Positive correlation between cytosolic free calcium and surfactant secretion in cultured rat alveolar type II cells

Spatio-temporal aspects, pathways and actions of Ca(2+) in surfactant secreting pulmonary alveolar type II pneumocytes

The type II cell of the pulmonary alveolus is a polarized epithelial cell that secretes surfactant into the alveolar space by regulated exocytosis of lamellar bodies (LBs). This process consists of multiple sequential steps and is correlated to elevations of the cytoplasmic Ca(2+) concentration ([Ca(2+)](c)) required for extended periods of secretory activity. Both chemical (purinergic) and mechanical (cell stretch or exposure to an air-liquid interface) stimuli give rise to complex Ca(2+) signals (such as Ca(2+) peaks, spikes and plateaus) that differ in shape, origin and spatio-temporal behavior. This review summarizes current knowledge about Ca(2+) channels, including vesicular P2X4 purinoceptors, in type II cells and associated signaling cascades within the alveolar microenvironment, and relates stimulus-dependent activation of these pathways with distinct stages of surfactant secretion, including pre- and postfusion stages of LB exocytosis.

Nucleotide-mediated airway clearance

A thin layer of airway surface liquid (ASL) lines the entire surface of the lung and is the first point of contact between the lung and the environment. Surfactants contained within this layer are secreted in the alveolar region and are required to maintain a low surface tension and to prevent alveolar collapse. Mucins are secreted into the ASL throughout the respiratory tract and serve to intercept inhaled pathogens, allergens and toxins. Their removal by mucociliary clearance (MCC) is facilitated by cilia beating and hydration of the ASL by active ion transport. Throughout the lung, secretion, ion transport and cilia beating are under purinergic control. Pulmonary epithelia release ATP into the ASL which acts in an autocrine fashion on P2Y(2) (ATP) receptors. The enzymatic network describes in Chap. 2 then mounts a secondary wave of signaling by surface conversion of ATP into adenosine (ADO), which induces A(2B) (ADO) receptor-mediated responses. This chapter offers a comprehensive description of MCC and the extensive ramifications of the purinergic signaling network on pulmonary surfaces.

A fluorescent microplate assay for exocytosis in alveolar type II cells

The authors describe a simple, reliable, and quantitative assay to monitor exocytotic fusion of lamellar bodies (LBs) in adherent rat alveolar type II (AT II) cells. The assay is based on fluorescence measurements of LB-plasma membrane (PM) fusions modified for the use in multiwell culture plates to obtain a high-sample throughput. In particular, it is based on the presence of a highly light-absorbing dye in the cell supernatants to increase the specificity of fluorescence signals and to yield pseudo-confocal information from the cells. When the assay was tested with agonist-(ATP) and phorbolester-induced stimulation of LB-PM fusions, the authors found a good correlation with direct microscopic investigations based on single cell recordings. To further validate the assay, they used Curosurf at 10 mg/ml. However, it influenced neither the basal nor the ATP-stimulated rate of LB-PM fusions. This was corroborated by the fact that Curosurf had no effect on resting Ca (2+) levels nor the ATP induced Ca (2+) signals. The results cast new light on previous findings that surfactant phospholipids decrease the rate of secretion in AT II cells in a dose-dependent way. The authors conclude that the inhibitory effect exerted by phospholipids might be due to action on a later step in exocytosis, probably associated with exocytotic fusion pore expansion and content release out of fused vesicles.

Exocytosis of lung surfactant: from the secretory vesicle to the air-liquid interface

Exocytosis is fundamental in biology and requires an orchestra of proteins and other constituents to fuse a vesicle with the plasma membrane. Although the molecular fusion machinery appears to be well conserved in evolution, the process itself varies considerably with regard to the diversity of physico-chemical and structural factors that govern the delay between stimulus and fusion, the expansion of the fusion pore, the release of vesicle content, and, finally, its extracellular dispersion. Exocytosis of surfactant is unique in many of these aspects. This review deals with the secretory pathway of pulmonary surfactant from the type II cell to the air-liquid interface, with focus on the distinct mechanisms and regulation of lamellar body (LB) fusion and release. We also discuss the fate of secreted material until it is rearranged into units that finally function to reduce the surface tension in the lung.

Ca2+ entry is essential for cell strain-induced lamellar body fusion in isolated rat type II pneumocytes

Using a new equibiaxial strain device, we investigated strain-induced Ca2+ signals and their relation to lamellar body (LB) exocytosis in single rat alveolar type II (AT II) cells. The strain device allows observation of single cells while inducing strain to the entire substratum. AT II cells tolerated high strain amplitudes up to 45% increase in cell surface area (Delta CSA) without release of lactate dehydrogenase or ATP. Strain exceeding a threshold of approximately 8% Delta CSA resulted in a transient rise of the cytoplasmic Ca2+ concentration in some cells. Higher strain levels increased the fraction of Ca2+-responding cells. The occurrence of strain-induced Ca2+ signals depended on cell-cell contacts, because lone cells (i.e., cells without cell-cell contacts) did not exhibit Ca2+ signals. Above threshold, the amplitude of the Ca2+ signal as well as the number of stimulated LB fusions correlated well with the amplitude of strain. Furthermore, stimulated LB fusions occurred only in cells exhibiting a Ca2+ signal; 50 microM Gd3+ in the bath affected neither Ca2+ signals nor fusions. Intracellular Ca2+ release was triggered at higher strain amplitudes and inhibited by thapsigargin. Removal of bath Ca2+ completely inhibited Ca2+ signals and fusions. We conclude that strain of AT II cells stimulates a Ca2+ entry pathway that is highly sensitive to strain and a prerequisite for subsequent Ca2+ release. Both mechanisms result in a graded response of fusions to strain. Our data also allow us to introduce the term "effective strain" as the physiologically relevant portion of the strain amplitude.

Fusion pore expansion is a slow, discontinuous, and Ca2+-dependent process regulating secretion from alveolar type II cells

In alveolar type II cells, the release of surfactant is considerably delayed after the formation of exocytotic fusion pores, suggesting that content dispersal may be limited by fusion pore diameter and subject to regulation at a postfusion level. To address this issue, we used confocal FRAP and N-(3-triethylammoniumpropyl)-4-(4-[dibutylamino]styryl) pyridinium dibromide (FM 1-43), a dye yielding intense localized fluorescence of surfactant when entering the vesicle lumen through the fusion pore (Haller, T., J. Ortmayr, F. Friedrich, H. Volkl, and P. Dietl. 1998. Proc. Natl. Acad. Sci. USA. 95:1579-1584). Thus, we have been able to monitor the dynamics of individual fusion pores up to hours in intact cells, and to calculate pore diameters using a diffusion model derived from Fick's law. After formation, fusion pores were arrested in a state impeding the release of vesicle contents, and expanded at irregular times thereafter. The expansion rate of initial pores and the probability of late expansions were increased by elevation of the cytoplasmic Ca2+ concentration. Consistently, content release correlated with the occurrence of Ca2+ oscillations in ATP-treated cells, and expanded fusion pores were detectable by EM. This study supports a new concept in exocytosis, implicating fusion pores in the regulation of content release for extended periods after initial formation.

Secretion in alveolar type II cells at the interface of constitutive and regulated exocytosis

Long-term, simultaneous, measurements of cytoplasmic free Ca(2+) concentrations and single exocytotic fusion events in surfactant-secreting type II cells were performed. All fusion (constitutive, phorbol ester-induced, and agonist-induced) was Ca(2+)-dependent. Kinetic analysis revealed that agonist (adenosine triphosphate [ATP])-induced fusion exhibited a kinetic pattern that correlated well with the Ca(2+) signal. The effects of Ca(2+) release from intracellular stores (early) and Ca(2+) entry (late) could be demonstrated for the first time by dissecting the slow (10-to-15-min) fusion response to ATP into these two components. Bath Ba(2+) or Sr(2+) could replace Ca(2+) to elicit a fusion response in thapsigargin-pretreated cells lacking ATP-induced Ca(2+) release from stores. Although the late response was partially inhibited by interrupting the phospholipase D-protein kinase C axis, a high Ca(2+) dependence of the entire secretory course was demonstrated by a significant correlation between the integrated Ca(2+) signal and the fusion response. There was also a highly significant correlation between constitutive and ATP-stimulated fusion activity in individual cells. We propose a common mechanistic model for all types of fusion in this slow secretory cell, in which constitutive and regulated forms of exocytosis are subject to the same principles of regulation.

Inhibition of ATP-induced surfactant exocytosis by dihydropyridine (DHP) derivatives: a non-stereospecific, photoactivated effect and independent of L-type Ca2+ channels

Purinergic stimulation of surfactant secretion via exocytosis of lamellar bodies is mediated by an elevation of the intracellular Ca2+ concentration ([Ca2+](i)). We tested the dihydropyridine (DHP) analogues isradipine (+/-enantiomers), nifedipine and Bay K 8644 (racemic forms) on ATP-induced surfactant secretion and [Ca2+](i) in single type II cells, using FM1-43 and fura-2 fluorescence. None of the DHPs (2 microM) had an effect on ATP-induced surfactant secretion in the dark. They did, however, inhibit secretion in a concentration-dependent manner during illumination, particularly with UV light. This effect was not stereospecific, because it was mimicked by (-)-isradipine. In addition, (+)- or (-)-isradipine, but not nifedipine or Bay K 8644, elicited a slow increase of [Ca2+](i) during illumination with UV light, which was reversible by exposure to dark. None of the DHPs inhibited the ATP-induced Ca2+ signal. In perforated patch clamp experiments, depolarizing voltage steps did not induce L-type Ca2+ (Sr(2+)) currents, even in the presence of the agonist Bay K 8644 (1 microM). We conclude that impairment of ATP-induced surfactant secretion by all tested DHPs and alterations of Ca2+ homeostasis by isradipine are photoactivated effects, independent of L-type Ca2+ channels.

[Ca(2+)](i) oscillations regulate type II cell exocytosis in the pulmonary alveolus

Pulmonary surfactant, a critical determinant of alveolar stability, is secreted by alveolar type II cells by exocytosis of lamellar bodies (LBs). To determine exocytosis mechanisms in situ, we imaged single alveolar cells from the isolated blood-perfused rat lung. We quantified cytosolic Ca(2+) concentration ([Ca(2+)](i)) by the fura 2 method and LB exocytosis as the loss of cell fluorescence of LysoTracker Green. We identified alveolar cell type by immunofluorescence in situ. A 15-s lung expansion induced synchronous [Ca(2+)](i) oscillations in all alveolar cells and LB exocytosis in type II cells. The exocytosis rate correlated with the frequency of [Ca(2+)](i) oscillations. Fluorescence of the lipidophilic dye FM1-43 indicated multiple exocytosis sites per cell. Intracellular Ca(2+) chelation and gap junctional inhibition each blocked [Ca(2+)](i) oscillations and exocytosis in type II cells. We demonstrated the feasibility of real-time quantifications in alveolar cells in situ. We conclude that in lung expansion, type II cell exocytosis is modulated by the frequency of intercellularly communicated [Ca(2+)](i) oscillations that are likely to be initiated in type I cells. Thus during lung inflation, type I cells may act as alveolar mechanotransducers that regulate type II cell secretion.

Threshold calcium levels for lamellar body exocytosis in type II pneumocytes

Pulmonary surfactant is secreted via exocytosis of lamellar bodies (LBs) by alveolar type II cells. Here we analyzed the dependence of LB exocytosis on intracellular Ca(2+) concentration ([Ca(2+)](i)). In fura 2-loaded cells, [Ca(2+)](i) was selectively elevated by flash photolysis of a cell-permeant caged Ca(2+) compound (o-nitrophenyl EGTA-AM) or by gradually enhancing cellular Ca(2+) influx. Simultaneously, surfactant secretion by single cells was analyzed with the fluorescent dye FM 1-43, enabling detection of exocytotic events with a high temporal resolution (T. Haller, J. Ortmayr, F. Friedrich, H. Volkl, and P. Dietl. Proc. Natl. Acad. Sci. USA 95: 1579-1584, 1998). Exocytosis was initiated at a threshold concentration near 320 nmol/l with both instantaneous or gradual [Ca(2+)](i) elevations. The exocytotic response to flash photolysis was highest during the first minute after the rise in [Ca(2+)](i) and thus almost identical to purinoceptor stimulation by ATP. Correspondingly, the effects of ATP on initial secretion could be sufficiently explained by its ability to mobilize Ca(2+). This was further demonstrated by the fact that exocytosis is significantly blocked by suppression of the ATP-induced Ca(2+) signal below approximately 300 nmol/l. Our results suggest a highly Ca(2+)-sensitive step in LB exocytosis.

Effects of eosinophil granule major basic protein on phosphatidylcholine secretion in rat type II pneumocytes

Eosinophils are involved in inflammatory diseases such as asthma. We previously reported that activated eosinophils increased the phosphatidylcholine (PC) secretion in primary cultures of rat type II pneumocytes. Increased PC secretion was confirmed to be partly mediated by superoxide anions released from activated eosinophils. However, the influence of eosinophil granule proteins on PC secretion is unknown at present. In this study, we determined whether eosinophil major basic protein (MBP) influences PC secretion. MBP dose dependently increased the PC secretion in rat type II pneumocytes without producing any cell damage. The MBP-induced increase in PC secretion was significantly reduced by preadministration of either H-7, a protein kinase inhibitor, or 1, 2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-AM, a chelator of intracellular Ca2+, but not by H-89, a protein kinase inhibitor. Our results suggest that the MBP-induced increase in PC secretion may provide mechanical stability and protect against lung atelectasis.

Exocytosis in alveolar type II cells revealed by cell capacitance and fluorescence measurements

Measurement of lamellar body (LB) exocytosis at high spatial and temporal resolution was recently enabled by fluorescence of the dye FM 1-43 (FFM1-43). Here, the capabilities of this method were further examined and extended by simultaneous measurement of the cell membrane capacitance (Cm) and laser-scanning confocal microscopy. Step increases in Cm were evoked by extracellular ATP (20 microM) or an elevated pipette Ca2+ concentration (>/=3 microM). The delay between the first Cm step and the increase in FFM1-43 was <1 s, indicating ready access of FM 1-43 to exocytosed LB contents. A specific Cm of 0.88 microF/cm2 for the membrane of an exocytosed LB was calculated. Compound exocytosis was occasionally observed. Decreases in Cm, indicative of transient fusion or endocytosis, did not occur within 20 min of stimulation. Exocytosis was stimulated by 160 microM guanosine 5'-O-(3-thiotriphosphate) in the pipette, but compound exocytosis was unaffected. The comparison of methods revealed that FM 1-43 is ideally suited to measure the onset of exocytosis and amount of secretion. Patch clamp is superior in resolving fusion events with the plasma membrane.

Regulatory mechanisms of surfactant secretion

Surfactant secretion is a critical regulated process in the metabolism of pulmonary surfactant. Presumably, because this process is vital to the survival of the organism, there are several independent pathways for stimulating secretion which work through different cell surface receptors and signaling mechanisms. In addition, there is apparent homeostatic regulation in that two components of surfactant, namely SP-A and dipalmitoylphosphatidylcholine, can inhibit secretion. Although secretion of surfactant has been studied for over two decades, there remains some important issues to be resolved. In vivo secretion can be stimulated by hyperventilation or even a single large breath. However, we do not know the biochemical mechanism for this physiologically important form of stimulation. In vitro, we know many of the proximal events in signaling, but we do not know how the lamellar bodies move within a cell or the docking mechanism at the plasma membrane. Many investigators have demonstrated that SP-A will inhibit secretion in vitro, but the mechanism is not known. Finally, there is a route of secretion of SP-A independent of lamellar bodies, but we do not know details of this pathway nor its regulation.

Inhibition of lung surfactant secretion from alveolar type II cells and annexin II tetramer-mediated membrane fusion by phenothiazines

We investigated the effects of phenothiazines on lung surfactant secretion from rat alveolar epithelial type II cells and on annexin II tetramer (Anx IIt)-mediated membrane fusion. Trifluoperazine and promethazine inhibited ATP-stimulated phosphatidylcholine (PC) secretion from type II cells in a dose-dependent manner. Concentrations that cause 50% inhibition (IC50) were approximately 3 and 25 microM for trifluoperazine and promethazine, respectively. Promethazine also inhibited PC secretion of type II cells stimulated by other secretagogues, including calcium ionophore A23187, phorbol 12-myristate 13-acetate, and terbutaline that are known to stimulate PC secretion via different signal transduction pathways. Since we have recently determined that Anx IIt is involved in PC secretion of type II cells, we examined whether phenothiazines influence Anx IIt's activity. Trifluoperazine and promethazine inhibited Anx IIt's ability to aggregate phosphatidylserine (PS) liposomes, to fuse PS/phosphatidylethanolamine (PE) liposomes, and to fuse PS/PE liposomes with lamellar bodies. These results suggest a relationship between lung surfactant secretion and Anx IIt-mediated membrane fusion.

Activation of protein kinase C by 1,4-dihydro-2,6-dimethyl-5-nitro-4-[2-(trifluoromethyl)-phenyl]-3-py rid ine carboxylic acid methyl ester (Bay K 8644), a calcium channel agonist, in alveolar type II cells

A role for calcium channels in the regulation of surfactant secretion is suggested by the observation that endothelin-1-stimulated surfactant secretion is inhibited by calcium channel blockers. 1,4-Dihydro-2,6-dimethyl-5-nitro-4-[2-(trifluoromethyl)-phenyl]-3-pyridi ne carboxylic acid methyl ester (Bay K 8644), a dihydropyridine derivative, stimulates voltage-dependent and non-voltage-dependent calcium channels in a number of cell types. This study demonstrates that Bay K 8644 increased phosphatidylcholine (PC) secretion in isolated lung epithelial type II cells in a time- and concentration-dependent manner with an EC50 of 100 +/- 8 nM (mean +/- SEM, N = 6). The secretagogue effect of Bay K 8644 was independently decreased in the absence of external calcium, or in the presence of nifedipine, a calcium channel antagonist, or inhibitors of protein kinase C (PKC). Bay K 8644 increased intracellular calcium from 130 +/- 8 to 230 +/- 14 nM (N = 6, P < 0.05), an effect that was blocked by nifedipine. Bay K 8644 also increased the membrane-associated PKC activity in a concentration-dependent manner. In the membranes from Bay K 8644-stimulated cells, the increase in calcium-dependent PKC was greater than that in the calcium-independent PKC, suggesting preferential translocation of calcium-dependent PKC to the membranes. We suggest that both elevated calcium and activation of PKC are required for calcium agonist Bay K 8644-induced surfactant secretion in type II cells.

Annexin II binds to the membrane of A549 cells in a calcium-dependent and calcium-independent manner

We investigated the nature of annexin II binding to the biological membranes using a lung epithelium-derived cell line A549. The cytosolic and membrane fractions of A549 cells were separated in the presence of 5 mM EGTA. Both fractions contain annexin II monomer and tetramer as evaluated by western blots using specific monoclonal antibodies against p36 and p11 subunits of annexin II. A substantial amount of annexin II was associated with the membrane fraction even after extensive washing with EGTA buffer, indicating the presence of two pools of annexin II. The EGTA-resistant membrane-bound annexin II could be partially extracted by 1% Triton X-100 or 60 mM n-octyl-beta-D-glucopyranoside, and completely by 30 mM CHAPS or 0.1% deoxycholate. This fraction of annexin II was also extracted by 0.1 M Na2CO3, pH 11 and partitioned into the aqueous phase after being treated with Triton X-114, demonstrating that the EGTA-resistant annexin II is a peripheral membrane protein. When the cells were lysed in varying concentrations of Ca2+, annexin II translocated from cytosolic fraction to membrane fraction at 4-25 microM Ca2+. To identify proteins closely associated with annexin II the membrane fraction was treated with the bifunctional chemical cross-linker disulfosuccinimidyl tartarate, followed by western blot analysis using anti-p36 or anti-p11 antibodies. We find that both p36 and p11 were cross-linked to a 51 kDa protein. In addition, p11 also binds to several proteins with molecular mass of 91, 65, 40 and 36 kDa. Our results suggest that annexin II may bind to the A549 cell membranes via specific membrane-associated proteins.

Cloning and sequencing of human gp330, a Ca(2+)-binding receptor with potential intracellular signaling properties

We present here the complete primary structure of human gp330, the human variant of the principal kidney autoantigen causing Heymann membranous glomerulonephritis in rats. The deduced 4655 amino acid residues give a calculated molecular mass of 519636 Da for the mature protein and consists of a probable 25-amino-acid N-terminal signal peptide sequence, an extracellular region of 4398 amino acids, a single transmembrane-spanning domain of 23 amino acids, and an intracellular C-terminal region of 209 amino acid residues. Three types of cysteine-rich repeats characteristic of the low density lipoprotein receptor (LDLR) superfamily are present in human gp330. In the extracellular region, there are a total of 36 LDLR ligand-binding repeats, comprising four distinct domains, 16 growth factor repeats separated by eight YWTD spacer regions, and one epidermal growth factor-like repeat. No consensus cleavage sequence for the processing endoprotease furin is detected in human gp330. The intracellular tail contains not only two copies of the F(X)NPXY coated-pit mediated internalization signal characteristic of LDLR superfamily members, but also intriguing and potentially functional motifs including several Src-homology 3 recognition motifs, one Src-homology 2 recognition motif for the p85 regulatory subunit of phosphatidylinositol 3-kinase, and additional sites for protein kinase C, casein kinase II and cAMP-/cGMP-dependent protein kinase. There is approximately 77% amino acid identity between human and rat gp330 with minor differences between the extracellular and intracellular regions. Recently gp330 has been implicated in Ca2+ regulation in the parathyroid, the placenta, and the renal tubule, but its overall physiological and pathological role still remains uncertain.

Lung annexin II promotes fusion of isolated lamellar bodies with liposomes

The role of annexin II in the secretion of lung surfactant was investigated using isolated lamellar bodies and/or liposomes as the model system for aggregation and fusion. We first compared membrane aggregation mediated by two forms of annexin II, annexin II monomer (Anx IIm) and annexin II tetramer (Anx IIt). Anx IIt required 20-fold less Ca2+ to mediate phosphatidylserine (PS) liposome aggregation compared to Anx IIm. Aggregation of lamellar bodies mediated by Anx IIt was 4-fold greater than that by Anx IIm at 1 mM Ca2+. These results suggest that Anx IIt may be the more active form in vivo. Fusion of lamellar bodies with PS liposomes was promoted by Anx IIt in a dose-dependent manner, with maximal fusion occurring at 10-15 micrograms/ml of Anx IIt. Fusion was dependent on Ca2+ and the phospholipid composition of liposomes. While the fusion of lamellar bodies with PS liposomes required 100 microM Ca2+, the fusion with PS/phosphatidylethanolamine (PE) (1:3) liposomes required only 10 microM Ca2+. Anx IIt-mediated lamellar body-liposome fusion was enhanced by arachidonic acid, a lung surfactant secretagogue and inhibited by 4.4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), an inhibitor of lung surfactant secretion. The data suggest that Anx IIt may play a role in the fusion of lamellar bodies with plasma membranes during lung surfactant secretion.

Inhibition of secretion from isolated rat alveolar epithelial type II cells by the cell permeant calpain inhibitor II (N-acetyl-leucyl-leucyl-methioninal)

Although several signal transduction pathways, including activation of specific protein kinases have been proposed and studied for the secretory processes of lung surfactant from alveolar epithelial type II cells, the role of proteolytic processing by calpains (calcium-activated neutral proteases) in secretion has not been investigated. Therefore, we examined the effect of cell permeable calpain inhibitor I (N-acetyl-leucyl-leucyl-norleucinal) and II (N-acetyl-leucyl-leucyl-methioninal) on secretion to test the hypothesis that calpains participate in the secretory processes of alveolar epithelial type II cells. Calpain inhibitor I preferentially inhibits micro (mu)-calpain while inhibitor II inhibits milli (m)-calpain. Isolated type II cells were prelabelled with [3H]-choline for 18-24 h. To measure secretion, [3H]-labelled disaturated phosphatidylcholine (DSPC) released in the medium was monitored. Basal secretion of DSPC was maximally (87%) depressed by the presence of 10 microM inhibitor II. Secretagogue-stimulated secretion was also modulated by inhibitor II treatment. Stimulation with calcium ionophore A23187 enhanced secretion 3-fold. However, cells pre-exposed to inhibitor II displayed a 90% reduction of calcium-stimulated secretion. Terbutaline (10 microM) and ATP (1 mM) each increased secretion 2- and 4-fold, respectively. However, the inhibitor-treated cells, exposed to the same stimuli, attained only 53 or 62% of these increases. Calpain inhibitor I, on the other hand, inhibited neither basal nor stimulated secretion. The results suggest that m-calpain, the major isozyme of lung calpain requiring mM calcium for activity in vitro, is involved in the secretory pathways of alveolar epithelial type II cells.

Arachidonic acid and eicosapentaenoic acid stimulate type II pneumocyte surfactant secretion

Arachidonic acid and its leukotriene metabolites have been shown to stimulate surfactant secretion by alveolar type II cells. The present study was undertaken to determine the effects of various unsaturated fatty acids, including eicosapentaenoic acid, on surfactant secretion. Surfactant secretion was expressed as the percent of [3H]choline-derived phospholipids released into culture medium by type II pneumocytes of adult rats. Consistent with the earlier findings, arachidonic acid stimulated secretion in a concentration-dependent fashion (3.5-21 microM), doubling baseline secretion at 21 microM. Eicosapentaenoic acid was found to be equally effective as arachidonic acid in stimulating secretion. A comparison with palmitic, oleic and linoleic acids revealed that highly unsaturated fatty acids stimulated secretion to the greatest extent. This was supported by a positive correlation between degree of unsaturation (i.e., 0, 1, 2, 4 and 5 double bonds) and stimulation of surfactant secretion. In the present study, exogenous leukotriene E4 (10(-13)-10(-6) did not stimulate surfactant secretion. Neither nordihydroguairetic acid (0.1 microM) nor indomethacin (0.1 microM) affected arachidonic acid-stimulated secretion. The stimulatory effects of arachidonic acid and eicosapentaenoic acid on surfactant secretion were related to the highly unsaturated nature of the fatty acids and were not mediated by increased levels of cyclic adenosine monophosphate or calcium.

Alpha 1-adrenergic and muscarinic receptors in adult and neonatal rat type II pneumocytes

Binding characteristics of the alpha 1-adrenergic radioloigand [3H]prazosin, and the muscarinic cholinergic radioligand, [3H]quinuclidinyl benzilate, were determined both in intact cell preparations of rat alveolar type II pneumocytes (TIIPs) and in membrane preparations of rat lung tissue. Binding in adult and neonatal (< 24 h postnatal age) rats was also compared. Binding affinities for both receptor classes on TIIPs and whole lung membrane preparations alike did not vary significantly with age. In lung membrane preparations, the concentrations of both receptor classes were higher in neonates than adults. In TIIPs, the alpha 1-adrenergic receptor concentration was higher in neonates, but muscarinic receptor concentration was higher in adults. To begin investigation of the functional significance of these receptors, the effects of alpha 1-adrenergic and muscarinic agonists on intracellular calcium ion concentration ([Ca2+]i) were also measured. Both agonists induced consistent increases in [Ca2+]i, which were blocked by respective antagonists. These data indicate the presence of receptors on TIIPs for alpha 1-adrenergic and muscarinic agonists that may influence cellular function via modulation of [Ca2+]i.

Carbamylcholine-induced morphological changes and spatial dynamics of [Ca2+]c in Harderian glands of guinea pigs: calcium-dependent lipid secretion and contraction of myoepithelial cells

To determine whether lipid-secreting cells have cytosolic Ca2+ concentration ([Ca2+]c)-related secretory mechanisms, morphological changes and intracellular calcium dynamics of Harderian glands of guinea pigs stimulated by secretagogue were studied by electron microscopy and Fura-2/AM digital image analysis. Control glandular cells contained large lipid vacuoles that were bordered by multi-layered membranes. Rough-surfaced endoplasmic reticulum, mitochondria, and smooth-surfaced endoplasmic reticulum may be involved in lipid vacuole formation. Myoepithelial cells surrounded alveoli. After carbamylcholine (CCh, 10(-6), 10(-5), and 10(-3) M) stimulation, lipid materials within the membranous structures were frequently discharged by an exocytotic mechanism. Conspicuous deformation of glandular cells caused by vigorous contraction of myoepithelial cells was observed in isolated alveoli after 10(-6) M CCh stimulation, whereas the deformities of glandular tissues perfused via vessels were small even after 10(-3) M CCh stimulation. Connective tissue between glandular alveoli inhibited unbridled myoepithelial-cell contraction. Fura-2/AM digital imaging analysis revealed that CCh stimulation caused an increase in [Ca2+]c in isolated alveoli. The morphological reactions and changes in [Ca2+]c were prevented by atropine. When extracellular calcium ions were absent, enhanced extrusion of lipid vacuoles, myoepithelial-cell contraction, and a rise in [Ca2+]c after CCh stimulation were not observed. Nicotine and catecholamines had no effect on the secretion or on the dynamics of [Ca2+]c. It can be concluded that acetylcholine elicits exocytosis in glandular cells and contraction of the myoepithelial cells of Harderian glands, accompanied by an increase in [Ca2+]c. The dynamics of [Ca2+]c of the gland alveoli are mostly dependent on extracellular Ca2+.

Low density lipoprotein- and high density lipoprotein-mediated signal transduction and exocytosis in alveolar type II cells

Low density lipoproteins (LDL) and high density lipoproteins (HDL) from serum stimulate signal-transduction pathways and exocytosis in rat alveolar type II cells. Both LDL and HDL stimulated primary cultures of type II cells to secrete phosphatidylcholine (PtdCho), the major phospholipid component of pulmonary surfactant. The effects on secretion were preceded temporally by stimulation of inositol phospholipid catabolism, calcium mobilization, and translocation of protein kinase C from cytosolic to membrane compartments. Heparin, which blocks the binding of ligands to the LDL receptor, completely inhibited the effects of LDL on signal transduction and PtdCho secretion but did not inhibit the effects of HDL. Unilamellar PtdCho liposomes the size of native LDL had no effect on type II cells; however, PtdCho complexes containing either apolipoproteins E or A-I stimulated both signal transduction and PtdCho secretion. LDL receptors were present in type II cell membranes by immunoblotting. In contrast to findings with hepatic membranes, type II cells exhibited two major bands of 130 kDa and 120 kDa and a minor band at 230 kDa that also was present under reducing conditions. These results are consistent with our hypothesis that the LDL-receptor pathway functions in vivo to deliver cholesterol to type II cells and that this process is coupled to surfactant assembly and secretion via signal-transduction pathway(s). HDL elicits similar responses independent of the LDL receptor, suggesting that type II cells may use the selective uptake pathway to obtain cholesterol or that HDL triggers signal transduction by mechanisms unrelated to lipid delivery.

Signal-transduction mechanisms of ATP-stimulated phosphatidylcholine secretion in rat type II pneumocytes: interactions between ATP and other surfactant secretagogues

ATP stimulates phosphatidylcholine secretion in type II cells, an effect that is mediated by both adenosine A2 receptors coupled to adenylate cyclase and P2 receptors coupled to phosphoinositide-specific phospholipase C. Activation of these effector enzymes leads to formation of cAMP, diacylglycerols and inositol trisphosphate (IP3). cAMP in turn activates cAMP-dependent protein kinase, diacylglycerols activate protein kinase C and IP3 promotes Ca2+ mobilization. To further investigate the signal-transduction mechanisms mediating the ATP effect, we examined its action in combination with that of other surfactant secretagogues: 5'(N-ethylcarboxyamido)adenosine (NECA), a A2 agonist that activates adenylate cyclase; TPA (12-O-tetradecanoylphorbol-13-acetate), a direct activator of protein kinase C; and ionomycin, an ionophore that increases intracellular Ca2+. The effects of NECA, TPA and ionomycin were additive and thus consistent with independent signaling mechanisms. However, the effects of all combinations of three or four secretagogues that contained ATP were 10-20% less than additive. This suggested that ATP and other secretagogues act via common mechanisms. Calmodulin antagonists decreased the effects of ionomycin and ATP by approx. 60% and 30%, respectively, but did not decrease the effects of NECA, terbutaline or TPA. Complete inhibition of the effect of ATP was achieved with a combination of a calmodulin antagonist, an A2 antagonist and a protein kinase C inhibitor. These and previous data suggest that the stimulatory effect of ATP on phosphatidylcholine secretion in type II cells is mediated by three signal-transduction mechanisms: activation of cAMP-dependent protein kinase; activation of protein kinase C; and a calmodulin-dependent mechanism.

Secretagogue-induced proteolysis of lung spectrin in alveolar epithelial type II cells

Incubation of isolated rat alveolar epithelial type II cells with secretagogues (calcium ionophore, ATP or terbutaline) resulted in rapid proteolysis of lung spectrin and appearance of multiple proteolytic products which showed immunoreactivity with an antibody against human erythrocyte spectrin. These proteolytic products were similar to those generated from erythrocyte spectrin or cultured lung tumor cells (A549 cells) incubated with purified calpain. Furthermore, incubation of alveolar type II cells with a calpain-specific inhibitor modulated the secretagogue-induced proteolysis of lung spectrin. Thus, stimulation of secretion appeared to activate endogenous calpain in type II cells, suggesting that calpain-mediated proteolysis of a submembranous cytoskeletal protein could play an important role in the secretory process.

Ventilation and secretion of pulmonary surfactant

Various factors are involved in the regulation of surfactant secretion: chemical agonist; local environmental factors such as mediators, locally produced hormones, and possibly pH; and finally, mechanical stress occurring during lung inflation. Here we suggest a model of regulation which is grouped into three levels: a basal autoregulatory mechanism with local factors being superimposed and a systemic level acting through hormones reaching the lung via the bloodstream. Depending on the situation, the different levels may vary in their importance. For the normal situation, in the absence of stress factors, we suggest the autoregulation of stretch-induced secretion and SP-A inhibition as indicated by in vitro experiments to be the prominent regulatory mechanism for surfactant secretion. From this model, mechanisms can be derived which indicate involvement of the surfactant system in, for example, obstructive lung disease. Support from the literature for this hypothesis is reviewed. Because quantitative measurement of the amount of surfactant-associated phospholipids cannot be done adequately at this time, we suggest testing the relatively risk-free application of exogenous surfactant in a pilot phase based on indications for its involvement and usefulness derived from animal and in vitro experiments.

Effects of extracellular ATP on surfactant secretion

Extracellular ATP is the most potent endogenous stimulus for surfactant phospholipid secretion from alveolar type II cells identified to date. This effect of ATP appears mediated via a P2-purinoceptor because the rank order of agonist potency is ATP greater than ADP greater than AMP = adenosine. Examination of other ATP analogues demonstrates a rank order of agonist potency of ATP = gamma S-ATP greater than AMPPNP greater than AMPPCP greater than AMPCPP for surfactant secretion, consistent with a P2y-purinoceptor mediating this effect. This hypothesis is further supported by experiments with reactive blue 2, which selectively inhibits ATP-stimulated surfactant phospholipid secretion and has been purported as a specific inhibitor at P2y-purinoceptors. Several second messenger systems are activated in the type II cell following agonist binding: intracellular Ca2+ is mobilized, prostaglandin levels increase, and protein kinase C is activated. Of these three second messengers, protein kinase C appears to be the most important for surfactant secretion because inhibition of protein kinase C activation blocks ATP-induced surfactant secretion whereas inhibition of Ca2+ mobilization and prostaglandin production does not affect ATP-induced surfactant secretion.

Calcium mobilization and exocytosis after one mechanical stretch of lung epithelial cells

Deep inflation of the lung stimulates surfactant secretion by unknown mechanisms. The hypothesis that mechanical distension directly stimulates type II cells to secrete surfactant was tested by stretching type II cells cultured on silastic membranes. The intracellular Ca2+ concentration was measured in single cells, before and after stretching. A single stretch of alveolar type II cells caused a transient (less than 60 seconds) increase in cytosolic Ca2+ followed by a sustained (15 to 30 minutes) stimulation of surfactant secretion. Both Ca2+ mobilization and exocytosis exhibited dose-dependence to the magnitude of the stretch-stimulus. Thus, mechanical factors can trigger complex cellular events in nonneuron, nonmuscle cells and may be involved in regulating normal lung functions.

P2-purinoceptor regulation of surfactant phosphatidylcholine secretion. Relative roles of calcium and protein kinase C

Ca2+ and protein kinase C have both been proposed as intracellular signals for subsequent phosphatidylcholine secretion by alveolar Type II cells. We have determined the relative roles of Ca2+ and protein kinase C in regulating surfactant phosphatidylcholine secretion by utilizing exogenous ATP and the phorbol ester TPA (12-O-tetradecanoylphorbol 13-acetate) as secretagogues, along with MAPTAM to chelate intracellular Ca2+ and sphingosine to inhibit endogenous protein kinase C. Exposure of Type II cells to the P2-purinoceptor agonist, ATP, results in a dose-dependent increase in surfactant phosphatidylcholine secretion from isolated alveolar Type II cells with an EC50 (concn. producing 50% of maximal response) of 2 microM. Administration of exogenous ATP to Type II cells also results in a dose-dependent increase in inositol trisphosphate production, Ca2+ mobilization and [3H]phorbol 12,13-dibutyrate ([3H]PDBu) binding as a measure of protein kinase C translocation. The EC50 in each case is 1-5 microM, indicating association of these events with surfactant phosphatidylcholine secretion. Loading Type II cells with non-hydrolysable GTP analogue (GTP[S]) inhibited ATP-induced Ca2+ mobilization, supporting the hypothesis that Type II cell P2-purinoceptors are coupled to phospholipase C via a GTP-binding protein. The ATP-induced elevation of cytosolic Ca2+ was also inhibited by MAPTAM (a cell-permeant EGTA analogue) by 90%, but MAPTAM was without effect on surfactant phosphatidylcholine secretion induced by ATP. Sphingosine inhibited both ATP- and TPA-induced surfactant phosphatidylcholine secretion as well as [3H]PDBu binding with a similar IC50 (concn. producing 50% of maximal inhibition) (10 microM). Sphingosine did not affect surfactant phosphatidylcholine secretion induced by terbutaline and did not have a significant effect on Ca2+ mobilization induced by exogenous ATP. These results are consistent with a prominent role for protein kinase C in regulation of P2-purinoceptor-induced surfactant phosphatidylcholine secretion, and indicate that Ca2+ mobilization is not a necessary step for ATP-induced surfactant phosphatidylcholine secretion.