Analysis of proteins in microsamples of rat airway surface fluid by capillary electrophoresis

A thin layer of airway surface fluid (ASF) lining the pulmonary airways plays an important role in the primary defense mechanisms of the lung against bacterial infection. However, little is known about the composition of ASF due to the thinness (typically 5-30 μm in healthy animals) of the fluid layer and its relative inaccessibility, which causes considerable difficulties in sample collection and subsequent analysis. We have used a novel technique of capillary sampling coupled with capillary electrophoresis (CE) to analyze the protein composition of rat ASF. CE analyses were performed under two different conditions: a berate buffer, pH 9.1, or a phosphate buffer, pH 2.5, with 0.5 mM spermine. The different selectivities afforded by the two methods aid in peal; identification, and quantitation of most of the major species was possible using both separation conditions. Albumin, transferrin and globulins are observed to be the major protein components in rat ASF, at concentrations of 28 mg ml^-1, 4.0 mg ml^-1 and 34 mg ml^-1 respectively, in comparison to 31 mg ml^-1, 3.1 mg ml^-1 and 40 mg ml^-1, respectively, in rat plasma.