Electrical resistance and ion diffusion through mesothelium

Since phospholipids (PHL) added on the luminal side of specimens of parietal pericardium of rabbits decrease diffusional permeability (P) to Na+, but not to Cl−, P to Rb+, a cation with hydrated radius similar to that of Cl− was measured. PRb+ was 13.1 (±1.1, S.E.)×10−5 cm/sec and it was not decreased by PHL. This suggests that PHL decrease size of intercellular ‘pores’ of mesothelium, and restrict diffusion of solutes with radius>0.2 nm. (2) Electrical resistance (Re) of pericardium specimens was measured without PHL, with PHL, and after mesothelium was scraped away, to obtain Re of connective tissue and, thus, to compute Re of mesothelium. Re of connective tissue was 1.0±0.2 Ω cm2; Re of mesothelium was 10.1±0.6 and 12.3±0.9 Ω cm2 without and with PHL, respectively. The fraction of electrical current carried by Na+ indicates that Na+ diffusion through mesothelium without PHL is nearly free. (3) Re of cultured mesothelial cell monolayers of rat visceral pleura was 6.1±0.2 Ω cm2, i.e. smaller than that of specimen mesothelium; it did not increase with PHL. PNa+ of cultured mesothelial cell monolayers was 20.0×10−5 cm/sec, i.e. greater than that of specimen mesothelium.