Abstract: | Diffusional permeability (P) to sucrose(Psuc) andNa+(PNa+)was determined in specimens of rabbit sternal parietal pericardium,which may be obtained without stripping. Specimens were mounted in anUssing apparatus with 3H-labeledsucrose and22Na+in a luminal (L) or interstitial (I) chamber.Psuc was 2.16 ± 0.44 for LI and 2.63 ± 0.45 (SE) × 105 cm/s for IL,i.e., ~10 times smaller than that previously obtained in strippedspecimens of pleura despite the similarity of intercellular junctionsin pericardium and pleural mesothelium of various species. Thesefindings suggest that previousPsuc wasoverestimated because stripping damages the mesothelium.PNa+ (×105 cm/s) was 7.07 ± 0.71 for LI and 7.37 ± 0.69 × 105 cm/s for IL.Measurements were also done with phospholipids, which are adsorbed onthe luminal side of mesothelium in vivo. With phospholipids in L,Psuc was 0.75 ± 0.10 and 0.65 ± 0.08 andPNa+was 3.80 ± 0.32 and 3.76 ± 0.15 × 105 cm/s for LI andIL, respectively, i.e., smaller than without phospholipids.With phospholipids in I (where they are not adsorbed), Psuc (2.33 ± 0.42 × 105 cm/s) andPNa+(7.01 ± 0.45 × 105 cm/s) were similar tothose values without phospholipids. Hence, adsorbed phospholipidsdecrease P of mesothelium. If themesothelium were scraped away from the specimen,Psuc of theconnective tissue would be 13.2 ± 0.76 × 105 cm/s.Psuc of themesothelium, computed fromPsuc of theunscraped and scraped specimens, corrected for the effect of unstirredlayers (2.54 and 19.4 × 105 cm/s, respectively),was 2.92 and 0.74 × 105 cm/s without and withphospholipids, respectively. Hence, most of the resistance to diffusionof the pericardium is provided by the mesothelium. |