Measurement of cytosolic free Ca2+ in individual pancreatic acini

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.

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.

The pancreas-specific form of secretory pathway calcium ATPase 2 regulates multiple pathways involved in calcium homeostasis

Acinar cell exocytosis requires spatiotemporal Ca²⁺ signals regulated through endoplasmic reticulum (ER) stores, Ca²⁺ATPases, and store-operated Ca²⁺ entry (SOCE). The secretory pathway Ca²⁺ATPase 2 (SPCA2) interacts with Orai1, which is involved in SOCE and store independent Ca²⁺ entry (SICE). However, in the pancreas, only a C-terminally truncated form of SPCA2 (termed SPAC2C) exists. The goal of this study was to determine if SPCA2C effects Ca²⁺ homeostasis in a similar fashion to the full-length SPCA2. Using epitope-tagged SPCA2C (SPCA2C^FLAG) expressed in HEK293A cells and Fura2 imaging, cytosolic [Ca²⁺] was examined during SICE, SOCE and secretagogue-stimulated signaling. Exogenous SPCA2C expression increased resting cytosolic [Ca²⁺], Ca²⁺ release in response to carbachol, ER Ca²⁺ stores, and store-mediated and independent Ca²⁺ influx. Co-IP detected Orai1-SPCA2C interaction, which was altered by co-expression of STIM1. Importantly, SPCA2C's effects on store-mediated Ca²⁺ entry were independent of Orai1. These findings indicate SPCA2C influences Ca²⁺ homeostasis through multiple mechanisms, some of which are independent of Orai1, suggesting novel and possibly cell-specific Ca²⁺ regulation.

Menadione-induced oxidative stress inhibits cholecystokinin-stimulated secretion of pancreatic acini by cell dehydration

The present study evaluated the effects of free radicals generated by menadione on morphology and function of pancreatic acinar cells focusing on enzyme secretion, stimulus-secretion coupling, and cell hydration. Various experiments evaluated morphology and function of isolated rat pancreatic acinar cells exposed to menadione. Menadione instantaneously generated free radicals (luminol and deoxyribose assays) followed by a time-dependent cell injury (uptake of trypan blue). Early ultrastructural changes included vacuolization and alterations of mitochondria, endoplasmic reticulum, and nucleus. Menadione caused a rapid glutathione oxidation followed by a depletion in reduced glutathione. An increase in lipid peroxides and a depletion of adenosine triphosphate were seen only after 30-60 minutes. Menadione markedly inhibited amylase release stimulated by cholecystokinin (CCK) and carbachol and simultaneously caused cell shrinkage after a few minutes. Similar degrees of cell shrinkage induced by hyperosmolar incubation and by menadione inhibited amylase secretion to a similar extent. CCK binding and its effect on calcium and inositol 1,4,5-trisphosphate (IP3) were not affected by menadione. Menadione (without CCK) induced an instantaneous increase of intracellular calcium followed by a slow constant increase. In single cells, menadione induced calcium oscillations with a frequency lower than that seen after CCK stimulation. Some morphologic and functional alterations owing to menadione-induced oxidative stress may be caused by adenosine triphosphate and glutathione depletion, lipid peroxidation, and changes in cytosolic calcium. The marked inhibition of secretagogue-stimulated enzyme secretion owing to menadione may be mediated to a large part by cell dehydration, whereas classical steps of stimulus-secretion coupling like receptor binding, calcium release, and IP3 generation remained unchanged.

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

Multiple kinetic components and the Ca2+ requirements of exocytosis

The use of caged-Ca2+ compounds to stimulate Ca(2+)-dependent exocytosis has substantially increased our understanding of this complex process. By this approach, the existence of multiple kinetic components of exocytosis has been established. These components may correspond to a series of sequential steps that lead to a single fusion-ready state (sequential mechanism) or, alternatively, to heterogeneity in secretory vesicles or in fusion-ready states (parallel mechanism). It is suggested that both of these mechanisms can underlie exocytosis of a single type of vesicle (mixed sequential-parallel mechanism). Studies with caged-Ca2+ compounds have also indicated that the Ca2+ requirement for exocytosis is substantially greater than that suggested by conventional methodologies. This discrepancy is mainly attributable to the underestimation, by imaging studies with high-affinity Ca2+ indicators (due to dye saturation), of the local increases in cytosolic Ca2+ concentration that trigger the exocytosis of individual vesicles. The effects of local saturation of such indicators are explored by means of a simple theory.

Synchronized calcium spiking resulting from spontaneous calcium action potentials in monolayers of NRK fibroblasts

The correlation between the intracellular Ca2+ concentration ([Ca2+]i) and membrane potential in monolayers of density-arrested normal rat kidney (NRK) fibroblasts was investigated. Using the fluorescent probe Fura-2, spontaneous repetitive spike-like increases in [Ca2+]i (Ca2+ spikes) were observed that were synchronised throughout the entire monolayer. Ca2+ spikes disappeared in Ca(2+)-free solutions and could be blocked by the L-type Ca2+ channel antagonist felodipine. Simultaneous measurements of [Ca2+]i and membrane potential showed that these Ca2+ spikes were paralleled by depolarisations of the plasma membrane. Using patch clamp measurements, action potential-like depolarisations consisting of a fast spike depolarisation followed by a plateau phase were seen with similar kinetics as the Ca2+ spikes. The action potentials could be blocked by L-type Ca2+ channel blockers and were dependent on extracellular Ca2+. The plateau phase was predominantly determined by a Cl- conductance and was dependent on intracellular Ca2+. The presence of voltage-dependent L-type Ca2+ channels in NRK cells was confirmed by patch clamp measurements in single cells. It is concluded that monolayers of density-arrested NRK fibroblasts exhibit spontaneous Ca2+ action potentials leading to synchronised Ca2+ spiking. This excitability of monolayers of fibroblasts may represent a novel Ca2+ signaling pathway in electrically coupled fibroblasts, cells that were hitherto considered to be inexcitable.

High concentrations of cholecystokinin octapeptide suppress protein kinase C activity in guinea pig pancreatic acini

In pancreatic acini, calcium-mobilizing agents increase intracellular calcium and stimulate the production of diacylglycerol, and then activate protein kinase C (PKC). However, there are few studies which have examined the activation of PKC in intact acini. To examine the activation of PKC in intact acini by calcium-mobilizing agents, we measured the binding of [3H]phorbol-12,13-dibutyrate (PDBu) to intact acini. Acini were incubated with 10 nM [3H]PDBu at 25 degrees C with or without agents. The binding reactions were terminated by filtration. The filters were counted by a scintillation counter after washing. Acini possessed a single class of binding sites to PDBu, with Kd = 70 nM. CCK-8 and carbachol upregulated the binding affinity of PKC to PDBu in the acini. The ability of calcium-mobilizing agents to increase binding of [3H]PDBu to the acini had a close correlation to their ability to stimulate the amylase secretion from the acini, and higher concentrations of CCK-8 for amylase secretion suppressed binding of [3H]PDBu to the acini. 8Br-cAMP, 8Br-cGMP, and calcium ionophore did not inhibit the maximal activation of PKC induced by CCK-8. The calmodulin inhibitor W7 did not reverse the inhibitory effect of higher concentrations of CCK-8 on PKC activation. These results indicate that calcium-mobilizing agents upregulate the binding affinity of PKC to PDBu in intact acini, and that higher concentrations of CCK-8 for amylase secretion may activate the intracellular mechanism that inhibits PKC activity in acini. This inhibitory mechanism was mediated by some other mechanism other than cAMP-, cGMP-, calcium- and calmodulin-dependent mechanisms.

Cellular and subcellular calcium signaling in gastrointestinal epithelium

Ca2+ is a critical second messenger in virtually all cell types, including the various epithelial cell types within the digestive system. When measured in cell populations, Ca2+ signals usually appear as a single transient or prolonged elevation. In individual epithelial cells, signaling patterns often vary from cell to cell and may contain more complex features such as Ca2+ oscillations. Subcellular Ca2+ signals show a further level of complexity, such as Ca2+ waves, and may relate to the polarized structure and function of epithelial cells. The approaches to detect cytosolic Ca2+ signals, the patterns and mechanisms of Ca2+ signaling, and the role of such signals in regulating the function of polarized epithelium within the gastrointestinal tract, pancreas, and liver are reviewed in this report.

Mechanisms and function of intercellular calcium signaling

Intercellular Ca2+ waves initiated by mechanical or chemical stimuli propagate between cells via gap junctions. The ability of a wide diversity of cells to display intercellular Ca2+ waves suggests that these Ca2+ waves may represent a general mechanism by which cells communicate. Although Ca2+ may permeate gap junctions, the intercellular movement of Ca2+ is not essential for the propagation of Ca2+ waves. The messenger that moves from one cell to the next through gap junctions appears to be IP3 and a regenerative mechanism for IP3 may be required to effect multicellular communication. Extracellularly mediated Ca2+ signaling also exists and this could be employed to supplement or replace gap junctional communication. The function of intercellular Ca2+ waves may be the coordination of cooperative cellular responses to local stimuli.

Receptor-operated Ca2+ signaling and crosstalk in stimulus secretion coupling

In the cells of higher eukaryotic organisms, there are several messenger pathways of intracellular signal transduction, such as the inositol 1,4,5-trisphosphate/Ca2+ signal, voltage-dependent and -independent Ca2+ channels, adenylate cyclase/cyclic adenosine 3',5'-monophosphate, guanylate cyclase/cyclic guanosine 3',5'-monophosphate, diacylglycerol/protein kinase C, and growth factors/tyrosine kinase/tyrosine phosphatase. These pathways are present in different cell types and impinge on each other for the modulation of the cell function. Ca2+ is one of the most ubiquitous intracellular messengers mediating transcellular communication in a wide variety of cell types. Over the last decades it has become clear that the activation of many types of cells is accompanied by an increase in cytosolic free Ca2+ concentration ([Ca2+]i) that is thought to play an important part in the sequence of events occurring during cell activation. The Ca2+ signal can be divided into two categories: receptor- and voltage-operated Ca2+ signal. This review describes and integrates some recent views of receptor-operated Ca2+ signaling and crosstalk in the context of stimulus-secretion coupling.

Effect of vasoactive intestinal peptide, carbachol and other agonists on the membrane voltage of pancreatic duct cells

The regulation of pancreatic exocrine secretion involves hormonal, neural and neurohormonal components. Many agonists are known to be effective in pancreatic acinar cells, but less is known about the ducts. Therefore, we wanted to investigate the influence of various agonists on isolated perfused pancreatic ducts and, as a physiological response, we measured the basolateral membrane voltage of the duct cells (Vbl) with microelectrodes. Pancreatic ducts were dissected from pancreas of normal rats and bathed in a HCO(3-)(-containing solution. Under control conditions, the average Vbl was between -50 and -70 mV. Vasoactive intestinal peptide (VIP) and carbachol (CCH) reversibly depolarized Vbl when applied to the bath. VIP (9 x 10(-9) mol/l) depolarized Vbl from -72 +/- 3 mV to -53 +/- 3 mV (n = 20) and CCH (10(-5) mol/l) from -62 +/- 3 to -35 +/- 4 mV (n = 10). Furthermore, a decrease of the Cl- concentration in the lumen led to an increase of VIP-induced depolarization of Vbl, suggesting that a luminal Cl- conductance was increased. Cholecystokinin (CCK, 10(-10)-10(-7) mol/l) and bombesin (10(-8), 10(-5) mol/l), which stimulate pancreatic exocrine secretion in acini or whole glands, showed no significant effect on Vbl of the duct cells tested in our preparation (n = 7, 6). Neurotensin (10(-8) mol/l) had a marked depolarizing effect in two out of ten cases; Vbl depolarized from about -65 mV to -29 mV and the effect was reversible. Substance P (2 x 10(-7) mol/l), alone or in combination with secretin, had no effect on Vbl of the tested duct cells (n = 11).(ABSTRACT TRUNCATED AT 250 WORDS)

Receptor-evoked Ca2+ mobilization in pancreatic acinar cells: evidence for a regulatory role of protein kinase C by a mechanism involving the transition of high-affinity receptors to a low-affinity state

In order to establish a regulatory role for phosphoproteins in the process of receptor-stimulated Ca2+ mobilization, isolated pancreatic acinar cells, loaded with fura-2, were stimulated with cholecystokinin-octapeptide (CCK8) in the presence of either staurosporine, a general inhibitor of protein kinase activity, or 12-O-tetradecanoylphorbol 13-acetate (TPA), an activator of protein kinase C. Staurosporine alone did not affect the average free cytosolic Ca2+ concentration ([Ca2+]i,av) in a suspension of acinar cells. However, in the presence of 1.0 microM staurosporine the stimulatory effect of submaximal concentrations of CCK8 was significantly enhanced. The potentiating effect of the inhibitor was paralleled by the increased production of inositol 1,4,5-trisphosphate. In addition, staurosporine evoked a transient increase in [Ca2+]i,av in cells prestimulated with a submaximal concentration of CCK8. The data obtained with staurosporine indicate that CCK8-stimulated phosphorylations exert a negative feedback role in the process of receptor-mediated Ca2+ mobilization. The involvement of protein kinase C was investigated by studying the effects of TPA on CCK8-induced Ca2+ mobilization. The phorbol ester induced a rightward shift of the dose/response curve for the CCK8-evoked increase in [Ca2+]i,av, which, in contrast to the unlimited shift obtained with the receptor antagonist D-lorglumide, reached a maximum of approximately one order of a magnitude at 10 nM TPA. The inhibitory effect of TPA was completely overcome by CCK8 at concentrations at or beyond 10 nM. This observation has led to the hypothesis that protein kinase C, directly or indirectly, converts the CCK receptor from a high-affinity state to a low-affinity state. Substantial evidence in favour of this hypothesis was provided by the observation that the increase in [Ca2+]i,av evoked by the CCK8 analogue JMV-180, which acts as an agonist at the high-affinity receptor, was completely blocked by TPA pretreatment. TPA also evoked a rightward shift of the dose/response curve for the carbachol-induced increase in [Ca2+]i,av, indicating that the protein-kinase-C-mediated transition of the affinity state of receptors is a more general phenomenon. In the presence of submaximal CCK8 concentrations, TPA dose-dependently decreased the poststimulatory elevated [Ca2+]i,av to the prestimulatory level, indicating that protein kinase C also inhibits the process of sustained Ca2+ mobilization. The effects of TPA were counteracted by staurosporine, suggesting that the effects of the inhibitor itself were indeed due to inhibition of the receptor-mediated activation of protein kinase C.(ABSTRACT TRUNCATED AT 400 WORDS)

Dose-dependent recruitment of pancreatic acinar cells during receptor-mediated calcium mobilization

Digital-imaging microscopy of Fura-2-loaded rabbit pancreatic acinar cells was used to simultaneously monitor the cholecystokinin-octapeptide (CCK8)-induced changes in free cytosolic Ca2+ concentration, [Ca2+]i, in large numbers of individual acinar cells. CCK8 typically evoked a switchlike increase in [Ca2+]i which was preceded by a concentration-dependent latency. The threshold concentration for the CCK8-induced rise in [Ca2+]i differed greatly among individual acinar cells, resulting in the dose-dependent recruitment of acinar cells in terms of CCK8-induced Ca2+ mobilization. The EC50 value for CCK8-induced cell-recruitment was estimated to be 15 pM. The hormone was equally potent in stimulating amylase secretion from acinar cells in suspension. At a CCK8 concentration of 100 pM, virtually all cells responded to the hormone with an increase in [Ca2+]i and the number of responding cells remained unchanged upon further increase of the CCK8 concentration. The dose-response curve for cell-recruitment coincides with that of the apparent [Ca2+]i increase in a suspension of acinar cells. This suggests that the most likely interpretation of the latter dose-response curve is not a generalized increase in [Ca2+]i but an increase in the number of responding cells. The initial rise in [Ca2+]i, which was transient by nature, was followed by repetitive [Ca2+]i transients of long duration. The dose-response curve for the effect of CCK8 on the percentage of acinar cells displaying these distinct [Ca2+]i oscillations was biphasic. A maximum of 99% of the cells showing oscillatory behaviour was reached at 100 pM CCK8, beyond which concentration the number of oscillating cells dose-dependently decreased again. The latter decrease was paralleled by a dose-dependent increase of the percentage responding but non-oscillating cells, indicating that beyond 100 pM CCK8 an increasing number of acinar cells became desensitized towards hormonal induction of oscillatory changes in [Ca2+]i. CCK8 was approximately 100-fold more potent in reducing the percentage of oscillating cells than in inhibiting amylase secretion. Oscillating acinar cells responded to a stepwise increase of the medium CCK8 concentration with a rapid change in amplitude and frequency of the oscillations. Thus, with increasing CCK8 concentration the frequency gradually increased, whereas the amplitude only slightly increased at first, reached a maximum, and decreased thereafter. In some cells full extinction was reached. Again, large differences in dose-dependency were observed among individual acinar cells. The observations presented demonstrate the existence of a marked functional heterogeneity among pancreatic acinar cells in terms of CCK8-induced Ca2+ mobilization.

Exendin-4, a new peptide from Heloderma suspectum venom, potentiates cholecystokinin-induced amylase release from rat pancreatic acini

We examined the actions of exendin-4, a new peptide isolated from Heloderma suspectum venom, on dispersed acini from rat pancreas. Exendin-4 caused a 3-fold increase in cAMP but did not alter cellular calcium concentration. Exendin-4-induced increases in cAMP were inhibited by an exendin-receptor antagonist, exendin (9-39)NH2, but not by VIP-receptor antagonists. Whereas up to 1 microM exendin-4 alone did not alter amylase release, potentiation of enzyme release was observed when the peptide (greater than 30 pM) was combined with cholecystokinin. Potentiation of amylase release was also observed when exendin-4 was combined with carbamylcholine, bombesin or a calcium ionophore, A23187. These results indicate that stimulation of exendin receptors on rat pancreatic acini causes an increase in cellular cAMP. Although this increase in cAMP alone does not result in amylase release, combination of exendin-4 with agents that increase cell calcium results in potentiation of amylase release.

Calcium oscillations in guinea-pig pancreatic acinar cells exposed to carbachol, cholecystokinin and substance P

1. Cytoplasmic Ca2+ ([Ca2+]i) responses were studied in guinea-pig pancreatic acinar cells during stimulation with cholecystokinin octapeptide (CCK-8), substance P (SP) and carbachol. 2. Individual cells exhibited [Ca2+]i responses to all three agonists. 3. In the absence of external Ca2+, all the agonists initiated [Ca2+]i peaks which, particularly at high agonist concentrations, rapidly declined. 4. SP induced repetitive monophasic [Ca2+]i transients which started from basal [Ca2+]i even after elevation of the external Ca2+ concentration. 5. CCK-8 triggered similar oscillations, which particularly at high agonist concentration or after elevating external Ca2+ became superimposed upon a sustained elevation of [Ca2+]i. 6. Carbachol-induced oscillations were more complex with [Ca2+]i transients superimposed on slower waves. 7. At high carbachol concentrations or elevation of external Ca2+ the slow waves fused into a sustained increase of [Ca2+]i. 8. The protein kinase C (PKC) activator 12-O-tetradecanoylphorbol-13-acetate attenuated the agonist-induced [Ca2+]i responses, and this effect was reversed by the PKC activator staurosporine. 9. The results indicate that oscillations of [Ca2+]i induced by SP, CCK-8 and carbachol involve intracellular mobilization of Ca2+. 10. CCK-8 and carbachol also cause a rise of [Ca2+]i by a mechanism more directly dependent on the presence of extracellular Ca2+. 11. In the case of carbachol the latter component is subject to oscillatory control. 12. The transition from oscillatory [Ca2+]i to sustained increase may be associated with inhibition of amylase release.

Cytoplasmic Ca2+ signals evoked by activation of cholecystokinin receptors: Ca(2+)-dependent current recording in internally perfused pancreatic acinar cells

The effects on the cytosolic Ca2+ concentration of activating cholecystokinin receptors on single mouse pancreatic acinar cells have been investigated using patch-clamp whole-cell recording of Ca(2+)-dependent Cl- current. We used the nonsulphated octapeptide of cholecystokinin (CCK8-NS) since the effects of even high concentrations were rapidly reversible which was not the case for the sulphated octapeptide. A submaximal concentration of CCK8-NS (10 nM) evoked a current response consisting of short-lasting (a few seconds) spikes, and some of these spikes were seen to trigger larger and longer (about half a minute) current pulses. At a higher concentration (100 nM) CCK8-NS evoked smooth and sustained responses. The effect of CCK8-NS was almost abolished when the internal perfusion solution contained a high concentration of the Ca2+ chelator EGTA (5 mM). The responses evoked by CCK8-NS were independent of the presence of Ca2+ in the external solution at least for the first 5 min of stimulation. Internal perfusion with GTP-gamma-S markedly potentiated the effect of CCK8-NS or at a higher concentration itself induced responses very similar to those normally evoked by CCK8-NS. Caffeine added to the external solution at a low concentration (0.2-1 mM) enhanced weak CCK8-NS responses, whereas high caffeine concentrations always inhibited the CCK8-NS-evoked responses. These inhibitory caffeine effects were quickly reversible. Forskolin evoked a similar inhibitory effect. Intracellular heparin (200 micrograms/ml) infusion markedly inhibited the response to CCK8-NS stimulation. We conclude that the primary effect of activating CCK receptors is to induce inositoltrisphosphate (IP3) production.(ABSTRACT TRUNCATED AT 250 WORDS)

PACAP-38, a novel peptide from ovine hypothalamus, is a potent modulator of amylase release from dispersed acini from rat pancreas

Despite studies indicating the presence of specific pancreatic acinar receptors for PACAP-38, a peptide that was recently isolated from ovine hypothalamus, the actions of the new peptide on pancreatic enzyme secretion have not been examined. The present study demonstrates that in terms of cAMP production and amylase release from dispersed acini from rat pancreatic acini, PACAP-38 and an N-terminal fragment, PACAP-27, have the same potency and efficacy as vasoactive intestinal peptide (VIP). As with VIP, these actions are potentiated by adding an inhibitor of cyclic nucleotide phosphodiesterase, and combination of PACAP-38 with bombesin, CCK-8, carbachol or the calcium ionophore A23187 results in 2-fold augmentation of the secretory actions of these agents. Inhibition of PACAP-38-induced cAMP production and amylase release by two VIP-receptor antagonists indicates that the secretory effects of PACAP-38 are mediated by interaction with VIP receptors. PACAP-38, a new brain-gut peptide, may be a physiological modulator of pancreatic enzyme secretion.

Synchronous transporting activity in epithelial cells in relation to intracellular calcium concentration

Cultured monolayers of human sweat-gland epithelia have been used to measure electrogenic sodium transport, as short-circuit current, and intracellular Ca2+ concentration ([Ca]i) from Fura-2 fluorescence. The sodium currents in response to the agonists lysylbradykinin, histamine and carbachol show oscillatory behaviour in the 1-2 per minute frequency range. The oscillations can be terminated either by using specific antagonists or with amiloride, which prevents sodium entry into the epithelium. Oscillatory behaviour is also seen when [Ca]i is measured and occurs in the same frequency range. Sodium transport in these cultured epithelia is thought to result from an increase in [Ca]i, which in turn activates calcium-sensitive potassium channels, so increasing the electrochemical gradient for sodium entry. The oscillatory behaviour implies that the epithelial cells behave in synchrony to increase [Ca]i, so inducing synchronous changes in sodium current. It is shown that the behaviour is not unique to sodium-absorbing epithelia, and the possible utility of synchronous behaviour in epithelial tissues is discussed.

Receptor-activated cytoplasmic Ca2+ spikes in communicating clusters of pancreatic acinar cells

The transmission of receptor-generated cytoplasmic Ca2+ signals between communicating pancreatic acinar cells has been investigated by comparing patch-clamp recordings of Ca(2+)-dependent Cl- current in internally perfused single cells and small multi-cell clusters. Acetylcholine (50 nM) generates shortlasting repetitive spikes of Ca(2+)-dependent current and these spikes are not transmitted to neighbouring cells. Cholecystokinin octapeptide (5 pM) also generates repetitive spikes, but a significant proportion of these trigger longer and larger pulses of Ca(2+)-dependent current and these waves can easily spread from cell to cell. In pancreatic acinar units it is therefore possible to observe both local Ca2+ signals confined to the cell of its origin as well as Ca2+ signals that spread through communicating junctions to all cells in the unit.

Characteristics of fluid secretion from isolated rat pancreatic ducts stimulated with secretin and bombesin

1. Micropuncture techniques were used to study the cellular mechanisms of fluid secretion by interlobular ducts isolated from the pancreas of copper-deficient rats. 2. Perifusing ducts with a calcium-free buffer containing 5 mM-EGTA reduced the volume of fluid secreted in the presence of 10 nM-bombesin by 62%, whereas fluid secretion measured in the presence of 10 nM-secretin was reduced by only 26%. 3. The anion selectivities of the fluid secretions evoked by secretin and bombesin were different. The anion sequence for secretin was: Br- = I- = NO3- = Cl- (1.0) much greater than thiocyanate = gluconate (0.3); whereas the sequence for bombesin was: Br- = Cl- (1.0) greater than I- = NO3- (0.6) greater than thiocyanate = gluconate (approximately 0.3). 4. SITS (4-acetamido-4'-isothiocyanatostilbene-2,2'-disulphonic acid; mM), reduced fluid secretion measured in the presence of bombesin by 61%, but had no effect on the response to secretin. 5. The K+ channel blockers, barium (3 mM) and tetraethylammonium (TEA; 10 mM), inhibited fluid secretion measured in the presence of both secretin and bombesin by between 52 and 66%. 6. From these results, we conclude that secretin and bombesin may utilize different intracellular signalling pathways and, furthermore, may activate different anion secretory mechanisms within the pancreatic ductal epithelium. However, the effect of the potassium channel blockers is consistent with both peptides activating secretory mechanisms which are electrogenic, and which depend for their operation on potassium efflux across the basolateral membrane of the duct cell.

Calcium regulation and homeostasis

The use of techniques to visualize the stimulus-induced changes in [Ca2+]i that occur at the single cell level has revealed that intracellular Ca2+ signals can be remarkably organized in space (waves), as well as in time (oscillations). New insights are beginning to emerge into how these complex Ca2+ signals may be generated, and into how Ca2+ signals may be transmitted from cell to cell.

Acetylcholine and cholecystokinin induce different patterns of oscillating calcium signals in pancreatic acinar cells

Receptor-activated cytoplasmic calcium (Ca2+) oscillations have been investigated in single pancreatic acinar cells by microfluorimetry (Fura-2 as indicator). At submaximal concentrations of the agonists acetylcholine (ACh) and cholecystokinin octapeptide (CCK-8), both give rise to oscillatory changes in the cytosolic free calcium concentration ([Ca2+]i). The patterns of oscillations are markedly and consistently different for each of these two agonists. The ACh induced oscillations are superimposed upon a median elevation in background [Ca2+]i. The CCK-8 induced oscillations are of longer duration with [Ca2+]i returning to prestimulus levels between the discrete spikes. The ACh induced oscillations are rapidly abolished upon removal of extracellular Ca2+ while the CCK-8 induced oscillations persist for many minutes in the absence of external Ca2+. The CCK-8, but not the ACh, induced oscillations are increased in duration by the protein kinase C (PKC) inhibitor staurosporine and abolished by the PKC activating phorbol ester PMA. It is clear that CCK-8 and ACh do not activate receptor transduction mechanisms in an identical manner to generate oscillating [Ca2+]i signals.

Organization of intracellular calcium signals generated by inositol lipid-dependent hormones

Recent studies at the single cell level have demonstrated hitherto unsuspected complexities in the organization of intracellular Ca2+ homeostasis in both the temporal and spatial domains. Activation of receptors coupled to the phosphoinositide signalling system has been shown to generate [Ca2+]i oscillations in many cell types. These oscillations display diverse patterns, with variations in oscillation amplitude, latency and frequency which are often tissue and/or agonist dose specific. Furthermore, increases in [Ca2+]i can either occur uniformly or originate from a specific region and propagate throughout the cell in the form of a Ca2+ wave. The significance and underlying mechanisms responsible for these phenomena are discussed.

Synchronized repetitive spikes in cytoplasmic calcium in confluent monolayers of human umbilical vein endothelial cells

Synchronized repetitive spikes in cytoplasmic free calcium concentration, [Ca2+]i, are evoked by histamine in confluent monolayers of human endothelial cells. The repetitive spikes, which are apparently dependent upon the establishment of cell coupling, are also induced by caffeine, indicating that they may be due to an oscillatory release of Ca2+ from the endoplasmic reticulum, and may not involve oscillations in inositol phosphates. It is suggested that synchronized repetitive spikes in [Ca2+]i might lead to oscillatory release of endothelial-derived substances such as prostacyclin, nitric oxide and endothelin, which have potent effects on the vascular system.

The molecular basis of enzyme secretion

Acinar cells are one of the best studied models of exocytotic secretion. A number of different hormones and neurotransmitters interact with specific membrane receptors, and it is commonly held that pancreatic secretagogues stimulate enzyme release via the elevation of either cytosolic free Ca2+ or cellular cyclic adenosine monophosphate. The discovery of the pivotal role played by phospholipid metabolism in the chain of events leading to secretion, together with the introduction of sensitive techniques to monitor cytosolic free Ca2+, has generated a series of studies that have challenged this classical model. Thus, several observations in pancreatic acini as well as other cell types have argued against the notion that a generalized increase in cytosolic free Ca2+ represents a sufficient and necessary stimulus for exocytosis in nonexcitable cells. Furthermore, the demonstration that a single agonist activates multiple transduction pathways has served to refute the schematic view that receptor agonists activate only one second messenger system. The aim of this article is to review the recent advances in understanding the molecular and cellular mechanisms of signal transduction, with particular emphasis on the inositol lipid pathway, and to integrate this information into a new working model of enzyme secretion from acinar cells.

Effect of vasoactive intestinal peptide, bombesin and substance P on fluid secretion by isolated rat pancreatic ducts

1. We have used micropuncture techniques to study the regulation of fluid secretion by interlobular ducts isolated from the pancreas of copper-deficient rats. 2. Ducts isolated from different strains of Wistar rats exhibited quantitative differences in basal fluid secretion; however, secretion rates measured in the presence of secretin were similar. 3. Vasoactive intestinal peptide had no effect on fluid transport. 4. Bombesin stimulated fluid secretion, and this effect was abolished by removal of extracellular bicarbonate. 5. Substance P inhibited basal secretion, and that stimulated by bombesin and secretin. These inhibitory effects were partially reversed by spantide. 6. Substance P also inhibited fluid secretion stimulated by dibutyryl cyclic AMP and forskolin. This places the site of inhibition mediated by substance P at a point in the secretory mechanism distal to the generation of cyclic AMP. 7. We conclude that rat pancreatic duct cells possess receptors for bombesin and substance P, in addition to 'secretin-preferring' receptors. Since VIP had no effect on fluid transport, it is unlikely that 'VIP-preferring' receptors are present on rat duct cells.

Cytosolic free calcium spiking affected by intracellular pH change

The characteristics underlying cytosolic free calcium oscillation were evaluated by superfused dual wave-length microspectrofluorometry of fura-2-loaded single acinar cells from rat pancreas. Application of a physiological concentration of cholecystokinin octapeptide (CCK) (20 pM) induced a small basal increase in cytosolic free calcium concentration ([Ca2+]i) averaging 34 nM above the prestimulation level (69 nM) with superimposed repetitive Ca2+ spike oscillation. The oscillation amplitude averaged 121 nM above the basal increase in [Ca2+]i and occurred at a frequency of one pulse every 49 s. Although extracellular Ca2+ was required for maintenance of high frequency and amplitude of the spikes with increase in basal [Ca2+]i, the primary source utilized for oscillation was intracellular. The threshold of the peak [Ca2+]i amplitude for causing synchronized and same-sized oscillations was less than 300 nM. The [Ca2+]i oscillation was sensitive to intracellular pH (pHi) change. This is shown by the fact that the large pHi shift toward acidification (delta pHi decrease, 0.95) led to a basal increase in [Ca2+]i to the spike peak level with inhibiting Ca2+ oscillation. The pHi shift toward alkalinization (delta pHi increase, 0.33) led to a basal decrease in [Ca2+]i to the prestimulation level, possibly due to reuptake of Ca2+ into the Ca2+ stores, with inhibiting Ca2+ oscillation. Whereas extracellular pH (pHo) change had only minimal effects on Ca2+ oscillation (and/or Ca2+ release from intracellular stores), the extra-Ca2+ entry process, which was induced by higher concentrations of CCK, was totally inhibited by decreasing pHo from 7.4 to 6.5. Thus the major regulatory sites by which H+ affects Ca2+ oscillation are accessible from the intracellular space.

Ca2(+)-sensitivity of inositol 1,4,5-trisphosphate-mediated Ca2+ release in permeabilized pancreatic acinar cells

Hormonal and phorbol ester pretreatment of pancreatic acinar cells markedly decreases the Ins(1,4,5)P3-induced release of actively stored Ca2+ [Willems, Van Den Broek, Van Os & De Pont (1989) J. Biol. Chem. 264, 9762-9767]. Inhibition occurred at an ambient free Ca2+ concentration of 0.1 microM, suggesting a receptor-mediated increase in Ca2(+)-sensitivity of the Ins(1,4,5)P3-operated Ca2+ channel. To test this hypothesis, the Ca2(+)-dependence of Ins(1,4,5)P3-induced Ca2+ release was investigated. In the presence of 0.2 microM free Ca2+, permeabilized cells accumulated 0.9 nmol of Ca2+/mg of acinar protein in an energy-dependent pool. Uptake into this pool increased 2.2- and 3.3-fold with 1.0 and 2.0 microM free Ca2+ respectively. At 0.2, 1.0 and 2.0 microM free Ca2+, Ins(1,4,5)P3 maximally released 0.53 (56%), 0.90 (44%) and 0.62 (20%) nmol of Ca2+/mg of acinar protein respectively. Corresponding half-maximal stimulatory Ins(1,4,5)P3 concentrations were calculated to be 0.5, 0.6 and 1.4 microM, suggesting that the affinity of Ins(1,4,5)P3 for its receptor decreases beyond 1.0 microM free Ca2+. The possibility that an inhibitory effect of sub-micromolar Ca2+ is being masked by the concomitant increase in size of the releasable store is excluded, since Ca2+ release from cells loaded in the presence of 0.1 or 0.2 microM free Ca2+ and stimulated at higher ambient free Ca2+ was not inhibited below 1.0 microM free Ca2+. At 2.0 and 10.0 microM free Ca2+, Ca2+, Ca2+ release was inhibited by approx. 30% and 75% respectively. The results presented show that hormonal pretreatment does not lead to an increase in Ca2(+)-sensitivity of the release mechanism. Such an increase in Ca2(+)-sensitivity to sub-micromolar Ca2+ is required to explain sub-micromolar oscillatory changes in cytosolic free Ca2+ by a Ca2(+)-dependent negative-feedback mechanism.

Convergence of cAMP and phosphoinositide pathways during rat parotid secretion

Rat parotid acinar cells were employed to investigate the mechanism by which receptor agonists that activate the phosphoinositide pathway enhance the stimulatory effects of adenosine 3',5'-cyclic monophosphate (cAMP) on amylase secretion. Norepinephrine (NE), which activates both alpha- and beta-adrenoceptors, evoked a secretory response that was greater than the sum of the responses obtained when NE was employed as a beta-agonist (in the presence of prazosin) and as an alpha-agonist (in the presence of propranolol). The enhancement of amylase secretion induced by NE was accompanied by an augmented rise in Ca2+ influx, as determined by fura-2 analysis. NE-induced cAMP production was comparable to that evoked by NE as a beta-agonist, and the accumulation of [3H]inositol 1,4,5-trisphosphate (IP3) evoked by NE was comparable to that elicited by NE as an alpha-agonist. The beta-adrenoceptor agonist isoproterenol potentiated the rise in cytosolic Ca2+ elicited by the muscarinic agonist carbachol, while possessing no stimulatory effect of its own. Isoproterenol had no effect on carbachol-induced stimulation of [3H]IP3 or 1,3,4,5-[3H]inositol tetrakisphosphate accumulation. Ionomycin and dibutyryl cAMP in combination produced a similar enhancing effect on the Ca2+ signal and amylase release as adrenergic and muscarinic receptor agonists. These results suggest that the synergism between the phosphoinositide and cAMP-signaling systems in parotid cells resides in enhanced Ca2+ availability.

Modulation of intracellular free Ca2+ concentration by IP3-sensitive and IP3-insensitive nonmitochondrial Ca2+ pools

Intracellular Ca2+ pools play an important role in the adjustment of cytosolic free Ca2+ concentrations. This review summarizes the recent knowledge on receptor-mediated Ca2+ release and Ca2+ uptake mechanisms in Ca2+ stores of exocrine cells taking the exocrine pancreas and the parotid gland as an example. The intracellular mediator for agonist-induced Ca2+ release is inositol 1,4,5-trisphosphate (IP3) which acts by opening Ca2+ channels from the endoplasmic reticulum or a more specialized organelle called 'calciosome'. This Ca2+ release is the major event to increase cytosolic free Ca2+ concentrations of exocrine glands from a resting level of approximately 10(-7) mol/l to approximately 10(-6) mol/l. Subsequently also Ca2+ influx from the extracellular fluid into the cell is increased which involves the action of inositol 1,3,4,5-tetrakisphosphate (IP4). Intracellular nonmitochondrial Ca2+ reuptake occurs into IP3-sensitive (IsCaP) as well as into IP3-insensitive Ca2+ pools Ca2+ pools (IisCaP). While Ca2+ uptake into the IisCaP is mediated by a vanadate-sensitive Ca2+ pump, Ca2+ uptake into the IsCaP is mediated by a Ca2+/H+ exchanger at the expense of an H+ gradient which is established by a vacuolar type H+ pump present in the same Ca2+ pool. During stimulation both Ca2+ pools, IsCaP and IisCaP, are probably connected, the nature of which has not yet been clarified. It is suggested that GTP and/or IP4 control Ca2+ conveyance between intracellular Ca2+ pools by forming Ca2+-carrying junctions between membranes. Other models propose that Ca2+, which is released by IP3, induces Ca2+ release from another Ca2+ pool. Taking into account that H+ transport is present in IP3-sensitive Ca2+ pools the possibility of pH-regulated Ca2+ channels in the IisCaP, located in close neighbourhood to the IsCaP, is also considered.

Calcium signalling in non-excitable cells: notes on oscillations and store refilling

Technical advances in studying cellular calcium concentrations, and discoveries about many aspects of signal transduction have transformed this field of biology since this Journal was launched a decade ago. At that time monitoring of the key variable, cytoplasmic free Ca2+ concentration [Ca2+]i, was possible with aequorin or arsenazo ill mainly in large invertebrate cells, though pioneering work with aequorin micro-injection into cardiac and smooth muscle had just started. At that time there was also intense activity by a few groups aiming to make Ca-selective micro-electrodes selective and sharp enough to measure [Ca2+]i in small cells. Also the use of electropermeabilized cells had begun to allow the defining of the concentrations of Ca2+ required to activate secretion in mammalian cells. Nearly all this work and all the relevant electrophysiology relating to calcium signalling had been done in excitable cells, basically muscle and nerve, and was aimed at understanding contraction, transmitter release and neurosecretion, and the control of membrane permeability. Recent advances have now allowed [Ca2+]i to be measured in non-excitable cells.