Lactoperoxidase and human airway host defense

The lactoperoxidase (LPO) antibiotic system is a well-characterized component of mammary and salivary gland secretions. Because LPO has been shown to function in ovine airways, human airway tissue and secretions were examined for the presence of LPO and its substrate, the anion thiocyanate (SCN-). In addition, human airway secretions were tested for LPO-mediated antibacterial activity, and LPO's activity was assessed against some human airway pathogens. The data showed that normal human airway secretions contained LPO enzyme activity (0.65 +/- 0.09 microg/mg secreted protein; n = 17), and Western blots of secretions demonstrated bands of the expected sizes for LPO. LPO mRNA was detected in trachea by sequencing PCR-amplified cDNA. SCN-, LPO's substrate, was present in undiluted airway secretions at concentrations sufficient for LPO catalysis (0.46 +/- 0.19 mM; n = 8), and diluted secretions contained antibacterial activity with LPO-like properties. Immunocytochemistry localized LPO to submucosal glands in human bronchi. Finally, as expected based on the known antibacterial spectrum of the LPO system, airway secretions showed LPO-dependent activity against Pseudomonas aeruginosa. In addition, the airway LPO system was shown to be effective against Burkholderia cepacia and Haemophilus influenzae. Thus, a functional LPO system exists in human airways and may contribute to airway host defense against infection.

Post-secretory fate of host defence components in mucus

Airway mucus is a complex mixture of secretory products that provide a multifaceted defence against infection. Among many antimicrobial substances, mucus contains a peroxidase identical to milk lactoperoxidase (LPO) that is produced by goblet cells and submucosal glands. Airway secretions contain the substrates for LPO, namely thiocyanate and hydrogen peroxide, at concentrations sufficient for production of the biocidal compound hypothiocyanite, a fact confirmed by us in vitro. In vivo, inhibition of airway LPO in sheep significantly inhibits bacterial clearance, suggesting that the LPO system is a major contributor to host defences. Since secretory products including LPO are believed to be steadily removed by mucociliary clearance, their amount and availability on the surface is thought to be controlled solely by secretion. In contrast to this paradigm, new data suggest that LPO and other substances are retained at the ciliary border of the airway epithelium by binding to surface-associated hyaluronan, thereby providing an apical, fully active enzyme pool. Thus, hyaluronan, secreted from submucosal gland cells, plays a previously unrecognized pivotal role in mucosal host defence by retaining LPO and possibly other substances important for first line host defence at the apical surface 'ready for use' and protected from ciliary clearance.

Hydrogen peroxide-scavenging properties of normal human airway secretions

To examine the antioxidant capacity of normal human airway secretions and to characterize its molecular components, tracheal lavages were obtained from eight patients intubated for elective surgery and free of lung disease. These samples (20 microl, approximately 6.8 microg of protein) scavenged 0.57 +/- 0.09 nmol of added 0.96 nmol hydrogen peroxide (H2O2) within 10 minutes at room temperature (n = 8). The scavenging activity was inhibited 60 +/- 4% by azide (an inhibitor of heme-containing peroxidases and catalase) and 42 +/- 9% by dapsone (an inhibitor of lactoperoxidase). Mercaptosuccinic acid (an inhibitor of glutathione peroxidase) did not significantly inhibit H2O2 scavenging by these secretions. Fourfold diluted secretions showed only nonenzymatic scavenging activity, but the addition of thiocyanate to these samples (0.4 mM; substrate for lactoperoxidase) restored their ability to scavenge H2O2. The addition of reduced glutathione (8 microM) only enhanced nonenzymatic scavenging activity. These data provide evidence that multiple enzymatic and nonenzymatic systems coexist in human airway secretions that contribute to H2O2 scavenging. It appears, however, that H2O2 is mainly consumed by the lactoperoxidase system.

Lactoperoxidase and hydrogen peroxide metabolism in the airway

Hydrogen peroxide (H2O2) is known to play an important role in airway homeostasis. For this reason its levels and thus its synthesis and consumption are important mechanisms for controlling airway functions. We have identified the major macromolecular consumer of H2O2 in sheep airway secretions to be lactoperoxidase (LPO), a heme peroxidase previously studied in milk and saliva. This enzyme uses H2O2 to oxidize the anion thiocyanate to an antibiotic compound that prevents growth of bacteria, fungi, and viruses. LPO was isolated from sheep airways and proved to be a major constituent comprising about 1% of the soluble protein in airway secretions. The isolated airway LPO was catalytically active and displayed the enzymatic characteristics previously described for the enzyme isolated from bovine milk. Airway LPO activity was shown to increase the rate of bacterial clearance from sheep airways. The role of this enzyme in the airway host defense strongly suggests that an active H2O2 production system exists to supply appropriate substrate for the enzyme. The identity of this H2O2 synthesis system is an important, yet unknown feature of airway oxygen radical metabolism.

Systemic ovalbumin sensitization downregulates norepinephrine uptake by rabbit aortic smooth muscle cells

Norepinephrine (NE) concentration at alpha-adrenergic receptors is partially regulated by steroid-sensitive, extraneuronal catecholamine uptake (uptake-2). Because alpha(1)-adrenergic agonist- and glucocorticosteroid (GS)-induced bronchial vasoconstriction is enhanced in individuals with asthma, atopy could be associated with decreased uptake-2 by vascular smooth muscle cells (SMCs). We therefore evaluated whether NE uptake and its specific transporter messenger RNA (mRNA) were reduced in aortic SMCs of rabbits systemically sensitized with ovalbumin (OVA). NE uptake was measured using a semiquantitative fluorescence microscopic method. Corticosterone and O-methyl-isoprenaline, but not desipramine, co-incubation (1 micro M each) for 20 min decreased NE uptake into SMCs, an inhibitor profile indicative of extraneuronal monoamine transporter (EMT). In OVA-sensitized rabbits, NE uptake was 25.9 +/- 4.5% (mean +/- SEM) lower than in control animals (P < 0.05). Sensitized serum had no effect on NE uptake into naive SMCs. EMT mRNA expression was measured in aortic smooth muscle, using multiplex reverse transcriptase-polymerase chain reaction. In OVA-sensitized rabbits, expression was 61.1 +/- 16.4% lower than in control animals (P < 0.05). These data demonstrate that NE uptake by aortic SMCs is impaired in atopic rabbits, and associated with a decreased transporter mRNA expression. The same mechanism may operate in bronchial arteries in individuals with asthma.

Effects of beta-agonists on airway epithelial cells

beta-Adrenergic receptor (betaAR) agonists exert a variety of effects on airway epithelial cells. Among their best known actions is their ability to increase ciliary beat frequency, mediated by cyclic adenosine monophosphate (cAMP) production, stimulation of protein kinase A (PKA), and phosphorylation of an outer dynein arm light chain. Submucosal glands express betaARs, and beta-agonists may stimulate secretion of mucus from airways, although human data are controversial. beta-Agonists may also affect ion transport across epithelial cells by opening apical ion channels such as the cystic fibrosis transmembrane regulator. This effect, likely to occur in submucosal glands, can influence water fluxes across the airway epithelium and may have profound influences on mucus hydration. betaAR activation can increase intracellular calcium in some ciliated cells, thereby stimulating ciliary beating and possibly influencing transepithelial ion transport. betaAR-mediated activation of cAMP-dependent protein kinase accelerates epithelial cell migration, thereby enhancing epithelial wound repair. beta-Agonists reduce the ultrastructural damage seen with infection and potentiate secretion of certain cytokines from epithelial cells while inhibiting secretion of others. Finally, beta-agonists may have effects on airway epithelial cells that are mediated through betaARs but do not require cAMP production. The signaling mechanisms of some beta-agonist effects are not well understood but are important to our understanding of airway epithelial cell growth, differentiation, and function.

Regulator of G-protein signaling protein 2 modulates purinergic calcium and ciliary beat frequency responses in airway epithelia

In ciliated airway epithelial cells, purinergic stimulation increases both intracellular calcium ([Ca(2+)](i)) and ciliary beat frequency (CBF). Because regulator of G-protein signaling protein 2 (RGS2) terminates Galphaq-mediated phospholipase C activation, we examined its role in regulating purinergic signaling in human and ovine airway epithelial cells. RT-PCR of both human and ovine epithelial cell RNA yielded fragments of expected size ( approximately 491 bp) and sequence, confirming RGS2 message. Immunofluorescence demonstrated RGS2 protein expression in cultured airway epithelial cells of both species. Overexpression of an EGFP-RGS2 fusion protein (increasing RGS2 protein levels 1.8 times control, n = 28 cells) resulted in a reduced [Ca(2+)](i) and CBF response to 10 micro M ATP (human: 58 +/- 9% and 49 +/- 8% lower, respectively; n = 8 measurements, 4 cells; ovine: 56 +/- 12% and 53 +/- 16% lower, respectively; n = 5 measurements, 4 cells). Reducing RGS2 protein levels using antisense oligonucleotides increased the response of both [Ca(2+)](i) and CBF to ATP in human cells by 57 +/- 10% and 47 +/- 11%, respectively (n = 10 measurements, 6 cells), and in ovine cells by 88 +/- 13% and 48 +/- 9%, respectively (n = 10 measurements, 5 cells). These data provide functional evidence that RGS2 modulates purinergic signaling in human and ovine ciliated airway epithelial cells.

Effects of albuterol enantiomers on ciliary beat frequency in ovine tracheal epithelial cells

beta(2)-Adrenergic agonists stimulate ciliary beat frequency (CBF), an integral part of mucociliary clearance. To evaluate the differential effects of albuterol enantiomers and their racemic mixture on ciliary function, CBF and intracellular calcium were measured at room temperature from single ovine airway epithelial cells with use of digital videomicroscopy. Baseline CBF was 7.2 +/- 0.2 (SE) Hz (n = 80 measurements). R-albuterol (10 microM to 1 mM) stimulated CBF in a dose-dependent manner to maximally 24.4 +/- 5.4% above baseline. Racemic albuterol stimulated CBF to maximally 12.8 +/- 3.6% above baseline, a significantly lower increase compared with R-albuterol alone, despite identical R-enantiomer amounts in both groups. Simultaneous recordings of intracellular calcium concentration and CBF from single cells indicated that the CBF increase in response to R-albuterol was mediated through beta-receptors and stimulation of protein kinase A, in a calcium-dependent and -independent fashion. S-albuterol had a negligible effect on CBF and did not change intracellular calcium. Together, these results suggest that R-albuterol is more efficacious than racemic albuterol in stimulating CBF. Thus S-albuterol may interfere with the ability of R-albuterol to increase CBF.

Prolonged increase in ciliary beat frequency after short-term purinergic stimulation in human airway epithelial cells

Stimulation of ovine airway epithelial cells with 10 microM ATP for 1 min at 25 degrees C transiently increased both cytoplasmic calcium (fura-2 epifluorescence microscopy) and ciliary beat frequency (CBF; differential interference contrast microscopy) with a similar time course. Identical purinergic stimulation of human airway epithelial cells at 25 or 35 degrees C, however, lead to an increase in CBF that outlasted the calcium transient at least 20 min. While a nitric oxide synthase inhibitor had no effect, pre-treatment of human cells with inhibitors of cAMP-dependent kinase (PKA), 10 microM myristoylated PKA-inhibitory peptide and 1 microM KT-5720, as well as an inhibitor of adenylyl cyclase, 1 mM SQ22536, blocked the prolonged, but not calcium-coupled CBF increase. Addition of PKA inhibitors after purinergic stimulation only partially reduced CBF from its elevated plateau. Prolonged CBF increases did not depend on adenosine production as 10 microM UTP had an effect similar to ATP and 8-sulphophenyl-theophylline did not block them. After increasing human CBF in a PKA-dependent manner to a stable plateau with forskolin (10 microM), ATP caused only a transient, calcium-coupled CBF increase. Calcium transients were necessary for both short-term and prolonged CBF changes as ATP failed to produce CBF increases after emptying calcium stores with 1 microM thapsigargin. These data suggest that in human, but not ovine airway epithelial cells, ATP-induced calcium transients activate a signalling cascade including adenylyl cyclase and PKA. The resulting prolonged CBF stimulation does not rely only on PKA activity, suggesting that the decay of CBF is influenced by ciliary phosphatase activity.

Hyaluronan serves a novel role in airway mucosal host defense

Enzymes secreted onto epithelial surfaces play a vital role in innate mucosal defense, but are believed to be steadily removed from the surface by mechanical actions. Thus, the amount and availability of enzymes on the surface are thought to be maintained by secretion. In contrast to this paradigm, we show here that enzymes are retained at the apical surface of the airway epithelium by binding to surface-associated hyaluronan, providing an apical enzyme pool 'ready for use' and protected from ciliary clearance. We have studied lactoperoxidase, which prevents bacterial colonization of the airway, and kallikrein, which mediates allergic bronchoconstriction that limits the inhalation of noxious substances. Binding to hyaluronan inhibits kallikrein, which is needed only in certain situations, whereas lactoperoxidase, useful at all times, does not change its activity. Hyaluronan itself interacts withthe receptor for hyaluronic acid-mediated motility (RHAMM or CD168) that is expressed at the apex of ciliated airway epithelial cells. Functionally, hyaluronan binding to RHAMM stimulates ciliary beating. Thus, hyaluronan plays a previously unrecognized pivotal role in mucosal host defense by stimulating ciliary clearance of foreign material while simultaneously retaining enzymes important for homeostasis at the apical surface so that they cannot be removed by ciliary action.

Steroid sensitivity of norepinephrine uptake by human bronchial arterial and rabbit aortic smooth muscle cells

We have shown that an inhaled glucocorticosteroid (GS) causes alpha(1)-adrenergic antagonist-blockable, rapid, and transient bronchial vasoconstriction in healthy and asthmatic subjects. Steroids inhibit norepinephrine (NE) uptake by non-neuronal cells, thereby increasing NE concentration at alpha-adrenergic receptor sites. This could explain the GS-induced bronchial vasoconstriction. We therefore studied expression of the steroid-sensitive extraneuronal monoamine transporter (EMT) and steroid sensitivity of NE uptake in human bronchial artery and rabbit aorta (as a substitute for the limited supply of human bronchial artery). NE uptake was measured using a semiquantitative, sucrose-potassium phosphate-glyoxylic acid fluorescence method that we newly adapted for use in single cells. Both human bronchial arteries and rabbit aorta expressed messenger RNA for EMT, and steroids blocked NE uptake into freshly dissociated human bronchial arterial and rabbit aortic smooth-muscle cells (SMCs). In the latter, inhibition of NE uptake by steroids was not altered, either by a protein synthesis inhibitor (cycloheximide) or by a transcription inhibitor (actinomycin D), and corticosterone made membrane-impermeant by conjugation to bovine serum albumin inhibited NE uptake equipotently. These data show that NE uptake into bronchial arterial and rabbit aortic SMCs is sensitive to steroids, possibly mediated by EMT, and suggest a mechanism for GS-induced bronchial vasoconstriction.

Lack of nitric oxide involvement in cholinergic modulation of ovine ciliary beat frequency

Ciliary beat frequency (CBF) is regulated, at least in part, by the cytoplasmic calcium concentration ([Ca(2+)](i)). Because Ca(2+) can stimulate nitric oxide (NO) production by nitric oxide synthase (NOS) and NO has been implicated in the regulation of CBF in some species, we examined whether NOS is present in cultured ovine ciliated epithelial cells and whether NO plays a role in the Ca(2+)-mediated muscarinic stimulation of CBF. Dissociated ovine tracheal epithelial cells were grown in culture for 2 to 14 days. Frequency from a single cilium was measured by on-line Fourier transform methods using video microscopy. [Ca(2+)](i) was determined with fura-2 using fluorescence ratio imaging from the same single cells. Ciliated cells contained NOS in culture as indicated by NADPH-diaphorase staining. Acetylcholine (ACh) increased CBF and [Ca(2+)](i) transiently as previously shown. Measurements with 2',7'-dichlorofluorescin diacetate indicated that reactive oxygen/nitrogen species were produced in these cells on ACh exposure. NOS inhibitors N(G)-nitro-L-arginine methyl ester (< or =10 mM), N(G)-nitro-L-arginine (< or =10 mM), and 7-nitro indazole (1 microM) were unable to block the CBF or [Ca(2+)](i) response to ACh. Furthermore, the NO donors sodium nitroprusside and S-nitroso-N-acetylpenicillamine (< or =1 mM) did not change CBF or [Ca(2+)](i). Above these concentrations, they both lead to a reversible decrease in CBF. The membrane-permeable cyclic guanosine monophosphate analogue 8-bromo-cyclic guanosine monophosphate had no effect on CBF, whereas 8-bromo-cyclic adenosine monophosphate stimulated CBF. Taken together, these results suggest that NO does not play a role in mediating the ACh-induced increase in CBF through [Ca(2+)](i). The role and targets for NO in ovine ciliated cells remains to be determined.

Hyaluronic acid in cultured ovine tracheal cells and its effect on ciliary beat frequency in vitro

Hyaluronic acid (hyaluronan, or HA) is secreted by submucosal glands, but its function in airway secretions other than influencing the rheology of mucus is not fully understood. HA is known to modulate cell behavior and to enhance sperm motility. Because sperm tails and cilia have the same microtubular structure, we studied the effect of HA on ciliary beat frequency (CBF) in vitro. CBF of cultured ovine airway epithelial cells was measured continuously by digital video microscopy. After removal of endogenous HA by hyaluronidase, cells were exposed to 50 to 100 microg/mL of HA at different times in culture. No change in CBF in response to HA was seen in cells cultured less than 7 days. After 7 days, however, 6 of 10 measured cells (from three different sheep) showed a transient CBF increase from a baseline of 6.4 +/- 0.3 Hz (mean +/- SE) to 7.4 +/- 0.4 Hz or 16% above baseline (p < 0.05). At these time points (but not before), cytochemical staining was positive for endogenous HA using a biotinylated HA-binding protein. These data suggest that HA can increase CBF of tracheal epithelial cells only late in culture when HA is able to bind to an unspecified cell surface structure. Because this binding has a physiological effect, we hypothesize that it is an HA-binding receptor, that is either transiently expressed late in culture or initially destroyed by the protease treatment for cell dispersion.

Agonist-stimulated calcium decreases in ovine ciliated airway epithelial cells: role of mitochondria

1. In ovine ciliated tracheal epithelial cells, acetylcholine (ACh) activates signal transduction pathways that not only transiently increase cytoplasmic Ca2+ ([Ca2+]i) but also actively lower [Ca2+]i. The pathway for decreasing [Ca2+]i is clearly revealed after depletion of intracellular Ca2+ stores by thapsigargin (Tg), 2,5-di-(tert-butyl)-1,4-benzohydroquinone or NiCl2. Measurements with microinjected fura-2 excluded a [Ca2+] measurement artefact. 2. A four-compartment model to simulate calcium transients in non-excitable cells (consisting of a plasma membrane Ca2+ pump and channel; Ca2+ store with pump and channel; and cytosolic Ca2+ buffer) could not account for the observed [Ca2+]i decrease. We therefore explored, by simulation and experimentation, several different mechanisms that could account for it. 3. The ACh-stimulated [Ca2+]i decrease was not due to an inhibition of Ca2+ influx (Ca2+ channel blockers or absence of extracellular calcium had no effect), activation of a plasma membrane Ca2+-ATPase (two inhibitors, vanadate (30 mM) and lanthanum (10 mM), had no effect) or inhibition of the Na+-Ca2+ exchanger (replacing extracellular Na+ with N-methylglucamine had no effect). 4. The application of mitochondrial uncouplers (5 microM CCCP or 5 microM FCCP), eliminated the ACh-induced [Ca2+]i decrease. Addition of CCCP at the nadir of the decrease restored intracellular calcium levels of Tg-treated cells to baseline faster than controls not exposed to mitochondrial uncouplers. CCCP application to naïve cells did not block the ACh-induced transient increase in [Ca2+]i. 5. These data suggest that ACh-induced [Ca2+]i decreases in ciliated cells are caused by stimulated Ca2+ uptake into mitochondria.

The lactoperoxidase system functions in bacterial clearance of airways

Airway mucus is a complex mixture of secretory products that provides a multifaceted defense against pulmonary infection. Mucus contains antimicrobial peptides (e.g., defensins) and enzymes (e.g., lysozyme) although the contribution of these to airway sterility has not been tested in vivo. We have previously shown that an enzymatically active, heme-containing peroxidase comprises 1% of the soluble protein in sheep airway secretions, and it has been hypothesized that this airway peroxidase may function as a biocidal system. In this study, we show that sheep airway peroxidase is identical to milk lactoperoxidase (LPO) and that sheep airway secretions contain thiocyanate (SCN(-)) at concentrations necessary and sufficient for a functional peroxidase system that can protect against infection. We also show that airway LPO, like milk LPO, produces the biocidal compound hypothiocyanite (OSCN(-)) in vitro. Finally, we show that in vivo inhibition of airway LPO in sheep leads to a significant decrease in bacterial clearance from the airways. The data suggest that the LPO system is a major contributor to airway defenses. This discovery may have significant implications for chronic airway colonization seen in respiratory diseases such as cystic fibrosis.

Mode of Ca2+ action on ciliary beat frequency in single ovine airway epithelial cells

1. We analysed the kinetics of coupling between cytoplasmic calcium ([Ca2+]i) and ciliary beat frequency (CBF) using simultaneous single cilium recording and single cell [Ca2+]i measurements from cultured ovine tracheal epithelial cells. 2. CBF and [Ca2+]i (indicated by fura-2) were measured at rest and in response to activation of the G-protein coupled M3 muscarinic receptor by 10 microM acetylcholine (ACh). 3. Fourier transform analysis of 3 s data segments of light intensity from phase-contrast microscopy showed no significant delay between changes in [Ca2+]i and CBF during a 2 min exposure to ACh and subsequent washout. 4. CBF time resolution was improved by computing instantaneous beat frequency. This revealed that CBF lagged the rapid increase in [Ca2+]i in response to ACh with a delay of less than 1 beat cycle (143 ms at 7 Hz). When CBF was estimated by an improved Fourier method, this delay was observed to be 70 +/- 30 ms (mean +/- s.e.m.; n = 20 cilia). During the slower return to baseline, a lag of 8 +/- 3.2 s was observed, indicative of hysteresis. 5. While calmodulin inhibitors (calmidazolium and W-7; each n = 5) decreased baseline CBF by an average of 1.1 +/- 0.1 Hz, they did not alter the kinetic relationship between [Ca2+]i and CBF. Similarly, phosphatase inhibitors (okadaic acid and cyclosporin A; each n = 5), changed neither baseline CBF nor the kinetic coupling between [Ca2+]i and CBF. 6. These data suggest that the timing of Ca2+ action on CBF in ovine airway epithelial cells, is unlikely to be determined by phosphorylation reactions involving calmodulin or kinase/phosphatase reactions. 7. A simple model for Ca2+ stimulation of CBF is presented. Fits of the model to the data suggest four or more Ca2+ ions bind cooperatively to speed up CBF.

Isolation and characterization of a peroxidase from the airway

Sheep airway mucus can potently scavenge hydrogen peroxide, an important mediator of airway inflammation. Here, the scavenging activity was identified as a peroxidase produced by goblet cells of the airway epithelium and secreted into the airway lumen. Ovine airway peroxidase activity was purified approximately 100-fold from airway lavage fluid in two steps, using cation exchange and lectin affinity chromatography, yielding an apparently homogeneous 82-kD glycoprotein. Ovine airway peroxidase represented about 1% of the total protein in airway mucus and thus was an abundant enzyme in airway secretions. The absorbance spectrum of the purified peroxidase showed a major peak at 412 nm indicative of a hemoprotein. The ratio of A412/A280 of the purified enzyme was 0.86. The absorption spectrum of ovine airway peroxidase, its ability to oxidize halides, its sensitivity to inhibitors and its apparent molecular mass on sodium dodecyl sulfate gels showed that airway peroxidase was similar to lactoperoxidase but distinguished from myeloperoxidase, eosinophil peroxidase as well as from glutathione peroxidases. Based on these observations, ovine airway peroxidase is a newly isolated and abundant enzyme of airway mucus which may function to control reactive oxygen species in the airway and to prevent infection by catalyzing the formation of biocidal compounds.

Muscarinic signaling in ciliated tracheal epithelial cells: dual effects on Ca2+ and ciliary beating

To examine cholinergic signal transduction pathways that modulate ciliary beat frequency (CBF), cultured ovine tracheal epithelial cells were imaged using a combination of phase-contrast (CBF) and fluorescence (Ca2+) microscopy techniques. In single cells, acetylcholine (ACh) transiently increased CBF and intracellular Ca2+ concentration ([Ca2+]i), mainly by Ca2+ release from internal stores, with a small delayed contribution from Ca2+ influx. Nicotinic agonists did not alter CBF or [Ca2+]i, whereas atropine blocked the ACh-stimulated transients, consistent with the involvement of muscarinic receptors. 4-Diphenylacetoxy-N-methylpiperidine methiodide was approximately 100 times more potent than pirenzepine in inhibiting the ACh-induced [Ca2+]i peaks, suggesting that the receptor is a pharmacologically defined (M3) subtype. Interestingly, after depletion of intracellular Ca2+ stores by thapsigargin, ACh caused a rapid transient decrease in both CBF and [Ca2+]i, again with an antagonist profile of M3 receptors. We conclude that activation of M3 muscarinic receptors initiates specific signaling pathways that act simultaneously to increase and decrease [Ca2+]i and CBF.

Hydrogen peroxide-scavenging properties of sheep airway mucus

Reactive oxygen species released from luminal phagocytes in the airway can potentially injure the airway epithelium. Naturally occurring oxygen radical scavengers must therefore exist to protect the epithelium. This study was designed to determine whether the high-molecular-weight fraction of normal sheep tracheal mucus has hydrogen peroxide (H2O2)-scavenging activity. Lyophilized mucus from 10 sheep was reconstituted in phosphate-buffered saline (PBS) or Krebs-Henseleit buffer. H2O2 was added to these mucus samples to a final concentration of 15 microM, and the level of H2O2 remaining was measured over a 10 min period. From a zero-time level of 17 +/- 1.8 microM (mean +/- SD), the H2O2 concentration fell within 10 min to 8 +/- 1.7 microM in 0.05%; to 3.9 +/- 2.2 microM in 0.1%; to 2.6 +/- 2.4 microM in 0.2%; and to 1.2 +/- 1.5 microM in 0.4% mucus reconstituted in PBS. The results obtained in Krebs-Henseleit buffer were similar. The disappearance of H2O2 was not due to the transformation into hydroxyl radicals. Heat and acid denaturation and cleavage of carbohydrate-free peptides from glycoproteins by pronase E treatment abolished the scavenging potential. Fractionation of 0.4% mucus samples according to molecular weight by gel filtration revealed that only one fraction with proteins of M(r) > 110 kD contained the active scavenger. Polyacrylamide gel electrophoresis and lectin blotting with Ulex europaeus I (UEAI) showed that both the whole mucus and the actively scavenging gel filtration fraction contained a glycoprotein that comigrated with a 205 kD molecular weight marker.(ABSTRACT TRUNCATED AT 250 WORDS)

Coupling of [Ca2+]i and ciliary beating in cultured tracheal epithelial cells

The molecular mechanisms responsible for the regulation of ciliary beating frequency (CBF) are only partially characterized. To determine whether elevation of intracellular Ca2+ ([Ca2+]i) can cause an increase in CBF, we measured CBF and Ca2+ in single cells. Ovine tracheal epithelial cells, obtained by dissociation with protease, were grown in primary culture for 1 to 28 days in a mucus-free system. CBF of a single cilium was measured by digital video phase-contrast microscopy and on-line Fourier-transform analysis. Changes in [Ca2+]i from single cells were determined with fura-2, using ratio imaging video microscopy. Activation of a muscarinic pathway with 10 microM ACh (acetylcholine) increased [Ca2+]i from 53 +/- 9 nM (mean +/- s.e.m.) to 146 +/- 12 nM or to 264 +/- 51% above initial baseline. In the same cells, ACh increased CBF from a baseline of 7 +/- 0.5 Hz to 9 +/- 0.2 Hz or to 31 +/- 2.8% above baseline (n = 14). The elevations of both [Ca2+]i and CBF were transient and relaxed back to an elevated plateau (10/14 cells) as long as ACh was present. To elevate [Ca2+]i by mechanisms independent of a G-protein-coupled receptor, we measured [Ca2+]i and CBF of the same cells in extracellular solutions with either 0 Ca2+ (+ 1 mM EGTA) or 10 mM Ca2+. Both signals rose and fell with similar kinetics in response to changing [Ca2+]0, suggesting that changes in [Ca2+]i alone can modulate CBF. In a second independent manipulation, cells were treated with 1 microM thapsigargin, an irreversible inhibitor of the endoplasmic reticulum Ca(2+)-ATPase. Upon thapsigargin addition, 37 of 42 cells showed a transient [Ca2+]i increase and, as measured in different experiments, 8 of 9 cells showed a transient increase in CBF. Interestingly, application of ACh after cells were treated with thapsigargin produced decreases in both [Ca2+]i and CBF in 8/8 cells. Lastly, after 1–3 days in culture, addition of 10 microM ACh often produced [Ca2+]i oscillations rather than transients in [Ca2+]i. Measurements of CBF in these cells showed frequency modulation of CBF with the same peak-to-peak time interval as the Ca2+ oscillation. These results show that: (1) CBF can be measured from a single cilium and monitored on-line to track changes; (2) CBF and [Ca2+]i can be measured in the same single cell; (3) transient changes in [Ca2+]i (induced by 4 different manipulations) are associated with kinetically similar changes in CBF; and (4) [Ca2+]i oscillations are coupled to frequency modulation of ciliary beating.(ABSTRACT TRUNCATED AT 400 WORDS)

Protein kinase C-dependent phosphorylation of a ciliary membrane protein and inhibition of ciliary beating

The present study examined whether protein kinase C phosphorylated a ciliary protein and whether this phosphorylation event was temporally correlated with a decrease in ciliary beat frequency. Activation of protein kinase C decreased ciliary beat frequency of sheep tracheal epithelium, an effect fully blockable by pretreatment of the tissue pieces with H-7, a protein kinase inhibitor. Using cilia removed from these epithelial surfaces and incubated in solutions containing stimulators of protein kinase C along with [gamma-32P]ATP or [gamma-35S]ATP, a single protein target of ciliary protein kinase C activity was identified. The protein is a polypeptide of molecular mass 37 kDa (p37) as estimated by SDS-polyacrylamide gel electrophoresis. Protein kinase C dependency of p37 phosphorylation was proven by showing that Calphostin C, a specific protein kinase C inhibitor, blocked label incorporation into p37 completely, and by demonstrating that purified protein kinase C phosphorylated p37. Inhibitors of cAMP-dependent kinase and calcium/calmodulin-dependent kinase did not change the phosphorylation of p37 in the presence of protein kinase C activators. p37 was recovered in a Triton X-100-extractable fraction of this ciliary preparation, suggesting that p37 is membrane associated. This hypothesis was further supported by the fact that p37 was present in a pellet representing reconstituted membranes. Thin-layer electrophoresis revealed that p37 was phosphorylated on serine and tyrosine residues, suggesting that the activation of protein kinase C also stimulated tyrosine kinase activity. p37 did not precipitate with annexin I or II antibodies. These results show that sheep tracheal cilia contain protein kinase C activity and that activated protein kinase C phosphorylates a membrane-associated ovine ciliary target, an effect temporally related to a protein kinase C-mediated decrease in ciliary beat frequency.

Cyclic AMP-dependent phosphorylation of a 26 kD axonemal protein in ovine cilia isolated from small tissue pieces

To study cyclic adenosine monophosphate (cAMP)-dependent phosphorylation events in ovine cilia in vitro, we adapted published axonemal isolation methods to obtain pure mammalian axonemal proteins from small ovine tracheal mucosa pieces with a surface area of only 1 cm2. The isolated axonemes could be reactivated in vitro upon ATP addition, thereby attesting to their functional integrity. The axonemal protein yield from these small mucosa pieces was high enough to allow protein concentration measurements of each sample and axonemal polypeptide analysis by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). cAMP is known to increase ciliary beat frequency, possibly through a phosphorylation event in the axoneme. To study cAMP-dependent phosphorylation events in ovine tracheal cilia, these axonemal preparations were exposed to [gamma-32P]ATP under conditions that stimulated or inhibited kinase activity. Analysis of axonemal polypeptides by SDS-PAGE and subsequent autoradiography showed that an axonemal protein with a M(r) of 26 kD is the only polypeptide consistently phosphorylated in a cAMP-dependent manner. The phosphorylation of this protein could be diminished by a highly specific inhibitor of cAMP-dependent protein kinase, KT-5720. The addition of calcium did not affect label incorporation into this protein during cAMP treatment. In the presence of cAMP and calcium, inhibitors of protein kinase C and calcium/calmodulin-dependent kinase did not change the level of phosphorylation of the 26 kD protein. We conclude that cAMP treatment of isolated mammalian cilia results in the phosphorylation of a single protein with a M(r) of 26 kD (p26).(ABSTRACT TRUNCATED AT 250 WORDS)