Macromolecule transfer through mesothelium and connective tissue.

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Abstract
Diffusional permeability ( P) to inulin ( P in), albumin ( P alb), and dextrans [70 ( P dx 70), 150 ( P dx 150), 550 ( P dx 550), and 2,000 ( P dx 2,000)] was determined in specimens of parietal pericardium of rabbits, which may be obtained with less damage than pleura. P in, P alb, P dx 70, P dx 150, P dx 550, and P dx 2,000 were 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 × 10−5cm/s, respectively. P in, P alb, and P dx 70 of connective tissue, obtained after removal of mesothelium from specimens, were 10.3 ± 1.42, 2.97 ± 0.38, and 2.31 ± 0.16 × 10−5 cm/s, respectively. Hence, P in, P alb, and P dx 70 of mesothelium were 0.54, 0.20, and 0.10 × 10−5 cm/s, respectively. Inulin (like small solutes) fitted the relationship P-solute radius for restricted diffusion with a 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, the addition of phospholipids on the luminal side (which decreases pore radius to ∼1.5 nm) halved P in but did not change P alband P dx 70. P in is roughly similar in mesothelium and capillary endothelium, whereas P to macromolecules is greater in mesothelium. The albumin diffusion coefficient through connective tissue was 17% of that in water. Mesothelium provides 92% of resistance to albumin diffusion through the pericardium.