Equivalent radius of paracellular "pores" of the mesothelium

Diffusional permeability (P) to water (P-w), Cl- (PCl-), and mannitol (P-man) was determined in specimens of rabbit parietal pericardium without and with phospholipids added on the luminal side, as previously done with sucrose and Na+. P to the above-mentioned molecules and to Na+ (PNa+) was also determined after mesothelium was scraped away from specimens. P-w, PCl-, PNa+, and P-man of connective tissue were the following (X10(-5) cm/s): 73.1 +/- 7.3 (SE), 59.5 +/- 4.5, 41.7 +/- 3.4, and 23.4 +/- 2.4, respectively. From these and corresponding data on integer pericardium, P-w, PCl-, PNa+, and P-man of mesothelium were computed. They were the following: 206, 17.9, 9.52, and 3.93, and 90.2, 14.4, 4.34, and 1.75 x 10(-5) cm/s without and with phospholipids, respectively. As previously found for P to sucrose, P to solutes is smaller in mesothelium than in connective tissue, although the latter is similar to 35-fold thicker; instead, P-w is higher in mesothelium, suggesting marked water diffusion through cell membrane. Equivalent radius of paracellular "pores" of mesothelium was computed with two approaches, disregarding P-w. The former, a graphical analysis on a P-molecular radius diagram, yielded 6.0 and 1.7 nm without and with phospholipids, respectively. The latter, on the basis of P-man, P to sucrose, and function for restricted diffusion, yielded 7.8 and 1.1 nm, respectively.