Intercellular calcium waves in rat pancreatic acini: mechanism of transmission

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.

Calcium imaging in intact mouse acinar cells in acute pancreas tissue slices

The physiology and pathophysiology of the exocrine pancreas are in close connection to changes in intra-cellular Ca²⁺ concentration. Most of our knowledge is based on in vitro experiments on acinar cells or acini enzymatically isolated from their surroundings, which can alter their structure, physiology, and limit our understanding. Due to these limitations, the acute pancreas tissue slice technique was introduced almost two decades ago as a complementary approach to assess the morphology and physiology of both the endocrine and exocrine pancreas in a more conserved in situ setting. In this study, we extend previous work to functional multicellular calcium imaging on acinar cells in tissue slices. The viability and morphological characteristics of acinar cells within the tissue slice were assessed using the LIVE/DEAD assay, transmission electron microscopy, and immunofluorescence imaging. The main aim of our study was to characterize the responses of acinar cells to stimulation with acetylcholine and compare them with responses to cerulein in pancreatic tissue slices, with special emphasis on inter-cellular and inter-acinar heterogeneity and coupling. To this end, calcium imaging was performed employing confocal microscopy during stimulation with a wide range of acetylcholine concentrations and selected concentrations of cerulein. We show that various calcium oscillation parameters depend monotonically on the stimulus concentration and that the activity is rather well synchronized within acini, but not between acini. The acute pancreas tissue slice represents a viable and reliable experimental approach for the evaluation of both intra- and inter-cellular signaling characteristics of acinar cell calcium dynamics. It can be utilized to assess many cells simultaneously with a high spatiotemporal resolution, thus providing an efficient and high-yield platform for future studies of normal acinar cell biology, pathophysiology, and screening pharmacological substances.

Modeling of Astrocyte Networks: Toward Realistic Topology and Dynamics

Neuronal firing and neuron-to-neuron synaptic wiring are currently widely described as orchestrated by astrocytes—elaborately ramified glial cells tiling the cortical and hippocampal space into non-overlapping domains, each covering hundreds of individual dendrites and hundreds thousands synapses. A key component to astrocytic signaling is the dynamics of cytosolic Ca²⁺ which displays multiscale spatiotemporal patterns from short confined elemental Ca²⁺ events (puffs) to Ca²⁺ waves expanding through many cells. Here, we synthesize the current understanding of astrocyte morphology, coupling local synaptic activity to astrocytic Ca²⁺ in perisynaptic astrocytic processes and morphology-defined mechanisms of Ca²⁺ regulation in a distributed model. To this end, we build simplified realistic data-driven spatial network templates and compile model equations as defined by local cell morphology. The input to the model is spatially uncorrelated stochastic synaptic activity. The proposed modeling approach is validated by statistics of simulated Ca²⁺ transients at a single cell level. In multicellular templates we observe regular sequences of cell entrainment in Ca²⁺ waves, as a result of interplay between stochastic input and morphology variability between individual astrocytes. Our approach adds spatial dimension to the existing astrocyte models by employment of realistic morphology while retaining enough flexibility and scalability to be embedded in multiscale heterocellular models of neural tissue. We conclude that the proposed approach provides a useful description of neuron-driven Ca²⁺-activity in the astrocyte syncytium.

Connexin43 and connexin50 channels exhibit different permeability to the second messenger inositol triphosphate

Gap junction channels made of different connexins have distinct permeability to second messengers, which could affect many cell processes, including lens epithelial cell division. Here, we have compared the permeability of IP₃ and Ca²⁺ through channels made from two connexins, Cx43 and Cx50, that are highly expressed in vertebrate lens epithelial cells. Solute transfer was measured while simultaneously monitoring junctional conductance via dual whole-cell/perforated patch clamp. HeLa cells expressing Cx43 or Cx50 were loaded with Fluo-8, and IP₃ or Ca²⁺ were delivered via patch pipette to one cell of a pair, or to a monolayer while fluorescence intensity changes were recorded. Cx43 channels were permeable to IP₃ and Ca²⁺. Conversely, Cx50 channels were impermeable to IP₃, while exhibiting high permeation of Ca²⁺. Reduced Cx50 permeability to IP₃ could play a role in regulating cell division and homeostasis in the lens.

Cholecystokinin (CCK) Regulation of Pancreatic Acinar Cells: Physiological Actions and Signal Transduction Mechanisms

Pancreatic acinar cells synthesize and secrete about 20 digestive enzymes and ancillary proteins with the processes that match the supply of these enzymes to their need in digestion being regulated by a number of hormones (CCK, secretin and insulin), neurotransmitters (acetylcholine and VIP) and growth factors (EGF and IGF). Of these regulators, one of the most important and best studied is the gastrointestinal hormone, cholecystokinin (CCK). Furthermore, the acinar cell has become a model for seven transmembrane, heterotrimeric G protein coupled receptors to regulate multiple processes by distinct signal transduction cascades. In this review, we briefly describe the chemistry and physiology of CCK and then consider the major physiological effects of CCK on pancreatic acinar cells. The majority of the review is devoted to the physiologic signaling pathways activated by CCK receptors and heterotrimeric G proteins and the functions they affect. The pathways covered include the traditional second messenger pathways PLC-IP3-Ca²⁺ , DAG-PKC, and AC-cAMP-PKA/EPAC that primarily relate to secretion. Then there are the protein-protein interaction pathways Akt-mTOR-S6K, the three major MAPK pathways (ERK, JNK, and p38 MAPK), and Ca²⁺ -calcineurin-NFAT pathways that primarily regulate non-secretory processes including biosynthesis and growth, and several miscellaneous pathways that include the Rho family small G proteins, PKD, FAK, and Src that may regulate both secretory and nonsecretory processes but are not as well understood. © 2019 American Physiological Society. Compr Physiol 9:535-564, 2019.

Ethanol suppresses carbamylcholine-induced intracellular calcium oscillation in mouse pancreatic acinar cells

Oscillation of intracellular calcium levels is closely linked to initiating secretion of digestive enzymes from pancreatic acinar cells. Excessive alcohol consumption is known to relate to a variety of disorders in the digestive system, including the exocrine pancreas. In this study, we have investigated the role and mechanism of ethanol on carbamylcholine (CCh)-induced intracellular calcium oscillation in murine pancreatic acinar cells. Ethanol at concentrations of 30 and 100 mM reversibly suppressed CCh-induced Ca²⁺ oscillation in a dose-dependent manner. Pretreatment of ethanol has no effect on the store-operated calcium entry induced by 10 μM of CCh. Ethanol significantly reduced the initial calcium peak induced by low concentrations of CCh and therefore, the CCh-induced dose-response curve of the initial calcium peak was shifted to the right by ethanol pretreatment. Furthermore, ethanol significantly dose-dependently reduced inositol 1,4,5-trisphosphate-induced calcium release from the internal stores in permeabilized acinar cells. These results provide evidence that excessive alcohol intake could impair cytosolic calcium oscillation through inhibiting calcium release from intracellular stores in mouse pancreatic acinar cells.

Role of Connexins and Pannexins in the Pancreas

The pancreas produces enzymes with a digestive function and hormones with a metabolic function, which are produced by distinct cell types of acini and islets, respectively. Within these units, secretory cells coordinate their functioning by exchanging information via signals that flow in the intercellular spaces and are generated either at distance (several neural and hormonal inputs) or nearby the pancreatic cells themselves (inputs mediated by membrane ionic-specific channels and by ionic- and metabolite-permeant pannexin channels and connexin "hemichannels"). Pancreatic secretory cells further interact via the extracellular matrix of the pancreas (inputs mediated by integrins) and directly with neighboring cells, by mechanisms that do not require extracellular mediators (inputs mediated by gap and tight junction channels). Here, we review the expression and function of the connexins and pannexins that are expressed by the main secretory cells of the exocrine and endocrine pancreatic cells. Available data show that the patterns of expression of these proteins differ in acini and islets, supporting distinct functions in the physiological secretion of pancreatic enzymes and hormones. Circumstantial evidence further suggests that alterations in the signaling provided by these proteins are involved in pancreatic diseases.

The role of Ca2+ influx in endocytic vacuole formation in pancreatic acinar cells

The inducers of acute pancreatitis trigger a prolonged increase in the cytosolic Ca(2+) concentration ([Ca(2+)]c), which is responsible for the damage to and eventual death of pancreatic acinar cells. Vacuolization is an important indicator of pancreatic acinar cell damage. Furthermore, activation of trypsinogen occurs in the endocytic vacuoles; therefore the vacuoles can be considered as 'initiating' organelles in the development of the cell injury. In the present study, we investigated the relationship between the formation of endocytic vacuoles and Ca(2+) influx developed in response to the inducers of acute pancreatitis [bile acid taurolithocholic acid 3-sulfate (TLC-S) and supramaximal concentration of cholecystokinin-8 (CCK)]. We found that the inhibitor of STIM (stromal interaction molecule)/Orai channels, GSK-7975A, effectively suppressed both the Ca(2+) influx (stimulated by inducers of pancreatitis) and the formation of endocytic vacuoles. Cell death induced by TLC-S or CCK was also inhibited by GSK-7975A. We documented the formation of endocytic vacuoles in response to store-operated Ca(2+) entry (SOCE) induced by thapsigargin [TG; inhibitor of sarcoplasmic/endoplasmic reticulum (ER) Ca(2+) pumps] and observed strong inhibition of TG-induced vacuole formation by GSK-7975A. Finally, we found that structurally-unrelated inhibitors of calpain suppress formation of endocytic vacuoles, suggesting that this Ca2+-dependent protease is a mediator between Ca(2+) elevation and endocytic vacuole formation.

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.

How to win ATP and influence Ca(2+) signaling

This brief review discusses recent advances in studies of mitochondrial Ca(2+) signaling and considers how the relationships between mitochondria and Ca(2+) responses are shaped in secretory epithelial cells. Perhaps the more precise title of this review could have been "How to win ATP and influence Ca(2+) signaling in secretory epithelium with emphasis on exocrine secretory cells and specific focus on pancreatic acinar cells". But "brevity is a virtue" and the authors hope that many of the mechanisms discussed are general and applicable to other tissues and cell types. Among these mechanisms are mitochondrial regulation of Ca(2+) entry and the role of mitochondria in the formation of localized Ca(2+) responses. The roles of Ca(2+) signaling in the physiological adjustment of bioenergetics and in mitochondrial damage are also briefly discussed.

Intercellular Ca(2+) waves: mechanisms and function

Intercellular calcium (Ca(2+)) waves (ICWs) represent the propagation of increases in intracellular Ca(2+) through a syncytium of cells and appear to be a fundamental mechanism for coordinating multicellular responses. ICWs occur in a wide diversity of cells and have been extensively studied in vitro. More recent studies focus on ICWs in vivo. ICWs are triggered by a variety of stimuli and involve the release of Ca(2+) from internal stores. The propagation of ICWs predominately involves cell communication with internal messengers moving via gap junctions or extracellular messengers mediating paracrine signaling. ICWs appear to be important in both normal physiology as well as pathophysiological processes in a variety of organs and tissues including brain, liver, retina, cochlea, and vascular tissue. We review here the mechanisms of initiation and propagation of ICWs, the key intra- and extracellular messengers (inositol 1,4,5-trisphosphate and ATP) mediating ICWs, and the proposed physiological functions of ICWs.

Low extracellular pH induces damage in the pancreatic acinar cell by enhancing calcium signaling

Low extracellular pH (pHe) occurs in a number of clinical conditions and sensitizes to the development of pancreatitis. The mechanisms responsible for this sensitization are unknown. Because abnormal Ca(2+) signaling underlies many of the early steps in the pathogenesis of pancreatitis, we evaluated the effect of decreasing pHe from 7.4 to 7.0 on Ca(2+) signals in the acinar cell. Low pHe significantly increased the amplitude of cerulein-induced Ca(2+) signals. The enhancement in amplitude was localized to the basolateral region of the acinar cell and was reduced by pretreatment with ryanodine receptor (RYR) inhibitors. Because basolateral RYRs also have been implicated in the pathogenesis of pancreatitis, we evaluated the effects of RYR inhibitors on pancreatitis responses in acidic conditions. RYR inhibitors significantly reduced the sensitizing effects of low pHe on zymogen activation and cellular injury. These findings suggest that enhanced RYR-mediated Ca(2+) signaling in the basolateral region of the acinar cell is responsible for the injurious effects of low pHe on the exocrine pancreas.

Intercellular calcium waves in the fire-diffuse-fire framework: Green's function for gap-junctional coupling

Calcium is a crucial component in a plethora of cellular processes involved in cell birth, life, and death. Intercellular calcium waves that can spread through multiple cells provide one form of cellular communication mechanism between various parts of cell tissues. Here we introduce a simple, yet biophysically realistic model for the propagation of intercellular calcium waves based on the fire-diffuse-fire type model for calcium dynamics. Calcium release sites are considered to be discretely distributed along individual linear cells that are connected by gap junctions and a solution of this model can be found in terms of the Green's function for this system. We develop the "sum-over-trips" formalism that takes into account the boundary conditions at gap junctions providing a generalization of the original sum-over-trips approach for constructing the response function for branched neural dendrites. We obtain the exact solution of the Green's function in the Laplace (frequency) domain for an infinite array of cells and show that this Green's function can be well approximated by its truncated version. This allows us to obtain an analytical traveling wave solution for an intercellular calcium wave and analyze the speed of solitary wave propagation as a function of physiologically important system parameters. Periodic and irregular traveling waves can be also sustained by the proposed model.

Exocytosis, dependent on Ca2+ release from Ca2+ stores, is regulated by Ca2+ microdomains

The relationship between the cellular Ca2+ signal and secretory vesicle fusion (exocytosis) is a key determinant of the regulation of the kinetics and magnitude of the secretory response. Here, we have investigated secretion in cells where the exocytic response is controlled by Ca2+ release from intracellular Ca2+ stores. Using live-cell two-photon microscopy that simultaneously records Ca2+signals and exocytic responses, we provide evidence that secretion is controlled by changes in Ca2+ concentration [Ca2+] in relatively large-volume microdomains. Our evidence includes: (1) long latencies (>2 seconds) between the rise in [Ca2+] and exocytosis, (2) observation of exocytosis all along the lumen and not clustered around Ca2+ release hot-spots, (3) high affinity (Kd=1.75 microM) Ca2+dependence of exocytosis, (4) significant reduction in exocytosis in the presence of cytosolic EGTA, (5) spatial exclusion of secretory granules from the cell membrane by the endoplasmic reticulum, and (6) inability of local Ca2+ responses to trigger exocytosis. These results strongly indicate that the control of exocytosis, triggered by Ca2+ release from stores, is through the regulation of cytosolic[Ca2+] within a microdomain.

Calcium signaling in liver

In hepatocytes, hormones linked to the formation of the second messenger inositol 1,4,5-trisphosphate (InsP3) evoke transient increases or spikes in cytosolic free calcium ([Ca2+]i), that increase in frequency with the agonist concentration. These oscillatory Ca2+ signals are thought to transmit the information encoded in the extracellular stimulus to down-stream Ca2+-sensitive metabolic processes. We have utilized both confocal and wide field fluorescence microscopy techniques to study the InsP3-dependent signaling pathway at the cellular and subcellular levels in the intact perfused liver. Typically InsP3-dependent [Ca2+]i spikes manifest as Ca2+ waves that propagate throughout the entire cytoplasm and nucleus, and in the intact liver these [Ca2+]i increases are conveyed through gap junctions to encompass entire lobular units. The translobular movement of Ca2+ provides a means to coordinate the function of metabolic zones of the lobule and thus, liver function. In this article, we describe the characteristics of agonist-evoked [Ca2+]i signals in the liver and discuss possible mechanisms to explain the propagation of intercellular Ca2+ waves in the intact organ.

Calcium Dynamics: Spatio‐Temporal Organization from the Subcellular to the Organ Level

Many essential physiological processes are controlled by calcium. To ensure reliability and specificity, calcium signals are highly organized in time and space in the form of oscillations and waves. Interesting findings have been obtained at various scales, ranging from the stochastic opening of a single calcium channel to the intercellular calcium wave spreading through an entire organ. A detailed understanding of calcium dynamics thus requires a link between observations at different scales. It appears that some regulations such as calcium-induced calcium release or PLC activation by calcium, as well as the weak diffusibility of calcium ions play a role at all levels of organization in most cell types. To comprehend how calcium waves spread from one cell to another, specific gap-junctional coupling and paracrine signaling must also be taken into account. On the basis of a pluridisciplinar approach ranging from physics to physiology, a unified description of calcium dynamics is emerging, which could help understanding how such a small ion can mediate so many vital functions in living systems.

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.

Calcium oscillations in a triplet of pancreatic acinar cells

We use a mathematical model of calcium dynamics in pancreatic acinar cells to investigate calcium oscillations in a ring of three coupled cells. A connected group of cells is modeled in two different ways: 1), as coupled point oscillators, each oscillator being described by a spatially homogeneous model; and 2), as spatially distributed cells coupled along their common boundaries by gap-junctional diffusion of inositol trisphosphate and/or calcium. We show that, although the point-oscillator model gives a reasonably accurate general picture, the behavior of the spatially distributed cells cannot always be predicted from the simpler analysis; spatially distributed diffusion and cell geometry both play important roles in determining behavior. In particular, oscillations in which two cells are in synchrony, with the third phase-locked but not synchronous, appears to be more dominant in the spatially distributed model than in the point-oscillator model. In both types of model, intercellular coupling leads to a variety of synchronous, phase-locked, or asynchronous behaviors. For some parameter values there are multiple, simultaneous stable types of oscillation. We predict 1), that intercellular calcium diffusion is necessary and sufficient to coordinate the responses in neighboring cells; 2), that the function of intercellular inositol trisphosphate diffusion is to smooth out any concentration differences between the cells, thus making it easier for the diffusion of calcium to synchronize the oscillations; 3), that groups of coupled cells will tend to respond in a clumped manner, with groups of synchronized cells, rather than with regular phase-locked periodic intercellular waves; and 4), that enzyme secretion is maximized by the presence of a pacemaker cell in each cluster which drives the other cells at a frequency greater than their intrinsic frequency.

Functional Mapping of Ca2+ Signaling Complexes in Plasma Membrane Microdomains of Polarized Cells

Many cells cluster signaling complexes in plasma membrane microdomains. Polarized secretory cells cluster all Ca2+ signaling proteins, including GPCRs, at the apical pole. The functional significance of such an arrangement is not known because of a lack of techniques for functional mapping of signaling complexes at plasma membrane patches. In the present work, we developed such a technique based on the use of two patch pipettes, a recording and a stimulating pipette (SP). Including 20% glycerol in the SP solution increased the viscosity and the hydrophobicity to prevent leakage and formation of tight seals on the plasma membrane. This allowed moving the SP between sites to stimulate multiple patches of the same cell and with the same agonist concentrations. Functional mapping of Ca2+ signaling in pancreatic acinar cells revealed that the M3, cholecystokinin, and bombesin signaling complexes at the apical pole are much more sensitive to stimulation than those at the basal pole. Furthermore, at physiological agonist concentrations, Ca2+ signals could be evoked only by stimulation of membrane patches at the apical pole. [Ca2+](i) imaging revealed that Ca2+ waves were invariably initiated at the site of apical membrane patch stimulation, suggesting that long range diffusion of second messengers is not obligatory to initiate and propagate apical-to-basal Ca2+ waves. The present studies reveal a remarkable heterogeneity in responsiveness of Ca2+ signaling complexes at membrane microdomains, with the most responsive complexes confined to the apical pole, probably to restrict the Ca2+ signals to the site of exocytosis and allow the polarized functions of secretory cells.

Chemical gating of gap junction channels; roles of calcium, pH and calmodulin

Both Ca(2+) and H(+) play a role in chemical gating of gap junction channels, but, with the possible exception of Cx46 hemichannels, neither of them is likely to induce gating by a direct interaction with connexins. Some evidence suggests that low pH(i) affects gating via an increase in [Ca(2+)](i); in turn, Ca(2+) is likely to induce gating by activation of CaM, which may act directly as a gating particle. The effective concentrations of both Ca(2+) and H(+) vary depending on cell type, type of connexin expressed and procedure employed to increase their cytosolic concentrations; however, pH(i) as high as 7.2 and [Ca(2+)](i) as low as 150 nM or lower have been reported to be effective in some cells. Some data suggest that Ca(2+) and H(+) affect gating by acting synergistically, but other data do not support synergism. Chemical gating follows the activation of a slow gate distinct from the fast V(j)-sensitive gate, and there is evidence that the chemical/slow gate is V(j)-sensitive. At the single channel level, the chemical/slow gate closes the channels slowly and completely, whereas the fast V(j) gate closes the channels rapidly and incompletely. At least three molecular models of channel gating have been proposed, but all of them are mostly based on circumstantial evidence.

Crosstalk between cAMP and Ca2+ signaling in non-excitable cells

An impressive array of cytosolic calcium ([Ca2+](i)) signals exert control over a broad range of physiological processes. The specificity and fidelity of these [Ca2+](i) signals is encoded by the frequency, amplitude, and sub-cellular localization of the response. It is believed that the distinct characteristics of [Ca2+](i) signals underlies the differential activation of effectors and ultimately cellular events. This "shaping" of [Ca2+](i) signals can be achieved by the influence of additional signaling pathways modulating the molecular machinery responsible for generating [Ca2+](i) signals. There is a particularly rich source of potential sites of crosstalk between the cAMP and the [Ca2+](i) signaling pathways. This review will focus on the predominant molecular loci at which these classical signaling systems interact to impact the spatio-temporal pattern of [Ca2+](i) signaling in non-excitable cells.

A model of calcium waves in pancreatic and parotid acinar cells

We construct a mathematical model of Ca(2+) wave propagation in pancreatic and parotid acinar cells. Ca(2+) release is via inositol trisphosphate receptors and ryanodine receptors that are distributed heterogeneously through the cell. The apical and basal regions are separated by a region containing the mitochondria. In response to a whole-cell, homogeneous application of inositol trisphosphate (IP(3)), the model predicts that 1), at lower concentrations of IP(3), the intracellular waves in pancreatic cells begin in the apical region and are actively propagated across the basal region by Ca(2+) release through ryanodine receptors; 2), at higher [IP(3)], the waves in pancreatic and parotid cells are not true waves but rather apparent waves, formed as the result of sequential activation of inositol trisphosphate receptors in the apical and basal regions; 3), the differences in wave propagation in pancreatic and parotid cells can be explained in part by differences in inositol trisphosphate receptor density; 4), in pancreatic cells, increased Ca(2+) uptake by the mitochondria is capable of restricting Ca(2+) responses to the apical region, but that this happens only for a relatively narrow range of [IP(3)]; and 5), at higher [IP(3)], the apical and basal regions of the cell act as coupled Ca(2+) oscillators, with the basal region partially entrained to the apical region.

Conducted dilations initiated by purines in arterioles are endothelium dependent and require endothelial Ca2+

The signaling pathways underlying the regulation of vascular resistance by purines in intact microvessels and particularly in communication of remote vasomotor responses are unclear. One process by which remote regions of arterioles communicate is via transmission of signals axially along the vessel wall. In this study, we identified a pathway for local and conducted dilations initiated by purines. Adenosine (Ado) or ATP (bind P1 and P2 purinergic receptors, respectively) was micropipette applied to arterioles (maximum diameter approximately 40 microm) in the cheek pouch of anesthetized hamsters. Observations were made at the site of stimulation (local) or approximately 1200 microm upstream along the same vessel. P2 antagonists (pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid tetrasodium and suramin) inhibited local constriction to ATP, whereas local and upstream dilations were unaffected. In contrast, during inhibition of P1 receptors (with xanthine amine congener) the local constriction was unchanged, whereas both local and upstream dilations to ATP were inhibited. Hydrolysis of ATP to Ado is implicated in the dilator response as blocking 5'-ectonucleotidase (with alpha,beta-methyleneadenosine 5'-diphosphate) attenuated ATP-induced dilations. After endothelium denudation, constriction to ATP was unchanged, but dilations to both ATP and Ado were inhibited, identifying endothelial cells (ECs) as the primary target for P1-mediated dilation. Purines increased EC Ca2+ locally and upstream. Chelation of EC Ca2+ (with BAPTA) abolished the local and upstream dilations to P1 receptor stimulation. Collectively, these data demonstrate that stimulation of P1 receptors on ECs produces a vasodilation that spreads to remote regions. There is an associated increase in EC Ca2+, which is a required signaling intermediate in the manifestation of both the local and axially communicated arteriolar dilations.

Coordination of mesangial cell contraction by gap junction--mediated intercellular Ca(2+) wave

Gap junction intercellular communication (GJIC) plays a fundamental role in mediating intercellular signals and coordinating multicellular behavior in various tissues and organs. Glomerular mesangial cells (MC) are rich in GJ, but the functional associations of these intercellular channels are still unclear. This study examines the potential role of GJ in the transmission of intercellular Ca(2+) signals and in the coordination of MC contraction. First, the presence of GJ protein Cx43 and functional GJIC was confirmed in MC by using immunochemical staining or transfer of Lucifer yellow (LY) after a single cell injection, respectively. Second, mechanical stimulation of a single MC initiated propagation of an intercellular Ca(2+) wave, which was preventable by the GJ inhibitor heptanol but was not altered by pretreatment of MC with ATP or addition of apyrase into the assay system. Third, the phospholipase C (PLC) inhibitor U73122 could largely eliminate the mechanically elicited propagation of intercellular Ca(2+) waves, suggesting a possibly mediating role of inositol trisphosphate (IP(3)) in the initiation and transmission of intercellular Ca(2+) signaling. Fourth, injection of IP(3) into a single cell caused contraction, not only in the targeted cell, but also in the adjacent cells, as indicated by the reduction of cellular planar area. Fifth, addition of two structurally unrelated GJ inhibitors, heptanol and alpha-glycyrrhetinic acid (GA), into MC embedded in collagen gels significantly attenuated the reduction of gel areas after exposure to serum. This study provides the first functional evidence supporting the critical role of GJIC in the synchronization of MC behaviors.

Basal and physiological Ca(2+) leak from the endoplasmic reticulum of pancreatic acinar cells. Second messenger-activated channels and translocons

We have studied the Ca(2+) leak pathways in the endoplasmic reticulum of pancreatic acinar cells by directly measuring Ca(2+) in the endoplasmic reticulum ([Ca(2+)](ER)). Cytosolic Ca(2+) ([Ca(2+)](C)) was clamped to the resting level by a BAPTA-Ca(2+) mixture. Administration of cholecystokinin within the physiological concentration range caused a graded decrease of [Ca(2+)](ER), and the rate of Ca(2+) release generated by 10 pm cholecystokinin is at least 3x as fast as the basal Ca(2+) leak revealed by inhibition of the endoplasmic reticulum Ca(2+)-ATPase. Acetylcholine also evokes a dose-dependent decrease of [Ca(2+)](ER), with an EC(50) of 0.98 +/- 0.06 microm. Inhibition of receptors for inositol 1,4,5-trisphosphate (IP(3)) by heparin or flunarizine blocks the effect of acetylcholine but only partly blocks the effect of cholecystokinin. 8-NH(2) cyclic ADP-ribose (20 microm) inhibits the action of cholecystokinin, but not of acetylcholine(.) The basal Ca(2+) leak from the endoplasmic reticulum is not blocked by antagonists of the IP(3) receptor, the ryanodine receptor, or the receptor for nicotinic acid adenine dinucleotide phosphate. However, treatment with puromycin (0.1-1 mm) to remove nascent polypeptides from ribosomes increases Ca(2+) leak from the endoplasmic reticulum by a mechanism independent of the endoplasmic reticulum Ca(2+) pumps and of the receptors for IP(3) or ryanodine.

Polarized calcium and calmodulin signaling in secretory epithelia

This review examines polarized calcium and calmodulin signaling in exocrine epithelial cells. The calcium ion is a simple, evolutionarily ancient, and universal second messenger. In exocrine epithelial cells, it regulates essential functions such as exocytosis, fluid secretion, and gene expression. Exocrine cells are structurally polarized, with the apical region usually dedicated to secretion. Recent advances in technology, in particular the development of videoimaging and confocal microscopy, have led to the discovery of polarized, subcellular calcium signals in these cell types. The properties of a rich variety of local and global calcium signals have now been described in secretory epithelial cells. Secretagogues stimulate apical-to-basal waves of calcium in many exocrine cell types, but there are some interesting exceptions to this rule. The shapes of intracellular calcium signals are determined by the distribution of calcium-releasing channels and mechanisms that limit calcium elevation. Polarized distribution of calcium-handling mechanisms also leads to transcellular calcium transport in exocrine epithelial cells. This transport can deliver considerable amounts of calcium into secreted fluids. Multicellular polarized calcium signals can coordinate the activity of many individual cells in epithelial secretory tissue. Certain particularly sensitive cells serve as pacemakers for initiation of intercellular calcium waves. Many calcium signaling pathways involve activation of calmodulin. This ubiquitous protein regulates secretion in exocrine cells and also activates interesting feedback interactions with calcium channels and transporters. Very recently it became possible to directly study polarized calcium-calmodulin reactions and to visualize the process of hormone-induced redistribution of calmodulin in live cells. The structural and functional polarity of secretory epithelia alongside the polarity of its calcium and calmodulin signaling present an interesting lesson in tissue organization.

Molecular basis for pacemaker cells in epithelia

Intercellular signaling is highly coordinated in excitable tissues such as heart, but the organization of intercellular signaling in epithelia is less clear. We examined Ca(2+) signaling in hepatoma cells expressing the hepatocyte gap junction protein connexin32 (cx32) or the cardiac gap junction protein cx43, plus a fluorescently tagged V(1a) vasopressin receptor (V(1a)R). Release of inositol 1,4,5-trisphosphate (InsP(3)) in wild type cells increased Ca(2+) in the injected cell but not in neighboring cells, while the Ca(2+) signal spread to neighbors when gap junctions were expressed. Photorelease of caged Ca(2+) rather than InsP(3) resulted in a small increase in Ca(2+) that did not spread to neighbors with or without gap junctions. However, photorelease of Ca(2+) in cells stimulated with low concentrations of vasopressin resulted in a much larger increase in Ca(2+), which spread to neighbors via gap junctions. Cells expressing tagged V(1a)R similarly had increased sensitivity to vasopressin, and could signal to neighbors via gap junctions. Higher concentrations of vasopressin elicited Ca(2+) signals in all cells. In cx32 or cx43 but not in wild type cells, this signaling was synchronized and began in cells expressing the tagged V(1a)R. Thus, intercellular Ca(2+) signals in epithelia are organized by three factors: 1) InsP(3) must be generated in each cell to support a Ca(2+) signal in that cell; 2) gap junctions are necessary to synchronize Ca(2+) signals among cells; and 3) cells with relatively increased expression of hormone receptor will initiate Ca(2+) signals and thus serve as pacemakers for their neighbors. Together, these factors may allow epithelia to act in an integrated, organ-level fashion rather than as a collection of isolated cells.

Modelling of simple and complex calcium oscillations. From single-cell responses to intercellular signalling

This review provides a comparative overview of recent developments in the modelling of cellular calcium oscillations. A large variety of mathematical models have been developed for this wide-spread phenomenon in intra- and intercellular signalling. From these, a general model is extracted that involves six types of concentration variables: inositol 1,4,5-trisphosphate (IP3), cytoplasmic, endoplasmic reticulum and mitochondrial calcium, the occupied binding sites of calcium buffers, and the fraction of active IP3 receptor calcium release channels. Using this framework, the models of calcium oscillations can be classified into 'minimal' models containing two variables and 'extended' models of three and more variables. Three types of minimal models are identified that are all based on calcium-induced calcium release (CICR), but differ with respect to the mechanisms limiting CICR. Extended models include IP3--calcium cross-coupling, calcium sequestration by mitochondria, the detailed gating kinetics of the IP3 receptor, and the dynamics of G-protein activation. In addition to generating regular oscillations, such models can describe bursting and chaotic calcium dynamics. The earlier hypothesis that information in calcium oscillations is encoded mainly by their frequency is nowadays modified in that some effect is attributed to amplitude encoding or temporal encoding. This point is discussed with reference to the analysis of the local and global bifurcations by which calcium oscillations can arise. Moreover, the question of how calcium binding proteins can sense and transform oscillatory signals is addressed. Recently, potential mechanisms leading to the coordination of oscillations in coupled cells have been investigated by mathematical modelling. For this, the general modelling framework is extended to include cytoplasmic and gap-junctional diffusion of IP3 and calcium, and specific models are compared. Various suggestions concerning the physiological significance of oscillatory behaviour in intra- and intercellular signalling are discussed. The article is concluded with a discussion of obstacles and prospects.

Stochastic effects in intercellular calcium spiking in hepatocytes

We carry out a Monte Carlo simulation of stochastic effects for two models of intercellular calcium wave propagation in rat hepatocytes. Both models involve gap junction diffusion by a second messenger. We find that, in general, the stochastic effects improve agreement with experiment, for a reasonable choice of model parameters. Both stochastic models exhibit baseline fluctuations and variations in the peak heights of Ca(2+). In addition, we find for one model that there is a distribution of latency times, rather than a single latency time, with a distribution width which is comparable to the experimental observation of spike widths. We also find for the other model with low gap junction diffusion that it is possible for cell multiplets to oscillate independently initially, but to subsequently become synchronized.

Ca(2+) regulation of gap junctional coupling in lens epithelial cells

The quantitative effects of Ca(2+) signaling on gap junctional coupling in lens epithelial cells have been determined using either the spread of Mn(2+) that is imaged by its ability to quench the fluorescence of fura 2 or the spread of the fluorescent dye Alexa Fluor 594. Gap junctional coupling was unaffected by a mechanically stimulated cell-to-cell Ca(2+) wave. Furthermore, when cytosolic Ca(2+) concentration (Ca) increased after the addition of the agonist ATP, coupling was unaffected during the period that Ca was maximal. However, coupling decreased transiently approximately 5-10 min after agonist addition when Ca returned to resting levels, indicating that this transient decrease in coupling was unlikely due to a direct action of Ca on gap junctions. An increase in Ca mediated by the ionophore ionomycin that was sustained for several minutes resulted in a more rapid and sustained decrease in coupling (IC(50) ~300 nM Ca(2+), Hill coefficient of 4), indicating that an increase in Ca alone could regulate gap junctions. Thus Ca increases that occurred during agonist stimulation and cell-to-cell Ca(2+) waves were too transient to mediate a sustained uncoupling of lens epithelial cells.

Intracellular signaling mechanisms activated by cholecystokinin-regulating synthesis and secretion of digestive enzymes in pancreatic acinar cells

The intracellular signaling mechanisms by which cholecystokinin (CCK) and other secretagogues regulate pancreatic acinar function are more complex than originally realized. CCK couples through heterotrimeric G proteins of the Gq family to lead to an increase in intracellular free Ca2+, which shows spatial and temporal patterns of signaling. The actions of Ca2+ are mediated in part by activation of a number of Ca2+-activated protein kinases and the protein phosphatase calcineurin. By the process of exocytosis the intracellular messengers Ca2+, diacylglycerol, and cAMP activate the release of the zymogen granule content in a manner that is poorly understood. This fusion event most likely involves SNARE and Rab proteins present on zymogen granules and cellular membrane domains. More likely related to nonsecretory aspects of cell function, CCK also activates three MAPK cascades leading to activation of ERKs, JNKs, and p38 MAPK. Although the function of these pathways is not well understood, ERKs are probably related to cell growth, and through phosphorylation of hsp27, p38 can affect the actin cytoskeleton. The PI3K (phosphatidylinositol 3-kinase)-mTOR (mammalian target of rapamycin) pathway is important for regulation of acinar cell protein synthesis because it leads to both activation of p70S6K and regulation of the availability of eIF4E in response to CCK. CCK also activates a number of tyrosyl phosphorylation events including that of p125FAK and other proteins associated with focal adhesions.

Investigation of the roles of Ca2+ and InsP3 diffusion in the coordination of Ca2+ signals between connected hepatocytes

Glycogenolytic agonists induce coordinated Ca2+ oscillations in multicellular rat hepatocyte systems as well as in the intact liver. The coordination of intercellular Ca2+ signals requires functional gap-junction coupling. The mechanisms ensuring this coordination are not precisely known. We investigated possible roles of Ca2+ or inositol 1,4,5-trisphosphate (InsP3) as a coordinating messengers for Ca2+ spiking among connected hepatocytes. Application of ionomycin or of supra-maximal concentrations of agonists show that Ca2+ does not significantly diffuse between connected hepatocytes, although gap junctions ensure the passage of small signaling molecules, as demonstrated by FRAP experiments. By contrast, coordination of Ca2+ spiking among connected hepatocytes can be favored by a rise in the level of InsP3, via the increase of agonist concentrations, or by a shift in the affinity of InsP3 receptor for InsP3. In the same line, coordination cannot be achieved if the InsP3 is rapidly metabolized by InsP3-phosphatase in one cell of the multiplet. These results demonstrate that even if small amounts of Ca2+ diffuse across gap junctions, they most probably do not play a significant role in inducing a coordinated Ca2+ signal among connected hepatocytes. By contrast, coordination of Ca2+ oscillations is fully dependent on the diffusion of InsP3 between neighboring cells.

A bifurcation analysis of two coupled calcium oscillators

In many cell types, asynchronous or synchronous oscillations in the concentration of intracellular free calcium occur in adjacent cells that are coupled by gap junctions. Such oscillations are believed to underlie oscillatory intercellular calcium waves in some cell types, and thus it is important to understand how they occur and are modified by intercellular coupling. Using a previous model of intracellular calcium oscillations in pancreatic acinar cells, this article explores the effects of coupling two cells with a simple linear diffusion term. Depending on the concentration of a signal molecule, inositol (1,4,5)-trisphosphate, coupling two identical cells by diffusion can give rise to synchronized in-phase oscillations, as well as different-amplitude in-phase oscillations and same-amplitude antiphase oscillations. Coupling two nonidentical cells leads to more complex behaviors such as cascades of period doubling and multiply periodic solutions. This study is a first step towards understanding the role and significance of the diffusion of calcium through gap junctions in the coordination of oscillatory calcium waves in a variety of cell types. (c) 2001 American Institute of Physics.

Intercellular Ca2+ wave propagation through gap-junctional Ca2+ diffusion: a theoretical study

Intercellular regenerative calcium waves in systems such as the liver and the blowfly salivary gland have been hypothesized to spread through calcium-induced calcium release (CICR) and gap-junctional calcium diffusion. A simple mathematical model of this mechanism is developed. It includes CICR and calcium removal from the cytoplasm, cytoplasmic and gap-junctional calcium diffusion, and calcium buffering. For a piecewise linear approximation of the calcium kinetics, expressions in terms of the cellular parameters are derived for 1) the condition for the propagation of intercellular waves, and 2) the characteristic time of the delay of a wave encountered at the gap junctions. Intercellular propagation relies on the local excitation of CICR in the perijunctional space by gap-junctional calcium influx. This mechanism is compatible with low effective calcium diffusivity, and necessitates that CICR can be excited in every cell along the path of a wave. The gap-junctional calcium permeability required for intercellular waves in the model falls in the range of reported gap-junctional permeability values. The concentration of diffusive cytoplasmic calcium buffers and the maximal rate of CICR, in the case of inositol 1,4,5-trisphosphate (IP3) receptor calcium release channels set by the IP(3) concentration, are shown to be further determinants of wave behavior.

The role of intracellular calcium signaling in premature protease activation and the onset of pancreatitis

The exocrine pancreas synthesizes and secretes large amounts of digestive proteases as inactive precursor zymogens. Under physiological conditions a variety of cellular defense mechanisms protect the pancreatic acinar cell against a premature and intracellular activation of these zymogens. When these defenses fail, pancreatic autodigestion is initiated and acute pancreatitis can develop. A number of experimental observations suggest that extra- as well as intracellular calcium concentrations play an important part in the initiation of pancreatic protease activation, but the intracellular signaling events that regulate this process are unknown. Using a model system in which we used pancreatic acini (freshly prepared functional units of living acinar cells), we were able to simulate the conditions found during experimental pancreatitis in rodents. By means of a cell permeant fluorescent trypsin substrate we could demonstrate in these acini that premature protease activation is initiated at the apical acinar cell pole and occurs only in the presence of secretagogue concentrations that exceed those required for a maximum secretory response. By combining this technique with fluorescence ratio imaging for the Ca(2+)-sensitive dye fura-2, we could further show that this protease activation is highly dependent on the spatial as well as the temporal distribution of the corresponding Ca(2+) release from stores within the same subcellular compartment and that it is not propagated to neighboring acinar cells.

Ruled by waves? Intracellular and intercellular calcium signalling

The field of calcium signalling has evolved rapidly the last 20 years. Physiologists had worked with cytosolic Ca2+ as the coupler of excitation and contraction of muscles and as a secretory signal in exocrine glands and in the synapses of the brain for several decades before the discovery of cellular calcium as a second messenger. Development of powerful techniques for measuring the concentration of cytosolic free calcium ions in cell suspensions and later in single cells and even in different cellular compartments, has resulted in an upsurge in the knowledge of the cellular machinery involved in intracellular calcium signalling. However, the focus on intracellular mechanisms might have led this field of study away from physiology. During the last few years there is an increasing evidence for an important role of calcium also as an intercellular signal. Via gap junctions calcium is able to co-ordinate cell populations and even organs like the liver. Here we will give an overview of the general mechanisms of intracellular calcium signalling, and then review the recent data on intercellular calcium signals. A functional coupling of cells in different tissues and organs by the way of calcium might be an important mechanism for controlling and synchronizing physiological responses

Selective permeability of different connexin channels to the second messenger inositol 1,4,5-trisphosphate

Intercellular propagation of signals through connexin32-containing gap junctions is of major importance in physiological processes like nerve activity-dependent glucose mobilization in liver parenchymal cells and enzyme secretion from pancreatic acinar cells. In these cells, as in other organs, more than one type of connexin is expressed. We hypothesized that different permeabilities towards second messenger molecules could be one of the reasons for connexin diversity. In order to investigate this, we analyzed transmission of inositol 1,4,5-trisphosphate-mediated calcium waves in FURA-2-loaded monolayers of human HeLa cells expressing murine connexin26, -32 or -43. Gap junction-mediated cell coupling in different connexin-transfected HeLa cells was standardized by measuring the spreading of microinjected Mn(2+) that led to local quenching of FURA-2 fluorescence. Microinjection of inositol 1,4,5-trisphosphate into confluently growing HeLa connexin32 transfectants induced propagation of a Ca(2+) wave from the injected cell to neighboring cells that was at least three- to fourfold more efficient than in HeLa Cx26 cells and about 2.5-fold more efficient than in HeLa Cx43 transfectants. Our results support the notion that diffusion of inositol 1,4,5-trisphosphate through connexin32-containing gap junctions is essential for the optimal physiological response, for example by recruiting liver parenchymal cells that contain subthreshold levels of this short lived second messenger.

Mechanism of receptor-oriented intercellular calcium wave propagation in hepatocytes

Intercellular calcium signals are propagated in multicellular hepatocyte systems as well as in the intact liver. The stimulation of connected hepatocytes by glycogenolytic agonists induces reproducible sequences of intracellular calcium concentration increases, resulting in unidirectional intercellular calcium waves. Hepatocytes are characterized by a gradient of vasopressin binding sites from the periportal to perivenous areas of the cell plate in hepatic lobules. Also, coordination of calcium signals between neighboring cells requires the presence of the agonist at each cell surface as well as gap junction permeability. We present a model based on the junctional coupling of several hepatocytes differing in sensitivity to the agonist and thus in the intrinsic period of calcium oscillations. In this model, each hepatocyte displays repetitive calcium spikes with a slight phase shift with respect to neighboring cells, giving rise to a phase wave. The orientation of the apparent calcium wave is imposed by the direction of the gradient of hormonal sensitivity. Calcium spikes are coordinated by the diffusion across junctions of small amounts of inositol 1,4, 5-trisphosphate (InsP(3)). Theoretical predictions from this model are confirmed experimentally. Thus, major physiological insights may be gained from this model for coordination and spatial orientation of intercellular signals.-Dupont, G., Tordjmann, T., Clair, C., Swillens, S., Claret, M., Combettes, L. Mechanism of receptor-oriented intercellular calcium wave propagation in hepatocytes.

Model of intercellular calcium oscillations in hepatocytes: synchronization of heterogeneous cells

Hepatocytes respond with repetitive cytosolic calcium spikes to stimulation by vasopressin and noradrenalin. In the intact liver, calcium oscillations occur in a synchronized fashion as periodic waves across whole liver lobules, but the mechanism of intercellular coupling remains unclear. Recently, it has been shown that individual hepatocytes can have very different intrinsic oscillation frequencies but become phase-locked when coupled by gap junctions. We investigate the gap junction hypothesis for intercellular synchronization by means of a mathematical model. It is shown that junctional calcium fluxes are effective in synchronizing calcium oscillations in coupled hepatocytes. An experimentally testable estimate is given for the junctional coupling coefficient required; it mainly depends on the degree of heterogeneity between cells. Intercellular synchronization by junctional calcium diffusion may occur also in other cell types exhibiting calcium-activated calcium release through InsP(3) receptors, if the gap junctional coupling is strong enough and the InsP(3) receptors are sufficiently sensitized by InsP(3).

The mechanism mediating regenerative intercellular Ca2+ waves in the blowfly salivary gland

Intercellular Ca2+ signaling in intact salivary glands of the blowfly Calliphora erythrocephala was studied by fluorimetric digital imaging combined with microinjection of putative messenger molecules. Iontophoretic injection of D-myo-inositol 1,4, 5-trisphosphate (InsP3) into salivary gland cells evoked regenerative intercellular Ca2+ waves that spread through the impaled cell and several rows of surrounding cells. Ca2+ increases induced by microinjection of Ca2+ ions were confined to the injected cells and their nearest neighbors. Depletion of intracellular Ca2+ stores by thapsigargin pre-treatment did not alter the time course of the Ca2+ increase caused by Ca2+ injection. However, activation of Ca2+ release became clearly evident when Ca2+ was injected in the presence of serotonin (5-HT). Under these conditions, injection of Ca2+ triggered intercellular Ca2+ waves that consecutively passed through >10 cells. The phospholipase C inhibitor U73122 blocked 5-HT-induced Ca2+ increases but did not affect InsP3-dependent Ca2+ spiking and intercellular Ca2+ wave propagation. The results demonstrate that propagation of agonist-evoked Ca2+ waves in the blowfly salivary gland requires supra-basal [InsP3] but does not depend on feedback activation of phospholipase C. We conclude that the intra- and intercellular transmission of these Ca2+ waves is mediated by diffusion of Ca2+ and Ca2+-induced Ca2+ release via the InsP3 receptor channel.

Agonist-dependent phosphorylation of the inositol 1,4,5-trisphosphate receptor: A possible mechanism for agonist-specific calcium oscillations in pancreatic acinar cells

The properties of inositol 1,4,5-trisphosphate (IP3)-dependent intracellular calcium oscillations in pancreatic acinar cells depend crucially on the agonist used to stimulate them. Acetylcholine or carbachol (CCh) cause high-frequency (10-12-s period) calcium oscillations that are superimposed on a raised baseline, while cholecystokinin (CCK) causes long-period (>100-s period) baseline spiking. We show that physiological concentrations of CCK induce rapid phosphorylation of the IP3 receptor, which is not true of physiological concentrations of CCh. Based on this and other experimental data, we construct a mathematical model of agonist-specific intracellular calcium oscillations in pancreatic acinar cells. Model simulations agree with previous experimental work on the rates of activation and inactivation of the IP3 receptor by calcium (DuFour, J.-F., I.M. Arias, and T.J. Turner. 1997. J. Biol. Chem. 272:2675-2681), and reproduce both short-period, raised baseline oscillations, and long-period baseline spiking. The steady state open probability curve of the model IP3 receptor is an increasing function of calcium concentration, as found for type-III IP3 receptors by Hagar et al. (Hagar, R.E., A.D. Burgstahler, M.H. Nathanson, and B.E. Ehrlich. 1998. Nature. 396:81-84). We use the model to predict the effect of the removal of external calcium, and this prediction is confirmed experimentally. We also predict that, for type-III IP3 receptors, the steady state open probability curve will shift to lower calcium concentrations as the background IP3 concentration increases. We conclude that the differences between CCh- and CCK-induced calcium oscillations in pancreatic acinar cells can be explained by two principal mechanisms: (a) CCK causes more phosphorylation of the IP3 receptor than does CCh, and the phosphorylated receptor cannot pass calcium current; and (b) the rate of calcium ATPase pumping and the rate of calcium influx from the outside the cell are greater in the presence of CCh than in the presence of CCK.

Calcium signaling in rat pancreatic acinar cells: a role for Galphaq, Galpha11, and Galpha14

Stimulus-secretion coupling in the pancreatic acinar cell is initiated by the secretagogues CCK and ACh and results in the secretion by exocytosis of the contents of zymogen granules. A key event in this pathway is the G protein-activated production of second messengers and the subsequent elevation of cytosolic-free Ca2+. The aim of this study was therefore to define the heterotrimeric G protein alpha-subunits present and participating in this pathway in rat pancreatic acinar cells. RT-PCR products were amplified from pancreatic acinar cell mRNA with primers specific for Galphaq, Galpha11, and Galpha14 but were not amplified with primers specific for Galpha15. The sequences of these PCR products confirmed them to be portions of the rat homologues of Galphaq, Galpha11, and Galpha14. The pancreatic-derived cell line AR42J similarly expressed Galphaq, Galpha11, and Galpha14; however, the Chinese hamster ovary (CHO) cell line only expressed Galpha11 and Galphaq. These data indicate that caution should be exercised when comparing signal transduction pathways between different cell types. The expression of these proteins in acinar cells was confirmed by immunoblotting samples of acinar membrane protein using specific antisera to the individual G protein alpha-subunits. The role of these proteins in Ca2+ signaling events was investigated by microinjecting a neutralizing antibody directed against a homologous sequence in Galphaq, Galpha11, and Galpha14 into acinar cells and CHO cells. Ca2+ signaling was inhibited in acinar cells and receptor-bearing CHO cells in response to both physiological and supermaximal concentrations of agonists. The inhibition was >75% in both cell types. These data indicate a role for Galphaq and/or Galpha11 in intracellular Ca2+ concentration signaling in CHO cells, and in addition to Galphaq and Galpha11, Galpha14 may also fulfill this role in rat pancreatic acinar cells.

Modulation of pancreatic acinar cell to cell coupling during ACh-evoked changes in cytosolic Ca2+

The temporal changes in cytosolic free Ca2+ ([Ca2+]i), Ca2+-dependent membrane currents (Im), and gap junctional current (Ij) elicited by acetylcholine (ACh) were measured in rat pancreatic acinar cells using digital imaging and dual perforated patch-clamp recording. ACh (50 nM-5 microM) increased [Ca2+]i and evoked Im currents without altering Ij in 19 of 37 acinar cell pairs. Although [Ca2+]i rose asynchronously in cells comprising a cluster, the delay of the [Ca2+]i responses decreased with increasing ACh concentrations. Perfusion of inositol 1,4,5-trisphosphate (IP3) into one cell of a cluster resulted in [Ca2+]i responses in neighboring cells that were not necessarily in direct contact with the stimulated one. This suggests that extensive coupling between acinar cells provides a pathway for cell-to-cell diffusion of Ca2+-releasing signals. Strikingly, maximal (1-5 microM) ACh concentrations reduced Ij by 69 +/- 15% (n = 9) in 25% of the cell pairs subjected to dual patch-clamping. This decrease occurred shortly after the Im peak and was prevented by incubating acinar cells in a Ca2+-free medium, suggesting that uncoupling was subsequent to the initiation of the Ca2+-mobilizing responses. Depletion of Ca2+-sequestering stores by thapsigargin resulted in a reduction of intercellular communication similar to that observed with ACh. In addition, ACh-induced uncoupling was prevented by blocking nitric oxide production with L-nitro-arginine and restored by exposing acinar cells to dibutyryl cGMP. The results suggest that ACh-induced uncoupling and capacitative Ca2+ entry are regulated concurrently. Closure of gap junction channels may occur to functionally isolate nearby cells differing in their intrinsic sensitivity to ACh and thereby to allow for sustained activity of groups of secreting cells.

Coupling between cytosolic and mitochondrial calcium oscillations: role in the regulation of hepatic metabolism

Mitochondria are strategically localized at sites of Ca2+ release, such that increases in cytosolic free Ca2+ ([Ca2+]c) from either internal Ca2+ stores or Ca2+ influx across the plasma membrane can be rapidly transported into the mitochondrial matrix. The consequent elevation in mitochondrial Ca2+ ([Ca2+]m) stimulates the Ca2+-sensitive intramitochondrial dehydrogenases, resulting in elevation of NAD(P)H. The preferential coupling between increases in [Ca2+]c and [Ca2+]m is one proposed mechanism to coordinate mitochondrial ATP production with cellular energy demand. In liver cells, hormones that act through the second messenger inositol 1,4, 5-trisphosphate (IP3) generate oscillatory [Ca2+]c signals, which result from a periodic Ca2+- and IP3-mediated activation/deactivation of intracellular Ca2+ release channels. The [Ca2+]c spiking frequency increases with agonist dose, whereas the amplitude of each [Ca2+]c spike is constant. This frequency modulation of [Ca2+]c spiking encodes the signal from the extracellular agonist, which is then decoded by the internal Ca2+-sensitive proteins such as the Ca2+-sensitive intramitochondrial dehydrogenases. Our studies have investigated the relationship between IP3-dependent [Ca2+]c signals and [Ca2+]m in primary cultured hepatocytes. In addition, the changes in cellular [Ca2+] levels have been correlated with the regulation of intramitochondrial NAD(P)H levels, pyruvate dehydrogenase activity and the magnitude of the mitochondrial proton motive force.

Novel paracrine signaling mechanism in the ocular ciliary epithelium

The ciliary body contains an epithelial bilayer consisting of an outer pigmented cell layer (PE) and an inner nonpigmented cell layer (NPE) responsible for aqueous humor secretion. Secretion may be mediated in part by cytosolic Ca2+ concentration ([Ca2+]i), but whether or how the two layers could coordinate their Ca2+ signals to regulate secretion is unclear. To investigate interactions between PE and NPE, we examined [Ca2+]i signaling in isolated intact ciliary epithelial bilayers using confocal microscopy. Phenylephrine selectively increased [Ca2+]i in PE and acetylcholine increased [Ca2+]i in NPE, but epinephrine increased [Ca2+]i in both layers. This increase spread from PE to NPE, and [Ca2+]i signaling across the bilayer remained coordinated during [Ca2+]i oscillations. All epinephrine-induced [Ca2+]i signaling was blocked by the alpha1-adrenergic antagonist prazosin, whereas signaling in the NPE but not PE was blocked by the beta-adrenergic antagonist propranolol, the gap junction blockers octanol and 18alpha-glycyrrhetinic acid, or the A kinase inhibitor Rp diastereomer of adenosine 3',5'-cyclic monophosphothioate. The beta-adrenergic agonist isoproterenol failed to increase Ca2+ by itself, but isoproterenol plus phenylephrine-induced [Ca2+]i signals across the bilayer similar to those induced by epinephrine. Finally, isoproterenol increased cell-to-cell spread of lucifer yellow via gap junctions, whereas cell-to-cell spread of [Ca2+]i signals could be induced by photorelease of caged inositol 1,4,5-trisphosphate. Thus, calcium signals are coordinated in the epithelial bilayer so that adrenergic stimulation can increase [Ca2+]i in NPE, but only if NPE are primed by activation of endogenous adenylyl cyclase, whereupon they receive stimulation from adjacent PE via gap junctions. This novel interplay between endocrine and paracrine pathways may coordinate [Ca2+]i signaling across the ciliary epithelial bilayer.