Characterization of sustained [Ca2+]i increase in pancreatic acinar cells and its relation to amylase secretion

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.

Dual Mode of Action of Acetylcholine on Cytosolic Calcium Oscillations in Pancreatic Beta and Acinar Cells In Situ

Cholinergic innervation in the pancreas controls both the release of digestive enzymes to support the intestinal digestion and absorption, as well as insulin release to promote nutrient use in the cells of the body. The effects of muscarinic receptor stimulation are described in detail for endocrine beta cells and exocrine acinar cells separately. Here we describe morphological and functional criteria to separate these two cell types in situ in tissue slices and simultaneously measure their response to ACh stimulation on cytosolic Ca2+ oscillations [Ca2+]c in stimulatory glucose conditions. Our results show that both cell types respond to glucose directly in the concentration range compatible with the glucose transporters they express. The physiological ACh concentration increases the frequency of glucose stimulated [Ca2+]c oscillations in both cell types and synchronizes [Ca2+]c oscillations in acinar cells. The supraphysiological ACh concentration further increases the oscillation frequency on the level of individual beta cells, inhibits the synchronization between these cells, and abolishes oscillatory activity in acinar cells. We discuss possible mechanisms leading to the observed phenomena.

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.

Betaine promotes cell differentiation of human osteoblasts in primary culture

BACKGROUND

Betaine (BET), a component of many foods, is an essential osmolyte and a source of methyl groups; it also shows an antioxidant activity. Moreover, BET stimulates muscle differentiation via insulin like growth factor I (IGF-I). The processes of myogenesis and osteogenesis involve common mechanisms with skeletal muscle cells and osteoblasts sharing the same precursor. Therefore, we have hypothesized that BET might be effective on osteoblast cell differentiation.

METHODS

The effect of BET was tested in human osteoblasts (hObs) derived from trabecular bone samples obtained from waste material of orthopedic surgery. Cells were treated with 10 mM BET at 5, 15, 60 min and 3, 6 and 24 h. The possible effects of BET on hObs differentiation were evaluated by real time PCR, western blot and immunofluorescence analysis. Calcium imaging was used to monitor intracellular calcium changes.

RESULTS

Real time PCR results showed that BET stimulated significantly the expression of RUNX2, osterix, bone sialoprotein and osteopontin. Western blot and immunofluorescence confirmed BET stimulation of osteopontin protein synthesis. BET stimulated ERK signaling, key pathway involved in osteoblastogenesis and calcium signaling. BET induced a rise of intracellular calcium by means of the calcium ions influx from the extracellular milieu through the L-type calcium channels and CaMKII signaling activation. A significant rise in IGF-I mRNA at 3 and 6 h and a significant increase of IGF-I protein at 6 and 24 h after BET stimulus was detected. Furthermore, BET was able to increase significantly both SOD2 gene expression and protein content.

CONCLUSIONS

Our study showed that three signaling pathways, i.e. cytosolic calcium influx, ERK activation and IGF-I production, are enhanced by BET in human osteoblasts. These pathways could have synergistic effects on osteogenic gene expression and protein synthesis, thus potentially leading to enhanced bone formation. Taken together, these results suggest that BET could be a promising nutraceutical therapeutic agent in the strategy to counteract the concomitant and interacting impact of sarcopenia and osteoporosis, i.e. the major determinants of senile frailty and related mortality.

Pancreatic acinar cells: molecular insight from studies of signal-transduction using transgenic animals

Pancreatic acinar cells are classical exocrine gland cells. The apical regions of clusters of coupled acinar cells collectively form a lumen which constitutes the blind end of a tube created by ductal cells - a structure reminiscent of a "bunch of grapes". When activated by neural or hormonal secretagogues, pancreatic acinar cells are stimulated to secrete a variety of proteins. These proteins are predominately inactive digestive enzyme precursors called "zymogens". Acinar cell secretion is absolutely dependent on secretagogue-induced increases in intracellular free Ca(2+). The increase in [Ca(2+)](i) has precise temporal and spatial characteristics as a result of the exquisite regulation of the proteins responsible for Ca(2+) release, Ca(2+) influx and Ca(2+) clearance in the acinar cell. This brief review discusses recent studies in which transgenic animal models have been utilized to define in molecular detail the components of the Ca(2+) signaling machinery which contribute to these characteristics.

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

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

Initiation site of Ca(2+) entry evoked by endoplasmic reticulum Ca(2+) depletion in mouse parotid and pancreatic acinar cells

PURPOSE

In non-excitable cells, which include parotid and pancreatic acinar cells, Ca(2+) entry is triggered via a mechanism known as capacitative Ca(2+) entry, or store-operated Ca(2+) entry. This process is initiated by the perception of the filling state of endoplasmic reticulum (ER) and the depletion of internal Ca(2+) stores, which acts as an important factor triggering Ca(2+) entry. However, both the mechanism of store-mediated Ca(2+) entry and the molecular identity of store-operated Ca(2+) channel (SOCC) remain uncertain.

MATERIALS AND METHODS

In the present study we investigated the Ca(2+) entry initiation site evoked by depletion of ER to identify the localization of SOCC in mouse parotid and pancreatic acinar cells with microfluorometeric imaging system.

RESULTS

Treatment with thapsigargin (Tg), an inhibitor of sarco/endoplasmic reticulum Ca(2+)-ATPase, in an extracellular Ca(2+) free state, and subsequent exposure to a high external calcium state evoked Ca(2+) entry, while treatment with lanthanum, a non-specific blocker of plasma Ca(2+) channel, completely blocked Tg-induced Ca(2+) entry. Microfluorometric imaging showed that Tg-induced Ca(2+) entry started at a basal membrane, not a apical membrane.

CONCLUSION

These results suggest that Ca2+ entry by depletion of the ER initiates at the basal pole in polarized exocrine cells and may help to characterize the nature of SOCC.

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

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

Agonist activation of arachidonate-regulated Ca2+-selective (ARC) channels in murine parotid and pancreatic acinar cells

ARC channels (arachidonate-regulated Ca(2+)-selective channels) are a novel type of highly Ca(2+)-selective channel that are specifically activated by low concentrations of agonist-induced arachidonic acid. This activation occurs in the absence of any depletion of internal Ca(2+) stores (i.e. they are 'non-capacitative'). Previous studies in HEK293 cells have shown that these channels provide the predominant pathway for the entry of Ca(2+) seen at low agonist concentrations where oscillatory [Ca(2+)](i) signals are typically produced. In contrast, activation of the more widely studied store-operated Ca(2+) channels (e.g. CRAC channels) is only seen at higher agonist concentrations where sustained 'plateau-type'[Ca(2+)](i) responses are observed. We have now demonstrated the presence of ARC channels in both parotid and pancreatic acinar cells and shown that, again, they are specifically activated by the low concentrations of appropriate agonists (carbachol in the parotid, and both carbachol and cholecystokinin in the pancreas) that are associated with oscillatory [Ca(2+)](i) signals in these cells. Uncoupling the receptor-mediated activation of cytosolic phospholipase A(2) (cPLA(2)) with isotetrandrine reduces the activation of the ARC channels by carbachol and, correspondingly, markedly inhibits the [Ca(2+)](i) signals induced by low carbachol concentrations, whilst those signals seen at high agonist concentrations are essentially unaffected. Interestingly, in the pancreatic acinar cells, activation by cholecystokinin induces a current through the ARC channels that is only approximately 60% of that seen with carbachol. This is consistent with previous reports indicating that carbachol-induced [Ca(2+)](i) signals in these cells are much more dependent on Ca(2+) entry than are the cholecystokinin-induced responses.

Cleavage of SNAP-25 and VAMP-2 impairs store-operated Ca2+entry in mouse pancreatic acinar cells

We recently reported that store-operated Ca2+entry (SOCE) in nonexcitable cells is likely to be mediated by a reversible interaction between Ca2+channels in the plasma membrane and the endoplasmic reticulum, a mechanism known as “secretion-like coupling.” As for secretion, in this model the actin cytoskeleton plays a key regulatory role. In the present study we have explored the involvement of the secretory proteins synaptosome-associated protein (SNAP-25) and vesicle-associated membrane protein (VAMP) in SOCE in pancreatic acinar cells. Cleavage of SNAP-25 and VAMPs by treatment with botulinum toxin A (BoNT A) and tetanus toxin (TeTx), respectively, effectively inhibited amylase secretion stimulated by the physiological agonist CCK-8. BoNT A significantly reduced Ca2+entry induced by store depletion using thapsigargin or CCK-8. In addition, treatment with BoNT A once SOCE had been activated reduced Ca2+influx, indicating that SNAP-25 is needed for both the activation and maintenance of SOCE in pancreatic acinar cells. VAMP-2 and VAMP-3 are expressed in mouse pancreatic acinar cells. Both proteins associate with the cytoskeleton upon Ca2+store depletion, although only VAMP-2 seems to be sensitive to TeTx. Treatment of pancreatic acinar cells with TeTx reduced the activation of SOCE without affecting its maintenance. These findings support a role for SNAP-25 and VAMP-2 in the activation of SOCE in pancreatic acinar cells and show parallels between this process and secretion in a specialized secretory cell type.

Nitric oxide and the pancreas: morphological base and role in the control of the exocrine pancreatic secretion

The distribution of nitric oxide synthase in both neuronal and non-neuronal pancreatic tissues and the role of nitric oxide in the control of exocrine pancreatic secretion are reviewed in this article. Earlier reports based on in vivo studies suggested that nitric oxide can affect the secretory activity of the exocrine pancreas through changes in pancreatic blood flow. More recently, the employment of either nitric oxide synthase inhibitors or nitric oxide donors in in vitro preparations has provided evidence that nitric oxide can exert a direct action on this gland independently on its vascular effects. Most research in this area seems to indicate that modulation of exocrine pancreatic function by nitric oxide is exerted via activation of guanylate cyclase and generation of cGMP, although other pathways cannot be excluded. Experiments performed over the last year in our laboratory reveal a novel and interesting mechanism based on the ability of nitric oxide to control the release of endogenous neurotransmitter in the pancreas and, subsequently, the nerve-mediated enzyme secretion.

Nonselective cation channel as a Ca(2+) influx pathway in pepsinogen-secreting cells of bullfrog esophagus

In pepsinogen-secreting cells of bullfrog (Rana catesbeiana), recent evidence suggests that Ca(2+) release from internal stores followed by Ca(2+) influx across the plasma membrane elicits pepsinogen secretion. Such a Ca(2+) influx could be carried by a background current, potentiated by bombesin, that was found in these cells using the whole cell patch-clamp technique. The permeability ratio of Cs(+)-Rb(+)-K(+)-Na(+)-Li(+)-N-methyl-D-glucamine(+)-Ca(2+) was 1.01:1:1:0.86:0.72:0.54:0.34. The current was almost totally blocked by the nonselective cation channel blockers La(3+) (0.1 mM) and Gd(3+) (0.1 mM) and was activated by intracellular Ca(2+). These properties demonstrated that the current, which was activated by bombesin, was a nonselective cation current. At the same time, Gd(3+) suppressed pepsinogen secretion by 29 +/- 5.6% in isolated pepsinogen-secreting glands. These results are in accord with the idea that a nonselective cation channel in pepsinogen-secreting cells plays a role as a Ca(2+) influx pathway leading to secretion of pepsinogen in bullfrog esophageal mucosa.

Calcium-magnesium interactions in pancreatic acinar cells

Although the role of calcium (Ca2+) in the signal transduction and pathobiology of the exocrine pancreas is firmly established, the role of magnesium (Mg2+) remains unclear. We have characterized the intracellular distribution of Mg2+ in response to hormone stimulation in isolated mouse pancreatic acinar cells and studied the role of Mg2+ in modulating Ca2+ signaling using microspectrofluorometry and digital imaging of Ca2+- or Mg2+-sensitive fluorescent dyes as well as Mg2+-sensitive intracellular microelectrodes. Our results indicate that an increase in intracellular Mg2+ concentrations reduced the cholecystokinin (CCK) -induced Ca2+ oscillations by inhibiting the capacitive Ca2+ influx. An intracellular Ca2+ mobilization, on the other hand, was paralleled by a decrease in [Mg2+]i, which was reversible upon hormone withdrawal independent of the electrochemical gradients for Mg2+, Ca2+, Na+, and K+, and not caused by Mg2+ efflux from acinar cells. In an attempt to characterize possible Mg2+ stores that would explain the reversible, hormone-induced intracellular Mg2+ movements, we ruled out mitochondria or ATP as potential Mg2+ buffers and found that the CCK-induced [Mg2+]i decrease was initiated at the basolateral part of the acinar cells, where most of the endoplasmic reticulum (ER) is located, and progressed from there toward the apical pole of the acinar cells in an antiparallel fashion to Ca2+ waves. These experiments represent the first characterization of intracellular Mg2+ movements in the exocrine pancreas, provide evidence for possible Mg2+ stores in the ER, and indicate that the spatial and temporal distribution of intracellular Mg concentrations profoundly affects acinar cell Ca2+ signaling.

Exocytosis and movement of zymogen granules observed by VEC-DIC microscopy in the pancreatic tissue en bloc

The dynamic aspects of exocytosis, especially in the normal acinar tissue en bloc, have remained unclear. We visualized exocytosis directly in the tissue of the exocrine pancreas of rodents by video-enhanced contrast-differential interference contrast (VEC-DIC) microscopy to investigate various exocytosis-related rates and the relationship between the movement of granules and exocytotic responses. Stimulation of the tissue with bethanechol or cholecystokinin caused many of the zymogen granules in the apical pole to disappear abruptly. The exocytotic transients of individual granules were completed in 0.48-0.65 s. Granules destined to participate in the exocytotic response moved randomly at velocities of approximately 0.06 microm/s or less during stimulation. In the tissue preparation, granules located far from the apical pole frequently moved back and forth for 1-7 microm without showing exocytosis. Colchicine suppressed this movement and the late phase of the secretory response. Real-time (VEC-DIC) observation of granule dynamics revealed that the initial step of exocytosis was not coupled directly with the microtubule-dependent translocation but with a continuous, slow Brownian fluctuation of granules.

Carbachol-induced [Ca2+]i increase, but not activation of protein kinase C, stimulates exocytosis in rat parotid acini

1. A column perfusion system was applied to rat parotid acinar cells to clarify the roles of Ca2+ and protein kinase C (PKC) in the mechanisms of carbachol (CCh)-induced amylase secretion. 2. CCh evoked a biphasic response of amylase secretion with an initial rapid and large peak that reached maximum at about 10 s followed by a sustained plateau. The time profile and the dose-response relationship paralleled with those of cytosolic free Ca2+ concentration ([Ca2+]i). 3. The CCh-induced sustained response of amylase secretion maintained by Ca2+ influx into cells was ATP dependent, while the initial peak response regulated by Ca2+ released from InsP3-sensitive stores was relatively ATP independent. 4. Restoration of extracellular Ca2+ during continuous stimulation with CCh in Ca(2+)-free medium evoked a second rapid and large peak of amylase secretion. 5. Phorbol 12,13-dibutyrate (PDBu) potentiated the CCh-induced amylase secretion in both the initial peak and the sustained plateau without enhancing CCh-induced [Ca2+]i changes. 6. PKC inhibitors such as Ro 31-8220 inhibited the potentiating effect of PDBu but only slightly reduced amylase secretion induced by CCh alone. 7. These results suggest that a CCh-induced rise in [Ca2+]i triggers the final fusion and/or exocytosis of amylase secretion. CCh also has some ability to promote ATP-dependent priming of secretory granules that, together with Ca2+ influxed into cells, contributes to the CCh-induced sustained plateau of amylase secretion. PDBu-induced activation of PKC promotes the priming of secretory granules, thereby enhancing the efficacy for Ca2+ to trigger fusion/exocytosis.

Modulation of calcium signals by intracellular pH in isolated rat pancreatic acinar cells

1. We have investigated the interactions between intracellular pH (pH1) and the intracellular free calcium concentration ([Ca2+]i) in isolated rat pancreatic acinar cells. The fluorescent dyes fura-2 and BCECF were used to measure [Ca2+]i and pHi, respectively. 2. Sodium acetate and ammonium chloride (NH4Cl) were used to acidify and alkalinize pHi, respectively. Cytosolic acidification had no effect on [Ca2+]i in resting pancreatic acinar cells, whereas cytosolic alkalinization released Ca2+ from intracellular stores. 3. Cytosolic acidification using either acetate or a CO2-HCO3(-)-buffered medium enhanced Ca2+ signals evoked by acetylcholine (ACh) and cholecystokinin (CCK). In contrast, both NH4Cl and trimethylamine (TMA) inhibited Ca2+ signals during stimulation with either ACh or CCK. This inhibitory effect was also observed in the absence of extracellular Ca2+, and was therefore not due to changes in Ca2+ entry. 4. Calcium oscillations evoked by physiological concentrations of CCK were enhanced by cytosolic acidification and inhibited by cytosolic alkalinization. 5. In order to determine the effects of pHi upon Ca2+ handling by intracellular Ca2+ stores, intraorganellar [Ca2+] was monitored using the low affinity Ca2+ indicator mag-fura-2 in permeabilized cells. Addition of NH4Cl, which is expected to alkalinize intraorganellar pH, did not alter intraorganellar [Ca2+] in permeabilized cells, suggesting that changing intraorganellar pH does not release Ca2+ from intracellular stores. Addition of NH4Cl or acetate also did not affect the rate of Ca2+ release induced by inositol 1,4,5-trisphosphate (InsP3). 6. Modification of extraorganellar ('cytosolic') pH did not affect the rate of ATP-dependent Ca2+ uptake into stores, but did modify the rate of Ca2+ release evoked by submaximal concentrations of InsP3. The rate of Ca2+ release was increased at more alkaline extraorganellar pHs. These results would suggest that manipulation of intraorganellar pH does not affect Ca2+ handling by the intracellular stores. In contrast, extraorganellar ('cytosolic') pH does affect InsP3-induced Ca2+ release from the stores. 7. In conclusion, changes in intracellular pH in pancreatic acinar cells can profoundly alter cytosolic [Ca2+]. This may shed light on earlier observations whereby cell-permeant weak acids and bases can modulate fluid secretion in epithelia.

Anion secretion induced by capacitative Ca2+ entry through apical and basolateral membranes of cultured equine sweat gland epithelium

1. Anion secretion induced by capacitative Ca2+ entry through apical and basolateral membranes of cultured equine sweat gland epithelium was studied using the short-circuit current (Isc) technique. 2. Thapsigargin induced an increase in Isc that could be inhibited when external Ca2+ was chelated by EGTA. 3. The inhibition of the thapsigargin-induced Isc could be reversed by re-addition of Ca2+ to apical or basolateral solutions. The magnitude of the reactivated Isc depended predominantly on basolateral Ca2+ concentration. 4. The magnitude of the reactivated Isc upon basolateral Ca2+ addition increased with the thapsigargin concentration, indicating its dependence on the emptied state of the Ca2+ store induced by thapsigargin. 5. The thapsigargin-induced Isc, as well as the Ca(2+)-dependent reactivation of Isc in EGTA-treated epithelia, was inhibitable by apical, but not basolateral, addition of flufenamate, and by basolateral addition of La3+. Other Ca2+ channel blockers, verapamil and nifedipine, had no effect when applied to either membrane. 6. The results suggest that thapsigargin-induced anion secretion by the equine sweat gland epithelial cells is crucially dependent upon the Ca2+ influx occurring primarily through the basolateral membrane, and that apical and basolateral membranes may possess different pathways for Ca2+ entry.

Calcium influx pathways in rat pancreatic ducts

A number of agonists increase intracellular Ca2+ activity, [Ca2+]i, in pancreatic ducts, but the influx/efflux pathways and intracellular Ca2+ stores in this epithelium are unknown. The aim of the present study was to characterise the Ca2+ influx pathways, especially their pH sensitivity, in native pancreatic ducts stimulated by ATP and carbachol, CCH. Under control conditions both agonists led to similar changes in [Ca2+]i. However, these Ca2+ transients, consisting of peak and plateau phases, showed different sensitivities to various experimental manoeuvres. In extracellular Ca2+-free solutions, the ATP-induced [Ca2+]i peak decreased by 25%, but the CCH-induced peak was unaffected; both plateaus were inhibited by 90%. Flufenamate inhibited the ATP-induced peak by 35%, but not the CCH-evoked peak; the plateaus were inhibited by 75-80%. La3+ inhibited the ATP-induced plateau fully, but that induced by CCH by 55%. In resting ducts, an increase in extracellular pH, pHe, by means of HEPES and HCO3-/CO2 buffers, increased [Ca2+]i; a decrease in pHe had the opposite effect. In stimulated ducts the pH-evoked effects on Ca2+ influx were more pronounced and depended on the agonist used. At pHe 6.5 both ATP- and CCH-evoked plateaus were inhibited by about 50%. At pH 8.0 the ATP-stimulated plateau was inhibited by 27%, but that stimulated by CCH was increased by 72%. Taken together, we show that CCH stimulates Ca2+ release followed by Ca2+ influx that is moderately sensitive to flufenamate, La3+, depolarisation, it is inhibited by low pH, but stimulated by high pH. ATP stimulates Ca2+ release and probably an early Ca2+ influx, which is more markedly sensitive to flufenamate and La3+, and is both inhibited by low and high pH. Thus our study indicates that there are at least two separate Ca2+ influx pathways in pancreatic ducts cells.

Phagosome-lysosome fusion is a calcium-independent event in macrophages

Phagosome-lysosome membrane fusion is a highly regulated event that is essential for intracellular killing of microorganisms. Functionally, it represents a form of polarized regulated secretion, which is classically dependent on increases in intracellular ionized calcium ([Ca2+]i). Indeed, increases in [Ca2+]i are essential for phagosome-granule (lysosome) fusion in neutrophils and for lysosomal fusion events that mediate host cell invasion by Trypanosoma cruzi trypomastigotes. Since several intracellular pathogens survive in macrophage phagosomes that do not fuse with lysosomes, we examined the regulation of phagosome-lysosome fusion in macrophages. Macrophages (M phi) were treated with 12.5 microM bis-(2-amino-S-methylphenoxy) ethane-N,N,N',N',-tetraacetic acid tetraacetoxymethyl ester (MAPT/AM), a cell-permeant calcium chelator which reduced resting cytoplasmic [Ca2+]; from 80 nM to < or = 20 nM and completely blocked increases in [Ca2+]i in response to multiple stimuli, even in the presence of extracellular calcium. Subsequently, M phi phagocytosed serum-opsonized zymosan, staphylococci, or Mycobacterium bovis. Microbes were enumerated by 4',6-diamidino-2-phenylindole, dihydrochloride (DAPI) staining, and phagosome-lysosome fusion was scored using both lysosome-associated membrane protein (LAMP-1) as a membrane marker and rhodamine dextran as a content marker for lysosomes. Confirmation of phagosome-lysosome fusion by electron microscopy validated the fluorescence microscopy findings. We found that phagosome-lysosome fusion in M phi occurs noramlly at very low [Ca2+]i (< or = 20 nM). Kinetic analysis showed that in M phi none of the steps leading from particle binding to eventual phagosome-lysosome fusion are regulated by [Ca2+]i in a rate-limiting way. Furthermore, confocal microscopy revealed no difference in the intensity of LAMP-1 immunofluorescence in phagolysosome membranes in calcium-buffered vs. control macrophages. We conclude that neither membrane recognition nor fusion events in the phagosomal pathway in macrophages are dependent on or regulated by calcium.

Ca2+ influx induced by store release and cytosolic Ca2+ chelation in Ht29 colonic carcinoma cells

Cl- secretion in HT29 cells is regulated by agonists such as carbachol, neurotensin and adenosine 5'-triphosphate (ATP). These agonists induce Ca2+ store release as well as Ca2+ influx from the extracellular space. The increase in cytosolic Ca2+ enhances the Cl- and K+ conductances of these cells. Removal of extracellular Ca2+ strongly attenuates the secretory response to the above-mentioned agonists. The present study utilises patch-clamp methods to characterise the Ca2+ influx pathway. Inhibitors which have been shown previously to inhibit non-selective cation channels, such as flufenamate (0.1 mmol.l-1, n = 6) and Gd3+ (10 micromol.l-1, n=6) inhibited ATP (0.1 mmol.l-1) induced increases in whole-cell conductance (Gm). When Cl- and K+ currents were inhibited by the presence of Cs2SO4 in the patch pipette and gluconate in the bath, ATP (0.1 mmol.l-1) still induced a significant increase in Gm from 1.2 +/- 0.3 nS to 4.7 +/- 1 nS (n = 24). This suggests that ATP induces a cation influx with a conductance of approximately 3-4 nS. This cation influx was inhibited by flufenamate (0.1 mmol.l-1, n = 6) and Gd3+ (10 micromol.l-1, n = 9). When Ba2+ (5 mmol.l-1) and 4,4'-diisothiocyanato-stilbene-2-2'-disulphonic acid (DIDS, 0.1 mmol.l-1) were added to the KCl/K-gluconate pipette solution to inhibit K+ and Cl- currents and the cells were clamped to depolarised voltages, ATP (0.1 mmol.l-1) reduced the membrane current (Im) significantly from 86 +/- 14 pA to 54 +/- 11 pA (n = 13), unmasking a cation inward current. In another series, the cation inward current was activated by dialysing the cell with a KCl/K-gluconate solution containing 5-10 mmol.l-1 1,2-bis-(2-aminoethoxy)ethane-N,N,N',N'-tetraacetic acid (EGTA) or 1,2-bis-(2-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid (BAPTA). The zero-current membrane voltage (Vm) and Im (at a clamp voltage of +10 mV) were monitored as a function of time. A new steady-state was reached 30-120 s after membrane rupture. Vm depolarised significantly from -33 +/- 2 mV to -12 +/- 1 mV, and Im fell significantly from 17 +/- 2 pA to 8.9 +/- 1.0 pA (n = 71). This negative current, representing a cation inward current, was activated when Ca2+ stores were emptied and was reduced significantly ( Im) when Ca2+ and/or Na+ were removed from the bathing solution: removal of Ca2+ in the absence of Na+ caused a Im of 5.0 +/- 1.2 pA (n = 12); removal of Na+ in the absence of Ca2+ caused a Im of 12.8 +/- 3.5 pA (n = 4). The cation inward current was also reduced significantly by La3+, Gd3+, and flufenamate. We conclude that store depletion induces a Ca2+/Na+ influx current in these cells. With 145 mmol.l-1 Na+ and 1 mmol.l-1 Ca2+, both ions contribute to this cation inward current. This current is an important component in the agonist-regulated secretory response.

Flufenamate and Gd3+ inhibit stimulated Ca2+ influx in the epithelial cell line CFPAC-1

The relevant influx pathway for stimulated Ca2+ entry into epithelial cells is largely unknown. Using flufenamate (Flu) and Gd3+, both known pharmacological blockers of non-selective cation currents in other epithelial preparations, we tested whether the stimulated Ca2+ entry in CFPAC-1 cells was inhibited by these agents. Transmembraneous Ca2+ influx into CFPAC-1 cells was stimulated by either ATP (10(-4) and 10(-5) mol/l), carbachol (CCH, 10(-4) mol/l) or thapsigargin (TG, 10(-8) mol/l). Three different experimental approaches were used. (1) Because the plateau phase of an agonist-induced [Ca2+]i transient reflects Ca2+ influx into these cells, we investigated the influence of Flu and Gd3+ on the level of the stimulated [Ca2+]i plateau. (2) The fura-2 Mn(2+)-quenching technique was used to visualise divalent cation entry and monitor its inhibition. (3) During the "refilling period" after agonist-induced discharge of the intracellular pools the putative influx inhibitors Flu and Gd3+ were given and subsequently the filling state of the agonist-sensitive intracellular stores tested. The results from the first experimental approach showed that both Flu and Gd3+ were potent inhibitors of the stimulated Ca2+ entry in CFPAC-1 cells. Flu reversibly decreased the ATP-induced [Ca2+]i plateau in a concentration dependent manner, with an IC50 value of 33 mumol/l (n = 6). Similar results were obtained for the CCH- (n = 5) and the TG-induced (n = 5) [Ca2+]i plateau. Gd3+ concentration dependently inhibited the stimulated Ca2+ plateau. A complete block of the ATP-induced [Ca2+]i plateau was seen at 0.5 mumol/l (ATP 10(-5) mol/l, n = 8).(ABSTRACT TRUNCATED AT 250 WORDS)

Activation of protein kinase C does not cause desensitization in rat and rabbit mandibular acinar cells

We have examined whether activation of protein kinase C by phorbol esters decreases the responsiveness of rat and rabbit mandibular, and rat lacrimal, acinar cells to muscarinic stimulation. Intracellular free calcium concentration ([Ca2+]i) was measured in isolated single acini and cell clusters by fura-2 microspectrofluorimetry. Accumulation of inositol phosphates was measured in acinar cell suspensions. All three cell types showed very similar changes in [Ca2+]i in response to acetylcholine (ACh), although mobilization of Ca2+ required somewhat higher ACh concentrations in rat lacrimal acinar cells than in mandibular acinar cells. There was no evidence for different dose dependencies of the peak and plateau phases of the [Ca2+]i response. The ACh-evoked [Ca2+]i increase in rabbit mandibular acinar cells exhibited desensitization, since it declined in magnitude when cells were stimulated repeatedly with a maximal dose of agonist. The phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) rapidly and irreversibly decreased the ACh-evoked [Ca2+]i signals in rat lacrimal acinar cells and reduced ACh-stimulated inositol phosphate accumulation. This inhibitory effect of TPA was most marked in cells stimulated with low doses of ACh, implying that TPA treatment shifted the ACh dose response curve to higher ACh concentrations. In contrast to the results obtained with lacrimal acinar cells, TPA had no effect on the [Ca2+]i and inositol phosphate responses to ACh in either rat or rabbit mandibular acinar cells. These results suggest that, although ACh-evoked [Ca2+]i signals, and hence presumably the stimulus-response coupling machinery, are very similar between different acinar cell types, acinar cells show marked differences in their sensitivity to phorbol esters. The insensitivity of mandibular acinar cell [Ca2+]i signals to TPA also suggests that the secretory tachyphylaxis observed in perfused rat and rabbit mandibular salivary glands is unlikely to be a consequence of negative feedback mediated by protein kinase C.

Suppression of Ca2+ oscillations induced by cholecystokinin (CCK) and its analog OPE in rat pancreatic acinar cells by low-level protein kinase C activation without transition of the CCK receptor from a high- to low-affinity state

Cholecystokinin (CCK) analogs, JMV-180 and OPE, release Ca2+ from intracellular stores and induce oscillations in the concentration of cytosolic Ca2+ ([Ca2+]i), but do not generate a detectable rise in inositol 1,4,5-trisphosphate (InsP3) levels. In contrast, high concentrations of CCK elevate InsP3, as well [Ca2+]i, to a peak which decreases to near basal levels without oscillations. The mechanisms which underlie inhibition of [Ca2+]i oscillations observed with high CCK concentrations are unclear, but are believed to involve a low-affinity CCK receptor state. Alternately, CCK analogs may be weak partial agonists of the phospholipase C pathway, whereas native CCK, as a full agonist of this pathway, stimulates low levels of protein kinase C (PKC) activity. Preincubation of acini with 1 nM 12 O-tetradecanoyl-phorbol 13-acetate (TPA) for 15 min at 37 degrees C did not affect OPE binding to acini, but abolished OPE-induced (at 1 microM) [Ca2+]i oscillations without affecting the initial [Ca2+]i spike. These transformed OPE-induced [Ca2+]i responses mimicked those induced by supramaximal CCK octapeptide (CCK-8) concentrations. Inhibition of [Ca2+]i oscillations by 1 nM TPA was reversed by the PKC inhibitor staurosporine (0.2 microM). After [Ca2+]i oscillations were induced with OPE or low concentrations of CCK-8 (20 pM), 1 nM TPA caused a gradual slowing of oscillation frequency over 15-20 min without affecting [Ca2+]i spike amplitude. In contrast, 1 microM TPA inhibited OPE binding and caused a more generalized inhibition of OPE- and CCK-evoked Ca2+ signals.(ABSTRACT TRUNCATED AT 250 WORDS)

A rat model to study hypercalcemia-induced acute pancreatitis

Hypercalcemia causes acute pancreatitis in humans, a phenomenon reproduced experimentally in cats and guinea pigs. Because the rat is the most frequently used animal for the study of experimental pancreatitis, the present studies were performed to evaluate the effects of hypercalcemia in the rat. In in vitro studies, pancreatic lobules were prepared from fasted Wistar rats (200-250 g) and incubated in HEPES bicarbonate-buffered medium (pH 7.4) containing 0, 0.6, 1.2, 2.5, 5, and 10 mM CaCl2 with or without carbachol 10(-6) M. Amylase was measured in the medium after 30 min to 3 h, and expressed as percent of total amylase. In in vivo studies, fasted male Wistar rats (300-400 g) received calcium (CaCl2; 0.6 mmol/kgh) into the tail vein for 12 h. Control animals received NaCl 0.9% infusion. Histologic slides (H&E-stained) were evaluated in a blinded fashion. Pancreatic lobules showed a higher basal amylase output when incubated in higher calcium medium. The largest, significant difference (2.6-fold) was between 0.6 and 5 mM medium CaCl2 (p < 0.05). Carbachol-stimulated amylase release was again higher with increasing medium calcium with the most pronounced difference (1.3-fold) between 0.6 and 2.5 mM CaCl2 (p < 0.05). In vivo calcium-treated animals showed accumulation of zymogen granules in the cytoplasm, cytoplasmic vacuolization, focal acinar cell depolarization, acinar necrosis, and edema. Calcium causes amylase release from rat pancreatic lobules in vitro. Higher medium calcium levels both significantly increase amylase release from unstimulated and carbachol stimulated lobules. Twelve-hour in vivo calcium infusion leads to accumulation of zymogen granules in acinar cells and acinar injury.(ABSTRACT TRUNCATED AT 250 WORDS)

Voltage-dependent Ca2+ influx in the epithelial cell line HT29: simultaneous use of intracellular Ca2+ measurements and nystatin perforated patch-clamp technique

Indirect evidence has accumulated indicating a voltage dependence of the agonist-stimulated Ca2+ influx into epithelial cells. Manoeuvres expected to depolarise the membrane voltage during agonist stimulation resulted in: (1) a decrease of the sustained phase of the adenosine triphosphate (ATP, 10(-5) mol/l)-induced intracellular Ca2+ transient, (2) a reduced fura-2 Mn(2+)-quenching rate, and (3) prevention of the refilling of the agonist-sensitive store. To quantify the change in intracellular Ca2+ as a function of membrane voltage, we measured simultaneously the intracellular Ca2+ activity ([Ca2+]i) with fura-2 and the electrical properties using the nystatin perforated patch-clamp technique in single HT29 cells. Ca2+ influx was either stimulated by ATP (10(-5) mol/l) or thapsigargin (TG, 10(-8) mol/l). After [Ca2+]i reached the sustained plateau phase we clamped the membrane voltage in steps of 10 mV in either direction. A stepwise depolarisation resulted in a stepwise reduction of [Ca2+]i. Similarly a stepwise hyperpolarisation resulted in a stepwise increase of [Ca2+]i (ATP: 27.5 +/- 10 nmol/l per 10 mV, n = 6; TG: 19 +/- 7.9 nmol/l per 10 mV, n = 12). The summarised data show a linear relationship between the delta fluorescence ratio 340/380 nm change and the applied holding voltage. In unstimulated cells the same voltage-clamp protocol did not change [Ca2+]i (n = 9). Under extracellular Ca(2+)-free conditions [Ca2+]i remained unaltered when changing the membrane voltage. These data provide direct evidence that the Ca2+ influx in epithelial cells is membrane voltage dependent. Our data indicate that small changes in membrane voltage lead to substantial changes in [Ca2+]i.(ABSTRACT TRUNCATED AT 250 WORDS)

Tauroursodeoxycholic acid stimulates hepatocellular exocytosis and mobilizes extracellular Ca++ mechanisms defective in cholestasis

To assess the effects of tauroursodeoxycholic acid (TUDCA) on bile excretory function, we examined whether TUDCA modulates vesicular exocytosis in the isolated perfused liver of normal rats in the presence of high (1.9 mM) or low (0.19 mM) extracellular Ca++ and in cholestatic rats 24 h after bile duct ligation. In addition, the effects of TUDCA on Ca++ homeostasis were compared in normal and in cholestatic hepatocytes. In the isolated perfused rat liver, TUDCA (25 microM) stimulated a sustained increase in the biliary excretion of horseradish peroxidase, a marker of the vesicular pathway, in the presence of high, but not low extracellular Ca++ or in the cholestatic liver. In contrast, TUDCA stimulated bile flow to the same extent regardless of the concentration of extracellular Ca++ or the presence of cholestasis. In indo-1-loaded hepatocytes, basal cytosolic free Ca++ ([Ca++]i) levels were not different between normal and cholestatic cells. However, in cholestatic cells [Ca++]i increases induced by TUDCA (10 microM) and its 7 alpha-OH epimer taurochenodeoxycholic acid (50 microM) were reduced to 22% and 26%, respectively, compared to normal cells. The impairment of TUDCA-induced [Ca++]i increase in cholestatic cells could be mimicked by exposing normal cells to low extracellular Ca++ (21%) or to the Ca++ channel blocker NiCl2 (23%). These data indicate that (a) dihydroxy bile acid-induced Ca++ entry may be of functional importance in the regulation of hepatocellular vesicular exocytosis, and (b) this Ca++ entry mechanism across the plasma membrane is impaired in cholestatic hepatocytes. We speculate that the beneficial effect of ursodeoxycholic acid in cholestatic liver diseases may be related to the Ca+(+)-dependent stimulation of vesicular exocytosis by its conjugate.

Agonist-induced intracellular Ca2+ transients in HT29 cells

In the present study we have investigated the mechanism of intracellular Ca2+ activity ([Ca2+]i) changes in HT29 cells induced by adenosine triphosphate (ATP), carbachol (CCH), and neurotensin (NT). [Ca2+]i was measured with the fluorescent Ca2+ indicator fura-2 at the single-cell level or in small cell plaques with high time resolution (1-40Hz). ATP and CCH induced not only a dose-dependent [Ca2+]i peak response, but also changes of the plateau phase. The [Ca2+]i plateau was inversely dependent on the ATP concentration, whereas the CCH-induced [Ca2+]i plateau increased at higher CCH concentrations. NT showed (from 10(-10) to 10(-7) mol/l) in most cases only a [Ca2+]i spike lasting 2-3 min. The [Ca2+]i plateau induced by ATP (10(-6) mol/l) and CCH (10(-5) mol/l) was abolished by reducing the Ca2+ activity in the bath from 10(-3) to 10(-4) mol/l (n = 7). In Ca(2+)-free bathing solution the [Ca2+]i peak value for all three agonists was not altered. Using fura-2 quenching by Mn2+ as an indicator of Ca2+ influx the [Ca2+]i peak was always reached before Mn2+ influx started. Every agonist showed this delayed stimulation of the Ca2+ influx with a lag time of 23 +/- 1.5 s (n = 15) indicating a similar mechanism in each case. Verapamil (10(-6)-10(-4) mol/l) blocked dose dependently both phases (peak and plateau) of the CCH-induced [Ca2+]i increase. Short pre-incubation with verapamil augmented the effect on the [Ca2+]i peak, whereas no further influence on the plateau was observed. Ni2+ (10(-3) mol/l) reduced the plateau value by 70%.