 150 000) via the jugular vein, temporal changes in the tracer concentration within the cross-illuminated portion of the muscle tissue were recorded as fluorescent intensity alterations. We calculated the diffusive (D) and convective (V) components of the extra-vasated tracer by best-fitting the experimental curve with the simulation model based on the “Pore theory”, in which macromolecules are assumed to pass across the vessel wall through the “Large pore” passively buy diffusion and convection. Diffusion permeability (P_d) and hydraulic conductivity (L_e) were determined from D and V values obtained at the centre and venular end of capillaries to compare their differences. The values of Pd at the venular ends were approximately 10 times greater than those at the centre but V values at the venular ends were only about two-fold greater than those of the central capillary. The mechanism bringing about such discrepancy was discussed.