Mode of inhibitory action of cholecystokinin in amylase release from isolated rat pancreatic acini--inhibition of secretory process post to protein kinase C-calcium ion systems

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.

Regulation of gap junctional coupling in isolated pancreatic acinar cell pairs by cholecystokinin-octapeptide, vasoactive intestinal peptide (VIP) and a VIP-antagonist

Cholecystokinin-octapeptide (CCK-OP) induces a time- and dose-dependent decrease of gap junctional conductance in isolated pairs of pancreatic acinar cells. In double whole-cell experiments, the time course could be described by the latency and the half-life time (t1/2) of cell-to-cell uncoupling. The latency shows a biphasic dependence on [CCK-OP] with a minimum of about 50 sec at 10(-9) M CCK-OP. In the presence of vasoactive intestinal peptide (VIP), the biphasic relationship is shifted to lower CCK-OP concentrations. The increase of latency at high concentrations of CCK-OP (> 10(-9) M) was blocked by addition of a VIP-antagonist. t1/2 decreases monophasically with increasing [CCK-OP]. Addition of GTP gamma S to the pipette solution suppresses the [CCK-OP] dependence of the latency and potentiates the uncoupling phase. The kinetic data are discussed in terms of CCK binding to receptors of high and low affinity. Evidence is presented that secretion and cell-to-cell coupling are not related by an all-or-none process, but that for physiological CCK-OP concentrations, gap junctional uncoupling follows secretion.

Mobilization of calcium from intracellular stores as one of the mechanisms underlying the antiopioid effect of cholecystokinin octapeptide

In enzymatically dissociated brain cells prepared from neonatal rats, KCl produced a significant increase in [Ca2+]i and this increase could be prevented by verapamil or nifedipine, known to block voltage-sensitive calcium channels. The opioid receptor agonists ohmefentanyl (OMF, mu agonist), [D-Pen2,D-Pen5]enkephalin (DPDPE, delta agonist), and 66A-078 (kappa agonist) produced a marked suppression of the Ca2+ influx induced by high K+ depolarization. The suppressive effect of OMF, DPDPE, and 66A-078 on the high K(+)-induced increase in [Ca2+]i was markedly reversed by their respective antagonists beta-funaltrexamine (beta-FNA), ICI174864, and nor-binaltorphimine (nor-BNI). Cholecystokinin octapeptide (CCK-8), at concentrations of 0.3, 3.0, and 30 nM, dose-dependently mobilized Ca2+ from intracellular stores. While CCK-8 30 nM did not affect significantly the increase of [Ca2+]i following high K+, it did reverse the suppression of the high K(+)-induced increase in [Ca2+]i by the mu agonist OMF and the kappa agonist 66A-078, but not that by the delta agonist DPDPE. The results suggested that while opioid ligands suppress [Ca2+]i by blocking voltage-operated Ca2+ influx, the antiopioid effect of CCK-8 seems to be operated via mobilization of Ca2+ from intracellular stores.

Protective effect of a microtubule stabilizer taxol on caerulein-induced acute pancreatitis in rat

The effect of taxol, which is a microtubule stabilizer, was examined in a model of acute edematous pancreatitis induced in rat by the administration of caerulein. Prophylactic administration of taxol ameliorated inhibition of pancreatic secretion, increased level of serum amylase, pancreatic edema, and histological alterations in this model. Immunofluorescence studies revealed that taxol stabilized the arrangement of microtubules by the action of promoting tubulin polymerization and prevented inhibition of pancreatic digestive enzyme secretion. In isolated rat pancreatic acini, taxol reversed the inhibition of amylase secretion induced by supramaximal concentrations of cholecystokinin octapeptide and did not affect the binding of cholecystokinin octapeptide to its receptor. The results obtained in this study suggest that microtubule disorganization is the initiating event in caerulein-induced pancreatitis and that the inhibition of pancreatic digestive enzyme secretion by interfering with intracellular vesicular transport due to microtubule disorganization causes caerulein-induced pancreatitis.

Effects of cholecystokinin, cholecystokinin JMV-180 and GTP analogs on enzyme secretion from permeabilized acini and chloride conductance in isolated zymogen granules of the rat pancreas

Previous studies have shown that hormonal activation of the Cl- conductance in pancreatic zymogen granules (ZG) is closely related to enzyme secretion from acinar cells. We have now examined the role of guanine nucleotides in stimulated and unstimulated protein secretion from isolated digitonin-permeabilized pancreatic acini and in the Cl- conductance of isolated ZG. Protein secretion from permeabilized isolated acini, measured at 0.1 mM Ca2+, increased with increasing cholecystokinin octapeptide (CCK-8) concentrations and decreased at high CCK-8 concentrations. The maximum secretion, approximately twice the control level, was reached at 1 nM CCK-8. The CCK analog, CCK JMV-180, which supposedly acts as an agonist on high-affinity CCK receptors and as an antagonist on low-affinity CCK receptors, stimulated maximum enzyme secretion at a CCK JMV-180 concentration of 0.1 microM and no decrease in secretion was observed at higher CCK JMV-180 concentrations, 0.1 mM guanosine 5'-[gamma-thio]triphosphate (GTP [S]) also increased the protein release by approximately twice that of the control and shifted the CCK-8 concentration causing maximum stimulation from 1 nM to 0.01 nM. GTP[S] concentrations greater than 0.1 mM inhibited protein release evoked by an optimal concentration of 1 nM CCK-8, 0.1 mM GTP[S] had no pronounced effect on the protein secretion stimulated by low concentrations of CCK JMV-180, but inhibited protein secretion evoked by CCK JMV-180 concentrations greater than 0.1 microM. This indicates that guanosine-nucleotide-binding proteins [G protein(s)] coupling to CCK receptors also mediate both CCK-induced increases and CCK-induced decreases of enzyme secretion at low and high CCK concentrations, respectively. ZG were prepared on a Percoll gradient from CCK-8-stimulated or CCK-JMV-180-stimulated and unstimulated acini. Their Cl- conductances were estimated in the absence of Ca2+ and in the presence of 1 mM EGTA from the rate of decrease in absorbance following addition of the K+ ionophore valinomycin as a measure of ZG osmotic lysis. The Cl- conductance in ZG from CCK-8-stimulated and CCK-JMV-180-stimulated acini was maximally activated at 1 pM and 10 nM respectively. At higher agonist concentrations, Cl- conductance was decreased. Direct addition of 10 microM GTP[S] to isolated ZG from unstimulated acini increased the rate of lysis by approximately 40% of the control value. This effect was approximately additive to that of CCK-8 or of CCK JMV-180 prestimulation.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of phorbol ester and cholecystokinin on the intracellular distribution of protein kinase C in rabbit pancreatic acini

Treatment of rabbit pancreatic acini with the phorbol ester, 12-O-tetradecanoylphorbol 13-acetate (TPA), resulted in a time- and dose-dependent decrease of soluble protein kinase C activity coinciding with an increase of protein kinase C activity in the particulate fraction. After 5 min, soluble protein kinase C activity had decreased to almost 10% of the corresponding control. Total extractable protein kinase C activity, however, remained unchanged, indicating that the decrease of soluble protein kinase C activity was not due to TPA-induced inactivation of the enzyme. The biologically inactive phorbol ester, 4 alpha-phorbol 12,13-didecanoate, did not induce such a translocation of protein kinase C. The half-maximal concentration for TPA-induced translocation of protein kinase C was 40 nM, and was equal to that for TPA-induced amylase secretion from isolated acini. This suggests that translocation of protein kinase C to the particulate fraction is an important step in TPA-induced activation of protein kinase C and enzyme secretion. On the other hand, cholecystokinin, a secretagogue of the calcium-mobilizing type, whose secretory action is thought to be mediated, at least in part, by protein kinase C, did not change the subcellular distribution of protein kinase C. In the presence of R59022 6-(2-[(4-fluorophenyl)phenylmethylene]-1-piperidinyl ) ethyl-7-methyl-5H-thiazolo[3,2-a]pyrimidin-5-one, an inhibitor of diacylglycerol kinase activity, cholecystokinin produced a small but significant translocation of protein kinase C, suggesting that the inability of the hormone to induce translocation is not due to a rapid conversion of the diacylglycerol formed into phosphatidic acid.

The ability of staurosporine to modulate pancreatic acinar cell desensitization by TPA, carbamylcholine and caerulein

The implication of protein kinase C in the phenomenon of pancreatic acinar cell desensitization to carbamylcholine, caerulein and the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) was investigated using a potent PKC inhibitor, staurosporine. At a concentration of 1 microM, staurosporine caused a maximum 64% inhibition of amylase release from rat pancreatic acini stimulated by 100 nM TPA. At 100 nM, staurosporine reduced by 50 to 55% amylase secretion elicited by maximal concentrations of carbamylcholine or caerulein without affecting their potency. Staurosporine was also able to prevent completely desensitization by TPA of the subsequent secretory response to carbamylcholine and caerulein. Furthermore, staurosporine also totally prevented desensitization by caerulein of the subsequent secretory response to caerulein. In contrast, staurosporine only partially prevented desensitization by carbamylcholine of the subsequent secretory response to carbamylcholine. These results indicate that staurosporine is a potent inhibitor of protein kinase C as it inhibited the secretory response to carbamylcholine, caerulein and TPA. They also suggest that desensitization of the secretory response induced by TPA and caerulein used a common pathway involving protein kinase C activation. Finally, desensitization by carbamylcholine is more complex as it is only partially prevented at staurosporine; therefore, protein kinase C activation seems to be one of the factors involved.

Dissimilar effects of the protein kinase C inhibitors, staurosporine and H-7, on cholecystokinin-induced enzyme secretion from rabbit pancreatic acini

The effects of two putative inhibitors of protein kinase C activity, staurosporine and H-7, on partially purified protein kinase C and amylase secretion from isolated rabbit pancreatic acini were investigated. Staurosporine dose-dependently inhibited amylase release stimulated by an optimal concentration of cholecystokinin C-terminal octapeptide. At a concentration of 100 nM, the drug inhibited the secretory response to the secretagogue by approximately 50%. At the same concentration, staurosporine inhibited 12-O-tetradecanoylphorbol 13-acetate-stimulated enzyme secretion by 90%. Moreover, the potentiating effect of this phorbol ester on cholecystokinin-induced amylase release was completely abolished in the presence of staurosporine. Interestingly, amylase release was decreased to the level observed with the combination of cholecystokinin and staurosporine. In contrast, H-7, potentiated rather than inhibited cholecystokinin-stimulated enzyme secretion, whereas the secretory response to 12-O-tetradecanoylphorbol 13-acetate was not affected by the drug. Both staurosporine and H-7, however, inhibited protein kinase C purified from exocrine pancreatic tissue. Kinetic analysis revealed that both compounds inhibited protein kinase C competitively with respect to ATP. The Ki value for staurosporine was 0.55 nM and for H-7 13.5 microM. Our results obtained with staurosporine are in line with a stimulatory role of protein kinase C in cholecystokinin-induced enzyme secretion from the exocrine pancreas. The results obtained with H-7 emphasize that care has to be taken in interpreting the biological effects of this drug.

The diacylglycerol kinase inhibitor, R59022, potentiates cholecystokinin-induced enzyme secretion from rabbit pancreatic acini

The putative inhibitor of diacylglycerol kinase activity, 6-(2-[(4-fluorophenyl)phenylmethylene]-1-piperidinyl)-ethyl-7-meth yl-5H- thiazolo[3,2-a]pyrimidin-5-one (R59022), markedly potentiated cholecystokinin-C-terminal-octapeptide(CCK-8-)stimulated enzyme secretion from isolated rabbit pancreatic acini. Maximal potentiation occurred when acini were stimulated in the presence of 5-10 microM R59022. Potentiation depended both on the concentration of R59022 and CCK-8. No potentiation was observed when acini were half-maximally stimulated, whereas the secretory response to maximal and supramaximal concentrations of secretagogue was increased by 50-60%. R59022 alone had no effect on basal enzyme secretion and the drug did not potentiate the secretory response to the Ca2+ ionophore A23187 or to the phorbol ester, 12-O-tetradecanoylphorbol 13-acetate. Moreover, no increase in basal secretion was observed when acini were incubated in the presence of both R59022 and forskolin. These observations strongly suggest that receptor-mediated activation of the inositol phospholipid pathway is required for R59022-induced potentiation. R59022 inhibited the CCK-8-stimulated incorporation of 32Pi into phosphatidic acid dose dependently, without affecting the CCK-8-stimulated hydrolysis of 32P-labelled phosphatidylinositol 4,5-bisphosphate. This is consistent with an inhibitory effect of R59022 on acinar cell diacylglycerol kinase activity. The potentiating effect of R59022 was mimicked by 12-O-tetradecanoylphorbol 13-acetate added simultaneously with CCK-8. Therefore, it is concluded that in the presence of 5-10 microM R59022 the receptor-mediated increase in acinar cell diacylglycerol content is enhanced leading to enhanced activation of protein kinase C and to potentiation of the secretory response. The fact that the secretory response to maximal and supramaximal concentrations of CCK-8 is potentiated by R59022 suggests that at these concentrations of secretagogue the diacylglycerol/protein kinase C branch of the signal-transduction route is rate-limiting.