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

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.

In vivo Ca2+ Imaging in Mouse Salivary Glands

Changes in intracellular calcium drive exocrine cell activity. In the salivary gland, acetylcholine released from parasympathetic neurons mobilizes endoplasmic reticulum calcium stores in acinar cells, which consequently initiates saliva secretion. However, our understanding of the signaling cascade is mainly based on ex vivo studies performed in enzymatically isolated cells. The dissociation process likely disrupts the extracellular matrix, removes neurons as the source of signal input, and disturbs the integrity of tight and gap junctional acinar connections. These alterations may affect the spatiotemporal properties of calcium signaling events. In vivo observations of calcium signals, where tissue organization is intact, are therefore important to establish the characteristics of physiological calcium signals that are crucial for the stimulation of fluid secretion. Here, we present a detailed protocol for in vivo imaging of calcium signaling events, following nervous stimulation by multi-photon microscopy in mouse salivary gland acinar cells, expressing the genetically encoded calcium indicator GCamp6F.

A KRAP(y) job: Defining the localization of active IP3R

A recent publication documented an exciting role for KRAP in tethering/immobilizing IP₃Rs to actin. This interaction "licenses" IP₃R activity as disrupting the partnership markedly diminishes Ca²⁺ puffs and global signals. These findings highlight a unique mechanism for regulating IP₃R activity.

TRPM4 links calcium signaling to membrane potential in pancreatic acinar cells

Transient receptor potential cation channel subfamily M member 4 (TRPM4) is a Ca²⁺-activated nonselective cation channel that mediates membrane depolarization. Although, a current with the hallmarks of a TRPM4-mediated current has been previously reported in pancreatic acinar cells (PACs), the role of TRPM4 in the regulation of acinar cell function has not yet been explored. In the present study, we identify this TRPM4 current and describe its role in context of Ca²⁺ signaling of PACs using pharmacological tools and TRPM4-deficient mice. We found a significant Ca²⁺-activated cation current in PACs that was sensitive to the TRPM4 inhibitors 9-phenanthrol and 4-chloro-2-[[2-(2-chlorophenoxy)acetyl]amino]benzoic acid (CBA). We demonstrated that the CBA-sensitive current was responsible for a Ca²⁺-dependent depolarization of PACs from a resting membrane potential of −44.4 ± 2.9 to −27.7 ± 3 mV. Furthermore, we showed that Ca²⁺ influx was higher in the TRPM4 KO- and CBA-treated PACs than in control cells. As hormone-induced repetitive Ca²⁺ transients partially rely on Ca²⁺ influx in PACs, the role of TRPM4 was also assessed on Ca²⁺ oscillations elicited by physiologically relevant concentrations of the cholecystokinin analog cerulein. These data show that the amplitude of Ca²⁺ signals was significantly higher in TRPM4 KO than in control PACs. Our results suggest that PACs are depolarized by TRPM4 currents to an extent that results in a significant reduction of the inward driving force for Ca²⁺. In conclusion, TRPM4 links intracellular Ca²⁺ signaling to membrane potential as a negative feedback regulator of Ca²⁺ entry in PACs.

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.

Spatial-temporal patterning of Ca2+ signals by the subcellular distribution of IP3 and IP3 receptors

The patterning of cytosolic Ca²⁺ signals in space and time underlies their ubiquitous ability to specifically regulate numerous cellular processes. Signals mediated by liberation of Ca²⁺ sequestered in the endoplasmic reticulum (ER) through inositol trisphosphate receptor (IP₃R) channels constitute a hierarchy of events; ranging from openings of individual IP₃ channels, through the concerted openings of several clustered IP₃Rs to generate local Ca²⁺ puffs, to global Ca²⁺ waves and oscillations that engulf the entire cell. Here, we review recent progress in elucidating how this hierarchy is shaped by an interplay between the functional gating properties of IP₃Rs and their spatial distribution within the cell. We focus in particular on the subset of IP₃Rs that are organized in stationary clusters and are endowed with the ability to preferentially liberate Ca²⁺.

Expression of BK channels and Na+-K+ pumps in the apical membrane of lacrimal acinar cells suggests a new molecular mechanism for primary tear-secretion

PURPOSE

Primary fluid secretion in secretory epithelia relies on the unidirectional transport of ions and water across a single cell layer. This mechanism requires the asymmetric apico-basal distribution of ion transporters and intracellular Ca²⁺ signaling. The primary aim of the present study was to verify the localization and the identity of Ca²⁺-dependent ion channels in acinar cells of the mouse lacrimal gland.

METHODS

Whole-cell patch-clamp-electrophysiology, spatially localized flash-photolysis of Ca²⁺ and temporally resolved digital Ca²⁺-imaging was combined. Immunostaining of enzymatically isolated mouse lacrimal acinar cells was performed.

RESULTS

We show that the Ca²⁺-dependent K⁺-conductance is paxilline-sensitive, abundant in the luminal, but negligible in the basal membrane; and co-localizes with Cl^--conductance. These data suggest that both Cl^- and K⁺ are secreted into the lumen and thus they account for the high luminal [Cl^-] (∼141 mM), but not for the relatively low [K⁺] (<17 mM) of the primary fluid. Accordingly, these results also imply that K⁺ must be reabsorbed from the primary tear fluid by the acinar cells. We hypothesized that apically-localized Na⁺-K⁺ pumps are responsible for K⁺-reabsorption. To test this possibility, immunostaining of lacrimal acinar cells was performed using anti-Na⁺-K⁺ ATP-ase antibody. We found positive fluorescence signal not only in the basal, but in the apical membrane of acinar cells too.

CONCLUSIONS

Based on these results we propose a new primary fluid-secretion model in the lacrimal gland, in which the paracellular pathway of Na⁺ secretion is supplemented by a transcellular pathway driven by apical Na⁺-K⁺ pumps.

Research Progress on the Relationship Between Acute Pancreatitis and Calcium Overload in Acinar Cells

Acute pancreatitis is a human disease with multiple causes that leads to autodigestion of the pancreas. There is sufficient evidence to support the key role of sustained increase in cytosolic calcium concentrations in the early pathogenesis of the disease. To clarify the mechanism of maintaining calcium homeostasis in the cell and pathological processes caused by calcium overload would help to research directly targeted therapeutic agents. We will specifically review the following: intracellular calcium homeostasis and regulation, the occurrence of calcium overload in acinar cells, the role of calcium overload in the pathogenesis of AP, the treatment strategy proposed for calcium overload.

Hydrogen peroxide inhibits Ca2+ efflux through plasma membrane Ca2+-ATPase in mouse parotid acinar cells

Intracellular Ca²⁺ mobilization is closely linked with the initiation of salivary secretion in parotid acinar cells. Reactive oxygen species (ROS) are known to be related to a variety of oxidative stress-induced cellular disorders and believed to be involved in salivary impairments. In this study, we investigated the underlying mechanism of hydrogen peroxide (H₂O₂) on cytosolic Ca²⁺ accumulation in mouse parotid acinar cells. Intracellular Ca²⁺ levels were slowly elevated when 1 mM H₂O₂ was perfused in the presence of normal extracellular Ca²⁺. In a Ca²⁺-free medium, 1 mM H₂O₂ still enhanced the intracellular Ca²⁺ level. Ca²⁺ entry tested using manganese quenching technique was not affected by perfusion of 1 mM H₂O₂. On the other hand, 10 mM H₂O₂ induced more rapid Ca²⁺ accumulation and facilitated Ca²⁺ entry from extracellular fluid. Ca²⁺ refill into intracellular Ca²⁺ store and inositol 1,4,5-trisphosphate (1 µM)-induced Ca²⁺ release from Ca²⁺ store was not affected by 1 mM H₂O₂ in permeabilized cells. Ca²⁺ efflux through plasma membrane Ca²⁺-ATPase (PMCA) was markedly blocked by 1 mM H₂O₂ in thapsigargin-treated intact acinar cells. Antioxidants, either catalase or dithiothreitol, completely protected H₂O₂-induced Ca²⁺ accumulation through PMCA inactivation. From the above results, we suggest that excessive production of H₂O₂ under pathological conditions may lead to cytosolic Ca²⁺ accumulation and that the primary mechanism of H₂O₂-induced Ca²⁺ accumulation is likely to inhibit Ca²⁺ efflux through PMCA rather than mobilize Ca²⁺ ions from extracellular medium or intracellular stores in mouse parotid acinar cells.

New saliva secretion model based on the expression of Na+-K+ pump and K+ channels in the apical membrane of parotid acinar cells

The plasma membrane of parotid acinar cells is functionally divided into apical and basolateral regions. According to the current model, fluid secretion is driven by transepithelial ion gradient, which facilitates water movement by osmosis into the acinar lumen from the interstitium. The osmotic gradient is created by the apical Cl^- efflux and the subsequent paracellular Na⁺ transport. In this model, the Na⁺-K⁺ pump is located exclusively in the basolateral membrane and has essential role in salivary secretion, since the driving force for Cl^- transport via basolateral Na⁺-K⁺-2Cl^- cotransport is generated by the Na⁺-K⁺ pump. In addition, the continuous electrochemical gradient for Cl^- flow during acinar cell stimulation is maintained by the basolateral K⁺ efflux. However, using a combination of single-cell electrophysiology and Ca²⁺-imaging, we demonstrate that photolysis of Ca²⁺ close to the apical membrane of parotid acinar cells triggered significant K⁺ current, indicating that a substantial amount of K⁺ is secreted into the lumen during stimulation. Nevertheless, the K⁺ content of the primary saliva is relatively low, suggesting that K⁺ might be reabsorbed through the apical membrane. Therefore, we investigated the localization of Na⁺-K⁺ pumps in acinar cells. We show that the pumps appear evenly distributed throughout the whole plasma membrane, including the apical pole of the cell. Based on these results, a new mathematical model of salivary fluid secretion is presented, where the pump reabsorbs K⁺ from and secretes Na⁺ to the lumen, which can partially supplement the paracellular Na⁺ pathway.

Region-specific proteolysis differentially regulates type 1 inositol 1,4,5-trisphosphate receptor activity

The inositol 1,4,5 trisphosphate receptor (IP₃R) is an intracellular Ca²⁺ release channel expressed predominately on the membranes of the endoplasmic reticulum. IP₃R1 can be cleaved by caspase or calpain into at least two receptor fragments. However, the functional consequences of receptor fragmentation are poorly understood. Our previous work has demonstrated that IP₃R1 channels, formed following either enzymatic fragmentation or expression of the corresponding complementary polypeptide chains, retain tetrameric architecture and are still activated by IP₃ binding despite the loss of peptide continuity. In this study, we demonstrate that region-specific receptor fragmentation modifies channel regulation. Specifically, the agonist-evoked temporal Ca²⁺ release profile and protein kinase A modulation of Ca²⁺ release are markedly altered. Moreover, we also demonstrate that activation of fragmented IP₃R1 can result in a distinct functional outcome. Our work suggests that proteolysis of IP₃R1 may represent a novel form of modulation of IP₃R1 channel function and increases the repertoire of Ca²⁺ signals achievable through this channel.

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.

Modelling the effects of calcium waves and oscillations on saliva secretion

An understanding of Ca(2+) signalling in saliva-secreting acinar cells is important, as Ca(2+) is the second messenger linking stimulation of cells to production of saliva. Ca(2+) signals affect secretion via the ion channels located both apically and basolaterally in the cell. By approximating Ca(2+) waves with periodic functions on the apical and basolateral membranes, we isolate individual wave properties and investigate them for their effect on fluid secretion in a mathematical model of the acinar cell. Mean Ca(2+) concentration is found to be the most significant property in signalling secretion. Wave speed was found to encode a range of secretion rates. Ca(2+) oscillation frequency and amplitude had little effect on fluid secretion.

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.

Phenotypic changes in mouse pancreatic stellate cell Ca2+ signaling events following activation in culture and in a disease model of pancreatitis

The specific characteristics of intracellular Ca 2+ signaling and the downstream consequences of these events were investigated in mouse pancreatic stellate cells (PSC) in culture and in situ using multiphoton microscopy in pancreatic lobules. PSC undergo a phenotypic transformation from a quiescent state to a myofibroblast-like phenotype in culture. This is believed to parallel the induction of an activated state observed in pancreatic disease such as chronic pancreatitis and pancreatic cancer. By day 7 in culture, the complement of cell surface receptors coupled to intracellular Ca 2+ signaling was shown to be markedly altered. Specifically, protease-activated receptors (PAR) 1 and 2, responsive to thrombin and trypsin, respectively, and platelet-derived growth factor (PDGF) receptors were expressed only in activated PSC (aPSC). PAR-1, ATP, and PDGF receptor activation resulted in prominent nuclear Ca 2+ signals. Nuclear Ca 2+ signals and aPSC proliferation were abolished by expression of parvalbumin targeted to the nucleus. In pancreatic lobules, PSC responded to agonists consistent with the presence of only quiescent PSC. aPSC were observed following induction of experimental pancreatitis. In contrast, in a mouse model of pancreatic disease harboring elevated K-Ras activity in acinar cells, aPSC were present under control conditions and their number greatly increased following induction of pancreatitis. These data are consistent with nuclear Ca 2+ signaling generated by agents such as trypsin and thrombin, likely present in the pancreas in disease states, resulting in proliferation of "primed" aPSC to contribute to the severity of pancreatic disease.

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

Morphological changes of parotid gland in experimental hyperlipidemia

Objective. The aim of this study was to investigate the role of hyperlipidemia in the microstructure of parotid gland and its possible amelioration through statin treatment on Wistar rats. Methods. Forty Wistar rats (111.06 ± 3.36 g) were divided into 4 groups (A1, A2 controls, B1, B2 experimental). Groups A1 and A2 consumed normal cereal rodents diet during the experimental hyperlipidemic mixture. The A2 and B2 were treated with simvastatin (Zocor) 40 mg/kg/daily p.o. for 3 months. Results. Total cholesterol, triglycerides, high density lipoproteins, and low density lipoproteins were increased in groups B1 and B2 while the parotid weight was decreased. The histological findings demonstrated changes in the parotid gland morphology of the B1 and B2, such as the presence of chronic inflammation, fibrosis, lipocytes, and foci of lymphocytic infiltration. Conclusions. The influence of statin tended to predominate over the chronic inflammation while the lipocytes were decreased and remodelling of the parotid's structure occurred.

A dynamic model of saliva secretion

We construct a mathematical model of the parotid acinar cell with the aim of investigating how the distribution of K(+) and Cl(-) channels affects saliva production. Secretion of fluid is initiated by Ca(2+) signals acting on Ca(2+) dependent K(+) and Cl(-) channels. The opening of these channels facilitates the movement of Cl(-) ions into the lumen which water follows by osmosis. We use recent results into both the release of Ca(2+) from internal stores via the inositol (1,4,5)-trisphosphate receptor (IP(3)R) and IP(3) dynamics to create a physiologically realistic Ca(2+) model which is able to recreate important experimentally observed behaviours seen in parotid acinar cells. We formulate an equivalent electrical circuit diagram for the movement of ions responsible for water flow which enables us to calculate and include distinct apical and basal membrane potentials to the model. We show that maximum saliva production occurs when a small amount of K(+) conductance is located at the apical membrane, with the majority in the basal membrane. The maximum fluid output is found to coincide with a minimum in the apical membrane potential. The traditional model whereby all Cl(-) channels are located in the apical membrane is shown to be the most efficient Cl(-) channel distribution.

Studying isoform-specific inositol 1,4,5-trisphosphate receptor function and regulation

Inositol 1,4,5-trisphosphate receptors (InsP3R) are a family of ubiquitously expressed intracellular Ca2+ channels. Isoform-specific properties of the three family members may play a prominent role in defining the rich diversity of the spatial and temporal characteristics of intracellular Ca2+ signals. Studying the properties of the particular family members is complicated because individual receptor isoforms are typically never expressed in isolation. In this article, we discuss strategies for studying Ca2+ release through individual InsP3R family members with particular reference to methods applicable following expression of recombinant InsP3R and mutant constructs in the DT40-3KO cell line, an unambiguously null InsP3R expression system.

Receptor-mediated signal transduction pathways and the regulation of pancreatic acinar cell function

PURPOSE OF REVIEW

Recent studies on pancreatic acinar cell function have led to a more detailed understanding of the signal transduction mechanisms regulating digestive enzyme synthesis and secretion as well as pancreatic growth. This review identifies and puts into context these recent studies, which further understanding in these areas.

RECENT FINDINGS

Receptors present on acinar cells, particularly those for cholecystokinin and secretin, have been better characterized as to the molecular nature of the ligand-receptor interaction. Other reports have described the receptors for natriuretic peptides and fibroblast growth factor on acini. Intracellular Ca(2+) signaling remains at the center of stimulus secretion coupling and its regulation by inositol 1,4,5-trisphosphate, nicotinic acid adenine dinucleotide phosphate and cyclic ADP-ribose has been further defined. Work downstream of intracellular mediators has focused on molecular mechanisms of exocytosis particularly involving small G proteins, soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins and cytoskeletal proteins. Considerable progress has been made defining the complex in acinar cells and its regulation. In addition to secretion, recent studies have further defined the regulation of pancreatic growth both in adaptive regulation to diet and hormones, particularly cholecystokinin, and in the regeneration that occurs after pancreatitis or partial pancreatectomy. This regulation involves calcineurin-nuclear factor of activated T cells, mammalian target of rapamycin, mitogen-activated protein kinase, Notch signaling pathways as well as various tyrosine kinases.

SUMMARY

Understanding the mechanisms that regulate pancreatic acinar cell function is contributing to our knowledge of normal pancreatic function and alterations in diseases such as pancreatitis and pancreatic cancer.