Regulation of the depth of surface liquid in bovine trachea

Abstract

The luminal surface of airways is lined by a thin film of airway surface liquid (ASL). Physiological regulation of the depth of ASL has not been reported previously. In this paper, we have used low-temperature scanning electron microscopy of rapidly frozen specimens of bovine tracheal epithelium to demonstrate alterations in the depth of ASL in response to the cholinergic agonist methacholine. We first established that methacholine selectively stimulated airway glands, with maximal secretion at ∼2 min and a return to baseline within ∼5 min. A 2-min exposure to methacholine increased the depth of ASL from 23 to 78 μm. Thereafter, depth decreased linearly with time, reaching 32 μm at 30 min. The initial increase in depth was blocked by bumetanide, an inhibitor of active chloride secretion, whereas the slow decline back to baseline was inhibited by amiloride, a blocker of active sodium absorption. We conclude that the methacholine-induced changes in ASL depth reflect transient gland secretion followed by liquid absorption across the surface epithelium.

Optical method for quantifying rates of mucus secretion from single submucosal glands

“…In an early study of individual gland secretions, powdered tantalum was placed on the airway surface to reveal hillocks of mucus that formed above gland ducts that were subsequently visualized with neutral red staining (27). This method is typically used to count secreting glands (41), but, by making assumptions about the shape of the hillocks, it can also be used to quantify secretion for individual glands (7,(13)(14)(15)28). For more accurate quantification of single gland secretion, Quinton (29) developed the oil-coating method and then collected secretions from excised tissues at timed intervals with constant-bore micropipettes.…”

“…The optical method also eliminates the natural interaction between gland secretions and the surface epithelium. For our present purposes, that simplification is useful, but it is also important to determine how the transport properties of surface epithelium and glands interact to determine the depth and composition of airway surface liquid, which in turn affects mucociliary clearance (40,41).…”

Noninvasive in vivo fluorescence measurement of airway-surface liquid depth, salt concentration, and pH

“…vious estimates (7,18). ASL thickness remained constant over more than 1 hour of observation and was insensitive to perfusion pressure in the range of 5-25 cm H 2 O. ASL thickness decreased to 8 µm when the humidified air was replaced by dry air, and increased rapidly upon addition of dry salt or mannitol to drive water influx osmotically (Figure 2b, bottom curves).…”

“…chea, salt concentrations of 45-138 mM have been found using filter paper-and micropipette-sampling methods (12)(13)(14)(15)(16)(17)(18). These invasive methods have been criticized because the sampled volumes are generally much greater than the fluid volume contained in the normally less than 80-µm-thick ASL (3,8,9,18).…”

“…These invasive methods have been criticized because the sampled volumes are generally much greater than the fluid volume contained in the normally less than 80-µm-thick ASL (3,8,9,18). Theoretically, the ASL could be hypotonic, isotonic, or hypertonic.…”