Thapsigargin defines the roles of cellular calcium in secretagogue-stimulated enzyme secretion from pancreatic acini

cAMP and Ca²⁺ signaling in secretory epithelia: crosstalk and synergism

The Ca(2+) and cAMP/PKA pathways are the primary signaling systems in secretory epithelia that control virtually all secretory gland functions. Interaction and crosstalk in Ca(2+) and cAMP signaling occur at multiple levels to control and tune the activity of each other. Physiologically, Ca(2+) and cAMP signaling operate at 5-10% of maximal strength, but synergize to generate the maximal response. Although synergistic action of the Ca(2+) and cAMP signaling is the common mode of signaling and has been known for many years, we know very little of the molecular mechanism and mediators of the synergism. In this review, we discuss crosstalk between the Ca(2+) and cAMP signaling and the function of IRBIT (IP3 receptors binding protein release with IP3) as a third messenger that mediates the synergistic action of the Ca(2+) and cAMP signaling.

Mechanism and synergism in epithelial fluid and electrolyte secretion

A central function of epithelia is the control of the volume and electrolyte composition of bodily fluids through vectorial transport of electrolytes and the obligatory H2O. In exocrine glands, fluid and electrolyte secretion is carried out by both acinar and duct cells, with the portion of fluid secreted by each cell type varying among glands. All acinar cells secrete isotonic, plasma-like fluid, while the duct determines the final electrolyte composition of the fluid by absorbing most of the Cl(-) and secreting HCO3 (-). The key transporters mediating acinar fluid and electrolyte secretion are the basolateral Na(+)/K(+) /2Cl(-) cotransporter, the luminal Ca(2+)-activated Cl(-) channel ANO1 and basolateral and luminal Ca(2+)-activated K(+) channels. Ductal fluid and HCO3 (-) secretion are mediated by the basolateral membrane Na(+)-HCO3 (-) cotransporter NBCe1-B and the luminal membrane Cl(-)/HCO3 (-) exchanger slc26a6 and the Cl(-) channel CFTR. The function of the transporters is regulated by multiple inputs, which in the duct include major regulation by the WNK/SPAK pathway that inhibit secretion and the IRBIT/PP1 pathway that antagonize the effects of the WNK/SPAK pathway to both stimulate and coordinate the secretion. The function of these regulatory pathways in secretory glands acinar cells is yet to be examined. An important concept in biology is synergism among signaling pathways to generate the final physiological response that ensures regulation with high fidelity and guards against cell toxicity. While synergism is observed in all epithelial functions, the molecular mechanism mediating the synergism is not known. Recent work reveals a central role for IRBIT as a third messenger that integrates and synergizes the function of the Ca(2+) and cAMP signaling pathways in activation of epithelial fluid and electrolyte secretion. These concepts are discussed in this review using secretion by the pancreatic and salivary gland ducts as model systems.

International Union of Pharmacology. LXVIII. Mammalian bombesin receptors: nomenclature, distribution, pharmacology, signaling, and functions in normal and disease states

The mammalian bombesin receptor family comprises three G protein-coupled heptahelical receptors: the neuromedin B (NMB) receptor (BB(1)), the gastrin-releasing peptide (GRP) receptor (BB(2)), and the orphan receptor bombesin receptor subtype 3 (BRS-3) (BB(3)). Each receptor is widely distributed, especially in the gastrointestinal (GI) tract and central nervous system (CNS), and the receptors have a large range of effects in both normal physiology and pathophysiological conditions. The mammalian bombesin peptides, GRP and NMB, demonstrate a broad spectrum of pharmacological/biological responses. GRP stimulates smooth muscle contraction and GI motility, release of numerous GI hormones/neurotransmitters, and secretion and/or hormone release from the pancreas, stomach, colon, and numerous endocrine organs and has potent effects on immune cells, potent growth effects on both normal tissues and tumors, potent CNS effects, including regulation of circadian rhythm, thermoregulation; anxiety/fear responses, food intake, and numerous CNS effects on the GI tract as well as the spinal transmission of chronic pruritus. NMB causes contraction of smooth muscle, has growth effects in various tissues, has CNS effects, including effects on feeding and thermoregulation, regulates thyroid-stimulating hormone release, stimulates various CNS neurons, has behavioral effects, and has effects on spinal sensory transmission. GRP, and to a lesser extent NMB, affects growth and/or differentiation of various human tumors, including colon, prostate, lung, and some gynecologic cancers. Knockout studies show that BB(3) has important effects in energy balance, glucose homeostasis, control of body weight, lung development and response to injury, tumor growth, and perhaps GI motility. This review summarizes advances in our understanding of the biology/pharmacology of these receptors, including their classification, structure, pharmacology, physiology, and role in pathophysiological conditions.

Adapter protein CRKII signaling is involved in the rat pancreatic acini response to reactive oxygen species

Recent studies demonstrate that reactive oxygen species (ROS) are important mediators of acute pancreatitis, whether induced experimentally or in necrotizing pancreatitis in humans; however, the cellular processes involved remain unclear. Adapter protein CrkII, plays a central role for convergence of cellular signals from different stimuli. Cholecystokinin (CCK), which induces pancreatitis, stimulates CrkII tyrosine phosphorylation and CrkII protein complexes, raising the possibility it can be important in the acinar cell responses to ROS. Therefore, our aim was to investigate whether CrkII signaling is involved in the biological response of rat pancreatic acini to H2O2 and the intracellular mediators implicated. Treatment of isolated rat pancreatic acini with H2O2 rapidly stimulates CrkII phosphorylation, measured as electrophoretic mobility shift and by using a phosphospecific antibody (pTyr221). Tyrosine kinase blocker B44 inhibits the higher phosphorylation state, demonstrating that it occurs mainly in tyrosine residues. H2O2-induced CrkII phosphorylation is time- and concentration-dependent, showing maximal effect with 3 mM H2O2 at 5 min. The intracellular pathways induced by H2O2 leading to CrkII tyrosine phosphorylation do not involve PKC, intracellular calcium, PI3-K or the actin cytoskeleton integrity. ROS generation clearly promotes the formation of protein complex CrkII-PYK2. In conclusion, ROS clearly affect the key adapter protein CrkII signaling by two ways: stimulation of CkII phosphorylation and a functional consequence: formation of CrkII-protein complexes. Because of its central role in activating more distal pathways, CrkII might likely play an important role in the ability of ROS to induce pancreatic cellular injury and pancreatitis.

Mitochondria play a critical role in shaping the exocytotic response of rat pancreatic acinar cells

We have previously demonstrated [M. Campos-Toimil, T. Bagrij, J.M. Edwardson, P. Thomas, Two modes of secretion in pancreatic acinar cells: involvement of phosphatidylinositol 3-kinase and regulation by capacitative Ca(2+) entry, Curr. Biol. 12 (2002) 211-215] that in rat pancreatic acinar cells, Gd(3+)-sensitive Ca(2+) entry is instrumental in governing which second messenger pathways control secretory activity. However, in those studies, we were unable to demonstrate a significant increase in cytoplasmic [Ca(2+)] during agonist application as a result of this entry pathway. In the present study, we combined pharmacology with ratiometric imaging of fura-2 fluorescence to resolve this issue. We found that 2 microM Gd(3+) significantly inhibits store-mediated Ca(2+) entry. Furthermore, both the protonophore, CCCP (5 microM) and the mitochondrial Ca(2+)-uptake blocker, RU360 (10 microM), led to an enhancement of the plateau phase of the biphasic Ca(2+) response induced by acetylcholine (1 microM). This enhancement was completely abolished by Gd(3+); and as has been previously shown for Gd(3+), RU360 led to a switch to a wortmannin-sensitive form of exocytosis. Using MitoTracker Red staining we found a close association of mitochondria with the lateral plasma membrane. We propose that in rat pancreatic acinar cells, capacitative Ca(2+) entry is targeted directly to mitochondria; and that as a result of Ca(2+) uptake, these mitochondria release "third" messengers which both enhance exocytosis and suppress phosphatidylinositol 3-kinase-dependent secretion.

Cholecystokinin-stimulated tyrosine phosphorylation of PKC-delta in pancreatic acinar cells is regulated bidirectionally by PKC activation

PKC-delta is important in cell growth, apoptosis, and secretion. Recent studies show its stability is regulated by tyrosine phosphorylation (TYR-P), which can be stimulated by a number of agents. Many of these stimuli also activate phospholipase C (PLC) cascades and little is known about the relationship between these cascades and PKC-delta TYR-P. Cholecystokinin (CCK) stimulates PKCs but it is unknown if it causes PKC-delta TYR-P and if so, the relationship between these cascades is unknown. In rat pancreatic acini, CCK-8 stimulated rapid PKC-delta TYR-P by activation of the low affinity CCK(A) receptor state. TPA had a similar effect. BAPTA did not decrease CCK-stimulated PKC-delta TYR-P but instead, increased it. A23187 did not stimulate PKC-delta TYR-P. Wortmannin and LY 294002 did not alter CCK-stimulated PKC-delta TYR-P. GF 109203X, at low concentrations, increased PKC-delta TYR-P stimulated by CCK or TPA and at higher concentrations, inhibited it. The cPKC inhibitors, Gö 6976 and safingol, caused a similar increase in TPA- and CCK-stimulated PKC-delta TYR-P. These results demonstrate that CCK(A) receptor activation causes PKC-delta TYR-P through activation of only one of its two receptor affinity states. This PKC-delta TYR-P is not directly influenced by changes in [Ca(2+)](i); however, the resultant activation of PKC-alpha has an inhibitory effect. Therefore, CCK activates both stimulatory and inhibitory PKC cascades regulating PKC-delta TYR-P and, hence, likely plays an important role in regulating PKC-delta degradation and cellular abundance.

Two modes of secretion in pancreatic acinar cells: involvement of phosphatidylinositol 3-kinase and regulation by capacitative Ca(2+) entry

In pancreatic acinar cells, muscarinic agonists stimulate both the release of Ca(2+) from intracellular stores and the influx of extracellular Ca(2+). The part played by Ca(2+) released from intracellular stores in the regulation of secretion is well established; however, the role of Ca(2+) influx in exocytosis is unclear. Recently, we observed that supramaximal concentrations of acetylcholine (>or=10 microM) elicited an additional component of exocytosis despite reducing Ca(2+) influx. In the present study, we found that supramaximal exocytosis was substantially inhibited (approximately 70%) by wortmannin (100 nM), an inhibitor of phosphatidylinositol 3-kinase. In contrast, exocytosis evoked by a lower concentration of acetylcholine (1 microM) was potentiated (approximately 45%) by wortmannin. Exocytosis stimulated by 1 microM acetylcholine in the absence of extracellular Ca(2+) was, like supramaximal exocytosis, inhibited by wortmannin. The switch to a wortmannin-inhibitable form of exocytosis depended upon a reduction in Ca(2+) entry through store-operated Ca(2+) channels, as the switch in exocytotic mode could also be brought about by the selective blockade of these channels by Gd(3+) (2 microM), but not by inhibition of noncapacitative Ca(2+) entry by SB203580 (10 microM). We conclude that supramaximal doses of acetylcholine lead to a switch in the mode of zymogen granule exocytosis by inhibiting store-dependent Ca(2+) influx.

Cholecystokinin activates PYK2/CAKbeta by a phospholipase C-dependent mechanism and its association with the mitogen-activated protein kinase signaling pathway in pancreatic acinar cells

PYK2/CAKbeta is a recently described cytoplasmic tyrosine kinase related to p125 focal adhesion kinase (p125(FAK)) that can be activated by a number of stimuli including growth factors, lipids, and some G protein-coupled receptors. Studies suggest PYK2/CAKbeta may be important for coupling various G protein-coupled receptors to the mitogen-activated protein kinase (MAPK) cascade. The hormone neurotransmitter cholecystokinin (CCK) is known to activate both phospholipase C-dependent cascades and MAPK signaling pathways; however, the relationship between these remain unclear. In rat pancreatic acini, CCK-8 (10 nM) rapidly stimulated tyrosine phosphorylation and activation of PYK2/CAKbeta by both activation of high affinity and low affinity CCK(A) receptor states. Blockage of CCK-stimulated increases in protein kinase C activity or CCK-stimulated increases in [Ca(2+)](i), inhibited by 40-50% PYK2/CAKbeta but not p125(FAK) tyrosine phosphorylation. Simultaneous blockage of both phospholipase C cascades inhibited PYK2/CAKbeta tyrosine phosphorylation completely and p125(FAK) tyrosine phosphorylation by 50%. CCK-8 stimulated a rapid increase in PYK2/CAKbeta kinase activity assessed by both an in vitro kinase assay and autophosphorylation. Total PYK2/CAKbeta under basal conditions was largely localized (77 +/- 7%) in the membrane fraction, whereas total p125(FAK) was largely localized (86 +/- 3%) in the cytosolic fraction. With CCK stimulation, both p125(FAK) and PYK2/CAKbeta translocated to the plasma membrane. Moreover CCK stimulation causes a rapid formation of both PYK2/CAKbeta-Grb2 and PYK2/CAKbeta-Crk complexes. These results demonstrate that PYK2/CAKbeta and p125(FAK) are regulated differently by CCK(A) receptor stimulation and that PYK2/CAKbeta is probably an important mediator of downstream signals by CCK-8, especially in its ability to activate the MAPK signaling pathway, which possibly mediates CCK growth effects in normal and neoplastic tissues.

Potentiation of carbachol-induced amylase release by propionate in guinea pig and vole pancreatic acini

The action of propionate, one of the major end products of microbial fermentation in herbivores was investigated in isolated, perifused pancreatic acini of guinea pigs, voles, and mice. With the use of guinea pig acini, 100 microM propionate had no effect, whereas 300 and 600 microM increased amylase release by six- and ninefold, respectively. Simultaneous perifusion of carbachol (CCh) 10 microM plus propionate 100 microM in guinea pig acini produced a potentiated secretory response that was 130% higher than the summated value obtained with CCh and propionate alone. The potentiation by propionate (100 microM) of CCh (10 microM)-induced amylase release was also obtained in vole pancreatic acini, but the mouse pancreatic preparation did not exhibit a similar potentiation. In contrast to CCh, propionate (100-600 microM) alone had no significant effect on intracellular Ca2+ concentration ([Ca2+]i) and did not alter [Ca2+]i elicited by CCh. Ca ionophore A23187 (5 microM)-induced amylase release in guinea pig acini was enhanced twofold by the addition of propionate. Cellular cAMP content was increased slightly by propionate, but did not alter dose dependently. The cAMP level with combinations of CCh and propionate was almost same as that with CCh alone and propionate alone. Staurosporine did not modify amylase secretion induced by a combination of CCh and propionate. These results suggest that propionate, in addition to a direct action on amylase release, potentiates CCh-induced amylase release in guinea pig and vole acini via a secretory pathway not associated with an increase in [Ca2+]i and cellular cAMP.

Secretagogues cause ubiquitination and down-regulation of inositol 1, 4,5-trisphosphate receptors in rat pancreatic acinar cells

BACKGROUND & AIMS

The action of several exocrine pancreas secretagogues depends on the second messenger inositol 1,4, 5-trisphosphate (IP3), which, via endoplasmic reticulum-located IP3 receptors, mobilizes intracellular Ca2+ stores. Signaling pathways like this one are regulated at multiple loci. To determine whether IP3 receptors are one of these loci, we measured IP3 receptor concentration, distribution, and modification in secretagogue-stimulated rat pancreatic acinar cells.

METHODS

Isolated rat pancreatic acinar cells were exposed to cholecystokinin and other secretagogues, or rats were injected intraperitoneally with cerulein. Then samples of cells or pancreata were probed for IP3 receptor content and distribution as well as for ubiquitin association with IP3 receptors.

RESULTS

Secretagogues rapidly down-regulated acinar cell IP3 receptors both in vitro and in vivo. They also elicited receptor redistribution and caused receptors to become ubiquitinated, indicating that the ubiquitin/proteasome proteolytic pathway contributes to the down-regulation. Surprisingly, however, proteasome inhibitors did not block IP3 receptor down-regulation, and phospholipase Cbeta1 and protein kinase C also were down-regulated. Thus, secretagogues simultaneously activate an additional proteolytic pathway.

CONCLUSIONS

Secretagogues rapidly down-regulate IP3 receptors and other proteins involved in intracellular signaling by a mechanism that involves, but is not limited to, the ubiquitin/proteasome pathway. Loss of these proteins may account for the disruption of Ca2+ mobilization that occurs in models of acute pancreatitis, and may contribute to cell adaptation under physiological conditions.

Secretagogue-induced digestive enzyme activation and cell injury in rat pancreatic acini

The mechanisms responsible for intrapancreatic digestive enzyme activation as well as the relationship between that activation and cell injury during pancreatitis are not understood. We have employed an in vitro system in which freshly prepared pancreatic acini are exposed to a supramaximally stimulating concentration of the CCK analog caerulein to explore these issues. We find that in vitro trypsinogen activation depends on the continued presence of Ca2+ in the suspending medium and that it is half-maximal in the presence of 0.3 mM Ca2+. Caerulein-induced trypsinogen activation can be halted by removal of Ca2+ from the suspending medium or by chelation of intracellular Ca2+. Increasing intracellular Ca2+ with either ionomycin or thapsigargin does not induce trypsinogen activation. We have monitored cell injury by measuring the leakage of lactate dehydrogenase (LDH) from acini and by quantitating intercalation of propidium iodide (PI) into DNA. Leakage of LDH and intercalation of PI in response to supramaximal stimulation with caerulein can be detected only after caerulein-induced trypsinogen activation has already occurred, and these indications of cell injury can be prevented by addition of a cell-permeant protease inhibitor. Our findings indicate that caerulein-induced intra-acinar cell activation of trypsinogen depends on a rise in intracellular Ca2+, which reflects entry of Ca2+ from the suspending medium. Intra-acinar cell activation of trypsinogen is an early as well as a critical event in pancreatitis. The subsequent cell injury in this model is mediated by activated proteases.

EGF stimulates tyrosine phosphorylation of focal adhesion kinase (p125FAK) and paxillin in rat pancreatic acini by a phospholipase C-independent process that depends on phosphatidylinositol 3-kinase, the small GTP-binding protein, p21rho, and the integrity of the actin cytoskeleton

Epidermal growth factor (EGF) is a potent mitogen in many cell types including pancreatic cells. Recent studies show that the effects of some growth factors on growth and cell migration are mediated by tyrosine phosphorylation of the cytosolic tyrosine kinase p125 focal adhesion kinase (p125FAK) and the cytoskeletal protein, paxillin. The aim of the present study was to determine whether EGF activates this pathway in rat pancreatic acini and causes tyrosine phosphorylation of each of these proteins, and to examine the intracellular pathways involved. Treatment of pancreatic acini with EGF induced a rapid, concentration-dependent increase in p125FAK and paxillin tyrosine phosphorylation. Depletion of the intracellular calcium pool or inhibition of PKC activation had no effect on the response to EGF. However, inhibition of the phosphatidylinositol 3-kinase (PI3-kinase) or inactivation of p21rho inhibited EGF-stimulated phosphorylation of p125FAK and paxillin by more than 70%. Finally, cytochalasin D, a selective disrupter of the actin filament network, completely inhibited EGF-stimulated tyrosine phosphorylation of both proteins. All these treatments did not modify EGF receptor autophosphorylation in response to EGF. These results identify p125FAK and paxillin as components of the intracellular pathways stimulated after EGF receptor occupation in rat pancreatic acini. Activation of this cascade requires activation of PI3-kinase and participation of p21rho, but not PKC activation and calcium mobilization.

Stimulatory effects of vanadate on amylase release from isolated rat pancreatic acini

The effects of vanadate on exocrine pancreatic function were examined in isolated rat pancreatic acini. Vanadate caused a concentration-dependent stimulation of amylase release above a concentration of 1 mM. Co-incubation of vanadate with vasoactive intestinal polypeptide, 8-bromoadenosine 3':5'-cyclic monophosphate, and the Ca2+ ionophore A23187 produced a synergistic pattern of amylase release, whereas co-incubation with cholecystokinin octapeptide (CCK-8), carbamylcholine, and 12-O-tetradecanoylphorbol 13-acetate produced an additive effect. Vanadate alone had no influence on acinar cyclic AMP content, Ca2+ efflux, or intracellular Ca2+ concentration. However, preincubation with vanadate prevented the plateau phase of CCK-8-induced Ca2+ transient increase from returning to baseline. Moreover, depletion of the intracellular Ca2+ pool by pretreatment of acini with CCK-8 in Ca2+-free medium (plus ethyleneglycol bis[beta-aminoethylether]-N,N'-tetraacetic acid) had no effect on subsequent stimulation by vanadate, although it abolished the response to both CCK-8 and carbamylcholine stimulation. The protein kinase C (PKC) inhibitors staurosporine and calphostin C significantly inhibited vanadate-stimulated amylase release, whereas the protein tyrosine kinase inhibitor genistein had no inhibitory effect. Moreover, vanadate caused a significant translocation of PKC from cytosol to membrane fraction in pancreatic acinar cells. This translocation was inhibited significantly by staurosporine and calphostin C but not by genistein. These results suggest that vanadate acts directly on pancreatic acini and stimulates amylase release by activating PKC without an effect on Ca2+ mobilization, cyclic AMP, or protein tyrosine kinase.

Depletion of intracellular calcium stores activates a calcium conducting nonselective cation current in mouse pancreatic acinar cells

Receptor-mediated Ca2+ release from inositol (1,4,5)-trisphosphate (IP3)-sensitive Ca2+ stores causes "capacitative calcium entry" in many cell types (Putney, J. W., Jr. (1986) Cell Calcium 7, 1-12; Putney, J. W., Jr. (1990) Cell Calcium 11, 611-624). We used patch-clamp and fluorescence techniques in isolated mouse pancreatic acinar cells to identify ion currents and cytosolic calcium concentrations under conditions in which intracellular Ca2+ stores were emptied. We found that depletion of Ca2+ stores activated a calcium-release-activated nonselective cation current (ICRANC) which did not discriminate between monovalent cations. ICRANC possessed a significant conductance for Ca2+ and Ba2+. It was not inhibited by La3+, Gd3+, Co2+, or Cd2+ but was completely abolished by flufenamic acid or genistein. In whole cell and cell-attached recordings, a 40-45 pS nonselective cation channel was identified which was activated by Ca2+ store depletion. Calcium entry as detected by single cell fluorescence measurements with fluo-3 or fura-2, showed the same pharmacological properties as ICRANC. We conclude that in mouse pancreatic acinar cells 40-45 pS nonselective cation channels serve as a pathway for capacitative Ca2+ entry. This entry pathway differs from the previously described ICRAC (Hoth, M., and Penner, R. (1992) Nature 355, 353-356) in its ion-selectivity, pharmacological profile, and single-channel conductance.

Inhibitory effect of caffeine on the response to carbachol in rat pancreatic acini

1. Caffeine did not affect by itself the amylase release, nor intracellular calcium concentration in rat pancreatic acini. 2. Concentration of caffeine higher than 1 mM inhibited the amylase released in response to the muscarinic agonist carbamylcholine. This inhibition was also seen on the intracellular calcium elevation: 20 mM caffeine inhibited by 80% the calcium elevation in response to 100 microM carbachol. 3. Caffeine also prevented the specific binding of [3H]-N-methylscopolamine ([3H]-NMS) on rat pancreatic muscarinic receptors. 4. We conclude that caffeine behaves as a muscarinic antagonist.

Gastric chief cells possess NK1 receptors which mediate pepsinogen secretion and are regulated by agents that increase cAMP and phospholipase C

In order to determine whether tachykinins alter the function of chief cells and to characterize the receptors mediating the effect, we investigated the abilities of various substance P (SP)-related peptides to inhibit the binding of 125I-Bolton-Hunter labeled substance P (125I-BH-SP) and their abilities to alter cell function in dispersed chief cells from guinea pig stomach. Binding of 125I-BH-SP was saturable, reversible, time- and temperature-dependent and was inhibited by several SP-related peptides with relative potencies of SP = physalaemin (IC50:0.19 nM) > SP methyl ester (SP-ME) (IC50:3.3 nM) > eledoisin (IC50:6.1 nM) > neurokinin A (NKA) (IC50: 65 nM) > neurokinin B (NKB) (IC50:80 nM). Analyses of these binding data demonstrated that chief cells possess a high and low affinity class of binding sites. Neither 125I-NKA nor [phenylalanyl-3,4,5-3H]senktide demonstrated saturable binding to chief cells. Acid stripping experiments demonstrated rapid ligand internalization with 55% of the bound radioligand internalized by 10 min. Phospholipase C activating agents (carbachol, CCK-8), adenylate cyclase activating agents (secretin, VIP), TPA and the calcium ionophore, A23187, all inhibited the binding of 125I-BH-SP and it was due to inhibition of ligand internalization with no change in surface bound parameters. SP (0.1 microM) stimulated pepsinogen secretion but was 4-times less efficacious than CCK-8 (10 nM) or carbachol (1 mM). 10 nM SP stimulated a rapid increase in cytoplasmic free calcium concentration ([Ca2+]i) followed by a sustained elevation lasting 2 min. Single cell spectroscopy demonstrated SP (10 pM to 1 microM) did not cause calcium oscillations. The NK1 receptor antagonist, CP96,345 specifically inhibited the SP-stimulated changes in [Ca2+]i and pepsinogen secretion. The relative potencies of SP-related peptides to stimulate pepsinogen secretion and [Ca2+]i demonstrated a close agreement with their abilities to inhibit the binding of 125I-BH-SP, and comparison of the dose-response curves suggests occupation of the low affinity sites mediate changes in biologic activity. In conclusion, the present study demonstrates that chief cells possess a NK1 subtype of tachykinin receptor, occupation of the low affinity sites of this receptor cause calcium mobilization and pepsinogen secretion, and that binding to this receptor is regulated by agents that activate phospholipase C, adenylate cyclase, protein kinase C and calcium mobilization.

Cholecystokinin-stimulated enzyme secretion from dispersed rabbit pancreatic acinar cells: phosphorylation-dependent changes in potency and efficacy

In order to establish a regulatory role for phosphoproteins in receptor-stimulated enzyme secretion, dispersed rabbit pancreatic acinar cells were stimulated with the COOH-terminal octapeptide of cholecystokinin (CCK8) in the absence and presence of staurosporine and/or 12-O-tetradecanoylphorbol 13-acetate (TPA) or forskolin. The dose/response curve for the stimulatory effect of CCK8 on amylase secretion was biphasic, with a mean half-maximal concentration (EC50) of 21 pM. Staurosporine (1 microM) did not affect secretion elicited by CCK8 concentrations below 0.1 nM, but reduced the response to CCK8 concentrations above 0.1 nM. As a result, the mean EC50 for CCK8 decreased to 8 pM and its efficacy to 70%. The phorbol ester TPA (0.1 microM) attenuated secretion evoked by CCK8 concentrations below 0.1 nM and potentiated the response to CCK8 concentrations above 0.1 nM. As a result, the mean EC50 for CCK8 increased to 0.14 nM and its efficacy to 300%. Staurosporine abolished both the inhibitory and the potentiating effect of TPA, thereby turning the inhibitory effect into a strong potentiating effect. As a result, the mean EC50 for CCK8 decreased to 3 pM, whereas its efficacy increased to 190%. Forskolin (30 microM) potentiated the response to both the lower and the higher CCK8 concentrations. As a result, the mean EC50 for CCK8 increased to 28 pM and its efficacy to 300%. Staurosporine enhanced the potentiating effect of forskolin at CCK8 concentrations below 0.1 nM, but abolished potentiation at CCK8 concentrations above 0.1 nM. As a result, the mean EC50 for CCK8 decreased to 1.4 pM, whereas its efficacy increased to 260%. The data presented demonstrate that the apparent sensitivity of dispersed pancreatic acinar cells to stimulation of the process of enzyme secretion by CCK8 decreases when kinases are activated and increases when kinases are inactivated. Moreover, they show that the efficacy of CCK8 increases by the action of kinases, both sensitive and insensitive to staurosporine.

Calcium signal induced by mechanical perturbation of osteoclasts

Multinucleated osteoclasts from rabbit long bone, 1-6 days in culture, respond to mechanical perturbation with a transient increase of intracellular calcium concentration ([Ca2+]i), as measured with the fluorescent indicator fluo-3 on a confocal laser scanning microscope. In experiments with different extracellular calcium concentrations (from 11.8 mM to calcium-free), the incidence, the magnitude, and the duration of [Ca2+]i responses decreases with decreasing bathing [Ca2+]. Following mechanical perturbation, a thapsigargin-induced [Ca2+]i response has a lower magnitude than the thapsigargin-induced response without mechanical perturbation. In thapsigargin-pretreated osteoclasts the mechanical perturbation-induced rise in [Ca2+]i is larger and longer than in control cells. Ni2+ inhibits the incidence and decreases both the magnitude and the duration of the responses, while nifedipine, verapamil, and Gd3+ have no effect. These measurements show that rabbit osteoclasts transduce a mechanical perturbation of the cell membrane into a [Ca2+]i signal via both a calcium influx and an internal calcium release.

Thapsigargin stimulates intracellular calcium mobilization and inhibits parathyroid hormone release

Ca2+ and other divalent cations like Sr2+, Ba2+, and Mg2+ stimulate rapid and sustained increases in intracellular Ca2+ ([Ca2+]i) and 1,4,5-inositol trisphosphate (1,4,5-InsP3) presumably by interacting with recently identified parathyroid cell membrane Ca2+ receptors. We used thapsigargin (THAPS), an inhibitor of the microsomal Ca(2+)-ATPase, to deplete InsP3-sensitive intracellular Ca2+ stores to determine whether sustained increases in [Ca2+]i due to divalent cations require intact cytosolic Ca2+ pools. In Fura 2-loaded parathyroid cells, THAPS produced a gradual increase in [Ca2+]i which reached a steady-state level by 2-3 minutes. The effect of THAPS (3 x 10(-6) M) was substantial with [Ca2+]i, rising from 281 +/- 27 nM at 0.5 mM Ca2+ to a peak value of 684 +/- 30 nM (p < 0.0001). The addition of Sr2+ to cells at 0.5 mM extracellular Ca2+ induced an immediate 2- to 3-fold increase in [Ca2+]i which stabilized at a [Ca2+]i above baseline for > or = 10 minutes. THAPS (3 x 10(-6) M) pretreatment for > or = 5 minutes blocked this sustained-phase increment in [Ca2+]i due to Sr2+. In the absence of extracellular Ca2+, there was a slight but nonsignificant effect of THAPS on [Ca2+]i. Incubation of cells with THAPS did not change the levels of 3H-inositol phosphates (InsP3, InsP2, and InsP1) or alter Sr(2+)-induced accumulation of InsP3, InsP2, and InsP1.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of the three different states of the cholecystokinin (CCK) receptor in pancreatic acini

By measuring binding of [125I]CCK-8 and [3H]L-364,718 to rat pancreatic acini we demonstrated directly that the pancreatic CCK receptor can exist in three different affinity states with respect to CCK--high affinity, low affinity and very low affinity. Binding of [125I]CCK-8 reflects interaction of the tracer with the high and low affinity states, whereas binding of [3H]L-364,718 reflects interaction of the tracer with the low and very low affinity states. Treating acini with carbachol abolished the high affinity state of the CCK receptor and converted approximately 25% of the low affinity receptors to the very low affinity state. Carbachol treatment was particularly useful in establishing the values of Kd for the high and low affinity states for different CCK receptor agonists and antagonists. Of the various CCK receptor agonists tested, CCK-8 had the highest affinity for the high affinity state (Kd approximately 1 nM), whereas CCK-JMV-180 had the highest affinity for the low (Kd 7 nM) and very low affinity (Kd 200 nM) states. Gastrin and de(SO4)CCK-8 had affinities for the high and low affinity states of the receptor that were 100- to 400-fold less than those of CCK-8 but had affinities for the very low affinity state that were only 3- to 10-fold less than that of CCK-8. CCK receptor antagonists showed several patterns in interacting with the different states of the CCK receptor. L-364,718 had the same affinity for each state of the CCK receptor. CR1409 and Bt2cGMP each had similar affinities for the high and low affinity states and lower affinity for the very low affinity state. L-365,260 and CCK-JMV-179 had the highest affinity for the low affinity state and lower affinities for the high and very low affinity states. Different CCK receptor agonists caused the same maximal stimulation of amylase secretion but showed different degrees of amplification in terms of the relationship between their abilities to stimulate amylase secretion and their abilities to occupy the low affinity state of the CCK receptor. When amplification was expressed quantitatively as the value of Kd for the low affinity state divided by the corresponding EC50 for stimulating amylase secretion the values were CCK-8 (1000), de(SO)CCK-8 (1500), gastrin (100) and CCK-JMV-180 (Menozzi, D., Vinayek, R., Jensen, R.T. and Gardner, J.D. (1991) J. Biol. Chem. 266, 10385-1091).(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of cholecystokinin (CCK) and other secretagogues on isoforms of protein kinase C (PKC) in pancreatic acini

We used rat pancreatic acini and measured the effects of various agents on digestive enzyme secretion, diacylglycerol (DAG) and the cellular distribution of protein kinase C (PKC) enzyme activity as well as isoforms of PKC determined by quantitative immunoblot analysis. TPA, but not CCK-8, caused translocation of PKC enzyme activity from the cytosol fraction to the membrane fraction. Immunoblot analysis detected PKC-alpha, PKC-delta, PKC-epsilon and PKC-zeta. PKC-beta, PKC-gamma and PKC-eta were not detected. TPA caused translocation of all isoforms from cytosol to membrane, whereas CCK-8 caused translocation of PKC-delta and PKC-epsilon, carbachol caused translocation of PKC-epsilon, and bombesin and secretin caused no detectable translocation of any isoform. Specific receptor antagonists could prevent, as well as reverse completely, the translocation of PKC isoforms caused by CCK-8 or carbachol. Agonists added in sequence with an interposed addition of a specific receptor antagonist caused cycling of PKC-epsilon between cytosol and membrane fractions. Each receptor-mediated agonist that caused translocation of PKC also increased DAG, and with CCK-8 and carbachol cycling of PKC-epsilon between cytosol and membrane was accompanied by corresponding cyclic changes in cellular DAG. CCK-JMV-180, bombesin and secretin increased DAG but did not cause translocation of any PKC isoform. Translocation of a PKC isoform could be accounted for by whether the increased DAG originated from PIP2 (accompanied by translocation) or from phosphatidylcholine (no accompanying translocation). Thus it appeared that DAG, in pancreatic acini, is functionally compartmentalized depending on the source of the lipid. Studies using CCK-8 and CCK-JMV-180 indicated that occupation of the low affinity state of the CCK receptor by either peptide increased DAG from phosphatidylcholine, whereas occupation of the very low affinity state by CCK-8 increased DAG from PIP2 and caused translocation of PKC-delta and PKC-epsilon. TPA stimulated amylase secretion, indicating that activation of PKC can stimulate enzyme secretion; however, with the various receptor-mediated secretagogues there was no consistent, unequivocal correlation between translocation of an isoform of PKC and accompanying changes in enzyme secretion.

Calcium release by cholecystokinin analogue OPE is IP3 dependent in single rat pancreatic acinar cells

Cholecystokinin (CCK) and carbachol raise intracellular Ca2+ concentration ([Ca2+]i) in pancreatic acinar cells by elevating inositol 1,4,5-trisphosphate (IP3). CCK analogues JMV-180 and OPE stimulate fully efficacious enzyme secretion and [Ca2+]i oscillations but release Ca2+ from intracellular stores by apparently IP3-independent mechanisms in permeabilized acinar cells. In the present study, we investigated whether OPE mobilizes Ca2+ from IP3-sensitive Ca2+ stores and whether IP3 mediates such responses in single intact cells. OPE and JMV-180 similarly elevated IP3 to low levels compared with those elicited by 10 nM CCK. Depletion of IP3-sensitive stores by elevation of intracellular IP3 using carbachol, microinjection of a nonmetabolizable IP3 analogue, or exposure to thapsigargin, in the absence of extracellular Ca2+, depleted the same Ca2+ stores that were sensitive to OPE. In converse experiments, OPE depleted carbachol- or thapsigargin-sensitive stores, indicating that carbachol-, thapsigargin-, IP3-, and OPE-sensitive Ca2+ stores overlap completely and that stores mobilized by OPE are IP3 sensitive. To determine whether IP3 mediates responses to OPE, cells were microinjected with low-molecular-weight heparin, a competitive inhibited the rise of [Ca2+]i in response to carbachol, OPE, or JMV-180, whereas de-N-sulfated heparin, an inactive heparin, was without effect. These results indicate that CCK analogues release Ca2+ from IP3-sensitive Ca2+ stores by mechanisms involving the IP3 receptor.

Supramaximal inhibition of cholecystokinin-induced pancreatic amylase release involves desensitization to cytoplasmic Ca2+

BACKGROUND

Cholecystokinin (CCK) is a major stimulant of pancreatic enzyme secretion. The dose-response relationship for CCK-induced secretion is bell-shaped, with a characteristic supramaximal inhibition. The mechanism for this inhibition has now been studied.

METHODS

The kinetics of amylase release and the changes of the cytoplasmic Ca2+ concentration ([Ca2+]i) were recorded during stimulation of guinea-pig pancreatic acinar cells with different concentrations of cholecystokinin octapeptide (CCK-8) and the Ca2+ ionophore ionomycin.

RESULTS

Individual cells reacted with [Ca2+]i oscillations at 10(-11)-10(-10) M CCK-8 and with an initial peak followed by a sustained suprabasal level at 10(-9)-10(-8) M of the agonist. The latter response was also seen in suspensions of acinar cells at all tested concentrations of CCK-8 and at 10(-6)-10(-5) M of ionomycin. With increases of extracellular Ca2+ from 0.5 to 5.0 mM there was a rise of [Ca2+]i during exposure to 10(-9)-10(-8) M CCK-8 or 10(-5) M ionomycin but a paradoxical decrease at lower concentrations of CCK-8 or ionomycin. A dose-dependent increase of amylase release was seen at CCK-8 concentrations from 10(-11) to 10(-9) M. At 10(-9)-10(-8) M CCK-8 secretion was characterized by an initial peak followed by a sustained phase. Whereas the initial peak of secretion remained unaffected by increasing CCK-8 from 10(-9) to 10(-8) M, the sustained phase was inhibited (supramaximal inhibition). Increasing extracellular Ca2+ from 0.5 to 5.0 mM transiently enhanced secretion in response to 10(-9) M but lacked effect during supramaximal inhibition of secretion by 10(-8) M CCK-8.

CONCLUSIONS

Both initial and sustained CCK-8-stimulated amylase release increase with [Ca2+]i. However, supramaximal inhibition of secretion was not due to a decrease of [Ca2+]i but was characterized by desensitization to the stimulatory effect of [Ca2+]i.

Effect of inhibition of microsomal Ca(2+)-ATPase on cytoplasmic calcium and enzyme secretion in pancreatic acini

We used thapsigargin (TG), 2,5-di-tert-butyl-1,4-benzohydroquinone (BHQ) and cyclopiazonic acid (CPA), each of which inhibits microsomal Ca(2+)-ATPase, to evaluate the effects of this inhibition on cytoplasmic free calcium ([Ca2+]i) and secretagogue-stimulated enzyme secretion in rat pancreatic acini. Using single-cell microspectrofluorimetry of fura-2-loaded acini we found that all three agents caused a sustained increase in [Ca2+]i by mobilizing calcium from inositol-(1,4,5)-trisphosphate-sensitive intracellular calcium stores and by promoting influx of extracellular calcium. Concentrations of all three agents that increased [Ca2+]i potentiated the stimulation of enzyme secretion caused by secretagogues that activate adenylate cyclase but inhibited the stimulation of enzyme secretion caused by secretagogues that activate phospholipase C. With BHQ, potentiation of adenylate cyclase-mediated enzyme secretion occurred immediately whereas inhibition of phospholipase C-mediated enzyme secretion occurred only after several min of incubation. In addition, the effects of BHQ and CPA on both [Ca2+]i and secretagogue-stimulated enzyme secretion were reversed completely by washing whereas the actions of TG could not be reversed by washing. Concentrations of BHQ in excess of those that caused maximal changes in [Ca2+]i inhibited all modes of stimulated enzyme secretion by a mechanism that was apparently unrelated to changes in [Ca2+]i. Finally, in contrast to the findings with TG and BHQ, CPA inhibited bombesin-stimulated enzyme secretion over a range of concentrations that was at least 10-fold lower than the range of concentrations over which CPA potentiated VIP-stimulated enzyme secretion.

Effect of thapsigargin on calcium homeostasis in Trypanosoma cruzi trypomastigotes and epimastigotes

By using the fluorescent calcium indicator fura-2, it was found that the concentration of free Ca2+ in the cytoplasm of Trypanosoma cruzi trypomastigotes incubated in the presence or absence of external calcium was maintained at very low levels (10-20 nM). When trypomastigotes were incubated in the presence of succinate and ATP and permeabilized with digitonin, they lowered the medium calcium concentration to a submicromolar level. In the presence of 1 microM FCCP the initial rate of Ca2+ sequestration by these permeabilized cells was very slow. When succinate alone was present, the initial rate of Ca2+ accumulation was slower than with ATP plus succinate, and the calcium set point was about 0.6 microM. The succinate dependence and FCCP sensitivity of the later Ca2+ uptake indicate that it may be exerted by the mitochondria. High concentrations of the tumor promoter thapsigargin slightly increased cytosolic Ca2+ in the presence of extracellular Ca2+ but had no effect on the FCCP- and oligomycin/antimycin A-insensitive Ca2+ pool. In addition, when used at those concentrations (4-20 microM), thapsigargin was shown to release Ca2+ from the mitochondria and to decrease the inner mitochondrial membrane potential of trypomastigotes and epimastigotes as measured using safranine O. Despite the presence of inositol phosphates as determined by [3H]inositol incorporation, no IP3-sensitive Ca2+ release could be detected in trypomastigotes.