Cytosolic Ca(2+) and Ca(2+)-activated Cl(-) current dynamics: insights from two functionally distinct mouse exocrine cells

Neuronal and hormonal control of Ca2+ signalling in exocrine glands: insight from in vivo studies

Fluid and enzyme secretion from exocrine glands is initiated by Ca2+ signalling in acinar cells and is activated by external neural or hormonal signals. A wealth of information has been derived from studies in acutely isolated exocrine cells but Ca2+ signalling has until recently not been studied in undisrupted intact tissue in live mice. Our in vivo observations using animals expressing genetically encoded Ca2+ indicators in specific cell types in exocrine glands revealed both similarities to and differences from the spatiotemporal characteristics previously reported in isolated cells. These in vivo studies facilitate further understanding of how both neuronal and hormonal input shapes Ca2+ signalling events in a physiological setting and how these signals are translated into the stimulation of fluid secretion and exocytosis.

Pacing intracellular Ca2+ signals in exocrine acinar cells

An increase in intracellular [Ca2+ ] in exocrine acinar cells resident in the salivary glands or pancreas is a fundamental event that drives fluid secretion and exocytosis of proteins. Stimulation with secretagogues initiates Ca2+ signals with precise spatiotemporal properties thought to be important for driving physiological output. Both in vitro, in acutely isolated acini, and in vivo, in animals expressing genetically encoded indicators, individual cells appear specialized to initiate Ca2+ signals upon stimulation. Furthermore, these signals appear to spread to neighbouring cells. These properties are present in the absence of a conventional pacemaker mechanism dependent on the cyclical activation of Ca2+ -dependent or Ca2+ -conducting plasma membrane ion channels. In this article, we propose a model for 'pacing' intracellular Ca2+ signals in acinar cells based on the enhanced sensitivity of a subpopulation of individual cells and the intercellular diffusion through gap junctions of inositol 1,4,5-trisphosphate and Ca2+ to neighbouring cells.

Ca2+ signals in pancreatic acinar cells in response to physiological stimulation in vivo

The exocrine pancreas secretes fluid and digestive enzymes in response to parasympathetic release of acetylcholine (ACh) via the vagus nerve and the gut hormone cholecystokinin (CCK). Both secretion of fluid and exocytosis of secretory granules containing enzymes and zymogens are dependent on an increase in the cytosolic [Ca²⁺ ] in acinar cells. It is thought that the specific spatiotemporal characteristics of the Ca²⁺ signals are fundamental for appropriate secretion and that these properties are disrupted in disease states in the pancreas. While extensive research has been performed to characterize Ca²⁺ signalling in acinar cells, this has exclusively been achieved in ex vivo preparations of exocrine cells, where it is difficult to mimic physiological conditions. Here we have developed a method to optically observe pancreatic acinar Ca²⁺ signals in vivo using a genetically expressed Ca²⁺ indicator and imaged with multi-photon microscopy in live animals. In vivo, acinar cells exhibited baseline activity in fasted animals, which was dependent on CCK1 receptors (CCK1Rs). Both stimulation of intrinsic nervous input and administration of systemic CCK induced oscillatory activity in a proportion of the cells, but the maximum frequencies were vastly different. Upon feeding, oscillatory activity was also observed, which was dependent on CCK1Rs. No evidence of a vago-vagal reflex mediating the effects of CCK was observed. Our in vivo method revealed the spatial and temporal profile of physiologically evoked Ca²⁺ signals, which will provide new insights into future studies of the mechanisms underlying exocrine physiology and that are disrupted in pathological conditions. KEY POINTS: In the exocrine pancreas, the spatiotemporal properties of Ca²⁺ signals are fundamentally important for the appropriate stimulation of secretion by the neurotransmitter acetylcholine and gut hormone cholecystokinin. These characteristics were previously defined in ex vivo studies. Here we report the spatiotemporal characteristics of Ca²⁺ signals in vivo in response to physiological stimulation in a mouse engineered to express a Ca²⁺ indicator in acinar cells. Specific Ca²⁺ 'signatures' probably important for stimulating secretion are evoked in vivo in fasted animals, by feeding, neural stimulation and cholecystokinin administration. The Ca²⁺ signals are probably the result of the direct action of ACh and CCK on acinar cells and not indirectly through a vago-vagal reflex.

Simulation of Calcium Dynamics in Realistic Three-Dimensional Domains

The cytosolic concentration of free calcium ions ([Ca2+]) is an important intracellular messenger in most cell types, and the spatial distribution of [Ca2+] is often critical. In a salivary gland acinar cell, a polarised epithelial cell, whose principal function is to transport water and thus secrete saliva, [Ca2+] controls the secretion of primary saliva, but increases in [Ca2+] are localised to the apical regions of the cell. Hence, any quantitative explanation of how [Ca2+] controls saliva secretion must take into careful account the spatial distribution of the various Ca2+ sources, Ca2+ sinks, and Ca2+-sensitive ion channels. Based on optical slices, we have previously constructed anatomically accurate three-dimensional models of seven salivary gland acinar cells, and thus shown that a model in which Ca2+ responses are confined to the apical regions of the cell is sufficient to provide a quantitative and predictive explanation of primary saliva secretion. However, reconstruction of such anatomically accurate cells is extremely time consuming and inefficient. Here, we present an alternative, mostly automated method of constructing three-dimensional cells that are approximately anatomically accurate and show that the new construction preserves the quantitative accuracy of the model.

Highly localized intracellular Ca2+ signals promote optimal salivary gland fluid secretion

Salivary fluid secretion involves an intricate choreography of membrane transporters to result in the trans-epithelial movement of NaCl and water into the acinus lumen. Current models are largely based on experimental observations in enzymatically isolated cells where the Ca²⁺ signal invariably propagates globally and thus appears ideally suited to activate spatially separated Cl and K channels, present on the apical and basolateral plasma membrane, respectively. We monitored Ca²⁺ signals and salivary secretion in live mice expressing GCamp6F, following stimulation of the nerves innervating the submandibular gland. Consistent with in vitro studies, Ca²⁺ signals were initiated in the apical endoplasmic reticulum. In marked contrast to in vitro data, highly localized trains of Ca²⁺ transients that failed to fully propagate from the apical region were observed. Following stimuli optimum for secretion, large apical-basal gradients were elicited. A new mathematical model, incorporating these data was constructed to probe how salivary secretion can be optimally stimulated by apical Ca²⁺ signals.

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.

A Multicellular Model of Primary Saliva Secretion in the Parotid Gland

We construct a three-dimensional anatomically accurate multicellular model of a parotid gland acinus to investigate the influence that the topology of its lumen has on primary fluid secretion. Our model consists of seven individual cells, coupled via a common lumen and intercellular signalling. Each cell is equipped with the intracellular calcium ([Formula: see text])-signalling model developed by Pages et al, Bull Math Biol 81: 1394-1426, 2019. https://doi.org/10.1007/s11538-018-00563-z and the secretion model constructed by Vera-Sigüenza et al., Bull Math Biol 81: 699-721, 2019. https://doi.org/10.1007/s11538-018-0534-z. The work presented here is a continuation of these studies. While previous mathematical research has proven invaluable, to the best of our knowledge, a multicellular modelling approach has never been implemented. Studies have hypothesised the need for a multiscale model to understand the primary secretion process, as acinar cells do not operate on an individual basis. Instead, they form racemous clusters that form intricate water and protein delivery networks that join the acini with the gland's ducts-questions regarding the extent to which the acinus topology influences the efficiency of primary fluid secretion to persist. We found that (1) The topology of the acinus has almost no effect on fluid secretion. (2) A multicellular spatial model of secretion is not necessary when modelling fluid flow. Although the inclusion of intercellular signalling introduces vastly more complex dynamics, the total secretory rate remains fundamentally unchanged. (3) To obtain an acinus, or better yet a gland flow rate estimate, one can multiply the output of a well-stirred single-cell model by the total number of cells required.

Role of NAADP for calcium signaling in the salivary gland

Coordination of intracellular Ca²⁺ signaling in parotid acini is crucial for controlling the secretion of primary saliva. Previous work from our lab has demonstrated acidic-organelle Ca²⁺ release as a participant in agonist-evoked signaling dynamics of the parotid acinar cell. Furthermore, results implicated a potential role for the potent Ca²⁺ releasing second messenger NAADP in these events. The current study interrogated a direct role of NAADP for Ca²⁺ signaling in the parotid salivary gland acinar cell. Use of live-cell Ca²⁺ imaging, patch-clamp methods, and confocal microscopy revealed for the first time NAADP can evoke or enhance Ca²⁺ dynamics in parotid acini. These results were compared with pancreatic acini, a morphologically similar cell type previously shown to display NAADP-dependent Ca²⁺ signals. Findings presented here may be relevant in establishing new therapeutic targets for those suffering from xerostomia produced by hypofunctioning salivary glands.

A Mathematical Model of Fluid Transport in an Accurate Reconstruction of Parotid Acinar Cells

Salivary gland acinar cells use the calcium ([Formula: see text]) ion as a signalling messenger to regulate a diverse range of intracellular processes, including the secretion of primary saliva. Although the underlying mechanisms responsible for saliva secretion are reasonably well understood, the precise role played by spatially heterogeneous intracellular [Formula: see text] signalling in these cells remains uncertain. In this study, we use a mathematical model, based on new and unpublished experimental data from parotid acinar cells (measured in excised lobules of mouse parotid gland), to investigate how the structure of the cell and the spatio-temporal properties of [Formula: see text] signalling influence the production of primary saliva. We combine a new [Formula: see text] signalling model [described in detail in a companion paper: Pages et al. in Bull Math Biol 2018, submitted] with an existing secretion model (Vera-Sigüenza et al. in Bull Math Biol 80:255-282, 2018. https://doi.org/10.1007/s11538-017-0370-6 ) and solve the resultant model in an anatomically accurate three-dimensional cell. Our study yields three principal results. Firstly, we show that spatial heterogeneities of [Formula: see text] concentration in either the apical or basal regions of the cell have no significant effect on the rate of primary saliva secretion. Secondly, in agreement with previous work (Palk et al., in J Theor Biol 305:45-53, 2012. https://doi.org/10.1016/j.jtbi.2012.04.009 ) we show that the frequency of [Formula: see text] oscillation has no significant effect on the rate of primary saliva secretion, which is determined almost entirely by the mean (over time) of the apical and basal [Formula: see text]. Thirdly, it is possible to model the rate of primary saliva secretion as a quasi-steady-state function of the cytosolic [Formula: see text] averaged over the entire cell when modelling the flow rate is the only interest, thus ignoring all the dynamic complexity not only of the fluid secretion mechanism but also of the intracellular heterogeneity of [Formula: see text]. Taken together, our results demonstrate that an accurate multiscale model of primary saliva secretion from a single acinar cell can be constructed by ignoring the vast majority of the spatial and temporal complexity of the underlying mechanisms.

Phosphatidylinositol 4,5-bisphosphate, cholesterol, and fatty acids modulate the calcium-activated chloride channel TMEM16A (ANO1)

The TMEM16A-mediated Ca²⁺-activated Cl^- current drives several important physiological functions. Membrane lipids regulate ion channels and transporters but their influence on members of the TMEM16 family is poorly understood. Here we have studied the regulation of TMEM16A by phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2), cholesterol, and fatty acids using patch clamp, biochemistry and fluorescence microscopy. We found that depletion of membrane PI(4,5)P2 causes a decline in TMEM16A current that is independent of cytoskeleton, but is partially prevented by removing intracellular Ca²⁺. On the other hand, supplying PI(4,5)P2 to inside-out patches attenuated channel rundown and/or partially rescued activity after channel rundown. Also, depletion (with methyl-β-cyclodextrin M-βCD) or restoration (with M-βCD+cholesterol) of membrane cholesterol slows down the current decay observed after reduction of PI(4,5)P2. Neither depletion nor restoration of cholesterol change PI(4,5)P2 content. However, M-βCD alone transiently increases TMEM16A activity and dampens rundown whereas M-βCD+cholesterol increases channel rundown. Thus, PI(4,5)P2 is required for TMEM16A function while cholesterol directly and indirectly via a PI(4,5)P2-independent mechanism regulate channel function. Stearic, arachidonic, oleic, docosahexaenoic, and eicosapentaenoic fatty acids as well as methyl stearate inhibit TMEM16A in a dose- and voltage-dependent manner. Phosphatidylserine, a phospholipid whose hydrocarbon tails contain stearic and oleic acids also inhibits TMEM16A. Finally, we show that TMEM16A remains in the plasma membrane after treatment with M-βCD, M-βCD+cholesterol, oleic, or docosahexaenoic acids. Thus, we propose that lipids and fatty acids regulate TMEM16A channels through a membrane-delimited protein-lipid interaction.

Ca2+-CaMKKβ pathway is required for adiponectin-induced secretion in rat submandibular gland

Adiponectin functions as a promoter of saliva secretion in rat submandibular gland via activation of adenosine monophosphate-activated protein kinase (AMPK) and increased paracellular permeability. Ca²⁺ mobilization is the primary signal for fluid secretion in salivary acinar cells. However, whether intracellular Ca²⁺ mobilization is involved in adiponectin-induced salivary secretion is unknown. Here, we found that full-length adiponectin (fAd) increased intracellular Ca²⁺ and saliva secretion in submandibular glands. Pre-perfusion with ethylene glycol-bis (2-aminoethylether)-N,N,N',N'-tetraacetic acid (EGTA) combined with thapsigargin (TG), an endoplasmic reticulum Ca²⁺-ATPase inhibitor, abolished fAd-induced salivary secretion, AMPK phosphorylation, and enlarged tight junction (TJ) width. Furthermore, in cultured SMG-C6 cells, co-pretreatment with EGTA and TG suppressed fAd-decreased transepithelial electrical resistance and increased 4-kDa FITC-dextran flux responses. Moreover, fAd increased phosphorylation of calcium/calmodulin-dependent protein kinase (CaMKKβ), a major kinase that is activated by elevated levels of intracellular Ca²⁺, but not liver kinase B1 phosphorylation. Pre-perfusion of the isolated gland with STO-609, an inhibitor of CaMKKβ, abolished fAd-induced salivary secretion, AMPK activation, and enlarged TJ width. CaMKKβ shRNA suppressed, whereas CaMKKβ re-expression rescued fAd-increased paracellular permeability. Taken together, these results indicate that adiponectin induced Ca²⁺ modulation in rat submandibular gland acinar cells. Ca²⁺-CaMKKβ pathway is required for adiponectin-induced secretion through mediating AMPK activation and increase in paracellular permeability in rat submandibular glands.

Conceptual and Experimental Tools to Understand Spatial Effects and Transport Phenomena in Nonlinear Biochemical Networks Illustrated with Patchy Switching

Many biochemical systems are spatially heterogeneous and exhibit nonlinear behaviors, such as state switching in response to small changes in the local concentration of diffusible molecules. Systems as varied as blood clotting, intracellular calcium signaling, and tissue inflammation are all heavily influenced by the balance of rates of reaction and mass transport phenomena including flow and diffusion. Transport of signaling molecules is also affected by geometry and chemoselective confinement via matrix binding. In this review, we use a phenomenon referred to as patchy switching to illustrate the interplay of nonlinearities, transport phenomena, and spatial effects. Patchy switching describes a change in the state of a network when the local concentration of a diffusible molecule surpasses a critical threshold. Using patchy switching as an example, we describe conceptual tools from nonlinear dynamics and chemical engineering that make testable predictions and provide a unifying description of the myriad possible experimental observations. We describe experimental microfluidic and biochemical tools emerging to test conceptual predictions by controlling transport phenomena and spatial distribution of diffusible signals, and we highlight the unmet need for in vivo tools.

Modeling calcium waves in an anatomically accurate three-dimensional parotid acinar cell

We construct a model of calcium waves in a three-dimensional anatomically accurate parotid acinar cell, constructed from experimental data. Gradients of inositol trisphosphate receptor (IPR) density are imposed, with the IPR density being greater closer to the lumen, which has a branched structure, and inositol trisphosphate (IP₃) is produced only at the basal membrane. We show (1) that IP₃ equilibrates so quickly across the cell that it can be assumed to be spatially homogeneous; (2) spatial separation of the sites of IP₃ action and IP₃ production does not preclude the formation of stable oscillatory Ca²⁺ waves. However, these waves are not waves in the mathematical sense of a traveling wave with fixed profile. They result instead from a time delay between the Ca²⁺ rise in the apical and basal regions; (3) the ryanodine receptors serve to reinforce the Ca²⁺ wave, but are not necessary for the wave to exist; (4) a spatially independent model is not sufficient to study saliva secretion, although a one-dimensional model might be sufficient. Our results here form the first stages of the construction of a multiscale and multicellular model of saliva secretion in an entire acinus.

Using concatenated subunits to investigate the functional consequences of heterotetrameric inositol 1,4,5-trisphosphate receptors

Inositol 1,4,5-trisphosphate receptors (IP3Rs) are a family of ubiquitous, ER localized, tetrameric Ca2+ release channels. There are three subtypes of the IP3Rs (R1, R2, R3), encoded by three distinct genes, that share ∼60-70% sequence identity. The diversity of Ca2+ signals generated by IP3Rs is thought to be largely the result of differential tissue expression, intracellular localization and subtype-specific regulation of the three subtypes by various cellular factors, most significantly InsP3, Ca2+ and ATP. However, largely unexplored is the notion of additional signal diversity arising from the assembly of both homo and heterotetrameric InsP3Rs. In the present article, we review the biochemical and functional evidence supporting the existence of homo and heterotetrameric populations of InsP3Rs. In addition, we consider a strategy that utilizes genetically concatenated InsP3Rs to study the functional characteristics of heterotetramers with unequivocally defined composition. This approach reveals that the overall properties of IP3R are not necessarily simply a blend of the constituent monomers but that specific subtypes appear to dominate the overall characteristics of the tetramer. It is envisioned that the ability to generate tetramers with defined wild type and mutant subunits will be useful in probing fundamental questions relating to IP3R structure and function.

Post-translational regulation of the type III inositol 1,4,5-trisphosphate receptor by miRNA-506

The type III isoform of the inositol 1,4,5-trisphosphate receptor (InsP3R3) is apically localized and triggers Ca(2+) waves and secretion in a number of polarized epithelia. However, nothing is known about epigenetic regulation of this InsP3R isoform. We investigated miRNA regulation of InsP3R3 in primary bile duct epithelia (cholangiocytes) and in the H69 cholangiocyte cell line, because the role of InsP3R3 in cholangiocyte Ca(2+) signaling and secretion is well established and because loss of InsP3R3 from cholangiocytes is responsible for the impairment in bile secretion that occurs in a number of liver diseases. Analysis of the 3'-UTR of human InsP3R3 mRNA revealed two highly conserved binding sites for miR-506. Transfection of miR-506 mimics into cell lines expressing InsP3R3-3'UTR-luciferase led to decreased reporter activity, whereas co-transfection with miR-506 inhibitors led to enhanced activity. Reporter activity was abrogated in isolated mutant proximal or distal miR-506 constructs in miR-506-transfected HEK293 cells. InsP3R3 protein levels were decreased by miR-506 mimics and increased by inhibitors, and InsP3R3 expression was markedly decreased in H69 cells stably transfected with miR-506 relative to control cells. miR-506-H69 cells exhibited a fibrotic signature. In situ hybridization revealed elevated miR-506 expression in vivo in human-diseased cholangiocytes. Histamine-induced, InsP3-mediated Ca(2+) signals were decreased by 50% in stable miR-506 cells compared with controls. Finally, InsP3R3-mediated fluid secretion was significantly decreased in isolated bile duct units transfected with miR-506, relative to control IBDU. Together, these data identify miR-506 as a regulator of InsP3R3 expression and InsP3R3-mediated Ca(2+) signaling and secretion.

Multiscale modelling of saliva secretion

We review a multiscale model of saliva secretion, describing in brief how the model is constructed and what we have so far learned from it. The model begins at the level of inositol trisphosphate receptors (IPR), and proceeds through the cellular level (with a model of acinar cell calcium dynamics) to the multicellular level (with a model of the acinus), finally to a model of a saliva production unit that includes an acinus and associated duct. The model at the level of the entire salivary gland is not yet completed. Particular results from the model so far include (i) the importance of modal behaviour of IPR, (ii) the relative unimportance of Ca(2+) oscillation frequency as a controller of saliva secretion, (iii) the need for the periodic Ca(2+) waves to be as fast as possible in order to maximise water transport, (iv) the presence of functional K(+) channels in the apical membrane increases saliva secretion, (v) the relative unimportance of acinar spatial structure for isotonic water transport, (vi) the prediction that duct cells are highly depolarised, (vii) the prediction that the secondary saliva takes at least 1mm (from the acinus) to reach ionic equilibrium. We end with a brief discussion of future directions for the model, both in construction and in the study of scientific questions.

Modelling the transition from simple to complex Ca²⁺ oscillations in pancreatic acinar cells

A mathematical model is proposed which systematically investigates complex calcium oscillations in pancreatic acinar cells. This model is based on calcium-induced calcium release via inositol trisphosphate receptors (IPR) and ryanodine receptors (RyR) and includes calcium modulation of inositol (1,4,5) trisphosphate (IP3) levels through feedback regulation of degradation and production. In our model, the apical and the basal regions are separated by a region containing mitochondria, which is capable of restricting Ca2+ responses to the apical region. We were able to reproduce the observed oscillatory patterns, from baseline spikes to sinusoidal oscillations. The model predicts that calcium-dependent production and degradation of IP3 is a key mechanism for complex calcium oscillations in pancreatic acinar cells. A partial bifurcation analysis is performed which explores the dynamic behaviour of the model in both apical and basal regions.

Studying the activation of epithelial ion channels using global whole-field photolysis

The production of saliva by parotid acinar cells is stimulated by Ca(2+) activation of Cl(-) and K(+) channels located in the apical plasma membrane of these polarized cells. Here we provide a detailed description of a flash photolysis experiment designed to give a global and relatively uniform photorelease of inositol 1,4,5-trisphosphate (InsP(3)) or Ca(2+) from caged precursors (NPE-InsP(3) or NP-EGTA) combined with the simultaneous measurement of whole-cell Ca(2+)-activated currents. The photolysis light source can be either an ultraviolet (UV) flash lamp or alternatively the output from a 375-nm diode laser, which is defocused to illuminate the entire field.

Apical Ca2+-activated potassium channels in mouse parotid acinar cells

Ca²⁺ activation of Cl and K channels is a key event underlying stimulated fluid secretion from parotid salivary glands. Cl channels are exclusively present on the apical plasma membrane (PM), whereas the localization of K channels has not been established. Mathematical models have suggested that localization of some K channels to the apical PM is optimum for fluid secretion. A combination of whole cell electrophysiology and temporally resolved digital imaging with local manipulation of intracellular [Ca²⁺] was used to investigate if Ca²⁺-activated K channels are present in the apical PM of parotid acinar cells. Initial experiments established Ca²⁺-buffering conditions that produced brief, localized increases in [Ca²⁺] after focal laser photolysis of caged Ca²⁺. Conditions were used to isolate K⁺ and Cl⁻ conductances. Photolysis at the apical PM resulted in a robust increase in K⁺ and Cl⁻ currents. A localized reduction in [Ca²⁺] at the apical PM after photolysis of Diazo-2, a caged Ca²⁺ chelator, resulted in a decrease in both K⁺ and Cl⁻ currents. The K⁺ currents evoked by apical photolysis were partially blocked by both paxilline and TRAM-34, specific blockers of large-conductance “maxi-K” (BK) and intermediate K (IK), respectively, and almost abolished by incubation with both antagonists. Apical TRAM-34–sensitive K⁺ currents were also observed in BK-null parotid acini. In contrast, when the [Ca²⁺] was increased at the basal or lateral PM, no increase in either K⁺ or Cl⁻ currents was evoked. These data provide strong evidence that K and Cl channels are similarly distributed in the apical PM. Furthermore, both IK and BK channels are present in this domain, and the density of these channels appears higher in the apical versus basolateral PM. Collectively, this study provides support for a model in which fluid secretion is optimized after expression of K channels specifically in the apical PM.

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.

Distinct contributions by ionotropic purinoceptor subtypes to ATP-evoked calcium signals in mouse parotid acinar cells

There is emerging consensus that P2X₄ and P2X₇ ionotropic purinoceptors (P2X₄R and P2X₇R) are critical players in regulating [Ca²⁺]i dynamics and fluid secretion in the salivary gland. In contrast, details regarding their compartmentalization and selective activation, contributions to the spatiotemporal properties of intracellular signals and roles in regulating protein exocytosis and ion channel activity have remained largely undefined. To address these concerns, we profiled mouse parotid acinar cells using live-cell imaging to follow the spatial and temporal features of ATP-evoked Ca²⁺ dynamics and exocytotic activity. Selective activation of P2X7Rs revealed an apical-to-basal [Ca²⁺]i signal that initiated at the sub-luminal border and propagated with a wave speed estimated at 17.3 ± 4.3 μm s⁻¹ (n =6). The evoked Ca²⁺ spike consisted of Ca²⁺ influx and Ca²⁺-induced Ca²⁺ release from intracellular Ca²⁺ channels. In contrast, selective activation of P2X₄Rs induced a Ca²⁺ signal that initiated basally and propagated toward the lumen with a wave speed of 4.3 ± 0.2 μm s⁻¹ (n =8) that was largely independent of intracellular Ca²⁺ channel blockade. Consistent with these observations, P2X₇R expression was enriched in the sub-luminal regions of acinar cells while P2X₄R appeared localized to basal areas. In addition, we showed that P2X₄R and P2X₇R activation evokes exocytosis in parotid acinar cells. Our studies also demonstrate that the P2X₄R-mediated [Ca²⁺]i rise and subsequent protein exocytosis was enhanced by ivermectin (IVR). Thus, in addition to furthering our understanding of salivary gland physiology, this study identifies P2X₄R as a potential target for treatment of salivary hypofunction diseases.

Regulation of Ca²⁺ release through inositol 1,4,5-trisphosphate receptors by adenine nucleotides in parotid acinar cells

Secretagogue-stimulated intracellular Ca(2+) signals are fundamentally important for initiating the secretion of the fluid and ion component of saliva from parotid acinar cells. The Ca(2+) signals have characteristic spatial and temporal characteristics, which are defined by the specific properties of Ca(2+) release mediated by inositol 1,4,5-trisphosphate receptors (InsP(3)R). In this study we have investigated the role of adenine nucleotides in modulating Ca(2+) release in mouse parotid acinar cells. In permeabilized cells, the Ca(2+) release rate induced by submaximal [InsP(3)] was increased by 5 mM ATP. Enhanced Ca(2+) release was not observed at saturating [InsP(3)]. The EC(50) for the augmented Ca(2+) release was ∼8 μM ATP. The effect was mimicked by nonhydrolysable ATP analogs. ADP and AMP also potentiated Ca(2+) release but were less potent than ATP. In acini isolated from InsP(3)R-2-null transgenic animals, the rate of Ca(2+) release was decreased under all conditions but now enhanced by ATP at all [InsP(3)]. In addition the EC(50) for ATP potentiation increased to ∼500 μM. These characteristics are consistent with the properties of the InsP(3)R-2 dominating the overall features of InsP(3)R-induced Ca(2+) release despite the expression of all isoforms. Finally, Ca(2+) signals were measured in intact parotid lobules by multiphoton microscopy. Consistent with the release data, carbachol-stimulated Ca(2+) signals were reduced in lobules exposed to experimental hypoxia compared with control lobules only at submaximal concentrations. Adenine nucleotide modulation of InsP(3)R in parotid acinar cells likely contributes to the properties of Ca(2+) signals in physiological and pathological conditions.

Efficiency of primary saliva secretion: an analysis of parameter dependence in dynamic single-cell and acinus models, with application to aquaporin knockout studies

Secretion from the salivary glands is driven by osmosis following the establishment of osmotic gradients between the lumen, the cell and the interstitium by active ion transport. We consider a dynamic model of osmotically driven primary saliva secretion and use singular perturbation approaches and scaling assumptions to reduce the model. Our analysis shows that isosmotic secretion is the most efficient secretion regime and that this holds for single isolated cells and for multiple cells assembled into an acinus. For typical parameter variations, we rule out any significant synergistic effect on total water secretion of an acinar arrangement of cells about a single shared lumen. Conditions for the attainment of isosmotic secretion are considered, and we derive an expression for how the concentration gradient between the interstitium and the lumen scales with water- and chloride-transport parameters. Aquaporin knockout studies are interpreted in the context of our analysis and further investigated using simulations of transport efficiency with different membrane water permeabilities. We conclude that recent claims that aquaporin knockout studies can be interpreted as evidence against a simple osmotic mechanism are not supported by our work. Many of the results that we obtain are independent of specific transporter details, and our analysis can be easily extended to apply to models that use other proposed ionic mechanisms of saliva secretion.

FKBP38 peptidylprolyl isomerase promotes the folding of cystic fibrosis transmembrane conductance regulator in the endoplasmic reticulum

FK506-binding protein 38 (FKBP38), a membrane-anchored, tetratricopeptide repeat (TPR)-containing immunophilin, associates with nascent plasma membrane ion channels in the endoplasmic reticulum (ER). It promotes the maturation of the human ether-à-go-go-related gene (HERG) potassium channel and maintains the steady state level of the cystic fibrosis transmembrane conductance regulator (CFTR), but the underlying mechanisms remain unclear. Using a combination of steady state and pulse-chase analyses, we show that FKBP38 knockdown increases protein synthesis but inhibits the post-translational folding of CFTR, leading to reduced steady state levels of CFTR in the ER, decreased processing, and impaired cell surface functional expression in Calu-3 human airway epithelial cells. The membrane anchorage of FKBP38 is necessary for the inhibition of protein synthesis but not for CFTR post-translational folding. In contrast, the peptidylprolyl cis/trans isomerase active site is utilized to promote CFTR post-translational folding but is not important for regulation of protein synthesis. Uncoupling FKBP38 from Hsp90 by substituting a conserved lysine in the TPR domain modestly enhances CFTR maturation and further reduces its synthesis. Removing the N-terminal glutamate-rich domain (ERD) slightly enhances CFTR synthesis but reduces its maturation, suggesting that the ERD contributes to FKBP38 biological activities. Our data support a dual role for FKBP38 in regulating CFTR synthesis and post-translational folding. In contrast to earlier prediction but consistent with in vitro enzymological studies, FKBP38 peptidylprolyl cis/trans isomerase plays an important role in membrane protein biogenesis on the cytoplasmic side of the ER membrane, whose activity is negatively regulated by Hsp90 through the TPR domain.

Mitochondrial superoxide flashes: metabolic biomarkers of skeletal muscle activity and disease

Mitochondrial superoxide flashes (mSOFs) are stochastic events of quantal mitochondrial superoxide generation. Here, we used flexor digitorum brevis muscle fibers from transgenic mice with muscle-specific expression of a novel mitochondrial-targeted superoxide biosensor (mt-cpYFP) to characterize mSOF activity in skeletal muscle at rest, following intense activity, and under pathological conditions. Results demonstrate that mSOF activity in muscle depended on electron transport chain and adenine nucleotide translocase functionality, but it was independent of cyclophilin-D-mediated mitochondrial permeability transition pore activity. The diverse spatial dimensions of individual mSOF events were found to reflect a complex underlying morphology of the mitochondrial network, as examined by electron microscopy. Muscle activity regulated mSOF activity in a biphasic manner. Specifically, mSOF frequency was significantly increased following brief tetanic stimulation (18.1 ± 1.6 to 22.3 ± 2.0 flashes/1000 μm²·100 s before and after 5 tetani) and markedly decreased (to 7.7 ± 1.6 flashes/1000 μm²·100 s) following prolonged tetanic stimulation (40 tetani). A significant temperature-dependent increase in mSOF frequency (11.9 ± 0.8 and 19.8 ± 2.6 flashes/1000 μm²·100 s at 23°C and 37°C) was observed in fibers from RYR1(Y522S/WT) mice, a mouse model of malignant hyperthermia and heat-induced hypermetabolism. Together, these results demonstrate that mSOF activity is a highly sensitive biomarker of mitochondrial respiration and the cellular metabolic state of muscle during physiological activity and pathological oxidative stress

Location of release sites and calcium-activated chloride channels relative to calcium channels at the photoreceptor ribbon synapse

Vesicle release from photoreceptor ribbon synapses is regulated by L-type Ca(2+) channels, which are in turn regulated by Cl(-) moving through calcium-activated chloride [Cl(Ca)] channels. We assessed the proximity of Ca(2+) channels to release sites and Cl(Ca) channels in synaptic terminals of salamander photoreceptors by comparing fast (BAPTA) and slow (EGTA) intracellular Ca(2+) buffers. BAPTA did not fully block synaptic release, indicating some release sites are <100 nm from Ca(2+) channels. Comparing Cl(Ca) currents with predicted Ca(2+) diffusion profiles suggested that Cl(Ca) and Ca(2+) channels average a few hundred nanometers apart, but the inability of BAPTA to block Cl(Ca) currents completely suggested some channels are much closer together. Diffuse immunolabeling of terminals with an antibody to the putative Cl(Ca) channel TMEM16A supports the idea that Cl(Ca) channels are dispersed throughout the presynaptic terminal, in contrast with clustering of Ca(2+) channels near ribbons. Cl(Ca) currents evoked by intracellular calcium ion concentration ([Ca(2+)](i)) elevation through flash photolysis of DM-nitrophen exhibited EC(50) values of 556 and 377 nM with Hill slopes of 1.8 and 2.4 in rods and cones, respectively. These relationships were used to estimate average submembrane [Ca(2+)](i) in photoreceptor terminals. Consistent with control of exocytosis by [Ca(2+)] nanodomains near Ca(2+) channels, average submembrane [Ca(2+)](i) remained below the vesicle release threshold (∼ 400 nM) over much of the physiological voltage range for cones. Positioning Ca(2+) channels near release sites may improve fidelity in converting voltage changes to synaptic release. A diffuse distribution of Cl(Ca) channels may allow Ca(2+) influx at one site to influence relatively distant Ca(2+) channels.

Ca2+-activated K channels in parotid acinar cells: The functional basis for the hyperpolarized activation of BK channels

Fluid secretion relies on a close interplay between Ca(2+)-activated Cl and K channels. Salivary acinar cells contain both large conductance, BK, and intermediate conductance, IK1, K channels. Physiological fluid secretion occurs with only modest (<500 nM) increases in intracellular Ca(2+) levels but BK channels in many cell types and in heterologous expression systems require very high concentrations for significant activation. We report here our efforts to understand this apparent contradiction. We determined the Ca(2+) dependence of IK1 and BK channels in mouse parotid acinar cells. IK1 channels activated with an apparent Ca(2+) affinity of about 350 nM and a Hill coefficient near 3. Native parotid BK channels activated at similar Ca(2+) levels unlike the BK channels in other cell types. Since the parotid BK channel is encoded by an uncommon splice variant, we examined this clone in a heterologous expression system. In contrast to the native parotid channel, activation of this expressed "parSlo" channel required very high levels of Ca(2+). In order to understand the functional basis for the special properties of the native channels, we analyzed the parotid BK channel in the context of the Horrigan-Aldrich model of BK channel gating. We found that the shifted activation of parotid BK channels resulted from a hyperpolarizing shift of the voltage dependence of voltage sensor activation and channel opening and included a large change in the coupling of these two processes.

Tumor necrosis factor-alpha-mediated regulation of the inositol 1,4,5-trisphosphate receptor promoter

Tumor necrosis factor-alpha (TNF-alpha), a proinflammatory cytokine, has been implicated as a central mediator in multiple homeostatic and pathologic processes. Signaling cascades downstream of its cellular cognate receptors, as well as the resultant transcriptional responses have received intense interest in regards to how such signals impact cellular physiology. Notably, TNF-alpha was shown to potentiate neuronal Ca(2+) signaling by enhancing type-1 inositol 1,4,5-trisphosphate receptor (IP(3)R) steady-state mRNA levels. In the present study, we sought to determine the promoter region ultimately responsive to TNF-alpha exposure. We report that a sequence encompassing a specificity protein 1 (SP-1) binding site is necessary for TNF-alpha regulation. Electrophoretic mobility shift analysis demonstrated specific binding to this sequence, while site-directed mutagenesis of this site abrogated both JNK-mediated regulation as well as transcription factor binding. Expression of a dominant-negative SP-1 eliminated both the enhanced promoter activity and the elevated IP(3)R-mediated Ca(2+) signals observed with TNF-alpha exposure. Overall, these data delineate a key pathway by which TNF-alpha in a neuronal environment modulates IP(3)R expression and intracellular Ca(2+) homeostasis.

Neural regulation of lacrimal gland secretory processes: relevance in dry eye diseases

The lacrimal gland is the major contributor to the aqueous layer of the tear film which consists of water, electrolytes and proteins. The amount and composition of this layer is critical for the health, maintenance, and protection of the cells of the cornea and conjunctiva (the ocular surface). Small changes in the concentration of tear electrolytes have been correlated with dry eye syndrome. While the mechanisms of secretion of water, electrolytes and proteins from the lacrimal gland differ, all three are under tight neural control. This allows for a rapid response to meet the needs of the cells of the ocular surface in response to environmental conditions. The neural response consists of the activation of the afferent sensory nerves in the cornea and conjunctiva to stimulate efferent parasympathetic and sympathetic nerves that innervate the lacrimal gland. Neurotransmitters are released from the stimulated parasympathetic and sympathetic nerves that cause secretion of water, electrolytes, and proteins from the lacrimal gland and onto the ocular surface. This review focuses on the neural regulation of lacrimal gland secretion under normal and dry eye conditions.

Visualizing form and function in organotypic slices of the adult mouse parotid gland

An organotypic slice preparation of the adult mouse parotid salivary gland amenable to a variety of optical assessments of fluid and protein secretion dynamics is described. The semi-intact preparation rendered without the use of enzymatic treatment permitted live-cell imaging and multiphoton analysis of cellular and supracellular signals. Toward this end we demonstrated that the parotid slice is a significant addition to the repertoire of tools available to investigators to probe exocrine structure and function since there is currently no cell culture system that fully recapitulates parotid acinar cell biology. Importantly, we show that a subpopulation of the acinar cells of parotid slices can be maintained in short-term culture and retain their morphology and function for up to 2 days. This in vitro model system is a significant step forward compared with enzymatically dispersed acini that rapidly lose their morphological and functional characteristics over several hours, and it was shown to be long enough for the expression and trafficking of exogenous protein following adenoviral infection. This system is compatible with a variety of genetic and physiological approaches used to study secretory function.

Physiology and pathophysiology of potassium channels in gastrointestinal epithelia

Epithelial cells of the gastrointestinal tract are an important barrier between the "milieu interne" and the luminal content of the gut. They perform transport of nutrients, salts, and water, which is essential for the maintenance of body homeostasis. In these epithelia, a variety of K(+) channels are expressed, allowing adaptation to different needs. This review provides an overview of the current literature that has led to a better understanding of the multifaceted function of gastrointestinal K(+) channels, thereby shedding light on pathophysiological implications of impaired channel function. For instance, in gastric mucosa, K(+) channel function is a prerequisite for acid secretion of parietal cells. In epithelial cells of small intestine, K(+) channels provide the driving force for electrogenic transport processes across the plasma membrane, and they are involved in cell volume regulation. Fine tuning of salt and water transport and of K(+) homeostasis occurs in colonic epithelia cells, where K(+) channels are involved in secretory and reabsorptive processes. Furthermore, there is growing evidence for changes in epithelial K(+) channel expression during cell proliferation, differentiation, apoptosis, and, under pathological conditions, carcinogenesis. In the future, integrative approaches using functional and postgenomic/proteomic techniques will help us to gain comprehensive insights into the role of K(+) channels of the gastrointestinal tract.

Decreased olfactory mucus secretion and nasal abnormality in mice lacking type 2 and type 3 IP3 receptors

Although nasal mucus is thought to play important roles in the mammalian olfactory system, the mechanisms of secretion of it and its physiological roles are poorly understood. Here we show that type 2 and type 3 IP3 receptors (IP3R2 and IP3R3) play critical roles in olfactory mucus secretion. Histological studies showed that IP3R2 and IP3R3 are predominantly expressed in two types of nasal glands, the anterior glands of the nasal septum and the lateral nasal glands (LNG), which contain mucosal proteins secreted to the main olfactory epithelium. We therefore examined LNG acinar cells, and found that acetylcholine-mediated calcium responses and fluid- and protein- secretion in the acinar cells were markedly decreased in IP3R2-R3 double-knockout (KO) mice. We also found nasal inflammation and a decrease in olfactory capacity in IP3R2-R3 KO mice. Despite intact signal transduction in the olfactory epithelium, IP3R2-R3 KO mice exhibited elevated threshold sensitivity to odorants on in vivo imaging of olfactory glomerular responses and behavioral tests. Our findings suggest that IP3R2 and IP3R3 mediate nasal mucus secretion, which is important for the maintenance of nasal tissue as well as the perception of odors.

Suppression of carbachol-induced oscillatory Cl- secretion by forskolin in rat parotid and submandibular acinar cells

Sympathetic stimulation induces weak salivation compared with parasympathetic stimulation. To clarify this phenomenon in salivary glands, we investigated cAMP-induced modulation of Ca(2+)-activated Cl(-) secretion from rat parotid and submandibular acinar cells because fluid secretion from salivary glands depends on the Cl(-) secretion. Carbachol (Cch), a Ca(2+)-increasing agent, induced hyperpolarization of the cells with oscillatory depolarization in the current clamp mode of the gramicidin-perforated patch recording. In the voltage clamp mode at -80 mV, Cch induced a bumetanide-sensitive oscillatory inward current, which was larger in rat submandibular acinar cells than in parotid acinar cells. Forskolin and IBMX, cAMP-increasing agents, did not induce any marked current, but they evoked a small nonoscillatory inward current in the presence of Cch and suppressed the Cch-induced oscillatory inward current in all parotid acinar cells and half (56%) of submandibular acinar cells. In the current clamp mode, forskolin + IBMX evoked a small nonoscillatory depolarization in the presence of Cch and reduced the amplitude of Cch-induced oscillatory depolarization in both acinar cells. The oscillatory inward current estimated at the depolarized membrane potential was suppressed by forskolin + IBMX. These results indicate that cAMP suppresses Ca(2+)-activated oscillatory Cl(-) secretion of parotid and submandibular acinar cells at -80 mV and possibly at the membrane potential during Cch stimulation. The suppression may result in the weak salivation induced by sympathetic stimulation.

Ca2+ release dynamics in parotid and pancreatic exocrine acinar cells evoked by spatially limited flash photolysis

Intracellular calcium concentration ([Ca(2+)](i)) signals are central to the mechanisms underlying fluid and protein secretion in pancreatic and parotid acinar cells. Calcium release was studied in natively buffered cells following focal laser photolysis of caged molecules. Focal photolysis of caged-inositol 1,4,5 trisphosphate (InsP(3)) in the apical region resulted in Ca(2+) release from the apical trigger zone and, after a latent period, the initiation of an apical-to-basal Ca(2+) wave. The latency was longer and the wave speed significantly slower in pancreatic compared with parotid cells. Focal photolysis in basal regions evoked only limited Ca(2+) release at the photolysis site and never resulted in a propagating wave. Instead, an apical-to-basal wave was initiated following a latent period. Again, the latent period was significantly longer under all conditions in pancreas than parotid. Although slower in pancreas than parotid, once initiated, the apical-to-basal wave speed was constant in a particular cell type. Photo release of caged-Ca(2+) failed to evoke a propagating Ca(2+) wave in either cell type. However, the kinetics of the Ca(2+) signal evoked following photolysis of caged-InsP(3) were significantly dampened by ryanodine in parotid but not pancreas, indicating a more prominent functional role for ryanodine receptor (RyR) following InsP(3) receptor (InsP(3)R) activation. These data suggest that differing expression levels of InsP(3)R, RyR, and possibly cellular buffering capacity may contribute to the fast kinetics of Ca(2+) signals in parotid compared with pancreas. These properties may represent a specialization of the cell type to effectively stimulate Ca(2+)-dependent effectors important for the differing primary physiological role of each gland.

Differential regulation of the apical plasma membrane Ca(2+) -ATPase by protein kinase A in parotid acinar cells

Cross-talk between intracellular calcium ([Ca(2+)](i)) signaling and cAMP defines the specificity of stimulus-response coupling in a variety of cells. Previous studies showed that protein kinase A (PKA) potentiates and phosphorylates the plasma membrane Ca(2+)-ATPase (PMCA) in a Ca(2+)-dependent manner in parotid acinar cells (Bruce, J. I. E., Yule, D. I., and Shuttleworth, T. J. (2002) J. Biol. Chem. 277, 48172-48181). The aim of this study was to further investigate the spatial regulation of [Ca(2+)](i) clearance in parotid acinar cells. Par-C10 cells were used to functionally isolate the apical and basolateral PMCA activity by applying La(3+) to the opposite side to inhibit the PMCA. Activation of PKA (using forskolin) differentially potentiated apical [Ca(2+)](i) clearance in mouse parotid acinar cells and apical PMCA activity in Par-C10 cells. Immunofluorescence of parotid tissue slices revealed that PMCA1 was distributed throughout the plasma membrane, PMCA2 was localized to the basolateral membrane, and PMCA4 was localized to the apical membrane of parotid acinar cells. However, in situ phosphorylation assays demonstrated that PMCA1 was the only isoform phosphorylated by PKA following stimulation. Similarly, immunofluorescence of acutely isolated parotid acinar cells showed that the regulatory subunit of PKA (RIIbeta) translocated to the apical region following stimulation. These data suggest that PKA-mediated phosphorylation of PMCA1 differentially regulates [Ca(2+)](i) clearance in the apical region of parotid acinar cells because of a dynamic translocation of PKA. Such tight spatial regulation of Ca(2+) efflux is likely important for the fine-tuning of Ca(2+)-dependent effectors close to the apical membrane important for the regulation of fluid secretion and exocytosis.

Measurement of Ca2+ signaling dynamics in exocrine cells with total internal reflection microscopy

In nonexcitable cells, such as exocrine cells from the pancreas and salivary glands, agonist-stimulated Ca2+ signals consist of both Ca2+ release and Ca2+ influx. We have investigated the contribution of these processes to membrane-localized Ca2+ signals in pancreatic and parotid acinar cells using total internal reflection fluorescence (TIRF) microscopy (TIRFM). This technique allows imaging with unsurpassed resolution in a limited zone at the interface of the plasma membrane and the coverslip. In TIRFM mode, physiological agonist stimulation resulted in Ca2+ oscillations in both pancreas and parotid with qualitatively similar characteristics to those reported using conventional wide-field microscopy (WFM). Because local Ca2+ release in the TIRF zone would be expected to saturate the Ca2+ indicator (Fluo-4), these data suggest that Ca2+ release is occurring some distance from the area subjected to the measurement. When acini were stimulated with supermaximal concentrations of agonists, an initial peak, largely due to Ca2+ release, followed by a substantial, maintained plateau phase indicative of Ca2+ entry, was observed. The contribution of Ca2+ influx and Ca2+ release in isolation to these near-plasma membrane Ca2+ signals was investigated by using a Ca2+ readmission protocol. In the absence of extracellular Ca2+, the profile and magnitude of the initial Ca2+ release following stimulation with maximal concentrations of agonist or after SERCA pump inhibition were similar to those obtained with WFM in both pancreas and parotid acini. In contrast, when Ca2+ influx was isolated by subsequent Ca2+ readmission, the Ca2+ signals evoked were more robust than those measured with WFM. Furthermore, in parotid acinar cells, Ca2+ readdition often resulted in the apparent saturation of Fluo-4 but not of the low-affinity dye Fluo-4-FF. Interestingly, Ca2+ influx as measured by this protocol in parotid acinar cells was substantially greater than that initiated in pancreatic acinar cells. Indeed, robust Ca2+ influx was observed in parotid acinar cells even at low physiological concentrations of agonist. These data indicate that TIRFM is a useful tool to monitor agonist-stimulated near-membrane Ca2+ signals mediated by Ca2+ influx in exocrine acinar cells. In addition, TIRFM reveals that the extent of Ca2+ influx in parotid acinar cells is greater than pancreatic acinar cells when compared using identical methodologies.

Calcium oscillations and waves generated by multiple release mechanisms in pancreatic acinar cells

We explore the dynamic behavior of a model of calcium oscillations and wave propagation in the basal region of pancreatic acinar cells [Sneyd, J., et al., Biophys. J. 85: 1392-1405, 2003]. Since it is known that two principal calcium release pathways are involved, inositol trisphosphate receptors (IPR) and ryanodine receptors (RyR), we study how the model behavior depends on the density of each receptor type. Calcium oscillations can be mediated either by IPR or RyR. Continuous increases in either RyR or IPR density can lead to the appearance and disappearance of oscillations multiple times, and the two receptor types interact via their common effect on cytoplasmic calcium concentration and the subsequent effect on the total amount of calcium inside the cell. Increases in agonist concentration can stimulate oscillations via the RyR by increasing calcium influx. Using a two time-scale approach, we explain these complex behaviors by treating the total amount of cellular calcium as a slow parameter. Oscillations are controlled by the shape of the slow manifold and where it intersects the nullcline of the slow variable. When calcium diffusion is included, the existence of traveling waves in the model equation is strongly dependent on the interplay between the total amount of calcium in the cell and membrane transport, a feature that can be experimentally tested. Our results help us understand the behavior of a model that includes both receptors in comparison to the properties of each receptor type in isolation.

A method for determining the dependence of calcium oscillations on inositol trisphosphate oscillations

In some cell types, oscillations in the concentration of free intracellular calcium ([Ca2+]) are accompanied by oscillations in the concentration of inositol 1,4,5-trisphosphate ([IP3]). However, in most cell types it is still an open question as to whether oscillations in [IP3] are necessary for Ca2+ oscillations in vivo, or whether they merely follow passively. Using a wide range of models, we show that the response to an artificially applied pulse of IP3 can be used to distinguish between these two cases. Hence, we show that muscarinic receptor-mediated, long-period Ca2+ oscillations in pancreatic acinar cells depend on [IP3] oscillations, whereas short-period Ca2+ oscillations in airway smooth muscle do not.

Ca2+ dynamics in salivary acinar cells: distinct morphology of the acinar lumen underlies near-synchronous global Ca2+ responses

In salivary acinar cells, the pattern of the Ca2+ signals that regulates fluid and enzyme secretion has yet to be resolved, as there are conflicting reports in the literature. We have used a two-photon technique to directly visualize the acinar cell lumen in living fragments of exocrine tissue and simultaneously recorded agonist-induced changes in intracellular Ca2+. We show near-synchronous global Ca2+ responses in submandibular acinar cells, distinct from the typical apical to basal Ca2+ wave usually seen in rodent pancreatic acinar cells. In an effort to explain the basis of these near-synchronous global Ca2+ responses we used immunocytochemical experiments to localize luminal proteins and inositol trisphosphate receptors (InsP3Rs) in tissue fragments. Zona occludens 1 (ZO-1), a tight junction protein, shows that individual submandibular acinar cells are often nearly completely encircled by a narrow luminal structure. By contrast, in pancreatic fragments, ZO-1 staining shows short luminal branches terminating abruptly at the apical pole of single acinar cells. Co-immunostaining of InsP3Rs type 2 and type 3 showed them in the same region as ZO-1 in both exocrine tissues. Functional experiments showed that the near-synchronous global Ca2+ responses were still observed in the absence of extracellular Ca2+ and also in the presence of ryanodine. We conclude that the elaborate luminal region of submandibular cells leads to a hitherto unrecognized extensive distribution of InsP3Rs in a band around the cell and that this underlies the near-synchronous global Ca2+ response to agonists. We suggest that this may be a structural adaptation in submandibular cells to support the copious amounts of fluid secreted.

Spatiotemporal analysis of exocytosis in mouse parotid acinar cells

Exocrine cells of the digestive system are specialized to secrete protein and fluid in response to neuronal and/or hormonal input. Although morphologically similar, parotid and pancreatic acinar cells exhibit important functional divergence in Ca(2+) signaling properties. To address whether there are fundamental differences in exocytotic release of digestive enzyme from exocrine cells of salivary gland versus pancreas, we applied electrophysiological and optical methods to investigate spatial and temporal characteristics of zymogen-containing secretory granule fusion at the single-acinar cell level by direct or agonist-induced Ca(2+) and cAMP elevation. Temporally resolved membrane capacitance measurements revealed that two apparent phases of exocytosis were induced by Ca(2+) elevation: a rapidly activated initial phase that could not be resolved as individual fusion events and a second phase that was activated after a delay, increased in a staircaselike fashion, was augmented by cAMP elevation, and likely reflected both sequential compound and multivesicular fusion of zymogen-containing granules. Optical measurements of exocytosis with time-differential imaging analysis revealed that zymogen granule fusion was induced after a minimum delay of approximately 200 ms, occurred initially at apical and basolateral borders of acinar cells, and under strong stimulation proceeded from apical pole to deeper regions of the cell interior. Zymogen granule fusions appeared to coordinate subsequent fusions and produced persistent structures that generally lasted several minutes. In addition, parotid gland slices were used to assess secretory dynamics in a more physiological context. Parotid acinar cells were shown to exhibit both similar and divergent properties compared with the better-studied pancreatic acinar cell regarding spatial organization and kinetics of exocytotic fusion of zymogen granules.

Calcium-activated chloride channels

Calcium-activated chloride channels (CaCCs) play important roles in cellular physiology, including epithelial secretion of electrolytes and water, sensory transduction, regulation of neuronal and cardiac excitability, and regulation of vascular tone. This review discusses the physiological roles of these channels, their mechanisms of regulation and activation, and the mechanisms of anion selectivity and conduction. Despite the fact that CaCCs are so broadly expressed in cells and play such important functions, understanding these channels has been limited by the absence of specific blockers and the fact that the molecular identities of CaCCs remains in question. Recent status of the pharmacology and molecular identification of CaCCs is evaluated.

Regulation of fluid and electrolyte secretion in salivary gland acinar cells

The secretion of fluid and electrolytes by salivary gland acinar cells requires the coordinated regulation of multiple water and ion transporter and channel proteins. Notably, all the key transporter and channel proteins in this process appear to be activated, or are up-regulated, by an increase in the intracellular Ca2+ concentration ([Ca2+]i). Consequently, salivation occurs in response to agonists that generate an increase in [Ca2+]i. The mechanisms that act to modulate these increases in [Ca2+]i obviously influence the secretion of salivary fluid. Such modulation may involve effects on mechanisms of both Ca2+ release and Ca2+ entry and the resulting spatial and temporal aspects of the [Ca2+]i signal, as well as interactions with other signaling pathways in the cells. The molecular cloning of many of the transporter and regulatory molecules involved in fluid and electrolyte secretion has yielded a better understanding of this process at the cellular level. The subsequent characterization of mice with null mutations in many of these genes has demonstrated the physiological roles of individual proteins. This review focuses on recent developments in determining the molecular identification of the proteins that regulate the fluid secretion process.

Muscarinic activation of Na+-dependent ion transporters and modulation by bicarbonate in rat submandibular gland acinus

To investigate the interaction between the ion channels and transporters in the salivary fluid secretion, we measured the membrane voltage (V(m)) and intracellular concentrations of Ca(2+), Na(+) ([Na(+)](c)), Cl(-), and H(+) (pH(i)) in rat submandibular gland acini (RSMGA). After a transient depolarization induced by a short application of acetylcholine (ACh; 5 muM, 20 s), RSMGA showed strong delayed hyperpolarization (V(h,ACh); -95 +/- 1.8 mV) that was abolished by ouabain. In the HCO(3)(-)-free condition, the V(h,ACh) was also blocked by bumetanide, a blocker of Na(+)-K(+)-2Cl(-) cotransporter (NKCC). In the presence of HCO(3)(-) (24 meq, bubbled with 5% CO(2)), however, the V(h,ACh) was not blocked by bumetanide, but it was suppressed by ethylisopropylamiloride (EIPA), a Na(+)/H(+) exchanger (NHE) inhibitor. Similarly, the ACh-induced increase in [Na(+)](c) was totally blocked by bumetanide in the absence of HCO(3)(-), but only by one-half in the presence of HCO(3)(-). ACh induced a prominent acidification of pH(i) in the presence of HCO(3)(-), and the acidification was further increased by EIPA treatment. Without HCO(3)(-), an application of ACh strongly accelerated the NKCC activity that was measured from the decay of pH(i) during the application of NH(4)(+) (20 mM). Notably, the ACh-induced activation of NKCC was largely suppressed in the presence of HCO(3)(-). In summary, the ACh-induced anion secretion in RSMGA is followed by the activation of NKCC and NHE, resulting an increase in [Na(+)](c). The intracellular Na(+)-induced activation of electrogenic Na(+)/K(+)-ATPase causes V(h,ACh). The regulation of NKCC and NHE by ACh is strongly affected by the physiological level of HCO(3)(-).

Local and global calcium signals and fluid and electrolyte secretion in mouse submandibular acinar cells

Polarized Ca(2+) signals that originate at and spread from the apical pole have been shown to occur in acinar cells from lacrimal, parotid, and pancreatic glands. However, "local" Ca(2+) signals, that are restricted to the apical pole of the cell, have been previously demonstrated only in pancreatic acinar cells in which the primary function of the Ca(2+) signal is to regulate exocytosis. We show that submandibular acinar cells, in which the primary function of the Ca(2+) signal is to drive fluid and electrolyte secretion, are capable of both Ca(2+) waves and local Ca(2+) signals. The generally accepted model for fluid and electrolyte secretion requires simultaneous Ca(2+)-activation of basally located K(+) channels and apically located Cl(-) channels. Whereas a propagated cell-wide Ca(2+) signal is clearly consistent with this model, a local Ca(2+) signal is not, because there is no increase in intracellular Ca(2+) concentration at the basal pole of the cell. Our data provide the first direct demonstration, in submandibular acinar cells, of the apical and basal location of the Cl(-) and K(+) channels, respectively, and confirm that local Ca(2+) signals do not Ca(2+)-activate K(+) channels. We reevaluate the model for fluid and electrolyte secretion and demonstrate that Ca(2+)-activation of the Cl(-) channels is sufficient to voltage-activate the K(+) channels and thus demonstrate that local Ca(2+) signals are sufficient to support fluid secretion.

Bile acids induce a cationic current, depolarizing pancreatic acinar cells and increasing the intracellular Na+ concentration

Biliary disease is a major cause of acute pancreatitis. In this study we investigated the electrophysiological effects of bile acids on pancreatic acinar cells. In perforated patch clamp experiments we found that taurolithocholic acid 3-sulfate depolarized pancreatic acinar cells. At low bile acid concentrations this occurred without rise in the cytosolic calcium concentration. Measurements of the intracellular Na(+) concentration with the fluorescent probe Sodium Green revealed a substantial increase upon application of the bile acid. We found that bile acids induce Ca(2+)-dependent and Ca(2+)-independent components of the Na(+) concentration increase. The Ca(2+)-independent component was resolved in conditions when the cytosolic Ca(2+) level was buffered with a high concentration of the calcium chelator 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA). The Ca(2+)-dependent component of intracellular Na(+) increase was clearly seen during stimulation with the calcium-releasing agonist acetylcholine. During acetylcholine-induced Ca(2+) oscillations the recovery of cytosolic Na(+) was much slower than the recovery of Ca(2+), creating a possibility for the summation of Na(+) transients. The bile-induced Ca(2+)-independent current was found to be carried primarily by Na(+) and K(+), with only small Ca(2+) and Cl(-) contributions. Measurable activation of such a cationic current could be produced by a very low concentration of taurolithocholic acid 3-sulfate (10 microm). This bile acid induced a cationic current even when applied in sodium- and bicarbonate-free solution. Other bile acids, taurochenodeoxycholic acid, taurocholic acid, and bile itself also induced cationic currents. Bile-induced depolarization of acinar cells should have a profound effect on acinar fluid secretion and, consequently, on transport of secreted zymogens.

cAMP potentiates ATP-evoked calcium signaling in human parotid acinar cells

In salivary acinar cells, intracellular calcium ([Ca(2+)](i)) signaling plays an important role in eliciting fluid secretion through the activation of Ca(2+)-activated ionic conductances. Ca(2+) and cAMP have synergistic effects on fluid secretion such that peak secretion is elicited following activation of both parasympathetic and sympathetic pathways. We have recently demonstrated that cAMP exerts effects on Ca(2+) release, through protein kinase A (PKA)-mediated phosphorylation of inositol 1,4,5-trisphosphate receptors (InsP(3)R) in mouse parotid acinar cells. To extend these findings, in the present study cross-talk between Ca(2+) signaling and cAMP pathways in human parotid acinar cells was investigated. In human parotid acinar cells, carbachol stimulation evoked increases in the [Ca(2+)](i) and the initial peak amplitude was enhanced following PKA activation, consistent with reports from mouse parotid. Stimulation with ATP also evoked an increase in [Ca(2+)](i). The ATP-evoked Ca(2+) elevation was largely dependent on extracellular Ca(2+), suggesting the involvement of the P2X family of purinergic receptors. Pharmacological elevation of cAMP resulted in a approximately 5-fold increase in the peak [Ca(2+)](i) change evoked by ATP stimulation. This enhanced [Ca(2+)](i) increase was not dependent on intracellular release from InsP(3)R or ryanodine receptors, suggesting a direct effect on P2XR. Reverse transcription-polymerase chain reaction and Western blot analysis confirmed the presence of P2X(4)R and P2X(7)R mRNA and protein in human parotid acinar cells. ATP-activated cation currents were studied using whole cell patch clamp techniques in HEK-293 cells, a null background for P2XR. Raising cAMP resulted in a approximately 4.5-fold enhancement of ATP-activated current in HEK-293 cells transfected with P2X(4)R DNA but had no effects on currents in cells expressing P2X(7)R. These data indicate that in human parotid acinar cells, in addition to modulation of Ca(2+) release, Ca(2+) influx through P2X(4)R may constitute a further locus for the synergistic effects of Ca(2+) and PKA activation.

Subpopulation of store-operated Ca2+ channels regulate Ca2+-induced Ca2+ release in non-excitable cells

Ca2+-induced Ca2+ release (CICR) is a well characterized activity in skeletal and cardiac muscles mediated by the ryanodine receptors. The present study demonstrates CICR in the non-excitable parotid acinar cells, which resembles the mechanism described in cardiac myocytes. Partial depletion of internal Ca2+ stores leads to a minimal activation of Ca2+ influx. Ca2+ influx through this pathway results in an explosive mobilization of Ca2+ from the majority of the stores by CICR. Thus, stimulation of parotid acinar cells in Ca2+ -free medium with 0.5 microm carbachol releases approximately 5% of the Ca2+ mobilizable by 1 mm carbachol. Addition of external Ca2+ induced the same Ca2+ release observed in maximally stimulated cells. Similar results were obtained by a short treatment with 2.5-10 microm cyclopiazonic acid, an inhibitor of the sarco/endoplasmic reticulum Ca2+ ATPase pump. The Ca2+ release induced by the addition of external Ca2+ was largely independent of IP(3)Rs because it was reduced by only approximately 30% by the inhibition of the inositol 1,4,5-trisphosphate receptors with caffeine or heparin. Measurements of Ca2+ -activated outward current and [Ca2+](i) suggested that most CICR triggered by Ca2+ influx occurred away from the plasma membrane. Measurement of the response to several concentrations of cyclopiazonic acid revealed that Ca2+ influx that regulates CICR is associated with a selective portion of the internal Ca2+ pool. The minimal activation of Ca2+ influx by partial store depletion was confirmed by the measurement of Mn2+ influx. Inhibition of Ca2+ influx with SKF96365 or 2-aminoethoxydiphenyl borate prevented activation of CICR observed on addition of external Ca2+. These findings provide evidence for activation of CICR by Ca2+ influx in non-excitable cells, demonstrate a previously unrecognized role for Ca2+ influx in triggering CICR, and indicate that CICR in non-excitable cells resembles CICR in cardiac myocytes with the exception that in cardiac cells Ca2+ influx is mediated by voltage-regulated Ca2+ channels whereas in non-excitable cells Ca2+ influx is mediated by store-operated channels.

Extracellular Zinc and ATP Restore Chloride Secretion across Cystic Fibrosis Airway Epithelia by Triggering Calcium Entry

Cystic fibrosis (CF) is caused by defective cyclic AMP-dependent cystic fibrosis transmembrane conductance regulator Cl(-) channels. Thus, CF epithelia fail to transport Cl(-) and water. A postulated therapeutic avenue in CF is activation of alternative Ca(2+)-dependent Cl(-) channels. We hypothesized that stimulation of Ca(2+) entry from the extracellular space could trigger a sustained Ca(2+) signal to activate Ca(2+)-dependent Cl(-) channels. Cytosolic [Ca(2+)](i) was measured in non-polarized human CF (IB3-1) and non-CF (16HBE14o(-)) airway epithelial cells. Primary human CF and non-CF airway epithelial monolayers as well as Calu-3 monolayers were used to assess anion secretion. In vivo nasal potential difference measurements were performed in non-CF and two different CF mouse (DeltaF508 homozygous and bitransgenic gut-corrected but lung-null) models. Zinc and ATP induced a sustained, reversible, and reproducible increase in cytosolic Ca(2+) in CF and non-CF cells with chemistry and pharmacology most consistent with activation of P2X purinergic receptor channels. P2X purinergic receptor channel-mediated Ca(2+) entry stimulated sustained Cl(-) and HCO(3)(-) secretion in CF and non-CF epithelial monolayers. In non-CF mice, zinc and ATP induced a significant Cl(-) secretory response similar to the effects of agonists that increase intracellular cAMP levels. More importantly, in both CF mouse models, Cl(-) permeability of nasal epithelia was restored in a sustained manner by zinc and ATP. These effects were reversible and reacquirable upon removal and readdition of agonists. Our data suggest that activation of P2X calcium entry channels may have profound therapeutic benefit for CF that is independent of cystic fibrosis transmembrane conductance regulator genotype.