Filter paper equilibration as a novel technique for in vitro studies of the composition of airway surface fluid.

We describe techniques which we developed to study the composition and regulation of airway surface fluid (ASF). ASF from isolated equine tracheal mucosa was absorbed onto or equilibrated with small strips of ashless filter paper after timed incubations in a water-saturated, 37 degrees C environment. After expression of the fluid from the paper, constant-volume aliquots of the samples were pipetted with a microvolumetric pipette on a filmed grid, together with standards containing known concentrations for Na, Cl, K, Ca, S, and P. Analysis of the microdroplets by dispersive X-ray microanalysis on a scanning electron microscope demonstrated that ASF from "unstimulated" tracheal tissue is significantly altered compared with the Ringer solution which was used to rinse the mucosal surface before incubation, in that Na is significantly decreased (approximately 130 mM), whereas K is significantly increased (approximately 16 mM). Furthermore, when a beta-adrenergic agonist (isoproterenol 10(-4) M) was added to the serosal surface, the Na concentration significantly increased to values which approached Ringer solution (146 +/- 5.3 mM) and were significantly different compared with the Na values obtained in unstimulated tissues. In contrast, Na remained unchanged after addition of a cholinergic agonist (metacholine, 10(-4) M) to the serosal side of the tissue; however, as with beta-adrenergic stimulation, most other minor elements and especially K significantly decreased compared with their values in unstimulated tissue. These results clearly suggest that ASF composition is clearly controlled by active transport of the airway epithelium and may be influenced by neurohumoral stimulation.