Diffusional permeability (P) to sucrose (Psuc) 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 3H-labeled sucrose and 22Na+ in a luminal (L) or interstitial (I) chamber. Psuc was 2.16 +/- 0.44 for L-->I and 2.63 +/- 0.45 (SE) x 10(-5) cm/s for I-->L, i.e., approximately 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 Psuc was overestimated because stripping damages the mesothelium. PNa+ (x10(-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, Psuc 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), Psuc (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, Psuc of the connective tissue would be 13.2 +/- 0.76 x 10(-5) cm/s. Psuc of the mesothelium, computed from Psuc 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.

SUMMARYPrevious indirect findings have suggested the occurrence of solute-coupled liquid absorption from the pleural space, consistent with Na+-Kt-ATPase on the interstitial side plus a Nat-Ht and Cl--HCO3-double exchange on the luminal side of the pleural mesothelium. To assess whether Nat-glucose cotransport also operates on the luminal side, the relationship between net rate of liquid absorption from the right pleural space (Vnet) and volume of liquid injected into this space (0.5, 1 or 2 ml) was determined in anaesthetized rabbits during hydrothoraces with phloridzin (10-3 M) or with phloridzin plus 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulphonic acid (SITS; 1.5 x 10-4 M). The relationship obtained during hydrothoraces with phloridzin was displaced downwards by 0.09 ml h-' relative to that in control hydrothoraces (P < 0.01). The decrease in Jnet was similar in hydrothoraces of various sizes. The relationship obtained in hydrothoraces with phloridzin plus SITS was displaced downwards by 0. 16 ml h-' relative to that in control hydrothoraces (P < 0.01), i.e. the decrease in Jnet was similar to the sum (0.17 ml h-') of the decreases in Jnet produced individually by phloridzin and by SITS (0.08 ml h-'). The decrease in Jnet was similar in hydrothoraces of differing size. The above findings are consistent with the occurrence of Nat-glucose cotransport on the luminal side of the pleural mesothelium, operating simultaneously with the double exchange also under physiological conditions.