In situ visualization of bronchial submucosal glands and their secretory response to acetylcholine

Abstract

Airway submucosal glands secrete both macromolecules and liquid, yet the mechanisms by which these substances are secreted are not well understood. In this study, a video microscope was used to directly visualize the submucosal glands in isolated porcine distal bronchi and to observe their responses to acetylcholine (ACh), a glandular secretagogue. Submucosal glands were classified as either "antral," "linear," or "convoluted" glands based on the morphology of their terminal collecting ducts. Because antral duct glands were most easily visualized, the response to ACh was studied in detail in this gland type. Within 5-10 s after addition of 10 microM ACh, the cross-sectional area of the gland duct openings to the airway surface increased severalfold but returned to pre-ACh dimensions within 1 min. Between 30 s and 10 min after ACh addition, spherical particles (1-10 microm) entered the antral ducts from distal acini and exited through the duct openings to the airway surface. Some of the particles were retained within the antral duct where they were kept in constant motion by the action of cilia present within the antral duct. The particles, which are likely to contain the macromolecular secretory products of mucous and/or serous cells, maintained their spherical shape within the gland duct, suggesting that the secretion product was membrane bound. To our knowledge, these studies provide the first description of airway submucosal gland secretion as viewed in situ.

Secretion of acid and base equivalents by intact distal airways

“…Addition of ACh to the bathing solution evoked both an alkalinization of pH lumen and a hyperpolarization of transepithelial PD, consistent with secretion of base equivalents. We previously used bioelectric techniques (18) and videomicroscopy (17) to show that ACh evoked both Cl Ϫ and HCO 3 Ϫ secretion across isolated, perfused distal bronchi and stimulated liquid and particulate secretion from the submucosal glands.…”

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

“…They measured single-gland secretion induced by autonomic stimulation (39), mechanical stimulation of the larynx (11), gastric irritation (10), and various autonomic mediators (22). In more recent studies, single-gland secretion in pig bronchi was studied with video microscopy with a water-immersion lens (18). Secretion rates were not quantified, but the latency of the rapid response to carbachol was estimated by dilation of the gland duct and emergence of particles from the gland.…”

“…Cystic fibrosis transmembrane conductance regulator (CFTR), the protein that is defective in cystic fibrosis, is heavily expressed in gland serous cells (9). Cholinergic stimulation induces gland secretion in porcine bronchi that is driven by Cl Ϫ and HCO 3 Ϫ (1,17,18,37) and involves CFTR (2). If serous cells depend on CFTR for fluid secretion, as indicated by studies of primary cultures of gland cells (19,42) and cell line models of serous cells (12,26,32), the resulting alterations in gland secretion might compromise mucosal defenses.…”

Aquaporin-5 Dependent Fluid Secretion in Airway Submucosal Glands

“…Serial color images were captured by a CCD camera interfaced to an image analysis system. Freshly secreted fluid droplets from individual gland openings were identified and imaged after intraperitoneal pilocarpine administration, similar to the methodology for studies of submucosal gland fluid in excised fragments of porcine (15,16) and human (8) airways. The rate of volume secretion from individual submucosal gland openings was computed from the time course of increasing fluid droplet diameter assuming semi-spherical droplet geometry.…”