Two types of chloride channel on duct cells cultured from human fetal pancreas

Effects of postnatal corticosteroids on lung development in newborn animals. A systematic review

BACKGROUND

Postnatal systemic corticosteroids reduce the risk of bronchopulmonary dysplasia but the effect depends on timing, dosing, and type of corticosteroids. Animal studies may provide valuable information on these variable effects. This systematic review summarizes the effects of postnatal systemic corticosteroids on lung development in newborn animals.

METHODS

A systematic search was performed in PubMed and Embase in December 2022. The protocol was published on PROSPERO (CRD42021177701).

RESULTS

Of the 202 eligible studies, 51 were included. Only newborn rodent studies met the inclusion criteria. Most studies used dexamethasone (98%). There was huge heterogeneity in study outcome measures and corticosteroid treatment regimens. Reporting of study quality indicators was mediocre and risk of bias was unclear due to poor reporting of study methodology. Meta-analysis showed that postnatal corticosteroids caused a decrease in body weight as well as persistent alveolar simplification. Subgroup analyses revealed that healthy animals were most affected.

CONCLUSION

In newborn rodents, postnatal systemic corticosteroids have a persistent negative effect on body weight and lung development. There was huge heterogeneity in experimental models, mediocre study quality, unclear risk of bias, and very small subgroups for meta-analysis which limited firm conclusions.

IMPACT

Postnatal corticosteroids reduce the risk of bronchopulmonary dysplasia but the effect depends on timing, dosing, and type of corticosteroids while the underlying mechanism of this variable effect is unknown. This is the first systematic review and meta-analysis of preclinical newborn animal studies reviewing the effect of postnatal systemic corticosteroids on lung development. In newborn rodent models, postnatal corticosteroids have a persistent negative effect on body weight and lung alveolarization, especially in healthy animals.

Role of CFTR in diabetes-induced pancreatic ductal fluid and HCO3 - secretion

Type 1 diabetes is a disease of the endocrine pancreas; however, it also affects exocrine function. Although most studies have examined the effects of diabetes on acinar cells, much less is known regarding ductal cells, despite their important protective function in the pancreas. Therefore, we investigated the effect of diabetes on ductal function. Diabetes was induced in wild-type and cystic fibrosis transmembrane conductance regulator (CFTR) knockout mice following an i.p. administration of streptozotocin. Pancreatic ductal fluid and HCO3 - secretion were determined using fluid secretion measurements and fluorescence microscopy, respectively. The expression of ion transporters was measured by real-time PCR and immunohistochemistry. Transmission electron microscopy was used for the morphological characterization of the pancreas. Serum secretin and cholecystokinin levels were measured by an enzyme-linked immunosorbent assay. Ductal fluid and HCO3 - secretion, CFTR activity, and the expression of CFTR, Na+ /H+ exchanger-1, anoctamine-1 and aquaporin-1 were significantly elevated in diabetic mice. Acute or chronic glucose treatment did not affect HCO3 - secretion, but increased alkalizing transporter activity. Inhibition of CFTR significantly reduced HCO3 - secretion in both normal and diabetic mice. Serum levels of secretin and cholecystokinin were unchanged, but the expression of secretin receptors significantly increased in diabetic mice. Diabetes increases fluid and HCO3 - secretion in pancreatic ductal cells, which is associated with the increased function of ion and water transporters, particularly CFTR. KEY POINTS: There is a lively interaction between the exocrine and endocrine pancreas not only under physiological conditions, but also under pathophysiological conditions The most common disease affecting the endocrine part is type-1 diabetes mellitus (T1DM), which is often associated with pancreatic exocrine insufficiency Compared with acinar cells, there is considerably less information regarding the effect of diabetes on pancreatic ductal epithelial cells, despite the fact that the large amount of fluid and HCO3 - produced by ductal cells is essential for maintaining normal pancreatic functions Ductal fluid and HCO3 - secretion increase in T1DM, in which increased cystic fibrosis transmembrane conductance regulator activation plays a central role. We have identified a novel interaction between T1DM and ductal cells. Presumably, the increased ductal secretion represents a defence mechanism in the prevention of diabetes, but further studies are needed to clarify this issue.

Structural identification of a selectivity filter in CFTR

The cystic fibrosis transmembrane conductance regulator (CFTR) is a chloride channel that regulates transepithelial salt and fluid homeostasis. CFTR dysfunction leads to reduced chloride secretion into the mucosal lining of epithelial tissues, thereby causing the inherited disease cystic fibrosis. Although several structures of CFTR are available, our understanding of the ion-conduction pathway is incomplete. In particular, the route that connects the cytosolic vestibule with the extracellular space has not been clearly defined, and the structure of the open pore remains elusive. Furthermore, although many residues have been implicated in altering the selectivity of CFTR, the structure of the "selectivity filter" has yet to be determined. In this study, we identify a chloride-binding site at the extracellular ends of transmembrane helices 1, 6, and 8, where a dehydrated chloride is coordinated by residues G103, R334, F337, T338, and Y914. Alterations to this site, consistent with its function as a selectivity filter, affect ion selectivity, conductance, and open channel block. This selectivity filter is accessible from the cytosol through a large inner vestibule and opens to the extracellular solvent through a narrow portal. The identification of a chloride-binding site at the intra- and extracellular bridging point leads us to propose a complete conductance path that permits dehydrated chloride ions to traverse the lipid bilayer.

Human molecular genetics and the long road to treating cystic fibrosis

The causative gene in cystic fibrosis was identified in 1989, three years before the publication of the first issue of Human Molecular Genetics. CFTR was among the first genes underlying a common inherited disorder to be cloned, and hence its subsequent utilization towards a cure for CF provides a roadmap for other monogenic diseases. Over the past 30 years the advances that built upon knowledge of the gene and the CFTR protein to develop effective therapeutics have been remarkable, and yet the setbacks have also been challenging. Technological progress in other fields has often circumvented the barriers. This review focuses on key aspects of CF diagnostics and current approaches to develop new therapies for all CFTR mutations. It also highlights the major research advances that underpinned progress towards treatments, and considers the remaining obstacles.

Clinical and molecular characterization of the R751L-CFTR mutation

Cystic fibrosis (CF) arises from mutations in the CF transmembrane conductance regulator (CFTR) gene, resulting in progressive and life-limiting respiratory disease. R751L is a rare CFTR mutation that is poorly characterized. Our aims were to describe the clinical and molecular phenotypes associated with R751L. Relevant clinical data were collected from three heterozygote individuals harboring R751L (2 patients with G551D/R751L and 1 with F508del/R751L). Assessment of R751L-CFTR function was made in primary human bronchial epithelial cultures (HBEs) and Xenopus oocytes. Molecular properties of R751L-CFTR were investigated in the presence of known CFTR modulators. Although sweat chloride was elevated in all three patients, the clinical phenotype associated with R751L was mild. Chloride secretion in F508del/R751L HBEs was reduced compared with non-CF HBEs and associated with a reduction in sodium absorption by the epithelial sodium channel (ENaC). However, R751L-CFTR function in Xenopus oocytes, together with folding and cell surface transport of R751L-CFTR, was not different from wild-type CFTR. Overall, R751L-CFTR was associated with reduced sodium chloride absorption but had functional properties similar to wild-type CFTR. This is the first report of R751L-CFTR that combines clinical phenotype with characterization of functional and biological properties of the mutant channel. Our work will build upon existing knowledge of mutations within this region of CFTR and, importantly, inform approaches for clinical management. Elevated sweat chloride and reduced chloride secretion in HBEs may be due to alternative non-CFTR factors, which require further investigation.

Ion Channel Signature in Healthy Pancreas and Pancreatic Ductal Adenocarcinoma

Pancreatic ductal adenocarcinoma (PDAC) is the fourth most common cause of cancer-related deaths in United States and Europe. It is predicted that PDAC will become the second leading cause of cancer-related deaths during the next decades. The development of PDAC is not well understood, however, studies have shown that dysregulated exocrine pancreatic fluid secretion can contribute to pathologies of exocrine pancreas, including PDAC. The major roles of healthy exocrine pancreatic tissue are secretion of enzymes and bicarbonate rich fluid, where ion channels participate to fine-tune these biological processes. It is well known that ion channels located in the plasma membrane regulate multiple cellular functions and are involved in the communication between extracellular events and intracellular signaling pathways and can function as signal transducers themselves. Hereby, they contribute to maintain resting membrane potential, electrical signaling in excitable cells, and ion homeostasis. Despite their contribution to basic cellular processes, ion channels are also involved in the malignant transformation from a normal to a malignant phenotype. Aberrant expression and activity of ion channels have an impact on essentially all hallmarks of cancer defined as; uncontrolled proliferation, evasion of apoptosis, sustained angiogenesis and promotion of invasion and migration. Research indicates that certain ion channels are involved in the aberrant tumor growth and metastatic processes of PDAC. The purpose of this review is to summarize the important expression, localization, and function of ion channels in normal exocrine pancreatic tissue and how they are involved in PDAC progression and development. As ion channels are suggested to be potential targets of treatment they are furthermore suggested to be biomarkers of different cancers. Therefore, we describe the importance of ion channels in PDAC as markers of diagnosis and clinical factors.

Increases in cytosolic Ca2+ induce dynamin- and calcineurin-dependent internalisation of CFTR

The cystic fibrosis transmembrane conductance regulator (CFTR) is a cAMP-regulated, apical anion channel that regulates ion and fluid transport in many epithelia including the airways. We have previously shown that cigarette smoke (CS) exposure to airway epithelia causes a reduction in plasma membrane CFTR expression which correlated with a decrease in airway surface hydration. The effect of CS on CFTR was dependent on an increase in cytosolic Ca2+. However, the underlying mechanism for this Ca2+-dependent, internalisation of CFTR is unknown. To gain a better understanding of the effect of Ca2+ on CFTR, we performed whole cell current recordings to study the temporal effect of raising cytosolic Ca2+ on CFTR function. We show that an increase in cytosolic Ca2+ induced a time-dependent reduction in whole cell CFTR conductance, which was paralleled by a loss of cell surface CFTR expression, as measured by confocal and widefield fluorescence microscopy. The decrease in CFTR conductance and cell surface expression were both dynamin-dependent. Single channel reconstitution studies showed that raising cytosolic Ca2+ per se had no direct effect on CFTR. In fact, the loss of CFTR plasma membrane activity correlated with activation of calcineurin, a Ca2+-dependent phosphatase, suggesting that dephosphorylation of CFTR was linked to the loss of surface expression. In support of this, the calcineurin inhibitor, cyclosporin A, prevented the Ca2+-induced decrease in cell surface CFTR. These results provide a hitherto unrecognised role for cytosolic Ca2+ in modulating the residency of CFTR at the plasma membrane through a dynamin- and calcineurin-dependent mechanism.

Most bicarbonate secretion by Calu-3 cells is mediated by CFTR and independent of pendrin

Bicarbonate plays an important role in airway host defense, however, its transport mechanisms remain uncertain. Here we examined the relative contributions of the anion channel CFTR (cystic fibrosis transmembrane conductance regulator, ABCC7) and the anion exchanger pendrin (SLC26A4) to HCO3- secretion by the human airway cell line Calu-3. Pendrin and CFTR were both detected in parental Calu-3 cells, although mRNA and protein expression appeared higher for CFTR than for pendrin. Targeting pendrin transcripts with lentiviral shRNA reduced pendrin detection by immunofluorescence staining but did not alter the rates of HCO3- or fluid secretion, HCO3- transport under pH-stat conditions, or net HCO3- flux across basolaterally permeabilized monolayers. Intracellular pH varied with step changes in apical Cl- and HCO3- concentrations in control and pendrin knockdown Calu-3 cells, but not in CFTR deficient cells. Exposure to the proinflammatory cytokine IL-4, which strongly upregulates pendrin expression in airway surface epithelia, had little effect on Calu-3 pendrin expression and did not alter fluid or HCO3- secretion. Similar results were obtained using air-liquid interface and submerged cultures, although CFTR and pendrin mRNA expression were both lower when cells were cultured under submerged conditions. While the conclusions cannot be extrapolated to other airway epithelia, the present results demonstrate that most HCO3- secretion by Calu-3 cells is mediated by CFTR.

Bicarbonate in cystic fibrosis

BACKGROUND

Cystic fibrosis (CF, mucoviscidosis) is caused by mutations in the gene encoding CF transmembrane conductance regulator (CFTR), which is a chloride and bicarbonate channel necessary for fluid secretion and extracellular alkalization. For a long time, research concentrated on abnormal Cl^- and Na⁺ transport, but neglected bicarbonate as a crucial factor in CF.

METHODS

The present short review reports early findings as well as recent insights into the role of CFTR for bicarbonate transport and its defects in CF.

RESULTS

The available data indicate impaired bicarbonate transport not only in pancreas, intestine, airways, and reproductive organs, but also in salivary glands, sweat duct and renal tubular epithelial cells. Defective bicarbonate transport is closely related to the impaired mucus properties and mucus blocking in secretory organs of CF patients, causing the life threatening lung disease.

CONCLUSIONS

Apart from the devastating lung disease, abrogated bicarbonate transport also leads to many other organ dysfunctions, which are outlined in the present review.

Role of CFTR in epithelial physiology

Salt and fluid absorption and secretion are two processes that are fundamental to epithelial function and whole body fluid homeostasis, and as such are tightly regulated in epithelial tissues. The CFTR anion channel plays a major role in regulating both secretion and absorption in a diverse range of epithelial tissues, including the airways, the GI and reproductive tracts, sweat and salivary glands. It is not surprising then that defects in CFTR function are linked to disease, including life-threatening secretory diarrhoeas, such as cholera, as well as the inherited disease, cystic fibrosis (CF), one of the most common life-limiting genetic diseases in Caucasian populations. More recently, CFTR dysfunction has also been implicated in the pathogenesis of acute pancreatitis, chronic obstructive pulmonary disease (COPD), and the hyper-responsiveness in asthma, underscoring its fundamental role in whole body health and disease. CFTR regulates many mechanisms in epithelial physiology, such as maintaining epithelial surface hydration and regulating luminal pH. Indeed, recent studies have identified luminal pH as an important arbiter of epithelial barrier function and innate defence, particularly in the airways and GI tract. In this chapter, we will illustrate the different operational roles of CFTR in epithelial function by describing its characteristics in three different tissues: the airways, the pancreas, and the sweat gland.

The Physiology and Pathophysiology of Pancreatic Ductal Secretion: The Background for Clinicians

The human exocrine pancreas consists of 2 main cell types: acinar and ductal cells. These exocrine cells interact closely to contribute to the secretion of pancreatic juice. The most important ion in terms of the pancreatic ductal secretion is HCO3. In fact, duct cells produce an alkaline fluid that may contain up to 140 mM NaHCO3, which is essential for normal digestion. This article provides an overview of the basics of pancreatic ductal physiology and pathophysiology. In the first part of the article, we discuss the ductal electrolyte and fluid transporters and their regulation. The central role of cystic fibrosis transmembrane conductance regulator (CFTR) is highlighted, which is much more than just a Cl channel. We also review the role of pancreatic ducts in severe debilitating diseases such as cystic fibrosis (caused by various genetic defects of cftr), pancreatitis, and diabetes mellitus. Stimulation of ductal secretion in cystic fibrosis and pancreatitis may have beneficial effects in their treatment.

Acid-base transport in pancreas-new challenges

Along the gastrointestinal tract a number of epithelia contribute with acid or basic secretions in order to aid digestive processes. The stomach and pancreas are the most extreme examples of acid (H(+)) and base (HCO(-) 3) transporters, respectively. Nevertheless, they share the same challenges of transporting acid and bases across epithelia and effectively regulating their intracellular pH. In this review, we will make use of comparative physiology to enlighten the cellular mechanisms of pancreatic HCO(-) 3 and fluid secretion, which is still challenging physiologists. Some of the novel transporters to consider in pancreas are the proton pumps (H(+)-K(+)-ATPases), as well as the calcium-activated K(+) and Cl(-) channels, such as KCa3.1 and TMEM16A/ANO1. Local regulators, such as purinergic signaling, fine-tune, and coordinate pancreatic secretion. Lastly, we speculate whether dys-regulation of acid-base transport contributes to pancreatic diseases including cystic fibrosis, pancreatitis, and cancer.

Neonatal steroids induce a down-regulation of tenascin-C and elastin and cause a deceleration of the first phase and an acceleration of the second phase of lung alveolarization

Pre- and postnatal corticosteroids are often used in perinatal medicine to improve pulmonary function in preterm infants. To mimic this clinical situation, newborn rats were treated systemically with dexamethasone (Dex), 0.1-0.01 mg/kg/day on days P1-P4. We hypothesized that postnatal Dex may have an impact on alveolarization by interfering with extracellular matrix proteins and cellular differentiation. Morphological alterations were observed on 3D images obtained by high-resolution synchrotron radiation X-ray tomographic microscopy. Alveolarization was quantified stereologically by estimating the formation of new septa between days P4 and P60. The parenchymal expression of tenascin-C (TNC), smooth muscle actin (SMA), and elastin was measured by immunofluorescence and gene expression for TNC by qRT-PCR. After Dex treatment, the first phase of alveolarization was significantly delayed between days P6 and P10, whereas the second phase was accelerated. Elastin and SMA expressions were delayed by Dex treatment, whereas TNC expression was delayed and prolonged. A short course of neonatal steroids impairs the first phase of alveolarization, most likely by altering the TNC and elastin expression. Due to an overshooting catch-up during the second phase of alveolarization, the differences disappear when the animals reach adulthood.

The cystic fibrosis of exocrine pancreas

The cystic fibrosis transmembrane conductance regulator (CFTR) protein is highly expressed in the pancreatic duct epithelia and permits anions and water to enter the ductal lumen. This results in an increased volume of alkaline fluid allowing the highly concentrated proteins secreted by the acinar cells to remain in a soluble state. This work will expound on the pathophysiology and pathology caused by the malfunctioning CFTR protein with special reference to ion transport and acid-base abnormalities both in humans and animal models. We will also discuss the relationship between cystic fibrosis (CF) and pancreatitis, and outline present and potential therapeutic approaches in CF treatment relevant to the pancreas.

Ion transport in human pancreatic duct epithelium, Capan-1 cells, is regulated by secretin, VIP, acetylcholine, and purinergic receptors

OBJECTIVES

The objective of the study was to establish a solid model of polarized epithelium for human pancreatic ducts, where electrical parameters could be measured as indicators of ion transport. Further, we aimed to determine functional expression of several receptors, in particular, purinergic receptors, and determine their effects on ion transport.

METHODS

Human adenocarcinoma cell line Capan-1 cells were grown on permeable supports and set in Ussing chambers for electrophysiological recordings. Transepithelial voltage (Vte), resistance, and short-circuit currents (Isc) were measured in response to agonists.

RESULTS

Secretin, vasoactive intestinal peptide (VIP), acetylcholine, forskolin, ionomycin, adenosine 5'-triphosphate (ATP), uridine 5'-triphosphate (UTP), 3'-O-(4-benzoyl)benzoyl ATP, and adenosine induced lumen negative Vte and Isc. These changes were consistent with anion secretion, as verified in forskolin-stimulated preparations. Extracellular nucleotides, ATP, and UTP, applied from luminal and basolateral sides, caused largest responses: Vte increased up to -5 mV, Isc increased to 20 to 30 μA/cm, and resistance decreased by up to 200 Ω·cm.

CONCLUSIONS

Transepithelial transport in human pancreatic duct epithelium, Capan-1 cells, is regulated by secretin, VIP, acetylcholine, adenosine, and purinergic P2 receptors; and this human model has a good potential for studies of physiology and pathophysiology of pancreatic duct ion transport.

Transepithelial bicarbonate secretion: lessons from the pancreas

Many cystic fibrosis transmembrane conductance regulator (CFTR)-expressing epithelia secrete bicarbonate (HCO(3)(-))-containing fluids. Recent evidence suggests that defects in epithelial bicarbonate secretion are directly involved in the pathogenesis of cystic fibrosis, in particular by building up hyperviscous mucus in the ductal structures of the lung and pancreas. Pancreatic juice is one of the representative fluids that contain a very high concentration of bicarbonate among bodily fluids that are secreted from CFTR-expressing epithelia. We introduce up-to-date knowledge on the basic principles of transepithelial bicarbonate transport by showing the mechanisms involved in pancreatic bicarbonate secretion. The model of pancreatic bicarbonate secretion described herein may also apply to other exocrine epithelia. As a central regulator of bicarbonate transport at the apical membrane, CFTR plays an essential role in both direct and indirect bicarbonate secretion. The major role of CFTR in bicarbonate secretion would be variable depending on the tissue and cell type. For example, in epithelial cells that produce a low concentration of bicarbonate-containing fluid (up to 80 mm), either CFTR-dependent Cl(-)/HCO(3)(-) exchange or CFTR anion channel with low bicarbonate permeability would be sufficient to generate such fluid. However, in cells that secrete high-bicarbonate-containing fluids, a highly selective CFTR bicarbonate channel activity is required. Therefore, understanding the molecular mechanism of transepithelial bicarbonate transport and the role of CFTR in each specific epithelium will provide therapeutic strategies to recover from epithelial defects induced by hyposecretion of bicarbonate in cystic fibrosis.

Basolateral chloride loading by the anion exchanger type 2: role in fluid secretion by the human airway epithelial cell line Calu-3

Anion exchanger type 2 (AE2 or SLC4A2) is an electroneutral Cl(-)/HCO(3)(-) exchanger expressed at the basolateral membrane of many epithelia. It is thought to participate in fluid secretion by airway epithelia. However, the role of AE2 in fluid secretion remains uncertain, due to the lack of specific pharmacological inhibitors, and because it is electrically silent and therefore does not contribute directly to short-circuit current (I(sc)). We have studied the role of AE2 in Cl(-) and fluid secretion by the airway epithelial cell line Calu-3. After confirming expression of its mRNA and protein, a knock-down cell line called AE2-KD was generated by lentivirus-mediated RNA interference in which AE2 mRNA and protein levels were reduced 90%. Suppressing AE2 increased the expression of the cystic fibrosis transmembrane conductance regulator (CFTR) by ∼70% without affecting the levels of NKCC1 (Na(+)-K(+)-2Cl(-) cotransporter) or NBCe1 (Na(+)-nHCO(3)(-) cotransporter). cAMP agonists stimulated fluid secretion by parental Calu-3 and scrambled shRNA cells >6.5-fold. In AE2-KD cells this response was reduced by ∼70%, and the secreted fluid exhibited elevated pH and [HCO(3)(-)] as compared with the control lines. Unstimulated equivalent short-circuit current (I(eq)) was elevated in AE2-KD cells, but the incremental response to forskolin was unaffected. The modest bumetanide-induced reductions in both I(eq) and fluid secretion were more pronounced in AE2-KD cells. Basolateral Cl(-)/HCO(3)(-) exchange measured by basolateral pH-stat in cells with permeabilized apical membranes was abolished in AE2-KD monolayers, and the intracellular alkalinization resulting from basolateral Cl(-) removal was reduced by ∼80% in AE2-KD cells. These results identify AE2 as a major pathway for basolateral Cl(-) loading during cAMP-stimulated secretion of Cl(-) and fluid by Calu-3 cells, and help explain the large bumetanide-insensitive component of fluid secretion reported previously in airway submucosal glands and some other epithelia.

CFTR-SLC26 transporter interactions in epithelia

Transport mechanisms that mediate the movements of anions must be coordinated tightly in order to respond appropriately to physiological stimuli. This process is of paramount importance in the function of diverse epithelial tissues of the body, such as, for example, the exocrine pancreatic duct and the airway epithelia. Disruption of any of the finely tuned components underlying the transport of anions such as Cl(-), HCO(3) (-), SCN(-), and I(-) may contribute to a plethora of disease conditions. In many anion-secreting epithelia, the interactions between the cystic fibrosis transmembrane conductance regulator (CFTR) and solute carrier family 26 (SLC26) transporters determine the final exit of anions across the apical membrane and into the luminal compartment. The molecular identification of CFTR and many SLC26 members has enabled the acquisition of progressively more detailed structural information about these transport molecules. Studies employing a vast array of increasingly sophisticated approaches have culminated in a current working model which places these key players within an interactive complex, thereby setting the stage for future work.

Molecular mechanism of pancreatic and salivary gland fluid and HCO3 secretion

Fluid and HCO(3)(-) secretion is a vital function of all epithelia and is required for the survival of the tissue. Aberrant fluid and HCO(3)(-) secretion is associated with many epithelial diseases, such as cystic fibrosis, pancreatitis, Sjögren's syndrome, and other epithelial inflammatory and autoimmune diseases. Significant progress has been made over the last 20 years in our understanding of epithelial fluid and HCO(3)(-) secretion, in particular by secretory glands. Fluid and HCO(3)(-) secretion by secretory glands is a two-step process. Acinar cells secrete isotonic fluid in which the major salt is NaCl. Subsequently, the duct modifies the volume and electrolyte composition of the fluid to absorb the Cl(-) and secrete HCO(3)(-). The relative volume secreted by acinar and duct cells and modification of electrolyte composition of the secreted fluids varies among secretory glands to meet their physiological functions. In the pancreas, acinar cells secrete a small amount of NaCl-rich fluid, while the duct absorbs the Cl(-) and secretes HCO(3)(-) and the bulk of the fluid in the pancreatic juice. Fluid secretion appears to be driven by active HCO(3)(-) secretion. In the salivary glands, acinar cells secrete the bulk of the fluid in the saliva that is driven by active Cl(-) secretion and contains high concentrations of Na(+) and Cl(-). The salivary glands duct absorbs both the Na(+) and Cl(-) and secretes K(+) and HCO(3)(-). In this review, we focus on the molecular mechanism of fluid and HCO(3)(-) secretion by the pancreas and salivary glands, to highlight the similarities of the fundamental mechanisms of acinar and duct cell functions, and to point out the differences to meet gland-specific secretions.

Regulation of CFTR chloride channel macroscopic conductance by extracellular bicarbonate

The CFTR contributes to Cl⁻ and HCO₃⁻ transport across epithelial cell apical membranes. The extracellular face of CFTR is exposed to varying concentrations of Cl⁻ and HCO₃⁻ in epithelial tissues, and there is evidence that CFTR is sensitive to changes in extracellular anion concentrations. Here we present functional evidence that extracellular Cl⁻ and HCO₃⁻ regulate anion conduction in open CFTR channels. Using cell-attached and inside-out patch-clamp recordings from constitutively active mutant E1371Q-CFTR channels, we show that voltage-dependent inhibition of CFTR currents in intact cells is significantly stronger when the extracellular solution contains HCO₃⁻ than when it contains Cl⁻. This difference appears to reflect differences in the ability of extracellular HCO₃⁻ and Cl⁻ to interact with and repel intracellular blocking anions from the pore. Strong block by endogenous cytosolic anions leading to reduced CFTR channel currents in intact cells occurs at physiologically relevant HCO₃⁻ concentrations and membrane potentials and can result in up to ∼50% inhibition of current amplitude. We propose that channel block by cytosolic anions is a previously unrecognized, physiologically relevant mechanism of channel regulation that confers on CFTR channels sensitivity to different anions in the extracellular fluid. We further suggest that this anion sensitivity represents a feedback mechanism by which CFTR-dependent anion secretion could be regulated by the composition of the secretions themselves. Implications for the mechanism and regulation of CFTR-dependent secretion in epithelial tissues are discussed.

Altered ion transport by thyroid epithelia from CFTR(-/-) pigs suggests mechanisms for hypothyroidism in cystic fibrosis

Subclinical hypothyroidism has been linked to cystic fibrosis, and the cystic fibrosis transmembrane conductance regulator (CFTR) shown to be expressed in the thyroid. The thyroid epithelium secretes Cl⁻ and absorbs Na(+) in response to cAMP. Chloride secretion may provide a counter-ion for the SLC26A4 (pendrin)-mediated I⁻ secretion which is required for the first step of thyroid hormonogenesis, thyroglobulin iodination. In contrast, few models exist to explain a role for Na(+) absorption. Whether CFTR mediates the secretory Cl⁻ current in thyroid epithelium has not been directly addressed. We used thyroids from a novel pig CFTR(-/-) model, generated primary pig thyroid epithelial cell cultures (pThECs), analysed these cultures for preservation of thyroid-specific transcripts and proteins, and monitored the following parameters: (1) the Cl⁻ secretory response to the cAMP agonist, isoprenaline; and (2) the amiloride-sensitive Na(+) current. Baseline short-circuit current (I(sc)) did not differ between CFTR(+/+) and CFTR(-/-) cultures. Serosal isoprenaline increased I(sc) in CFTR(+/+), but not CFTR(-/-), monolayers. Compared with CFTR(+/+) thyroid cultures, amiloride-sensitive Na(+) absorption measured in CFTR(-/-) pThECs represented a greater fraction of the resting I(sc). However, levels of transcripts encoding epithelial sodium channel (ENaC) subunits did not differ between CFTR(+/+) and CFTR(-/-) pThECs. Immunoblot analysis verified ENaC subunit protein expression, but quantification indicated no difference in expression levels. Our studies definitively demonstrate that CFTR mediates cAMP-stimulated Cl⁻ secretion in a well-differentiated thyroid culture model and that knockout of CFTR promotes increased Na(+) absorption by a mechanism other than increased ENaC expression. These findings suggest several models for the mechanism of cystic fibrosis-associated hypothyroidism.

Molecular and functional characterization of the cystic fibrosis transmembrane conductance regulator from the Australian common brushtail possum, Trichosurus vulpecula

Unlike eutherian mammals, the colon of the Australian common brushtail possum, Trichosurus vulpecula, a metatherian mammal, is incapable of electrogenic Cl(-) secretion and has elevated levels of electrogenic Na(+) absorption, while the ileum secretes HCO (3) (-) rather than Cl(-). In eutherian mammals, the cystic fibrosis transmembrane conductance regulator (CFTR) is essential for both Cl(-) and HCO (3) (-) secretion and the regulation of Na(+) absorption. Therefore, we have sequenced possum (p)CFTR, described its distribution and characterized the properties of cloned pCFTR expressed in Fischer rat thyroid (FRT) cells. pCFTR (GenBank accession No. AY916796) has a 1,478 amino acid open reading frame, which has >90% identity with CFTR from other marsupials and >80% identity with non-rodent eutherian mammals. In pCFTR, there is a high level of conservation of the transmembrane and nucleotide binding domains although, with the exception of other marsupials, there is considerable divergence from other species in the R domain. FRT cells transfected with pCFTR express mature CFTR protein which functions as a small Cl(-) channel activated by cAMP-dependent phosphorylation. In whole-cell recordings it has a linear, time and voltage-independent conductance, with a selectivity sequence P(Br) > P(Cl) > P(I) > P(HCO)(3) >> P(Gluconate). pCFTR transcript is present in a range of epithelia, including the ileum and the colon. The presence of pCFTR in the ileum and its measured HCO (3) (-) permeability suggest that it may be involved in ileal HCO (3) (-) secretion. Why the possum colon does not secrete Cl(-) and has elevated electrogenic Na(+) absorption, despite the apparent expression of CFTR, remains to be determined.

The angiogenic factor midkine is regulated by dexamethasone and retinoic acid during alveolarization and in alveolar epithelial cells

Background

A precise balance exists between the actions of endogenous glucocorticoids (GC) and retinoids to promote normal lung development, in particular during alveolarization. The mechanisms controlling this balance are largely unknown, but recent evidence suggests that midkine (MK), a retinoic acid-regulated, pro-angiogenic growth factor, may function as a critical regulator. The purpose of this study was to examine regulation of MK by GC and RA during postnatal alveolar formation in rats.

Methods

Newborn rats were treated with dexamethasone (DEX) and/or all-trans-retinoic acid (RA) during the first two weeks of life. Lung morphology was assessed by light microscopy and radial alveolar counts. MK mRNA and protein expression in response to different treatment were determined by Northern and Western blots. In addition, MK protein expression in cultured human alveolar type 2-like cells treated with DEX and RA was also determined.

Results

Lung histology confirmed that DEX treatment inhibited and RA treatment stimulated alveolar formation, whereas concurrent administration of RA with DEX prevented the DEX effects. During normal development, MK expression was maximal during the period of alveolarization from postnatal day 5 (PN5) to PN15. DEX treatment of rat pups decreased, and RA treatment increased lung MK expression, whereas concurrent DEX+RA treatment prevented the DEX-induced decrease in MK expression. Using human alveolar type 2 (AT2)-like cells differentiated in culture, we confirmed that DEX and cAMP decreased, and RA increased MK expression.

Conclusion

We conclude that MK is expressed by AT2 cells, and is differentially regulated by corticosteroid and retinoid treatment in a manner consistent with hormonal effects on alveolarization during postnatal lung development.

Functional coupling of apical Cl-/HCO3- exchange with CFTR in stimulated HCO3- secretion by guinea pig interlobular pancreatic duct

Pancreatic ductal epithelium produces a HCO(3)(-)-rich fluid. HCO(3)(-) transport across ductal apical membranes has been proposed to be mediated by both SLC26-mediated Cl(-)/HCO(3)(-) exchange and CFTR-mediated HCO(3)(-) conductance, with proportional contributions determined in part by axial changes in gene expression and luminal anion composition. In this study we investigated the characteristics of apical Cl(-)/HCO(3)(-) exchange and its functional interaction with Cftr activity in isolated interlobular ducts of guinea pig pancreas. BCECF-loaded epithelial cells of luminally microperfused ducts were alkalinized by acetate prepulse or by luminal Cl(-) removal in the presence of HCO(3)(-)-CO(2). Intracellular pH recovery upon luminal Cl(-) restoration (nominal Cl(-)/HCO(3)(-) exchange) in cAMP-stimulated ducts was largely inhibited by luminal dihydro-DIDS (H(2)DIDS), accelerated by luminal CFTR inhibitor inh-172 (CFTRinh-172), and was insensitive to elevated bath K(+) concentration. Luminal introduction of CFTRinh-172 into sealed duct lumens containing BCECF-dextran in HCO(3)(-)-free, Cl(-)-rich solution enhanced cAMP-stimulated HCO(3)(-) secretion, as calculated from changes in luminal pH and volume. Luminal Cl(-) removal produced, after a transient small depolarization, sustained cell hyperpolarization of approximately 15 mV consistent with electrogenic Cl(-)/HCO(3)(-) exchange. The hyperpolarization was inhibited by H(2)DIDS and potentiated by CFTRinh-172. Interlobular ducts expressed mRNAs encoding CFTR, Slc26a6, and Slc26a3, as detected by RT-PCR. Thus Cl(-)-dependent apical HCO(3)(-) secretion in pancreatic duct is mediated predominantly by an Slc26a6-like Cl(-)/HCO(3)(-) exchanger and is accelerated by inhibition of CFTR. This study demonstrates functional coupling between Cftr and Slc26a6-like Cl(-)/HCO(3)(-) exchange activity in apical membrane of guinea pig pancreatic interlobular duct.

CLC-0 and CFTR: Chloride Channels Evolved From Transporters

CLC-0 and cystic fibrosis transmembrane conductance regulator (CFTR) Cl−channels play important roles in Cl−transport across cell membranes. These two proteins belong to, respectively, the CLC and ABC transport protein families whose members encompass both ion channels and transporters. Defective function of members in these two protein families causes various hereditary human diseases. Ion channels and transporters were traditionally viewed as distinct entities in membrane transport physiology, but recent discoveries have blurred the line between these two classes of membrane transport proteins. CLC-0 and CFTR can be considered operationally as ligand-gated channels, though binding of the activating ligands appears to be coupled to an irreversible gating cycle driven by an input of free energy. High-resolution crystallographic structures of bacterial CLC proteins and ABC transporters have led us to a better understanding of the gating properties for CLC and CFTR Cl−channels. Furthermore, the joined force between structural and functional studies of these two protein families has offered a unique opportunity to peek into the evolutionary link between ion channels and transporters. A promising byproduct of this exercise is a deeper mechanistic insight into how different transport proteins work at a fundamental level.

Maternal and intra-amniotic corticosteroid effects on lung morphometry in preterm lambs

We investigated the effects of intra-amniotic (IA) betamethasone or budesonide injections on lung structure 2 or 7 d after treatment in preterm fetal sheep. Pregnant ewes received intra-amniotic betamethasone (2 mg/kg fetal weight), budesonide (2 mg/kg), saline or maternal intramuscular betamethasone (0.5 mg/kg maternal weight), or saline. Lambs were delivered 2 or 7 d later at 124 d of gestation. Morphometric analysis was conducted on the right upper lung lobe. Intra-amniotic corticosteroids, 2 or 7 d before delivery, resulted in higher average alveolar volumes than controls (2 d: 25%-35%, 7d: 15%-25%). All corticosteroid treatments resulted in thinning of alveolar walls 7 d after treatment and a higher proportion of alveolar ducts and a lower alveolar wall fraction relative to controls 2 or 7 d after treatment. The changes in structural lung indices correlated with the improved lung function at 2 d. Structural lung indices and increased surfactant correlated with the improved lung function at 7 d. Similar structural changes induced by intra-amniotic corticosteroids and maternal intramuscular betamethasone were associated with improvements in lung function at 2 and 7 d.

Sendai virus-mediated CFTR gene transfer to the airway epithelium

The potential for gene therapy to be an effective treatment for cystic fibrosis has been hampered by the limited gene transfer efficiency of current vectors. We have shown that recombinant Sendai virus (SeV) is highly efficient in mediating gene transfer to differentiated airway epithelial cells, because of its capacity to overcome the intra- and extracellular barriers known to limit gene delivery. Here, we have identified a novel method to allow the cystic fibrosis transmembrane conductance regulator (CFTR) cDNA sequence to be inserted within SeV (SeV-CFTR). Following in vitro transduction with SeV-CFTR, a chloride-selective current was observed using whole-cell and single-channel patch-clamp techniques. SeV-CFTR administration to the nasal epithelium of cystic fibrosis (CF) mice (Cftr(G551D) and Cftr(tm1Unc)TgN(FABPCFTR)#Jaw mice) led to partial correction of the CF chloride transport defect. In addition, when compared to a SeV control vector, a higher degree of inflammation and epithelial damage was found in the nasal epithelium of mice treated with SeV-CFTR. Second-generation transmission-incompetent F-deleted SeV-CFTR led to similar correction of the CF chloride transport defect in vivo as first-generation transmission-competent vectors. Further modifications to the vector or the host may make it easier to translate these studies into clinical trials of cystic fibrosis.

Acute inflammation alters bicarbonate transport in mouse ileum

T-cell mediated acute inflammation of the ileum may occur during Crohn's disease exacerbations. During ileal inflammation, absorption of nutrients and electrolytes by villus cells is decreased with a concomitant increase in crypt and/or villus fluid secretion. These alterations lead to fluid accumulation and the subsequent diarrhoea. Net intestinal fluid secretion consists of HCO3--rich plasma-like fluid. However, the regulation and mechanisms of HCO3- secretion in normal and acutely inflamed ileum are not clearly understood. To study this phenomenon, anti-CD3 monoclonal antibody (mAb)- induced in vivo ileal inflammatory mouse models was used for in vitro functional studies with Ussing chamber and pH stat techniques. Three hours after anti-CD3 mAb injection, ileal mucosa stripped of muscular and serosal layers showed a significant increase in short circuit current (Isc) (0.58+/-0.07 microEq h(-1) cm2 versus 1.63+/-0.14 microEq h(-1) cm2). The cAMP-stimulated Isc component was sensitive to glibenclamide but not to DIDS, suggesting that a cystic fibrosis transmembrane conductance regulator (Cftr)-mediated anion conductance was responsible. Basal Cl--dependent HCO3- secretion, measured using a pH stat technique, was decreased significantly in anti-CD3-injected mice, with a simultaneous increase in Cl--independent HCO3- secretion that was also inhibited by glibenclamide. Experiments using Cftr-/- mice showed neither an increase in Isc nor an increase in HCO3- secretion, confirming the role for Cftr protein in stimulating anion secretion following anti-CD3 treatment. Western blot analysis indicated that Cftr protein levels were unaltered by anti-CD3 treatment, at least acutely. Finally, an immunoassay for cAMP showed significant increases in intracellular cAMP in villus cells, but not in crypt cells. These studies therefore suggest a shift from a predominantly electroneutral Cl-HCO3- exchange in normal mice, to a predominantly electrogenic anion secretion including HCO3- that occurs via functional Cftr during anti-CD3-mediated acute inflammation.

An outwardly rectifying chloride channel in human atrial cardiomyocytes

INTRODUCTION

Among a range of chloride channels, outwardly rectifying Cl- channels have been reported in the heart of various species. Although the anionic current carried by this channel has been subjected to intense electrophysiological investigations, paradoxically no examination of single-channel currents has been reported for human cardiomyocytes.

METHODS AND RESULTS

Using the cell-attached and cell-free configurations of the patch-clamp technique, we have characterized the properties of an outwardly rectifying chloride current (ORCC) at the unitary level in freshly isolated human atrial cardiomyocytes. In excised inside-out patches, the channel presented a nonlinear I/V relationship with a conductance of 76.5 +/- 14.7 pS in the positive voltage range and 8.1 +/- 2 pS in the negative voltage range, indicating an outward rectification. Preincubation with the protein kinase C activator phorbol 12-myristate 13-acetate (PMA) significantly increased the number of spontaneously active channels observed. The channel was Cl- selective (Cl- to Na+ permeability ratio, PCl/PNa= 18) with the permeability sequence I- > Br- > Cl- > F- > gluconate. It was blocked by the classical Cl- channels blockers glibenclamide, NPPB, SITS, and DIDS. Channel activity was not dependent upon internal calcium concentration. In the cell-attached configuration, ORCC channel activation was observed under perfusion of a hypotonic solution.

CONCLUSION

Human atrial myocytes express an outwardly rectifying Cl- channel that is sensitive to PKC activation. This channel shares biophysical and pharmacological properties with the swelling-activated chloride current implicated in cardiac pathologies such as myocardial ischemia and dilated cardiopathies.

Histochemical analyses of altered fetal lung development following single vs multiple courses of antenatal steroids

A single course of antenatal steroids is widely used during preterm labor to promote fetal lung maturation. However, little is known regarding efficacy and safety of multiple courses of antenatal steroids. In animal models and clinical trials, treatment with glucocorticoids can inhibit growth. The present study of single- vs multiple-course steroids in pregnant ewes analyzes the effects of steroids vs placebo on fetal lung histopathology. Single-course groups received dexamethasone (Dex) 6 mg or normal saline every 12 hr for 48 hr at 104-106 days of gestation (term = 150 days). Multiple-course groups received the first course at 76-78 days; this was repeated weekly for 5 weeks. At 108 days, lungs were analyzed using immunohistochemistry for alpha-smooth muscle actin, a myofibroblast marker and proliferating cell nuclear antigen. Cell injury/death was evaluated using TdT-mediated dUTP digoxigenin nick end labeling (TUNEL) analysis. Although fetal growth was restricted by either single or multiple courses of Dex, alveolar development was accelerated as measured by mean linear intercepts. Alveolar walls were thinner, developing septa were longer, and septal myofibroblasts were increased for both Dex groups compared with controls. Cell proliferation increased following multiple steroid courses, especially in the distal parenchyma, with a corresponding decrease in apoptosis. These observations suggest that Dex promotes alveolarization, whether given in single or multiple courses.

Pulmonary function and structure following mild preterm birth in lambs

Our objective was to determine whether postnatal respiratory function, lung growth, and lung structure are affected by preterm birth which did not require neonatal respiratory support. Two groups of preterm (P) lambs were delivered 2 weeks before term, at 133 days of gestational age (GA). Tissue was collected at term equivalent age (TEA, 147 days GA) in one P group and at 6 weeks post-TEA in the other. Tissue was also collected from control (C) lambs soon after term birth (TEA) and at 6 weeks post-TEA. Lung function was assessed at TEA and 6 weeks post-TEA. Respiratory system compliance (Crs/kg BWT) was not different between P and C groups at TEA, but was higher (P = 0.02) in P lambs at 6 weeks post-TEA. Pulmonary resistance was 62% higher in P lambs than controls (P = 0.07) at TEA, and remained higher at 6 weeks post-TEA. Lung weights (wet and dry) were greater (P < 0.05) in preterm animals at both ages; when adjusted for body weight, only dry lung weight remained higher at 6 weeks post-TEA. Alveoli were more numerous (P = 0.05) and smaller (P = 0.05) in preterm lambs compared to controls at both ages. Alveolar septa were 33% thicker and the blood-air barrier was 26% thicker in P lambs than in controls at TEA, and remained thicker at 6 weeks post-TEA. In P lambs, the airway epithelium was thicker at TEA and 6 weeks post-TEA. At TEA, pulmonary tropoelastin expression was 27% lower in P lambs. At 6 weeks post-TEA, dry lung weight and lung protein content were approximately 50% greater in preterm lambs than in controls (P < 0.05), whereas lung DNA, elastin, and collagen contents were similar in the two groups. We conclude that mild preterm birth per se leads to both transient and persistent changes in lung development. Persistent increases in lung protein content and in the thickness of the airway epithelium, and a greater number of smaller alveolar, may alter later lung function.

Surfactant Palmitoylmyristoylphosphatidylcholine Is a Marker for Alveolar Size during Disease

Two common lung-related complications in the neonate are respiratory distress syndrome, which is associated with a failure to generate low surface tension at the air-liquid interface because of pulmonary surfactant insufficiency, and bronchopulmonary dysplasia (BPD), a chronic lung injury with reduced alveolarization. Surfactant phosphatidylcholine (PC) molecular species composition during alveolarization has not been examined. Mass spectrometry analysis of bronchoalveolar lavage fluid of rodents and humans revealed significant changes in surfactant PC during alveolar development and BPD. In rats, total PC content rose during alveolarization, which was caused by an increase in palmitoylmyristoyl-PC (16:0/14:0PC) concentration. Furthermore, two animal models of BPD exhibited a specific reduction in 16:0/14:0PC content. In humans, 16:0/14:0PC content was specifically decreased in patients with BPD and emphysema compared with patients without alveolar pathology. Palmitoylmyristoyl-PC content increased with increasing intrinsic surfactant curvature, suggesting that it affects surfactant function in the septating lung. The changes in acyl composition of PC were attributed to type II cells producing an altered surfactant during alveolar development. These data are compatible with extracellular surfactant 16:0/14:0PC content being an indicator of alveolar architecture of the lung.

Mechanisms of bicarbonate secretion in the pancreatic duct

In many species the pancreatic duct epithelium secretes HCO3- ions at a concentration of around 140 mM by a mechanism that is only partially understood. We know that HCO3- uptake at the basolateral membrane is achieved by Na+-HCO3- cotransport and also by a H+-ATPase and Na+/H+ exchanger operating together with carbonic anhydrase. At the apical membrane, the secretion of moderate concentrations of HCO3- can be explained by the parallel activity of a Cl-/HCO3- exchanger and a Cl- conductance, either the cystic fibrosis transmembrane conductance regulator (CFTR) or a Ca2+-activated Cl- channel (CaCC). However, the sustained secretion of HCO3- into a HCO- -rich luminal fluid cannot be explained by conventional Cl-/HCO3- exchange. HCO3- efflux across the apical membrane is an electrogenic process that is facilitated by the depletion of intracellular Cl-, but it remains to be seen whether it is mediated predominantly by CFTR or by an electrogenic SLC26 anion exchanger.

The patch-clamp and planar lipid bilayer techniques: powerful and versatile tools to investigate the CFTR Cl− channel

Using the patch-clamp (PC) and planar lipid bilayer (PLB) techniques the molecular behaviour of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel can be visualised in real-time. The PC technique is a highly powerful and versatile method to investigate CFTR's mechanism of action, interaction with other proteins and physiological role. Using the PLB technique, the structure and function of CFTR can be investigated free from the influence of other proteins. Here we discuss how these techniques are employed to investigate the CFTR Cl- channel with special emphasis on its permeation, conduction and gating properties.

Fetal growth restriction has long-term effects on postnatal lung structure in sheep

We have previously shown that fetal growth restriction (FGR) during late gestation in sheep affects lung development in the near-term fetus and at 8 wk after birth. In the present study, our aim was to determine the effects of FGR on the structure of the lungs at 2 y after birth; our hypothesis was that changes observed at 8 wk after birth would persist until maturity. FGR was induced in sheep by umbilicoplacental embolization, which was maintained from 120 d until delivery at term (approximately 147 d); birth weights of FGR lambs were 41% lower than in controls. At 2 y after birth, body and lung weights were not different, but there were 28% fewer alveoli and alveoli were significantly larger than in controls; hence there was a 10% reduction in the internal surface area relative to lung volume in FGR sheep compared with controls. The lungs of FGR sheep, compared with controls, had thicker interalveolar septa as a result of increased extracellular matrix deposition; the alveolar blood-air barrier was also thicker, largely because of an 82% increase in basement membrane thickness. These changes are qualitatively similar to those observed at 8 wk. Our data show that structural alterations in the lungs induced by FGR that were apparent at 8 wk were still evident at 2 y after birth, indicating that FGR may result in permanent changes in the structure of the lungs of the offspring and may affect respiratory health and lung aging later in life.

β-Adrenergic receptors couple to CFTR chloride channels of intercalated mitochondria-rich cells in the heterocellular toad skin epithelium

In the heterocellular toad skin epithelium the beta-adrenergic receptor agonist isoproterenol activates cyclic AMP-dependent Cl(-) channels that are not located in the principal cells. With four experimental approaches, in the present study, we tested the hypothesis that the signalling pathway targets apical CFTR-chloride channels of mitochondria-rich cells. (i) Serosal application of isoproterenol (log(10)EC50=-7.1+/0.2; Hill coefficient=1.1+/0.2), as well as nor-adrenaline, activated an anion pathway with an apical selectivity sequence, G(Cl)>G(Br)> or =G(NO(3))>G(I), comparable to the published selectivity sequence of cloned human CFTR expressed in Xenopus oocytes. (ii) Known modulators of human CFTR, glibenclamide (200 micromol/l) and genistein (50 micromol/l), depressed and activated, respectively, the receptor-stimulated G(Cl). Genistein did not modify the anion selectivity. (iii) Transcellular voltage clamp studies of single isolated mitochondria-rich cells revealed functional beta-adrenergic receptors on the basolateral membrane. With approximately 60,000 mitochondria-rich cells per cm(2), the saturating activation of 11.9+/-1.6 ns/cell accounted for the measured isoproterenol-activated transepithelial conductance of 600-900 micros/cm(2). In forskolin-stimulated cells, glibenclamide (200 micromol/l) reversibly inhibited the transcellular conductance by 9.6+/1.6 ns/cell. (iv) A nucleotide sequence of one third of the Bufo bufo CFTR gene corresponding to the R-domain and part of the first nucleotide binding domain (NBD1) including its Walker motif was amplified from gallbladder epithelium. Somewhat smaller sequences of the BbCFTR were cloned from lung and isolated skin epithelium. The above new results taken together with our previously identified small-conductance CFTR-like Cl(-) channel in the apical membrane of isolated mitochondria-rich cells provide compelling evidence that the toad's CFTR gene codes for a functional Cl(-) channel in the apical plasma membrane of this minority cell type.

Where have all the Na+ channels gone? In search of functional ENaC in exocrine pancreas

Many epithelia express specific Na(+) channels (ENaC) together with the cystic fibrosis regulator (CFTR) Cl(-) channels. Pancreatic ducts secrete HCO(3)(-)-rich fluid and express CFTR. However, the question whether they possess ENaC has not been consistently addressed. The aim of the present study was to investigate if pancreatic ducts express functional ENaC. Membrane voltages (V) of ducts isolated from rat pancreas were measured with microelectrodes or whole-cell patch-clamp technique. Amiloride and benzamil given from bath or luminal sides did not hyperpolarize V. Lowering of extracellular Na(+) concentrations had effects that were not consistent with a simple Na(+) conductance, but rather with a Na(+)/Ca(2+) exchange. Acute or long-lasting treatment of pancreatic ducts with mineralocorticoids had no effect on V of unstimulated or secretin-stimulated preparations. Furthermore, pre-treatment of animals with glucocorticoids had no effect on pancreatic fluid secretion evoked from ducts, or from acini. Hence, our study shows that pancreas especially pancreatic ducts do not express functional ENaC.

Membrane potential and bicarbonate secretion in isolated interlobular ducts from guinea-pig pancreas

The interlobular duct cells of the guinea-pig pancreas secrete HCO(3)(-) across their luminal membrane into a HCO(3)(-)-rich (125 mM) luminal fluid against a sixfold concentration gradient. Since HCO(3)(-) transport cannot be achieved by luminal Cl-/HCO(3)(-) exchange under these conditions, we have investigated the possibility that it is mediated by an anion conductance. To determine whether the electrochemical potential gradient across the luminal membrane would favor HCO(3)(-) efflux, we have measured the intracellular potential (V(m)) in microperfused, interlobular duct segments under various physiological conditions. When the lumen was perfused with a 124 mM Cl- -25 mM HCO(3)(-) solution, a condition similar to the basal state, the resting potential was approximately -60 mV. Stimulation with dbcAMP or secretin caused a transient hyperpolarization (approximately 5 mV) due to activation of electrogenic Na+-HCO(3)(-) cotransport at the basolateral membrane. This was followed by depolarization to a steady-state value of approximately -50 mV as a result of anion efflux across the luminal membrane. Raising the luminal HCO(3)(-) concentration to 125 mM caused a hyperpolarization (approximately 10 mV) in both stimulated and unstimulated ducts. These results can be explained by a model in which the depolarizing effect of Cl- efflux across the luminal membrane is minimized by the depletion of intracellular Cl- and offset by the hyperpolarizing effects of Na+-HCO(3)(-) cotransport at the basolateral membrane. The net effect is a luminally directed electrochemical potential gradient for HCO(3)(-) that is sustained during maximal stimulation. Our calculations indicate that the electrodiffusive efflux of HCO(3)(-) to the lumen via CFTR, driven by this gradient, would be sufficient to fully account for the observed secretory flux of HCO(3)(-).

Ion channels in secretory granules of the pancreas and their role in exocytosis and release of secretory proteins

Regulated secretion in exocrine and neuroendocrine cells occurs through exocytosis of secretory granules and the subsequent release of stored small molecules and proteins. The introduction of biophysical techniques with high temporal and spatial resolution, and the identification of Ca(2+)-dependent and -independent "docking" and "fusion" proteins, has greatly enhanced our understanding of exocytosis. The cloning of families of ion channel proteins, including intracellular ion channels, has also revived interest in the role of secretory granule ion channels in exocytotic secretion. Thus secretory granules of pancreatic acinar cell express a ClC-2 Cl(-) channel, a HCO-permeable member of the CLCA Ca(2+)-dependent anion channel family, and a KCNQ1 K(+) channel. Evidence suggests that these channels may facilitate the release of digestive enzymes and/or prevent exocytosed granules from collapsing during "kiss and run" recycling. In pancreatic beta-cells, a granular ClC-3 Cl(-) channel provides a shunt pathway for a vacuolar-type H(+)-ATPase. Acidification "primes" the granules for Ca(2+)-dependent exocytosis and release of insulin. In summary, secretory granules are equipped with specific sets of ion channels, which modulate regulated exocytosis and the release of macromolecules. These channels could represent excellent targets for therapeutic interventions to control exocytotic secretion in relevant diseases, such as pancreatitis, cystic fibrosis, or diabetes mellitus.

An alternate pathway of cAMP-stimulated Cl secretion across the NKCC1-null murine duodenum

BACKGROUND & AIMS

Adenosine 3',5'-cyclic monophosphate (cAMP)-stimulated anion secretion across the duodenal epithelium requires the cystic fibrosis transmembrane conductance regulator (CFTR) in the apical membrane and anion uptake proteins in the basolateral membrane. NKCC1, the epithelial Na(+)/K(+)/2Cl(-) cotransporter, is the major protein responsible for Cl(-) uptake. In this study, we evaluate the role of NKCC1 in determining the relative rates of transepithelial Cl(-) and HCO(3)(-) secretion during cAMP stimulation of the duodenum.

METHODS

Bicarbonate and chloride secretion across duodenal mucosa was measured in Ussing chambers by pH stat and (36)Cl flux methods using mice with either gene-targeted deletion of NKCC1 (NKCC1-/-) or bumetanide blockade of NKCC1.

RESULTS

Total anion secretion stimulated by forskolin treatment of NKCC1-null duodenum resulted from approximately equivalent rates of electrogenic chloride, electrogenic bicarbonate, and electroneutral bicarbonate secretion. Evaluation of the alternate chloride secretory pathway indicated chloride uptake by a basolateral membrane anion exchange process with characteristics consistent with the anion exchanger isoform AE2.

CONCLUSIONS

Chloride uptake by basolateral anion exchanger activity (AE2) supports intracellular cAMP-stimulated chloride secretion in the NKCC1-null duodenum. A model for the alternate chloride secretion pathway is proposed whereby chloride uptake via AE2 is coupled to basolateral NaHCO(3) cotransport to support CFTR-mediated chloride and bicarbonate secretion.

Beta-adrenoceptor-mediated control of apical membrane conductive properties in fetal distal lung epithelia

Distal lung epithelial cells isolated from fetal rats were cultured (48 h) on permeable supports so that transepithelial ion transport could be quantified electrometrically. Unstimulated cells generated a short-circuit current (I(sc)) that was inhibited (~80%) by apical amiloride. The current is thus due, predominantly, to the absorption of Na(+) from the apical solution. Isoprenaline increased the amiloride-sensitive I(sc) about twofold. Experiments in which apical membrane Na(+) currents were monitored in basolaterally permeabilized cells showed that this was accompanied by a rise in apical Na(+) conductance (G(Na(+))). Isoprenaline also increased apical Cl- conductance (G(Cl-)) by activating an anion channel species sensitive to glibenclamide but unaffected by 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS). The isoprenaline-evoked changes in G(Na(+)) and G(Cl(minus sign)) could account for the changes in I(sc) observed in intact cells. Glibenclamide had no effect upon the isoprenaline-evoked stimulation of I(sc) or G(Na(+)) demonstrating that the rise in G(Cl-) is not essential to the stimulation of Na(+) transport.

Multiple courses of antenatal corticosteroids are associated with early severe lung disease in preterm neonates

OBJECTIVE

Determine whether the increased neonatal mortality following repeated courses of antenatal corticosteroids (ANCS), observed in the Thyrotropin-Releasing Hormone (TRH) Trial, was related to confounding maternal risk factors or specific preterm morbidities.

STUDY DESIGN

A post hoc analysis of 595 TRH trial neonates, 26 to 32 weeks' gestation, studied the association between > or =3 courses ANCS and mortality. Potential confounding maternal factors and preterm morbidities were evaluated using logistic regression and log likelihood modeling.

RESULTS

Mortality was 9.2% after > or =3 courses (13/141) vs. 4.8% after 1 or 2 courses (22/454). This association was not explained by maternal factors, or other common preterm morbidities. However, 15/141 infants receiving > or =3 courses (10.6%) had early severe lung disease (ESLD) with 10 deaths, compared to 16/454 of the 1- to 2-course infants (3.5%) with 7 deaths (odds ratio 3.5, p<0.001).

CONCLUSIONS

ESLD, but not maternal risk factors, was associated with increased mortality in preterm infants after > or =3 courses ANCS.

CFTR modulates programmed cell death by decreasing intracellular pH in Chinese hamster lung fibroblasts

To study the potential influence of cystic fibrosis conductance regulator (CFTR) on intracellular pH regulation during apoptosis induction, we used PS120 Chinese hamster lung fibroblasts devoid of the Na(+)/H(+) exchanger (NHE1 isoform) transfected with constructs, allowing the expression of CFTR and/or NHE1. Kinetics of lovastatin-induced apoptosis were measured by orcein staining, double staining with Hoechst-33258, propidium iodide, DNA fragmentation, and annexin V labeling. In PS120 control cells, the percentage of apoptotic cells after 40 h of lovastatin treatment was 23 +/- 3%, whereas in PS120 CFTR-transfected cells, this percentage was 40 +/- 4%. In PS120 NHE1 cells, the transfection with CFTR did not modify the percentage of apoptotic cells after 40 h (control: 19 +/- 3%, n = 8; CFTR: 17 +/- 1%, n = 8), indicating that blocking intracellular acidification by overexpressing the Na(+)/H(+) exchanger inhibited the enhancement of apoptosis induced by CFTR. In all cell lines, the initial pH values were identical (pH = 7.46 +/- 0.04, n = 9), and treatment with lovastatin led to intracellular acidification. However, the pH value after 40 h was lower in PS120 CFTR-transfected cells (pH = 6.85 +/- 0.02, n = 10) than in PS120 cells (pH = 7.15 +/- 0.03, n = 10). To further investigate the origin of this increased intracellular acidification observed in CFTR-transfected cells, the activity of the DIDS-inhibitable Cl(-)/HCO exchanger was studied. 8-Bromoadenosine 3',5'-cyclic monophosphate incubation resulted in Cl(-)/HCO exchanger activation in PS120 CFTR-transfected cells but had no effect on PS120 cells. Together, our results suggest that CFTR can enhance apoptosis in Chinese hamster lung fibroblasts, probably due to the modulation of the Cl(-)/HCO exchanger, resulting in a more efficient intracellular acidification.

Lung Morphometry after Repetitive Antenatal Glucocorticoid Treatment in Preterm Sheep

Antenatal glucocorticoids are thought to be less effective when delivery occurs more than 7 d after initiation of treatment; therefore, repeat courses are often administered. We examined lung structure after single or repetitive antenatal glucocorticoid injections in fetal sheep. Pregnant ewes received single or repetitive doses of 0.5 mg/kg betamethasone at 7-d intervals by maternal or fetal injection, beginning at D104 or D114 with delivery at D125, D135, or D146 gestation (term = 150 d). Changes in lung structure were more pronounced after repetitive versus single injections. Repetitive fetal or maternal injections beginning at D104 (delivery at D125) resulted in comparable structural changes: alveolar volume increased by 50 to 80%, alveolar numerical density decreased by 30 to 40%, and pleural and interlobular septal volumes decreased by as much as 70%. Similar changes were seen in animals delivered at D135 after repetitive maternal injections beginning at D114. There were no structural differences between control and repetitive betamethasone animals when delivery was delayed until D146, indicating that betamethasone induced structural changes were reversible.

Voltage-dependent flickery block of an open cystic fibrosis transmembrane conductance regulator (CFTR) channel pore

1. Fast flickery block of the cystic fibrosis transmembrane conductance regulator (CFTR) was studied with cell-attached and whole-cell patch-clamp recordings from mouse NIH3T3 cells stably expressing a mutant CFTR channel, K1250A-CFTR. This mutant CFTR channel, once open, can stay open for minutes. Within a prolonged opening, the kinetics of fast flickery closures can be readily quantified. 2. Flickering block of K1250A-CFTR channels was voltage dependent since the open probability within an opening burst decreased as the membrane was hyperpolarized. 3. Mean open time (tau(o)) and mean closed time (tau(c)), obtained from single-channel kinetic analysis, were corrected for missed events. Our data show that corrected tau(c) was voltage dependent while corrected tau(o) exhibited little voltage dependence. Results from whole-cell current relaxation upon voltage jump further indicate that tau(c) at a membrane potential of -100 mV was at least 10-fold longer than that at +100 mV. 4. tau(c), but not tau(o), was sensitive to external permeant anions. After complete replacement of external Cl(-) with impermeant anions, tau(c) showed little voltage dependence and approximated a value observed under strong hyperpolarization in the presence of high external permeant anions. These results suggest that the resident time of the blocker is prolonged by conditions (i.e. hyperpolarization or the absence of external permeant anions) that deplete Cl(-) in the CFTR pore. 5. Results from macroscopic current noise analysis of both wild-type CFTR and K1250A-CFTR channels further confirm the voltage dependence and Cl(-) sensitivity of the fast flickery block observed with single-channel analysis. 6. We conclude that the voltage dependence of the flickery block in CFTR is mainly due to the voltage-dependent occupancy of an anion-binding site in the channel pore by trans-anions. The blocker acquires a voltage-dependent off rate through an electrostatic interaction with Cl(-) in the pore.

The stoichiometry of the electrogenic sodium bicarbonate cotransporter pNBC1 in mouse pancreatic duct cells is 2 HCO(3)(-):1 Na(+)

The electrogenic sodium bicarbonate cotransporter pNBC1 is believed to play a major role in the secretion of bicarbonate by pancreatic duct cells, by transporting bicarbonate into the cell across the basolateral membrane. Thermodynamics predict that this function can be achieved only if the reversal potential of the cotransporter is negative to the cell's membrane potential, or equivalently that the HCO3-:Na+ stoichiometry is not larger then 2:However, there are no data available on either the reversal potential or the HCO3-:Na+ stoichiometry of pNBC1 in pancreatic cells. We studied pNBC1 function in mouse pancreatic duct cells. RT-PCR analysis of total RNA revealed that these cells contain the message for pNBC1, but not for kNBC1, NBC2 or NBC3. To measure cotransporter activity, mouse pancreatic duct cells were grown to confluence on a porous substrate, mounted in an Ussing chamber, and the apical plasma membrane permeabilized with amphotericin B. Ion flux through pNBC1 was achieved by applying Na+ concentration gradients across the basolateral plasma membrane. The current through the cotransporter was isolated as the difference current due to the reversible inhibitor dinitrostilbene disulfonate (DNDS). Current-voltage relationships for the cotransporter, measured at three different Na+ concentration gradients, were linear over a range of about 100 mV. The reversal potential data, obtained from these current-voltage relationships, all corresponded to a 2 HCO3-:1 Na+ stoichiometry. The data indicate that pNBC1 is functionally expressed in mouse pancreatic duct cells. The cotransporter operates with a 2 HCO3-:1 Na+ stoichiometry in these cells, and mediates the transport of bicarbonate into the cell across the basolateral membrane.

Antenatal endotoxin and glucocorticoid effects on lung morphometry in preterm lambs

In utero inflammation may accelerate fetal lung maturation but may also play a role in the pathogenesis of chronic lung disease. We examined the impact of endotoxin, a potent proinflammatory stimulus, on structural and functional maturation of preterm sheep lungs. Date bred ewes received 20 mg Escherichia coli endotoxin or saline by ultrasound guided intra-amniotic injection at 119 d gestation. A comparison group of animals received 0.5 mg/kg betamethasone, a known maturational agent, at 118 d gestation. Lambs were delivered by cesarean section at 125 d (term = 150 d) and ventilated for 40 min. Lung function data are reported elsewhere. Total and differential white cell counts were performed on amniotic fluid and fetal lung fluid samples. Morphometric analyses were performed on inflation fixed right upper lobes. Total cell count increased slightly but not significantly in both amniotic fluid and fetal lung fluid. Both endotoxin and betamethasone had similar effects on alveolarization: average alveolar volume increased by approximately 20% and total alveolar number decreased by almost 30%. Both treatments led to thinning of alveolar walls, although this was statistically significant in the betamethasone-treated group only. Although antenatal endotoxin leads to striking improvements in postnatal lung function, this may be at the expense of normal alveolar development.