Macromolecule transfer through mesothelium and connective tissue

Diffusional permeability (P) to inulin, albumin, and dextrans was determined in specimens of parietal pericardium of rabbits, which may be obtained with less damage than pleura. Pin, Palb, Pdx,70, Pdx,150, Pdx,550 and Pdx,2000 were (x 10-5 cm / s): 0.51 ± 0.06 (SE); 0.18 ± 0.03; 0.097 ± 0.021; 0. 047 ± 0.011; 0.025 ± 0.004 and 0.021 ± 0.005, respectively. Pin, Palb, and Pdx,70 of connective tissue, obtained after having removed mesothelium from specimens, were (x 10-5 cm / s): 10.3 ± 1.42; 2.97 ± 0.38 and 2.31 ± 0.16, respectively. Hence, Pin, Palb, and Pdx,70 of mesothelium were (x 10-5 cm / s): 0.54; 0.20 and 0.10, respectively. Inulin (like small solutes) fitted the relationship P - solute radius for restricted diffusion with 6 nm “pore” radius, whereas macromolecules were much above it. Hence, macromolecule transfer mainly occurs through “large pores” and / or transcytosis. In line with this, addition of phospholipids on luminal side (which decreases “pore” radius to  1.5 nm) halved Pin, but did not change Palb and Pdx,70. Pin is roughly similar in mesothelium and capillary endothelium, whereas P to macromolecules is greater in mesothelium. Albumin diffusion coefficient through connective tissue was 17 % of that in water. Mesothelium provides 92 % of resistance to albumin diffusion through pericardium.