Restricted dextran transport in the sheep lung lymph preparation.
The ease with which plasma proteins and other large solutes enter sheep lung lymph had led previous investigators to conclude that such large molecules probably move through a pathway that has transport properties similar to pores 100 nm or larger in radius. To test this hypothesis, the transport of some very large solutes into lung lymph was measured. Dextran fractions having Stokes-Einstein radii of 10--150 nm were prepared and labeled with fluorescein isothiocyanate (FITC). Five sheep were given 2--2.5 g FITC-dextran 24--36 h prior to cannulation of the efferent duct of the caudal mediastinal lymph node. Analysis of paired plasma and lymph samples for fluorescence as a function of molecular size gave the following results. FITC-dextran 10 nm in radius appeared in sheep lung lymph at 0.30 +/- 0.06 of its plasma concentration. Larger FITC-dextran molecules appeared at still lower concentrations until, above 35 nm, fluorescence could no longer be detected in lymph. The restriction to transport of FITC-dextran was found to be inconsistent with values predicted by a pore pathway 100 nm or larger. These data are compatible with a pore pathway 22--35 nm in radius.