Diffusional permeability of rabbit mesothelium

Diffusional permeability (P) to sucrose (P_suc) and Na⁺($P_{{\text{Na}}^{+}}$ ) was determined in specimens of rabbit sternal parietal pericardium, which may be obtained without stripping. Specimens were mounted in an Ussing apparatus with ³H-labeled sucrose and²²Na⁺in a luminal (L) or interstitial (I) chamber.P_suc was 2.16 ± 0.44 for L→I and 2.63 ± 0.45 (SE) × 10⁻⁵ cm/s for I→L, i.e., ∼10 times smaller than that previously obtained in stripped specimens of pleura despite the similarity of intercellular junctions in pericardium and pleural mesothelium of various species. These findings suggest that previousP_suc was overestimated because stripping damages the mesothelium.$P_{{\text{Na}}^{+}}$ (×10⁻⁵ cm/s) was 7.07 ± 0.71 for L→I and 7.37 ± 0.69 × 10⁻⁵ cm/s for I→L. Measurements were also done with phospholipids, which are adsorbed on the luminal side of mesothelium in vivo. With phospholipids in L,P_suc was 0.75 ± 0.10 and 0.65 ± 0.08 and$P_{{\text{Na}}^{+}}$ was 3.80 ± 0.32 and 3.76 ± 0.15 × 10⁻⁵ cm/s for L→I and I→L, respectively, i.e., smaller than without phospholipids. With phospholipids in I (where they are not adsorbed),P_suc (2.33 ± 0.42 × 10⁻⁵ cm/s) and$P_{{\text{Na}}^{+}}$ (7.01 ± 0.45 × 10⁻⁵ cm/s) were similar to those values without phospholipids. Hence, adsorbed phospholipids decrease P of mesothelium. If the mesothelium were scraped away from the specimen,P_suc of the connective tissue would be 13.2 ± 0.76 × 10⁻⁵ cm/s.P_suc of the mesothelium, computed fromP_suc of the unscraped and scraped specimens, corrected for the effect of unstirred layers (2.54 and 19.4 × 10⁻⁵ cm/s, respectively), was 2.92 and 0.74 × 10⁻⁵ cm/s without and with phospholipids, respectively. Hence, most of the resistance to diffusion of the pericardium is provided by the mesothelium.