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

Neuronal and hormonal control of Ca2+ signalling in exocrine glands: insight from in vivo studies

Fluid and enzyme secretion from exocrine glands is initiated by Ca2+ signalling in acinar cells and is activated by external neural or hormonal signals. A wealth of information has been derived from studies in acutely isolated exocrine cells but Ca2+ signalling has until recently not been studied in undisrupted intact tissue in live mice. Our in vivo observations using animals expressing genetically encoded Ca2+ indicators in specific cell types in exocrine glands revealed both similarities to and differences from the spatiotemporal characteristics previously reported in isolated cells. These in vivo studies facilitate further understanding of how both neuronal and hormonal input shapes Ca2+ signalling events in a physiological setting and how these signals are translated into the stimulation of fluid secretion and exocytosis.

Pacing intracellular Ca2+ signals in exocrine acinar cells

An increase in intracellular [Ca2+ ] in exocrine acinar cells resident in the salivary glands or pancreas is a fundamental event that drives fluid secretion and exocytosis of proteins. Stimulation with secretagogues initiates Ca2+ signals with precise spatiotemporal properties thought to be important for driving physiological output. Both in vitro, in acutely isolated acini, and in vivo, in animals expressing genetically encoded indicators, individual cells appear specialized to initiate Ca2+ signals upon stimulation. Furthermore, these signals appear to spread to neighbouring cells. These properties are present in the absence of a conventional pacemaker mechanism dependent on the cyclical activation of Ca2+ -dependent or Ca2+ -conducting plasma membrane ion channels. In this article, we propose a model for 'pacing' intracellular Ca2+ signals in acinar cells based on the enhanced sensitivity of a subpopulation of individual cells and the intercellular diffusion through gap junctions of inositol 1,4,5-trisphosphate and Ca2+ to neighbouring cells.

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

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

Time-resolved quantitative analysis of CCK1 receptor-induced intracellular calcium increase

Cholecystokinin (CCK) is a gastrointestinal hormone, which regulates many physiological functions such as satiety by binding to the CCK receptor (CCKR). Molecules, which recognize this receptor can mimic or block CCK signaling and thereby influence CCKR-mediated processes. We have set up a quantitative heterologous assay with CHO cells over-expressing the rat CCK1 receptor to screen for such candidate molecules. Receptor activation, induced by agonist binding, is followed by an intracellular calcium increase, which was monitored using a fluorescent sensor dye. For quantification of the calcium increase, a population average technique using a fluorescence plate reader was optimized and subsequently compared with a single-cell approach using confocal microscopy. With both strategies, dose-response curves were generated for the natural agonist CCK-8S, the partial agonist JMV-180 as well as the antagonist lorglumide. Significant differences were found between the ligands and a strong correspondence was observed between both methods in terms of maximum response and median effect concentrations. Both highly sensitive methods proved complementary: whereas the plate reader assay allowed faster, high throughput screening, the confocal microscopy identified single-cell variations and revealed factors that reduce specificity and sensitivity.

Somatic CTG*CAG repeat instability in a mouse model for myotonic dystrophy type 1 is associated with changes in cell nuclearity and DNA ploidy

Background

Trinucleotide instability is a hallmark of degenerative neurological diseases like Huntington's disease, some forms of spinocerebellar ataxia and myotonic dystrophy type 1 (DM1). To investigate the effect of cell type and cell state on the behavior of the DM1 CTG•CAG repeat, we studied a knock-in mouse model for DM1 at different time points during ageing and followed how repeat fate in cells from liver and pancreas is associated with polyploidization and changes in nuclearity after the onset of terminal differentiation.

Results

After separation of liver hepatocytes and pancreatic acinar cells in pools with 2n, 4n or 8n DNA, we analyzed CTG•CAG repeat length variation by resolving PCR products on an automated PAGE system. We observed that somatic CTG•CAG repeat expansion in our DM1 mouse model occurred almost uniquely in the fraction of cells with high cell nuclearity and DNA ploidy and aggravated with aging.

Conclusion

Our findings suggest that post-replicative and terminal-differentiation events, coupled to changes in cellular DNA content, form a preconditional state that influences the control of DNA repair or recombination events involved in trinucleotide expansion in liver hepatocytes and pancreatic acinar cells.

Oxidant-impaired intracellular Ca2+ signaling in pancreatic acinar cells: role of the plasma membrane Ca2+-ATPase

Pancreatitis is an inflammatory disease of pancreatic acinar cells whereby intracellular calcium concentration ([Ca(2+)](i)) signaling and enzyme secretion are impaired. Increased oxidative stress has been suggested to mediate the associated cell injury. The present study tested the effects of the oxidant, hydrogen peroxide, on [Ca(2+)](i) signaling in rat pancreatic acinar cells by simultaneously imaging fura-2, to measure [Ca(2+)](i), and dichlorofluorescein, to measure oxidative stress. Millimolar concentrations of hydrogen peroxide increased cellular oxidative stress and irreversibly increased [Ca(2+)](i), which was sensitive to antioxidants and removal of external Ca(2+), and ultimately led to cell lysis. Responses were also abolished by pretreatment with (sarco)endoplasmic reticulum Ca(2+)-ATPase inhibitors, unless cells were prestimulated with cholecystokinin to promote mitochondrial Ca(2+) uptake. This suggests that hydrogen peroxide promotes Ca(2+) release from the endoplasmic reticulum and the mitochondria and that it promotes Ca(2+) influx. Lower concentrations of hydrogen peroxide (10-100 muM) increased [Ca(2+)](i) and altered cholecystokinin-evoked [Ca(2+)](i) oscillations with marked heterogeneity, the severity of which was directly related to oxidative stress, suggesting differences in cellular antioxidant capacity. These changes in [Ca(2+)](i) also upregulated the activity of the plasma membrane Ca(2+)-ATPase in a Ca(2+)-dependent manner, whereas higher concentrations (0.1-1 mM) inactivated the plasma membrane Ca(2+)-ATPase. This may be important in facilitating "Ca(2+) overload," resulting in cell injury associated with pancreatitis.

R-Ras alters Ca2+ homeostasis by increasing the Ca2+ leak across the endoplasmic reticular membrane

Evidence in the literature implicating both Ras-like Ras (R-Ras) and intracellular Ca(2+) in programmed cell death and integrin-mediated adhesion prompted us to investigate the possibility that R-Ras alters cellular Ca(2+) handling. Chinese hamster ovary cells expressing the cholecystokinin (CCK)-A receptor were loaded with indo-1 to study the effects of constitutively active V38R-Ras and dominant negative N43R-Ras on the kinetics of the thapsigargin (Tg)- and CCK(8)-induced Ca(2+) rises using high speed confocal microscopy. In the absence of extracellular Ca(2+), both 1 microm Tg, a potent and selective inhibitor of the Ca(2+) pump of the intracellular Ca(2+) store, and 100 nm CCK(8) evoked a transient rise in Ca(2+), the size of which was decreased significantly after expression of V38R-Ras. At 0.1 nm, CCK(8) evoked periodic Ca(2+) rises. The frequency of these Ca(2+) oscillations was reduced significantly in V38R-Ras-expressing cells. In contrast to V38R-Ras, N43R-Ras did not alter the kinetics of the Tg- and CCK(8)-induced Ca(2+) rises. The present findings are compatible with the idea that V38R-Ras expression increases the passive leak of Ca(2+) of the store leading to a decrease in Ca(2+) content of this store, which, in turn, leads to a decrease in frequency of the CCK(8)-induced cytosolic Ca(2+) oscillations. The effect of V38R-Ras on the Ca(2+) content of the intracellular Ca(2+) store closely resembles that of the antiapoptotic protein Bcl-2 observed earlier. Together with reports on the role of dynamic Ca(2+) changes in integrin-mediated adhesion, this leads us to propose that the reduction in endoplasmic reticulum Ca(2+) content may underlie the antiapoptotic effect of R-Ras, whereas the decrease in frequency of stimulus-induced Ca(2+) oscillations may play a role in the inhibitory effect of R-Ras on stimulus-induced cell detachment and migration.

Native LDL potentiate TNF alpha and IL-8 production by human mononuclear cells

Native LDL (nLDL) increases expression of adhesion molecules on endothelial cells through induction of Ca(2+) mobilization. Ca(2+) mobilization is also involved in the induction of proinflammatory cytokines, important mediators involved in atherogenesis. The aim of the study was to evaluate the capacity of nLDL to affect spontaneous and lipopolysaccharide (LPS)-stimulated cytokine production. Preincubation of human peripheral blood mononuclear cells (PBMC) with nLDL for 24 h did not influence spontaneous production of tumor necrosis factor alpha (TNF alpha) or interleukin-8 (IL-8), but significantly potentiated LPS-induced production of these cytokines. nLDL preincubation of PBMC did not increase the expression of the LPS receptors Toll-like receptor-4, CD14, or CD11c/CD18. Potentiation of cytokine production by nLDL was mediated through induction of Ca(2+) mobilization, because: a) nLDL induced a sustained pattern of repetitive Ca(2+) transients in human PBMC; b) the Ca(2+) chelator fura 2-acetoxymethyl ester, 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, an intracellular Ca(2+) chelator, inhibited the potentiating effect of nLDL on LPS-induced cytokine synthesis; c) induction of Ca(2+) mobilization by thapsigargin potentiated LPS-induced cytokine production. nLDL are able to potentiate LPS-induced production of cytokines by human PBMC, and this effect is probably mediated through induction of Ca(2+) mobilization. This may represent an important pathogenetic mechanism in atherogenesis induced by hyperlipoproteinemia.

ATP-stimulated Ca(2+)-activated K(+) efflux pathway and differentiation of human placental cytotrophoblast cells

The aim of this study was to determine whether extracellular ATP ([ATP](o)) stimulated a Ca(2+)-activated K(+) efflux in trophoblast cells that was dependent on extracellular Ca(2+) ([Ca(2+)](o)). Cytotrophoblast cells, isolated from human placenta, were examined following 18 h (relatively undifferentiated) and 66 h (multinucleate cells) of culture. Potassium efflux was measured using (86)Rb as a trace marker. Intracellular Ca(2+) ([Ca(2+)](i)) was examined by microfluorometry using fura 2. [ATP](o) significantly increased (86)Rb efflux to a peak that declined to control (18-h cells) or an elevated plateau (66-h cells) and was inhibited by 100 nM charybdotoxin. Removing [Ca(2+)](o) significantly reduced (86)Rb efflux in both groups as did application of 150 microM GdCl(3). [ATP](o) significantly increased [Ca(2+)](i) in both groups of cells. The response was reduced by removing [Ca(2+)](o) and applying 150 microM GdCl(3). For both (86)Rb efflux and microfluorometry experiments, the response to [ATP](o) was more dependent on [Ca(2+)](o) in 66-h cells compared with 18-h cells (approximately 70% greater). Cytotrophoblast cells exhibit an [ATP](o)-stimulated Ca(2+)-activated K(+) efflux. The dependency of this pathway on [Ca(2+)](o) is greater in the 66-h multinucleate syncytiotrophoblast-like cells, suggesting that the mechanism for Ca(2+) entry may be altered during differentiation of trophoblast cells.

Oxidized low-density lipoprotein induces calcium influx in polymorphonuclear leukocytes

Polymorphonuclear leukocytes (PMN) have been suggested to play a role in atherosclerosis, but intracellular signaling after stimulation with oxidized low-density lipoprotein (LDL) is unknown. We investigated mechanistic aspects of oxidized LDL-induced superoxide production by human PMN, with special emphasis on intracellular Ca(2+) concentration ([Ca(2+)](i)). Oxidized LDL, but not native LDL, evoked an early but sustained increase in [Ca(2+)](i) and a delayed production of superoxide. The increase in [Ca(2+)](i) could be reduced by fucoidan and completely prevented by U73122, suggesting involvement of the scavenger receptor and coupling to the phospholipase C signal transduction pathway. Furthermore, we provide evidence that the increase in [Ca(2+)](i) partly results from protein kinase C-dependent Ca(2+) influx. The relevance of this Ca(2+) entry for oxidized LDL-stimulated effects is illustrated by the finding that superoxide production was markedly reduced in the absence of external Ca(2+). Finally, inhibition of phagocytosis by cytochalasin B abolished oxidized LDL-stimulated superoxide production without affecting, however, the Ca(2+) mobilization. These effects of oxidized LDL on [Ca(2+)](i) and on respiratory burst of PMN may underlie the occurrence of elevated levels of [Ca(2+)](i) of resting PMN in hypercholesterolemia and represent a mechanism by which PMN can amplify processes in the early phase of atherosclerosis.

Pharmacological evaluation of the role of cytochrome P450 in intracellular calcium signalling in rat pancreatic acinar cells

We have investigated whether the cytochrome P450 system is involved in Ca(2+) signalling in rat pancreatic acinar cells. Intracellular free [Ca(2+)] ([Ca(2+)](i)) was measured in collagenase-isolated cells using fura-2 microspectrofluorimetry and imaging. The imidazole P450 inhibitor ketoconazole (5 - 50 microM) inhibited [Ca(2+)](i) oscillations induced by cholecystokinin octapeptide (CCK). However, ketoconazole also raised baseline [Ca(2+)](i) when applied in the absence of CCK. These effects were mimicked by 5 - 50 microM SKF96365, an imidazole widely used as an inhibitor of Ca(2+) entry. The non-imidazole P450 inhibitor proadifen (SKF525A) inhibited CCK-induced [Ca(2+)](i) oscillations at a concentration of 10 - 50 microM. Proadifen alone caused intracellular Ca(2+) release at 25 or 50 microM, but not at 10 microM. Octadecynoic acid and 1-aminobenzotriazole, structurally-unrelated non-imidazole P450 inhibitors, did not alter baseline [Ca(2+)](i) or CCK-evoked oscillations. We compared cumulative CCK dose-response relationship in control cells and in cells where P450 had been induced by prior injection of animals with beta-naphthoflavone. Only minor differences were apparent, with induced cells showing some decrease in responsiveness at moderate and higher concentration of CCK (30 pM - 3 nM). Direct assessment of depletion-activated Ca(2+) entry showed no clear differences between control and induced cells. In conclusion, we could find no compelling evidence for a role of P450 in controlling Ca(2+) signalling generally, or Ca(2+) entry in particular, in pancreatic acinar cells. Induction of P450 is therefore probably toxic to acinar cells via a Ca(2+)-independent mechanism.

Inhibitory and stimulatory interactions between endogenous gonadotropin-releasing hormones in the African catfish (Clarias gariepinus)

In the brain of all vertebrate classes, chicken (c) GnRH-II ([His(5), Trp(7),Tyr(8)]GnRH, cGnRH-II) is expressed in the mesencephalon. In addition, at least one other form of GnRH is expressed in the preoptical area/hypothalamus. In the human pituitary stalk and the mouse median eminence, cGnRH-II is present together with mammalian GnRH. Similarly, in the pituitary of several teleost fish (e.g., goldfish and eel, but not salmon or trout), a teleost GnRH is found together with cGnRH-II. These GnRHs are not colocalized in the same cells. Hence, these GnRH peptides may differentially regulate gonadotropin secretion and, in addition, may exert their effects simultaneously. The current study therefore investigated the effects of combinations of the two forms of GnRH present in the African catfish (Clarias gariepinus) pituitary-cGnRH-II and catfish GnRH ([His(5),Asn(8)]GnRH, cfGnRH)-on the cytosolic free calcium concentration ([Ca(2+)](i)) in single, Fura-2-loaded catfish gonadotrophs, as well as their effects on both in vitro and in vivo LH secretion. Both inhibitory and stimulatory effects of combinations of cfGnRH and cGnRH-II on [Ca(2+)](i) were observed, which were mirrored by their effects on both in vitro and in vivo LH secretion. The following pattern became apparent. The effect of intermediate or maximal effective cfGnRH doses was inhibited by the simultaneous presence of subthreshold or borderline effective cGnRH-II doses. Conversely, subthreshold or borderline effective concentrations of cfGnRH enhanced the effects of intermediate and maximal concentrations of cGnRH-II. In addition, combinations of cfGnRH and cGnRH-II concentrations that were equally active when tested separately showed an additive effect. The observed interactions between the two GnRHs may be of particular physiological relevance in the control of seasonal LH levels in the African catfish, as well as in other teleost species. Moreover, the occurrence of mutual inhibitory and stimulatory interactions between endogenous GnRHs may be a widespread aspect of GnRH action in vertebrates.

Molecular and functional identification of a Ca2+ (polyvalent cation)-sensing receptor in rat pancreas

The balance between the concentrations of free ionized Ca2+ and bicarbonate in pancreatic juice is of critical importance in preventing the formation of calcium carbonate stones. How the pancreas regulates the ionic composition and the level of Ca2+ saturation in an alkaline environment such as the pancreatic juice is not known. Because of the tight cause-effect relationship between Ca2+ concentration and lithogenicity, and because hypercalcemia is proposed as an etiologic factor for several pancreatic diseases, we have investigated whether pancreatic tissues express a Ca2+-sensing receptor (CaR) similar to that recently identified in parathyroid tissue. Using reverse transcriptase-polymerase chain reaction and immunofluorescence microscopy, we demonstrate the presence of a CaR-like molecule in rat pancreatic acinar cells, pancreatic ducts, and islets of Langerhans. Functional studies, in which intracellular free Ca2+ concentration was measured in isolated acinar cells and interlobular ducts, show that both cell types are responsive to the CaR agonist gadolinium (Gd3+) and to changes in extracellular Ca2+ concentration. We also assessed the effects of CaR stimulation on physiological HCO3- secretion from ducts by making measurements of intracellular pH. Luminal Gd3+ is a potent stimulus for HCO3- secretion, being equally as effective as raising intracellular cAMP with forskolin. These results suggest that the CaR in the exocrine pancreas monitors the Ca2+ concentration in the pancreatic juice, and might therefore be involved in regulating the level of Ca2+ in the lumen, both under basal conditions and during hormonal stimulation. The failure of this mechanism might lead to pancreatic stone formation and even to pancreatitis.

The insulin receptor tyrosine kinase domain in a chimaeric epidermal growth factor-insulin receptor generates Ca2+ signals through the PLC-gamma1 pathway

The receptors for insulin (IR) and epidermal growth factor (EGFR) are members of the tyrosine kinase receptor (TKR) family. Despite homology of their cytosolic TK domains, both receptors induce different cellular responses. Tyrosine phosphorylation of insulin receptor substrate (IRS) molecules is a specific IR post-receptor response. The EGFR specifically activates phospholipase C-gamma1 (PLC-gamma1). Recruitment of substrate molecules with Src homology 2 (SH2) domains or phosphotyrosine binding (PTB) domains to phosphotyrosines in the receptor is one of the factors creating substrate specificity. In addition, it has been shown that the TK domains of the IR and EGFR show preferences to phosphorylate distinct peptides in vitro, suggesting additional mechanisms of substrate recognition. We have examined to what extent the substrate preference of the TK domain contributes to the specificity of the receptor in vivo. For this purpose we determined whether the IR TK domain, in situ, is able to tyrosine-phosphorylate substrates normally used by the EGFR. A chimaeric receptor, consisting of an EGFR in which the juxtamembrane and tyrosine kinase domains were exchanged by their IR counterparts, was expressed in CHO-09 cells lacking endogenous EGFR. This receptor was found to activate PLC-gamma1, indicating that the IR TK domain, in situ, is able to tyrosine phosphorylate substrates normally used by the EGFR. These findings suggest that the IR TK domain, in situ, has a low specificity for selection and phosphorylation of non-cognate substrates.

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

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

The linear C-terminal regions of epidermal growth factor (EGF) and transforming growth factor-alpha bind to different epitopes on the human EGF receptor

Epidermal growth factor (EGF) and transforming growth factor-alpha (TGFalpha) bind with similar affinities in a competitive fashion to the human EGF receptor, and basically induce similar mitogenic responses. In spite of the fact that EGF and TGFalpha are structurally alike, it is still not clear if the two growth factors bind the receptor in an identical manner. The observation that the 13A9 antibody blocks binding of TGFalpha, but not that of EGF, to the human EGF receptor [Winkler, O'Connor, Winget and Fendly (1989) Biochemistry 28, 6373-6378] suggests that their binding characteristics are not identical. In the present study we have made use of a set of EGF/TGFalpha chimaeric molecules to show that the 13A9 antibody blocks receptor binding of ligands with TGFalpha sequences, but not of ligands with EGF sequences, in their C-terminal linear regions. Using HaCaT human keratinocyte cells in culture, it was determined that ligands that are able to bind the EGF receptor in the presence of 13A9 are also able to induce calcium release from intracellular stores in these cells, indicating that these ligands have the ability to activate the EGF receptor in the presence of the antibody. From these data it is concluded that the flexible C-terminal linear domains of EGF and TGFalpha bind to separate sequences on the EGF receptor, such that the binding domain of TGFalpha, but not that of EGF, overlaps with the binding epitope of the 13A9 antibody.

Expression and trans-synaptic regulation of P2x4 and P2z receptors for extracellular ATP in parotid acinar cells. Effects of parasympathetic denervation

Trans-synaptic regulation of muscarinic, peptidergic, and purinergic responses after denervation has been reported previously in rat parotid acinar cells (McMillian, M. K., Soltoff, S. P., Cantley, L. C., Rudel, R., and Talamo, B. R. (1993) Br. J. Pharmacol. 108, 453-461). Characteristics of the ATP-mediated responses and the effects of parasympathetic denervation were further analyzed through assay of Ca2+ influx, using fluorescence ratio imaging methods, and by analysis of P2x receptor expression. ATP activates both a high affinity and a low affinity response with properties corresponding to the recently described P2x4 and the P2z (P2x7)-type purinoceptors, respectively. Reverse transcription-polymerase chain reaction analysis reveals mRNA for P2x4 as well as P2x7 subtypes but not P2x1, P2x2, P2x3, P2x5, or P2x6. P2x4 protein also is detected by Western blotting. Distribution of the two types of ATP receptor responses on individual cells was stochastic, with both high and low affinity responses on some cells, and only a single type of response on others. Sensitivity to P2x4-type activation also varied even among cells responsive to low concentrations of ATP. Parasympathetic denervation greatly enhanced responses, tripling the proportion of acinar cells with a P2x4-type response and increasing the fraction of highly sensitive cells by 7-fold. Moreover, P2x4 mRNA is significantly increased following parasympathetic denervation. These data indicate that sensitivity to ATP is modulated by neurotransmission at parasympathetic synapses, at least in part through increased expression of P2x4 mRNA, and suggest that similar regulation may occur at other sites in the nervous system where P2x4 receptors are widely expressed.

Mutational analysis of the potential phosphorylation sites for protein kinase C on the CCK(A) receptor

1. Many G protein-coupled receptors contain potential phosphorylation sites for protein kinase C (PKC), the exact role of which is poorly understood. In the present study, a mutant cholecystokininA (CCK(A)) receptor was generated in which the four consensus sites for PKC action were changed in an alanine. Both the wild-type (CCK(A)WT) and mutant (CCK(A)MT) receptor were stably expressed in Chinese hamster ovary (CHO) cells. 2. Binding of [3H]-cholecystokinin-(26-33)-peptide amide (CCK-8) to membranes prepared from CHO-CCK(A)WT cells and CHO-CCK(A)MT cells revealed no difference in binding affinity (Kd values of 0.72 nM and 0.86 nM CCK-8, respectively). 3. The dose-response curves for CCK-8-induced cyclic AMP accumulation and inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) formation were shifted to the left in CHO-CCK(A)MT cells. This leftward shift was mimicked by the potent inhibitor of protein kinase activity, staurosporine. However, the effect of staurosporine was restricted to CHO-CCK(A)WT cells. This demonstrates that attenuation of CCK-8-induced activation of adenylyl cyclase and phospholipase C-beta involves a staurosporine-sensitive kinase, which acts directly at the potential sites of PKC action on the CCK(A) receptor in CCK-8-stimulated CHO-CCK(A)WT cells. 4. The potent PKC activator, 12-O-tetradecanoylphorbol 13-acetate (TPA), evoked a rightward shift of the dose-response curve for CCK-8-induced cyclic AMP accumulation in CHO-CCK(A)WT cells but not CHO-CCK(A)MT cells. This is in agreement with the idea that PKC acts directly at the CCK(A) receptor to attenuate adenylyl cyclase activation. 5. In contrast, TPA evoked a rightward shift of the dose-response curve for CCK-8-induced Ins(1,4,5)P3 formation in both cell lines. This demonstrates that high-level PKC activation inhibits CCK-8-induced Ins(1,4,5)P3 formation also at a post-receptor site. 6. TPA inhibition of agonist-induced Ca2+ mobilization was only partly reversed in CHO-CCK(A)MT cells. TPA also inhibited Ca2+ mobilization in response to the G protein activator, Mas-7. These findings are in agreement with the idea that partial reversal of agonist-induced Ca2+ mobilization is due to the presence of an additional site of PKC inhibition downstream of the receptor and that the mutant receptor itself is not inhibited by the action of PKC. 7. The data presented demonstrate that the predicted sites for PKC action on the CCK(A) receptor are the only sites involved in TPA-induced uncoupling of the receptor from its G proteins. In addition, the present study unveils a post-receptor site of PKC action, the physiological relevance of which may be that it provides a means for the cell to inhibit phospholipase C-beta activation by receptors that are not phosphorylated by PKC.

Enhanced secretion of amylase from exocrine pancreas of connexin32-deficient mice

To determine whether junctional communication between pancreatic acinar cells contributes to their secretory function in vivo, we have compared wild-type mice, which express the gap junctional proteins connexin32 (Cx32) and connexin26, to mice deficient for the Cx32 gene. Pancreatic acinar cells from Cx32 (-/-) mice failed to express Cx32 as evidenced by reverse transcription-PCR and immunolabeling and showed a marked reduction (4.8- and 25-fold, respectively) in the number and size of gap junctions. Dye transfer studies showed that the extent of intercellular communication was inhibited in Cx32 (-/-) acini. However, electrical coupling was detected by dual patch clamp recording in Cx32 (-/-) acinar cell pairs. Although wild-type and Cx32 (-/-) acini were similarly stimulated to release amylase by carbamylcholine, Cx32 (-/-) acini showed a twofold increase of their basal secretion. This effect was caused by an increase in the proportion of secreting acini, as detected with a reverse hemolytic plaque assay. Blood measurements further revealed that Cx32 (-/-) mice had elevated basal levels of circulating amylase. The results, which demonstrate an inverse relationship between the extent of acinar cell coupling and basal amylase secretion in vivo, support the view that the physiological recruitment of secretory acinar cells is regulated by gap junction mediated intercellular communication.

Control of Ca2+ wave propagation in mouse pancreatic acinar cells

We have investigated control mechanisms involved in the propagation of agonist-induced Ca2+ waves in isolated mouse pancreatic acinar cells. Using a confocal laser-scanning microscope, we were able to show that maximal stimulation of cells with acetylcholine (ACh, 500 nM) or bombesin (1 nM) caused an initial Ca2+ release of comparable amounts with both agonists at the luminal cell pole. Subsequent Ca2+ spreading to the basolateral membrane was faster with ACh (17.3 +/- 5.4 microns/s) than with bombesin (8.0 +/- 2.2 microns/s). The speed of bombesin-induced Ca2+ waves could be increased up to the speed of ACh-induced Ca2+ waves by inhibition of protein kinase C (PKC). Activation of PKC significantly decreased the speed of ACh-induced Ca2+ waves but had only little effect on bombesin-evoked Ca2+ waves. Within 3 s after stimulation, production of inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] was higher in the presence of ACh compared with bombesin, whereas bombesin induced higher levels of diacylglycerol (DAG) than ACh. These data suggest that the slower propagation speed of bombesin-induced Ca2+ waves is due to higher activation of PKC in the presence of bombesin compared with ACh. The higher increase in bombesin-compared with ACh-induced DAG production is probably due to activation of phospholipase D (PLD). Inhibition of the PLD-dependent DAG production by preincubation with 0.3% butanol led to an acceleration of the bombesin-induced Ca2+ wave. In further experiments, we could show that ruthenium red (100 microM), an inhibitor of Ca(2+)-induced Ca2+ release in skeletal muscle, also decreased the speed of ACh-induced Ca2+ waves. The effect of ruthenium red was not additive to the effect of PKC activation. From the data, we conclude that, following Ins(1,4,5)P3-induced Ca2+ release in the luminal cell pole, secondary Ca2+ release from stores, which are located in series between the luminal and the basal plasma membrane, modifies Ca2+ spreading toward the basolateral cell side by Ca(2+)-induced Ca2+ release. Activation of PKC leads to a reduction in Ca2+ release from these stores and therefore could explain the slower propagation of Ca2+ waves in the presence of bombesin compared with ACh.

Protein kinase C-mediated inhibition of transmembrane signalling through CCK(A) and CCK(B) receptors

1. The rat CCK(A) and CCK(B) receptors were stably expressed in Chinese hamster ovary (CHO-09) cells in order to compare modes of signal transduction and effects of protein kinase C (PKC) thereupon. 2. Spectrofluorophotometry of Fura-2-loaded cells revealed that both receptors retained their pharmacological characteristics following expression in CHO cells. Sulphated cholecystokinin-(26-33)-peptide amide (CCK-8-S) increased the cytosolic Ca2+ concentration ([Ca2+]i) in CCK(A) cells, measured as an increase in Fura-2 fluorescence emission ratio, 1000 fold more potently than its non-sulphated form (CCK-8-NS) (EC50 values of 0.19 nM and 0.18 microM, respectively). By contrast, CCK-8-S and CCK-8-NS were equally potent in CCK(B) cells (EC50 values of 0.86 nM and 1.18 nM, respectively). The CCK(A) receptor agonist JMV-180 increased [Ca2+]i only in CCK(A) cells. Likewise, pentagastrin increased [Ca2+]i only in CCK(B) cells. Finally, CCK-8-S-induced Ca2+ signalling through the CCK(A) receptor was most potently inhibited by the CCK(A) receptor antagonist L364,718, whereas the CCK(B) receptor antagonist L365,260 was more potent in CCK(B) cells. 3. Receptor-mediated activation of adenylyl cyclase was measured in the presence of the inhibitor of cyclic nucleotide phosphodiesterase activity, 3-isobutyl-1-methylxanthine. CCK-8-S and, to a lesser extent, CCK-8-NS, but not JMV-180 or pentagastrin, stimulated the accumulation of cyclicAMP in CCK(A) cells. By contrast, none of these agonists increased cyclicAMP in CCK(B) cells. 4. Short-term (3 min) pretreatment with the PKC activator 12-O-tetradecanoylphorbol 13-acetate (TPA) evoked a rightward shift of the dose-response curve for the Ca2+ mobilizing effect of CCK-8-S in both cell lines. In addition, short-term TPA pretreatment markedly reduced CCK-8-S-induced cyclicAMP accumulation in CCK(A) cells. In both cases, the inhibitory effect of TPA was abolished by the PKC inhibitors, GF-109203X and staurosporine, whereas no inhibition was observed with the inactive phorbol ester, 4-alpha-phorbol 12-myristate 13-acetate. 5. During prolonged TPA treatment, the cells gradually recovered from phorbol ester inhibition and in the case of CCK-8-S-induced Ca2+ mobilization complete recovery was achieved after 24 h of TPA treatment. Western blot analysis revealed that this recovery was paralleled by down-regulation of PKC-alpha, suggesting the involvement of this PKC isotype in the inhibitory action of TPA. 6. This study demonstrates that following expression in CHO cells (i) both CCK(A) and CCK(B) receptors are coupled to Ca2+ mobilization, (ii) only CCK(A) receptors are coupled to cyclicAMP formation and (iii) with both receptors signalling is inhibited by PKC.

Synchronized Ca2+ signaling by intercellular propagation of Ca2+ action potentials in NRK fibroblasts

The intercellular propagation of Ca2+ waves by diffusion of inositol trisphosphate has been shown to be a general mechanism by which nonexcitable cells communicate. Here, we show that monolayers of normal rat kidney (NRK) fibroblasts behave like a typical excitable tissue. In confluent monolayers of these cells, Ca2+ action potentials can be generated by local depolarization of the monolayer on treatment with either bradykinin or an elevation of the extracellular K+ concentration. These electronically propagating action potentials travel intercellularly over long distances in an all-or-none fashion at a speed of approximately 6.1 mm/s and can be blocked by L-type Ca2+ channel blockers. The action potentials are generated by depolarizations beyond the threshold value for L-type Ca2+ channels of about -15 mV. The result of these locally induced, propagating Ca2+ action potentials is an almost synchronous, transient increase in the intracellular Ca2+ concentration in large numbers of cells. These data show that electrically coupled fibroblasts can form an excitable syncytium, and they elucidate a novel mechanism of intercellular Ca2+ signaling in these cells that may coordinate synchronized multicellular responses to local stimuli.

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

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

Coxsackievirus protein 2B modifies endoplasmic reticulum membrane and plasma membrane permeability and facilitates virus release

Digital-imaging microscopy was performed to study the effect of Coxsackie B3 virus infection on the cytosolic free Ca2+ concentration and the Ca2+ content of the endoplasmic reticulum (ER). During the course of infection a gradual increase in the cytosolic free Ca2+ concentration was observed, due to the influx of extracellular Ca2+. The Ca2+ content of the ER decreased in time with kinetics inversely proportional to those of viral protein synthesis. Individual expression of protein 2B was sufficient to induce the influx of extracellular Ca2+ and to release Ca2+ from ER stores. Analysis of mutant 2B proteins showed that both a cationic amphipathic alpha-helix and a second hydrophobic domain in 2B were required for these activities. Consistent with a presumed ability of protein 2B to increase membrane permeability, viruses carrying a mutant 2B protein exhibited a defect in virus release. We propose that 2B gradually enhances membrane permeability, thereby disrupting the intracellular Ca2+ homeostasis and ultimately causing the membrane lesions that allow release of virus progeny.

Mutational analysis of the putative devazepide binding site of the CCK(A) receptor

Recently a molecular model was proposed for the binding site of the antagonist 3S(-)-N-(2,3-dihydro-1-methyl-2-oxo-5-phenyl-1H-1,4-benzodiazepine-3-yl) -1H-indole-2-carboxamide (devazepide) on the cholecystokinin-A (CCK(A)) receptor (Van der Bent et al., 1994. Drug Design Discov. 12, 129-148). Fifteen amino acids were identified, including hydrophilic ones such as Ser139, Asn349 and Ser379, that might interact with the carboxamide moiety in devazepide. To provide mutational evidence for this model, wild-type and mutant receptors (S139A, N349A and S379A) were transiently expressed and compared with respect to the ability of devazepide to inhibit binding of radiolabelled cholecystokinin-(26-33)-peptide amide (CCK-8) and CCK-8-evoked Ca2+ mobilization. The data presented suggest the involvement of the three residues in antagonist binding, although to a different extent. However, it does not seem likely that hydrogen bonds are the driving force in view of the relatively minor changes in receptor affinity and activity.

Characterization of a selective antagonist of neuropeptide Y at the Y2 receptor. Synthesis and pharmacological evaluation of a Y2 antagonist

Neuropeptide Y (NPY) is a potent inhibitor of neurotransmitter release through the Y2 receptor subtype. Specific antagonists for the Y2 receptors have not yet been described. Based on the concept of template-assembled synthetic proteins we have used a cyclic template molecule containing two beta-turn mimetics for covalent attachment of four COOH-terminal fragments RQRYNH2 (NPY 33-36), termed T4-[NPY(33-36)]4. This structurally defined template-assembled synthetic protein has been tested for binding using SK-N-MC and LN319 cell lines that express the Y1 and Y2 receptor, respectively. T4-[NPY(33-36)]4 binds to the Y2 receptor with high affinity (IC50 = 67.2 nM) and has poor binding to the Y1 receptor. This peptidomimetic tested on LN319 cells at concentrations up to 10 microM shows no inhibitory effect on forskolin-stimulated cAMP levels (IC50 for NPY = 2.5 nM). Furthermore, we used confocal microscopy to examine the NPY-induced increase in intracellular calcium in single LN319 cells. Preincubation of the cells with T4-[NPY(33-36)]4 shifted to the right the dose-response curves for intracellular mobilization of calcium induced by NPY at concentrations ranging from 0.1 nM to 10 microM. Finally, we assessed the competitive antagonistic properties of T4-[NPY(33-36)]4 at presynaptic peptidergic Y2 receptors modulating noradrenaline release. the compound T4-[NPY(33-36)]4 caused a marked shift to the right of the concentration-response curve of NPY 13-36, a Y2-selective fragment, yielding a pA2 value of 8.48. Thus, to our best knowledge, T4-[NPY(33-36)]4 represents the first potent and selective Y2 antagonist.

Recovery from TPA inhibition of receptor-mediated Ca2+ mobilization is paralleled by down-regulation of protein kinase C-alpha in CHO cells expressing the CCK-A receptor

Digital-imaging microscopy of Fura-2-loaded Chinese hamster ovary cells, stably expressing the cholecystokinin-A receptor, revealed that both the C-terminal octapeptide of cholecystokinin (CCKB) and its analogue JMV-180, which acts as an agonist at the high-affinity CCK-A receptor, recruited CHO-CCK-A cells dose-dependently in terms of receptor-mediated Ca2+ mobilization. Agonist-evoked cell recruitment was inhibited by short-term (10 min) pretreatment with 0.1 microM 12-O-tetradecanoylphorbol 13-acetate (TPA). In the case of CCKB, inhibition was overcome with increasing of the hormone concentration. In contrast, increasing of the JMV-180 concentration did not reverse the inhibitory action of TPA. CHO-CCK-A cells gradually regained their responsiveness to JMV-180 during prolonged TPA pretreatment. Complete recovery was observed within 1 h following addition of TPA. Western blot analysis using antibodies directed against the various PKC isotypes revealed that recovery was paralleled by the disappearance of PKC-alpha. Surprisingly, short-term (10 min) TPA pretreatment virtually completely inhibited the formation of inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] in response to CCKB concentrations at which the effect on cell recruitment was not affected by short term phorbol ester pretreatment. Together with the finding that JMV-180 does not detectably increase the cellular Ins(1,4,5)P3 content, this suggests a large overproduction of this second messenger by CCKB concentrations supramaximal in terms of cell recruitment. Again, full responsiveness was observed after long term TPA pretreatment. The present observations are in agreement with the idea that in CHO-CCK-A cells activation of PKC-alpha leads to inhibition of agonist-evoked Ca2+ mobilization through inhibition of receptor-stimulated Ins(1,4,5)P3 formation.

beta-Adrenergic relaxation in mesenteric resistance arteries of spontaneously hypertensive and Wistar-Kyoto rats: the role of precontraction and intracellular Ca2+

An attenuated beta-adrenergic vasodilation of small arteries may help explain the increased peripheral resistance in hypertension. To investigate this, we compared the isoprenaline-induced relaxation of mesenteric resistance arteries of spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY) using a small vessel myograph. The arteries had similar diameters, but the contractile force induced by cumulative addition of K+ (10-130 mM) was 1.3-fold higher for the SHR. The beta-adrenoceptor-mediated relaxation of arteries, precontracted with 40 mM K+, was significantly less in SHR (41 +/- 3%, n = 11) than in WKY (56 +/- 3%, n = 15, p = 0.003), and the pD2 value for isoprenaline was significantly lower in SHR (7.13 +/- 0.09 vs. 7.41 +/- 0.07, p = 0.02). In contrast, when precontracted with phenylephrine (PE, alpha 1-adrenoceptor agonist, 3-10 microM), isoprenaline relaxation was almost complete in both SHR and WKY, and the pD2 value for isoprenaline did not differ between strains. Forskolin induced complete relaxation of both precontractions. Because the beta-adrenergic relaxation of the mesenteric resistance arteries was attenuated only after K(+)-precontraction, we conclude that alterations in this precontracting mechanism in SHR rather than a defect in the beta-adrenoceptor system may provide an explanation for the decreased relaxation in these vessels. Intracellular Ca2+ measurements and a review of the literature support this conclusion.

Protein kinase C activation inhibits receptor-evoked inositol trisphosphate formation and induction of cytosolic calcium oscillations by decreasing the affinity-state of the cholecystokinin receptor in pancreatic acinar cells

Digital-imaging microscopy of Fura-2-loaded pancreatic acinar cells revealed that the C-terminal octapeptide of cholecystokinin (CCK8) dose-dependently recruited 94% of freshly isolated acinar cells in terms of receptor-evoked Ca2+ mobilization. Maximal and half-maximal cell-recruitment were reached with 0.1 nM and 16.8 pM CCK8, respectively. The upstroke of the dose-recruitment curve consisted of cells displaying oscillatory changes in free cytosolic Ca2+ concentration ([Ca2+]i). After having reached its maximum, the percentage oscillating cells dose-dependently decreased upon further increasing of the CCK8 concentration. Pretreatment of the acinar cells with 0.1 microM TPA caused a rightward shift of the dose-recruitment curve but did not change the maximal effect of CCK8 on the recruitment of oscillating cells. Half-maximal recruitment was obtained with 287 pM CCK8. This observation demonstrates that high levels of protein kinase C activation do not inhibit Ca2+ oscillations at a level downstream to receptor activation. Moreover, this observation demonstrates that protein kinase C-mediated inhibition of Ca2+ oscillations evoked by submaximal CCK8 concentrations occurs at the receptor level, converting it from a high-affinity state into a low-affinity state. This conclusion is supported by the observation that TPA completely inhibited the recruitment of acinar cells in response to the high-affinity receptor agonist JMV-180. The inhibitory action of TPA on CCK8-evoked cell-recruitment was paralleled by an inhibitory effect of the phorbol ester on the CCK8-evoked peak increase in average inositol trisphosphate concentration in a population of acinar cells. This observation indicates that low concentrations of CCK8 interact with the high-affinity CCK receptor to increase [Ca2+]i through the intermediation of inositol trisphosphate.

Lung surfactant protein A (SP-A) activates a phosphoinositide/calcium signaling pathway in alveolar macrophages

Lung surfactant protein A (SP-A), the main protein component of lung surfactant which lines the alveoli, strongly enhances serum-independent phagocytosis of bacteria by rat alveolar macrophages. We tested if the effect of SP-A is due to interaction with the macrophages or to opsonization of the bacteria. In phagocytosis assays with fluorescein isothiocyanate labeled bacteria, SP-A had no opsonic effect on Escherichia coli, Pseudomonas aeruginosa and Staphylococcus aureus, but enhanced phagocytosis by acting only on the macrophages. We characterized this activation mechanism. With single cell measurements of fura-2 loaded cells we demonstrate that SP-A raises the intracellular free calcium ion concentration 6 to 8 seconds after addition. This calcium mobilization is dose-dependent in that increased SP-A concentrations lead to a higher percentage of responding cells. Additionally, SP-A leads to a dose-dependent and transient generation of inositol 1,4.5-trisphosphate. Release of intracellular stored calcium by SP-A is a prerequisite for its stimulatory effect on phagocytosis, since SP-A-induced enhancement of phagocytosis can be impaired by prior addition of thapsigargin, a Ca(2+)-ATPase inhibitor that leads to depletion of intracellular calcium stores. We conclude that SP-A activates a phosphoinositide/calcium signaling pathway in alveolar macrophages leading to enhanced serum-independent phagocytosis of bacteria.

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.

The relationship between receptor-effector unit heterogeneity and the shape of the concentration-effect profile: pharmacodynamic implications

The apparent concentration-effect relationship is the ensemble of many effector units (such as individual cells or channels) that do not always exhibit a uniform stimulus-effect relationship. This concept is substantiated by many observations of heterogeneity in receptor-effector populations including hormone secreting cells, response to hormonal stimuli, activity pattern of second messengers, stimulus-evoked synaptic currents, and single ion channels. The relationship between drug concentration and magnitude of pharmacologic response is commonly described by the sigmoidal Emax model which was derived from the Hill equation. The sigmoidicity factor (N) in this model is assumed to be a pure mathematical parameter without physiological connotations. This work demonstrates that the numerical value of N (measured empirically) is the product of two factors: (i) the degree of heterogeneity of the effector subunits, i.e., the elemental component that upon drug stimulus contributes its pharmacological effect independently and does not interact with other subunits (it could range from a single receptor up to a whole tissue), and (ii) value of N*--the shape factor of the subunits' concentration-effect relationship. A special case of this approach occurs when N* > 5, which is an on-off case. Here N is determined by the distribution (density equation) of the subunit values. In case of heterogeneity of the microparameters of the effector subunits the apparent N will always have a lower value than N*. According to this theory it can be concluded that without knowledge of the distribution of the microparameters no mechanistic interpretation can be deduced from the apparent N value. If in the future N* can be determined by theoretical or experimental methods, the distribution function relating N* to N can be calculated. The relevance of this theory is increased in view of the progress being made in advanced research techniques which may enable us to determine the concentration-effect relationship at the level of the individual effector unit.

Heterogeneity and contact-dependent regulation of amylase release by individual acinar cells

We have used a reverse hemolytic plaque assay to investigate the amylase release of single and aggregated pancreatic acinar cells. We have found that a minority of single acinar cells released detectable amounts of amylase under basal conditions and were modestly stimulated, in a dose-dependent manner, during a 30-min exposure to concentrations of carbamylcholine (CCh) ranging from 10(-8) to 10(-5) M. This stimulation was largely accounted for by the recruitment of additional secreting cells, rather than by a significant increase in their individual secretory output. We have also observed that aggregates comprising two to five acinar cells secreted more frequently and released more amylase than single acinar cells in the presence of each of the CCh concentrations tested. Under both basal conditions and following CCh stimulation, the proportion of secreting aggregates and their amylase output increased linearly with the aggregate size. Under basal conditions as well as in the presence of secretagogue concentrations in the 10(-8) - 10(-7) M range, individual cells contributed similarly to amylase secretion whether they were single or part of aggregates. By contrast, following stimulation by 10(-6) - 10(-5) M CCh, aggregated cells showed a much higher average secretion than single cells. Investigating the mechanism of this contact-dependent effect, we found that 10(-3) M heptanol did not significantly modify the secretion of single cells and markedly promoted the basal amylase release of acinar cell pairs. This effect was associated with a marked reduction in gap junctional communication between acinar cells, as evaluated by microinjection of Lucifer yellow, and was not observed during exposure to high concentrations of CCh, which also reduced junctional communication. These data show that pancreatic acinar cells are intrinsically heterogeneous in their ability to release amylase and that their basal as well as stimulated secretion are promoted by the establishment of direct intercellular contacts. Our experiments also suggest that junctional coupling contributes to the contact-dependent mechanism which enhances the recruitment of secreting cells and their individual output. These observations strengthen the view that direct interactions between acinar cells are essential in the control of pancreatic secretion.

Induction of Ca2+ oscillations by selective, U73122-mediated, depletion of inositol-trisphosphate-sensitive Ca2+ stores in rabbit pancreatic acinar cells

The effect of the putative inhibitor of phospholipase C activity, U73122, on the Ca2+ sequestering and releasing properties of internal Ca2+ stores was studied in both permeabilized and intact rabbit pancreatic acinar cells. U73122 dose dependently inhibited ATP-dependent Ca2+ uptake in the inositol (1,4,5)-trisphosphate-[Ins(1,4,5)P3]-sensitive, but not the Ins(1,4,5)P3-insensitive, Ca2+ store in acinar cells permeabilized by saponin treatment. In a suspension of intact acinar cells, loaded with the fluorescent Ca2+ indicator, Fura-2, U73122 alone evoked a transient increase in average free cytosolic Ca2+ concentration ([Ca2+]i,av), which was largely independent of external Ca2+. Addition of U73122 to cell suspensions prestimulated with either cholecystokinin octapeptide or JMV-180 revealed an inverse relationship in size between the U73122- and the agonist-evoked [Ca2+]i,av transient. Moreover, thapsigargin-induced inhibition of intracellular Ca(2+)-ATPase activity resulted in a [Ca2+]i,av transient, the size of which was not different following maximal prestimulation with either U73122 or agonist. These observations suggest that U73122 selectively affects the Ins(1,4,5)P3- casu quo agonist-sensitive internal Ca2+ store, whereas thapsigargin affects both the Ins(1,4,5)P3-sensitive and -insensitive Ca2+ store. Digital-imaging microscopy of Fura-2-loaded acinar cells demonstrated that U73122, in contrast to thapsigargin, evoked sustained oscillatory changes in [Ca2+]i. The U73122-evoked oscillations were abolished in the absence of external Ca2+. The ability of U73122 to generate external Ca(2+)-dependent Ca2+ oscillations suggests that depletion of the agonist-sensitive store leads to an increase in Ca2+ permeability of the plasma membrane and that the Ins(1,4,5)P3-insensitive Ca2+ pool is necessary for the Ca2+ oscillations.

Spatial and temporal signalling by calcium

Recent research has shown the importance of the spatial and temporal aspects of calcium signals, which depend upon regenerative properties of the inositol trisphosphate and ryanodine receptors that regulate the release of calcium from internal stores. Initiation sites have been found to spontaneously release calcium, recognized as 'hot spots' or 'sparks', and can trigger a wave that spreads through a process of calcium-induced calcium release.

Two different spatiotemporal patterns for Ca2+ oscillations in pancreatic acinar cells: evidence of a role for protein kinase C in Ins(1,4,5)P3-mediated Ca2+ signalling

The oscillations in cytosolic Ca2+ evoked in pancreatic exocrine acinar cells by submaximal concentrations of the two phosphoinositidase-coupled agonists acetylcholine (ACh) and cholecystokinin octapeptide (CCK-8) have very different temporal patterns. In the present study we use digital video imaging of Fura-2 fluorescence to map the spatial distribution of Ca2+ during the oscillating responses to these two agonists. The spatial patterns induced are very different for each of these agonists. ACh oscillations are sinusoidal and initiated at the secretory pole of these morphologically and functionally polarized cells. As they spread across the cell, pronounced gradients in Ca2+ develop that persist throughout the oscillating response. CCK-8 induces a series of discrete Ca2+ transients of longer duration and lower frequency. These elevations in Ca2+ arise slowly, throughout the cells and without any detectable gradients in Ca2+. We consider that the different spatiotemporal patterns can be explained on the basis of a physiologically relevant interaction between Ins(1,4,5)P3 and protein kinase C in second messenger-mediated Ca2+ signalling.

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

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