Neurohormonal receptors and cyclic AMP-binding proteins in rabbit tracheal mucosa-submucosa

Proteinases release mucin from airways goblet cells

The mucin-release effect of proteinases on airways epithelium was assessed in vitro. Using explants of rabbit tracheal mucosa-submucosa we determined that elastase and alkaline proteinase from Pseudomonas aeruginosa, pancreatic trypsin and elastase and the microbial proteinases subtilisin, thermolysin and pronase, all stimulate mucin release from goblet cells. On the other hand Streptomyces caespitosus proteinase pancreatic chymotrypsin and collagenase fail to trigger mucin release. Bovine trachea and human nasal polyp epithelium also release mucins in response to proteinases. Mucin release activity is dependent on proteolytic activity of enzymes which have a fairly broad, but generally similar, substrate specificity. The cellular mechanism of action is not known. We propose that mucin secretion in response to proteinases represents a useful defence mechanism but also forms the basis for hypersecretory states and airways obstruction in chronic endobronchial inflammatory states.

Nucleotide regulation of paracellular Cl- permeability in natural rabbit airway epithelium

In this study, we demonstrate a novel regulatory mechanism by which mucosal nucleotides via P2Y receptors decrease paracellular Cl(-) ion permeability in natural rabbit airway epithelium (in addition to a decrease in active Na(+) absorption). In contrast to primary cultures, the natural airway epithelium is a low-resistance epithelium, and an equivalent circuit model predicts that changes of more than approximately 12% in transepithelial conductance (G (t)) must include an effect on paracellular conductance (G (s)). Mucosal P2Y receptor stimulation with uridine triphosphate (UTP; 200 microM) decreased G (t) by up to 50% (average, 24%) and simultaneously decreased the paracellular Cl(-) permeability (mucosa-to-serosa Cl(-) flux) by 16%, but had no effect on mannitol permeability. The G (t) response to UTP was mimicked and attenuated by ionomycin (1 microM), suggesting a dependence on Ca(2+) (i). Amiloride (100 microM) and hyperosmolarity (+75 mM mannitol) also decreased G (t), indicating a role of cell shrinkage. Elevation of cAMP with forskolin (8 microM) or isoproterenol (10 microM) increased G (t) by 55 and 32%, and forskolin increased paracellular Cl(-) permeability by 37% without affecting mannitol permeability. The opposite effects of Ca(2+) (i) and cAMP on G (t) suggest an autocrine nucleotide signaling sequence where P2Y-dependent decrease in passive, paracellular Cl(-) transport is succeeded by a reversion of this effect due to P1-receptor-stimulated cAMP formation by adenosine originating from a time-dependent breakdown of mucosal ATP.

Differentiated properties of rabbit tracheal epithelial cells in primary culture

The purpose of this study was to establish whether rabbit tracheal epithelial cells, grown in primary cell culture, retain neurohormonal receptor and mediator activity. Epithelial cells were isolated by enzymatic treatment and cultured on a collagen matrix. The culture medium consisted of a 1:1 mixture of Dulbecco's modified Eagle medium and Ham's F12 supplemented with 5% fetal calf serum, epidermal growth factor, insulin, transferrin, fibronectin, hydrocortisone, and trace elements. Cultures had a population doubling time of 48 h. Mucus-containing cells and cilia were not observed after 7 days of incubation. Positive immunofluorescent staining with monoclonal antibodies to keratins established the epithelial nature of the cell cultures. Primary cell cultures responded to beta-adrenergic agonists with a dose- and time-dependent increase in intracellular adenosine 3',5'-cyclic monophosphate (cAMP) levels. Propranolol, a beta-adrenergic antagonist, blocked the effects of the beta-adrenergic agonist. Adrenergic agonists also mediated a dose-dependent increase in the generation and release of prostaglandin E2 (PGE2). PGE2 caused an increase in cAMP levels. The results demonstrate that primary cultures of rabbit tracheal epithelial cells retain hormonal responses of the isolated epithelium and tracheal mucosa-submucosa.

Synthesis of mucous glycoproteins by rabbit tracheal cells in vitro. Modulation by substratum, retinoids and cyclic AMP

One function of airway epithelium is the secretion of mucins, which comprise an important component of the mucous lining layer. We demonstrate that rabbit tracheal epithelial cells grown in primary culture incorporate [3H]glucosamine into material released into the medium which is characterized as mucin by the following criteria: high Mr, monosaccharide composition, ion-exchange behaviour different from that of glycosaminoglycans and oligosaccharides attached via N-acetylgalactosamine. The production of mucin by the cells requires growth on a substratum of collagen gel and is enhanced by retinoids in the extracellular medium. In the presence of retinoids, 8-bromo cyclic AMP and factors present in medium from 3T3 fibroblasts each further stimulate mucin production. These results indicate that an isolated epithelial-cell culture system, in the absence of nervous, mesenchymal or other tissue types, can be used to answer questions about the regulation of mucin production at the cellular level.

Na+ transport in cystic fibrosis respiratory epithelia. Abnormal basal rate and response to adenylate cyclase activation

The transepithelial potential difference (PD) of cystic fibrosis (CF) airway epithelium is abnormally raised and the Cl- permeability is low. We studied the contribution of active Na+ absorption to the PD and attempted to increase the Cl- permeability of CF epithelia. Nasal epithelia from CF and control subjects were mounted in Ussing chambers and were short-circuited. The basal rate of Na+ absorption was raised in CF polyps compared with control tissues. Whereas beta agonists induced Cl- secretion in normal and atopic epithelia, beta agonists further increased the rate of Na+ absorption in CF epithelia without inducing Cl- secretion. This unusual effect is not due to an abnormal CF beta receptor because similar effects were induced by forskolin, and because cAMP production was similar in normal and CF epithelia. We conclude that CF airway epithelia absorb Na+ at an accelerated rate. The abnormal response to beta agonists may reflect a primary abnormality in a cAMP-modulated path, or a normal cAMP-modulated process in a Cl- impermeable epithelial cell.

Proteinases of Pseudomonas aeruginosa evoke mucin release by tracheal epithelium

We have determined the potential of exoproducts from pathogenic bacteria to stimulate the release of high molecular weight mucins from goblet cells of airway epithelium in a rabbit tracheal explant system. Culture supernatants from proteolytic strains of Pseudomonas aeruginosa and Serratia marcescens, but not supernatants from a number of non-proteolytic strains, released mucins from goblet cells. Highly purified elastase and alkaline proteinase from P. aeruginosa stimulated goblet cell mucin release in a dose-dependent fashion. Lipopolysaccharide, exotoxin A, and alginate of P. aeruginosa did not possess mucin release properties. Proteolytic activity was required for mucin release by P. aeruginosa elastase, but such release in goblet cells was not mediated by cyclic AMP. Morphologic studies suggested rapid release of mucins from goblet cells was response to elastase by a process resembling apocrine secretion. Several nonbacterial proteinases mimicked the effect of Pseudomonas proteases. These studies provide support for the hypothesis that bacterial and other play a role in the pathogenesis of mucus hypersecretion in acute and chronic lung infections.

Physiological responsiveness of isolated rabbit tracheal epithelial cells

Surface tracheal epithelial cells (tracheocytes) from rabbit were isolated by treating intact tissue with chelators and proteolytic enzymes. The cells were viable as assessed by the following criteria: fluorescent viability staining, sequestration of lactate dehydrogenase, and maintenance of constant ATP levels. Radiolabeled Na+ was transported into cells with a rate constant of 0.06/min and an initial velocity of 1.6 nmol X 10(6) cells-1 X min-1 X beta-adrenergic agonists increased adenosine 3',5'-cyclic monophosphate (cAMP) levels in a time- and dose-dependent manner. The beta-adrenergic effects were potentiated by the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine and blocked by propranolol. The tracheocytes retained the capacity to respond to beta-adrenergic agonists for at least 90 min after isolation. Two major cAMP binding proteins of apparent molecular weights of 50,000 and 54,000 were identified in tracheocytes with the photoaffinity label 8-N3-[32P]cAMP. Agents that increased cAMP levels in intact cells and unlabelled cAMP added to homogenates of cells that were not exposed to drugs decreased photoaffinity labeling. The two proteins correspond in electrophoretic mobility to the regulatory subunits of cAMP-dependent protein kinases I and II, respectively. The results demonstrate that the beta-adrenergic receptors and cAMP binding proteins identified in rabbit tracheal mucosa submucosa are present on tracheocytes, suggesting a role for these receptors in the regulation of tracheocyte physiological events.

Chloride and sodium transport across bovine tracheal epithelium: effects of secretagogues and indomethacin

The regulation of ion transport in bovine tracheal epithelium was studied in vitro. In the absence of exogenous modifiers of ion transport, average values for transepithelial electrical potential difference (psi t), short-circuit-current (Isc) and tissue resistance (Rt) were 35.4 mV (lumen negative), 5.4 muEq X h-1 X cm-2 and 187 omega X cm2 respectively; net Cl secretion (3.2 muEq X h-1 X cm-2) and net Na absorption (1.3 muEq X h-1 X cm2) accounted for 82% of the Isc. Amiloride reduced psi t and Isc, and increased Rt. The values of psi t, Rt and Isc obtained following addition of theophylline, epinephrine or prostaglandin E1 (PGE1) were not different from control values. Theophylline also did not alter Na and Cl fluxes but it increased tissue cAMP content 3-fold. Indomethacin did not affect psi t but it increased Rt and net Na absorption, and decreased Isc and net Cl secretion; it did not significantly reduce tissue cAMP. When added to indomethacin-treated tissues, epinephrine restored Isc, Rt and Na and Cl fluxes to control levels and increased tissue cAMP 3-fold. Similarly, when PGE1 was added to indomethacin-treated tissues, Isc and Rt were restored to control levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Beta-adrenergic modulation of mucin secretion in cat trachea

The mechanism of beta-adrenergic regulation of mucin secretion was investigated in cat trachea in vitro. beta-Adrenergic agonists increased the release of [35S]SO4-radiolabeled mucin and mucosa-submucosal adenosine 3',5'-cyclic monophosphate (cAMP) levels in a dose- and time-dependent manner. The relative potencies and efficacies of l-isoproterenol, l-epinephrine, l-norepinephrine, terbutaline, and dobutamine for physiological and biochemical events were similar. The effect of these agonists were blocked by d-l-propranolol. 3-Isobutyl-l-methylxanthine (IBMX) and 8-bromo-cAMP mimicked the effects of the agonists on mucin release. IBMX increased cAMP levels and potentiated the increase in cAMP levels effected by beta-adrenergic agonists. The half-maximal effects of l-isoproterenol on cAMP levels and mucin release were attained at 1.6 and 8.8 min, respectively. Three major mucosa-submucosal proteins of apparent molecular weights of 49,000, 54,000, and 59,000 daltons displayed reduced binding of the photoaffinity label 8-N3-[32P]cAMP when endogenous cAMP levels were increased with l-isoproterenol and/or IBMX. The first two proteins correspond in electrophoretic mobility to the regulatory subunits of type I and type II cAMP-dependent protein kinases, respectively. The 59,000-dalton cAMP binding protein may be the phosphorylated form of the regulatory subunit of type II cAMP-dependent protein kinase. These data are consistent with the hypothesis that beta-adrenergic modulation of tracheal mucin release is mediated by cAMP and suggest that activation of cAMP-dependent protein kinases may be involved in the neurohormonal effects.