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1 Istituto di Fisiologia Umana I and 2 Dipartimento di Fisiologia e Biochimica Generali, Università di Milano, 20133 Milan, Italy
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) × 10
5 cm/s for IL,
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 previous
Psuc was
overestimated 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 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 × 105 cm/s for LI and
IL, respectively, i.e., smaller than without phospholipids.
With phospholipids in I (where they are not adsorbed), Psuc (2.33 ± 0.42 × 105 cm/s) and
PNa+
(7.01 ± 0.45 × 105 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 × 105 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 × 105 cm/s, respectively),
was 2.92 and 0.74 × 105 cm/s without and with
phospholipids, respectively. Hence, most of the resistance to diffusion
of the pericardium is provided by the mesothelium.
connective tissue; diffusional permeability to sucrose; passive sodium flux; pericardium; phospholipids
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