The fluid that covers the surface of conducting airways (airway surface fluid, ASF) is a critical component of one of the first defense mechanisms of the lung against microbial and other environmental insults. Despite its physiologic importance, ASF is one of the only fluids in the human body whose composition remains poorly defined and understood. Attempts to analyze ASF have been hampered greatly by the fact that it exists only as a very thin layer covering the mucosal surface of airway epithelia. To overcome some of these limitations, we have applied ultramicroanalytic techniques to microsamples collected in human airways in vivo. In contrast to previous thinking from studies on sputum samples, ASF collected from healthy airways contains much less Na and Cl (approximately 45% less) and much more K (around 600% more) than extracellular fluid or plasma (ECF), which shows that steep ion gradients exist across normal airway epithelia. These differences also show that ASF composition must be regulated and maintained by active electrolyte transport processes of airway epithelia and that it is not merely the evaporated residue of isotonic secretions or extracellular fluid exudate. However, in patients with sustained airway irritation, infection, or cystic fibrosis, we find that ASF composition appears to become more isotonic with respect to plasma and much more hypotonic in patients with asthma.

We demonstrate that electrogenic glucose coupled Na+ absorption accounts for about 20% of the residual, amiloride and Cl- insensitive, Na+ transport in isolated (equine) trachea. Either glucose removal from the mucosal side or addition of 10(-4) M phloridzin, a known pharmacological inhibitor of Na-glucose cotransport in small intestine and kidney proximal tubule, results in depolarization (lumen referenced to bath) of the transepithelial potential difference (Vt) and in a significant decrease of the equivalent short circuit current Ieq by 4 mV and 4 microA/cm2, respectively, without affecting the transepithelial resistance (Rt). Neither glucose removal nor the application of phloridzin on the basolateral side have a significant or consistent effect on the bioelectric parameters.

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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.

The reabsorptive duct of the eccrine sweat gland has a large transepithelial conductance consisting mainly of a high conductance to Cl- and a smaller, amiloride-blockable Na+ conductance (Bijman and Frömter 1986; Quinton 1986). Cells have been cultured from sweat ducts and their properties previously studied in Ussing chambers (Pedersen 1988) and with microelectrodes (Jones et al. 1988). We have now studied the ion channels present in excised, inside-out patches of human cultured sweat duct cells, and find a marked predominance of linear, 15 pS, amiloride-blockable, low selectivity, Na+ channels. Such channels were seen in 54/92 (59%) of the patches, with up to 7 channels recorded in a single patch. Other channel types were seen at much lower densities. The prevalence of an amiloride-blockable Na+ channel in cultured duct cells clearly distinguishes these cells from cultured sweat gland secretory cells, which lack such a channel.