Liquid volume, Na+ and mannitol concentration in a hypertonic mannitol-Ringer hydrothorax

In anesthetised rabbits with a 2 ml hypertonic mannitol-Ringer hydrothorax in the right space 30 mM/L mannitol were required for an unchanged volume of the hydrothorax after 60 min. [Na+] in the pleural liquid 10, 30 and 60 min after this hydrothorax was 8, 7 and 5 mEq/L, respectively, lower (P less than 0.01) than the initial one and that in a Ringer-hydrothorax. This seems due to the active transport of Na+ out of the pleural space followed by little water because of the osmotic pressure exerted by mannitol. This finding provides further evidence for an active transport without using inhibitors, and implies that the mesothelium offers an appreciable resistance to small solute diffusion. Mannitol concentration, measured at corresponding times from the activity of labeled mannitol, was 76, 68 and 56%, respectively, of the initial one (24.5 mM/L). From 30 to 60 min 6.5 microM of mannitol left the right space mainly by diffusion. The diffusional permeability of the mesothelium was indirectly assessed from the diffusional outflux of mannitol, the surface of the pleural space, and an estimate of mannitol concentration in the interstitium next to the mesothelium: it is smaller than that found in vitro.