Diffusional permeability of rabbit mesothelium

Diffusional permeability (P) to sucrose (P-suc) and Na+ (PNa+) was determined in specimens of rabbit sternal parietal pericardium, which may be obtained without stripping. Specimens were mounted in an Ussing apparatus with H-3-labeled sucrose and Na-22(+) in a luminal (L) or interstitial (I) chamber. P-suc was 2.16 +/- 0.44 for L --> I and 2.63 +/- 0.45 (SE) x 10(-5) cm/s for I --> L, i.e., similar to 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 previous P-suc was overestimated because stripping damages the mesothelium. PNa+ (x 10(-5) cm/s) was 7.07 +/- 0.71 for L --> I and 7.37 +/- 0.69 x 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 PNa+ was 3.80 +/- 0.32 and 3.76 +/- 0.15 x 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 x 10(-5) cm/s) and PNa+ (7.01 +/- 0.45 x 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 x 10(-5) cm/s. P-suc of the mesothelium, computed from P-suc of the unscraped and scraped specimens, corrected for the effect of unstirred layers (2.54 and 19.4 x 10(-5) cm/s, respectively), was 2.92 and 0.74 x 10(-5) cm/s without and with phospholipids, respectively. Hence, most of the resistance to diffusion of the pericardium is provided by the mesothelium.