The in vivo transalveolar transport of three large polar solutes, sucrose, inulin, and dextran (mol wt 60,000-90,00), was compared with the transport of urea in saline-filled dog lung. Apparent permeability coefficents (p', in cm X sec-1 X 10(6)) were as follows; urea: 2.4 +/- 0.28 (SD) greater than sucrose: 0.64 +/- 0.31 (P less than 0.001) greater than inulin: 0.12 +/- .05 (P less than 0.001)--not different from dextran (mol wt 60,000-90,000): 0.08 +/- .02 (P greater then .01). Calculation of the resistance of the alveolar epithelium compared to total barrier resistance for the various solutes indicates that approximately 90% of the total resistance resides in the alveolar epithelium. Comparison of the ratio of permeability coefficients to the ratio of free-diffusion coefficients in water shows similar values for the three large polar solutes, suggesting that permeation through the alveolar epithelium occurs by means of water-filled channels. The values for permeability coefficients of alveolar epithelium fit into the spectrum of values reported for other epithelial structures (including gall bladder, frog skin, and toad bladder); it seems to have a system of channels with a small number of wide "pores" (greater than 80 A) that permit permeation of large polar solutes and is not a relatively homogeneous structure.

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