On the use of absorbing discs to sample mucosal surface liquids.

A common technique to sample airway mucosal 'surface' liquids is with absorbing discs of filter paper. The present study examined the efficacy of this technique by analysing tracheal liquids of control and capsaicin (0.1 nmol)-exposed guinea-pig airways. Mucosal fluids, obtained by topically applied discs or by a specific lavage procedure, and tracheal tissue were sampled. The animals had received FITC-dextran (MW 70 kDa) intravenously and this specific plasma tracer was analysed in the sampled material. Under control conditions significantly more FITC-dextran was found in the discs than in the tracheal lavage fluids (P less than 0.001) despite the fact that the lavaged mucosal surface was much larger than that covered by the discs. Capsaicin significantly increased the content of FITC-dextran in all fluids sampled as well as in the airway tissue. In all cases concentrations of FITC-dextran in the disc fluids did not differ much from that in the tissue samples. These data suggest that absorbing discs severely disturb the epithelial-barrier function and sample subepithelial fluid and solutes including macromolecules. As demonstrated in this study by the elevated content of a plasma tracer molecule an inflammatory process may, nevertheless, be traced in the mixture of surface and tissue fluids that is sampled by the discs.