Calmodulin antagonists inhibit dihydropyridine calcium channel activator (BAY-K-8644) induced cGMP synthesis in pituitary tumor cells

The dihydropyridine calcium channel activator, BAY-K-8644, stimulates cGMP formation in ACTH-secreting mouse AtT-20 clonal corticotrophs. The recent report that calmodulin antagonists could inhibit dihydropyridine binding in several tissues suggested that these agents might also affect the cyclic nucleotide response to BAY-K-8644. In fact, TMB-8, trifluoperazine, and melittin, described as in vitro antagonists of calmodulin-dependent enzyme activities, all inhibited BAY-K-8644 induced cGMP synthesis in a concentration-dependent manner. The antagonists had no effect on cGMP formation stimulated by sodium nitroprusside or sodium azide. The calcium channel antagonist, nifedipine, did not stimulate cGMP formation nor did it alter the effect of BAY-K-8644 on accumulation of the nucleotide; one explanation thus is that the cyclase involved in cGMP formation is coupled to a low affinity binding site for BAY-K-8644, which is less accessible to other dihydropyridines. The relation of cyclic GMP formation to the function of the calcium channel in AtT-20 cells remains unknown.     

Opposing actions of the enantiomers of BAY-K-8644 on calcium currents and ACTH secretion in clonal pituitary corticotrophs

BAY-K-8644 in low concentrations is known to stimulate, and in higher concentrations, to depress calcium-dependent ACTH secretion from mouse clonal (tumor) pituitary corticotrophs, AtT-20/D16-16 (AtT-20). In the present study, voltage-dependent inward calcium currents in these cells were potentiated by low concentrations of this compound and depressed by higher concentrations consistent with its actions on ACTH secretion. A similar relationship was demonstrated for a different but related compound, CGP 28,392. Each of BAY-K-8644's enantiomers, BAY-R(-)5417 and BAY-R(+)4407, had opposing effects upon these inward calcium currents and ACTH secretion. The (+)isomer antagonized both inward calcium currents and ACTH secretion. In contrast, the (-)enantiomer was responsible for the stimulatory effects of BAY-K-8644. Nevertheless, some antagonistic properties were noted with high concentrations of this latter enantiomer. The stimulation of ACTH secretion in AtT-20 cells by low concentrations of BAY-K-8644 can be attributed to a potentiation of voltage-activated calcium currents by one of its enantiomers, BAY-R-(-)5417. In contrast, the depression of secretion that occurs at higher concentrations is likely to be the result of the reduction of these currents by the other enantiomer (BAY-R(+)4407).     

BAY-K-8644, a calcium channel agonist, induces a rise in cytoplasmic free calcium and iodide discharge in thyroid cells

BAY-K-8644, a calcium channel agonist, induces a rise in cytoplasmic free calcium and iodide discharge in cultured porcine thyroid cells. The cytoplasmic free calcium concentration, [Ca2+]i, was measured using aequorin, a calcium-sensitive photoprotein. BAY-K-8644, a dihydropyridine derivative, acts as a Ca channel agonist and induces a rise in [Ca2+]i and iodide discharge; 0.5 nM BAY-K-8644 is a minimal dose to effect a rise in [Ca2+]i and iodide discharge and 50 nM BAY-K-8644 produces the maximal effect. The data indicate that BAY-K-8644-induced iodide discharge is mediated by a rise in [Ca2+]i.     

Generation of H2O2 is regulated by cytoplasmic free calcium in cultured porcine thyroid cells

Effects of calcium ionophore A23187 and BAY-K-8644, a calcium channel agonist, on cytoplasmic free calcium ([Ca2+]i) and H2O2 generation were studied in cultured porcine thyroid cells. We monitored continuously the effects of A23187 and BAY-K-8644 on [Ca2+]i and H2O2 generation, using the intracellularly trapped fluorescent Ca2+ indicator, fura-2, and homovanillic acid, respectively. A23187 and BAY-K-8644 induce an immediate increase in [Ca2+]i and H2O2 generation. The A23187- and BAY-K-8644-induced [Ca2+]i responses and H2O2 generation occur immediately, reach a maximum within several seconds, and then slowly decline. The minimum doses of A23187 or BAY-K-8644 to increase [Ca2+]i stimulate H2O2 generation. H2O2 generation is regulated by [Ca2+]i.     

Dihydropyridine Modulation of the Chromaffin Cell Secretory Response

Prolonged perfusion of cat adrenal glands with Krebs-bicarbonate solutions containing nicotine, muscarine, or excess K rapidly increased the rate of catecholamine output proportional to the concentrations of secretagogue used. The secretory responses to nicotine or high K reached a peak and declined to almost basal rates of secretion after about 10 min of stimulation. The dihydropyridine Ca channel agonist Bay K 8644 potentiated markedly the secretory responses to 1 microM nicotine and to 17.7 mM K but not to higher concentrations of these secretagogues. The muscarinic response did not decrease with time and was modestly potentiated by Bay K 8644. Similar curves were obtained with 17.7 mM K plus Bay K 8644 and with 59 mM K alone. CGP28392, another agonist, was about 10 times less potent than Bay K 8644 in potentiating the secretory responses to 17.7 mM K. Bay K 8644 also potentiated the release of [3H]noradrenaline evoked by stimulation of cultured bovine adrenal chromaffin cells with 17.7 mM K or 2 microM nicotine but not with higher concentrations of K or nicotine. Dihydropyridine Ca channel antagonists reversed the effects of Bay K 8644 with the following order of potency: niludipine greater than nifedipine = nimodipine greater than nitrendipine. The secretory rates from intact chromaffin cells treated with the Ca ionophores X537A or A23187, or those evoked by Ca-EGTA buffers from digitonin-permeabilized cells, were not affected by Bay K 8644. These results are compatible with the following conclusions: Bay K 8644 selectively potentiates catecholamine secretory responses mediated through the activation of voltage-sensitive Ca channels; during nicotine or high-K stimulation, Ca gains access to the cell interior through a common permeability pathway, the Ca channel.(ABSTRACT TRUNCATED AT 250 WORDS)     

The influence of exogenous cyclic nucleotides and theophylline on the hydrokinetic and ecbolic function of the isolated perfused canine pancreas (author's transl)

The influence of exogenous cyclic nucleotides or theophylline either on basal or stimulated volume and protein secretion is studied on the isolated perfused canine pancreas in dependence on varied extracellular calcium concentrations. Bt2cAMP or theophylline do not influence basal secretory rates of pancreatic juice but potentiate secretin-stimulated volume output. They additionally increase basal protein secretion under exclusive secretin stimulation and potentiate dose-dependently CCK- or acetylcholine-induced protein output. The hydrokinetic and ecbolic effects of Bt2cAMP and theophylline persist in a calcium-free medium but fail in normalizing inhibited protein secretion during calcium deprivation. Bt2cGMP neither increases basal nor stimulated volume and protein secretion. The demonstrated influence of Bt2cAMP and theophylline on ductal volume and acinar protein secretion accomplishes two criteria, as suggested by Sutherland, for cAMP as second messenger for secretin and CCK or acetylcholine as well.     

Characteristics of pancreatic exocrine secretion produced by venom from the Brazilian scorpion, Tityus serrulatus.

The influence of venom (TSV) from the Brazilian scorpion, Tityus serrulatus, on exocrine pancreatic secretion was studied in relation to known cholinergic and peptidergic secretagogue activity. Pulse-labeling followed by chase incubation in the presence of secretagogues and various pharmacological agents revealed unique physiological characteristics of TSV in guinea pig pancreatic lobules. Exocytotic discharge of newly synthesized 3H-labeled proteins during a 3-h chase incubation showed a marked increase over basal discharge levels using logarithmic TSV doses of 0.10 to 100 micrograms/ml. This stimulation was comparable to maximal values elicited by carbachol, cholecystokinin-octapeptide (CCK-8) or caerulein and discharge kinetics were similar. TSV-mediated secretion was ATP and calcium dependent and partially inhibited by atropine. Only tetrodotoxin completely blocked TSV stimulation of newly synthesized protein discharge. Both botulinum toxin and curare had no effect on venom stimulation, indicating that TSV interaction with exocrine pancreatic cells occurs postsynaptically. Verapamil, a calcium channel antagonist, produced a moderate inhibition of TSV stimulation. When antagonists to the cholecystokinin (CCK) receptor were incubated with TSV, no change in secretory activity occurred. Therefore, TSV does not bind to CCK receptors and probably operates through its own receptor which may be an ion channel. Additionally, morphological studies in vitro revealed a high level of pancreatic secretory activity as evidenced by dense secretory acinar luminal content, reduction in zymogen granule (ZG) population, and development of exocytotic images.     

Unstimulated amylase secretion is proteoglycan-dependent in rat parotid acinar cells

It is well-known that amylase is secreted in response to extracellular stimulation from the acinar cells. However, amylase is also secreted without stimulation. We distinguished vesicular amylase as a newly synthesized amylase from the accumulated amylase in secretory granules by short time pulse and chased with ³⁵S-amino acid. The newly synthesized amylase was secreted without stimulation from secretory vesicles in rat parotid acinar cells. The secretion process did not include microtubules, but was related to microfilaments. p-Nitrophenyl β-xyloside, an inhibitor of proteoglycan synthesis, inhibited the newly synthesized amylase secretion. This indicated that the newly synthesized amylase was secreted from secretory vesicles, not via the constitutive-like secretory route, which includes the immature secretory granules, and that proteoglycan synthesis was required for secretory vesicle formation.     

Forskolin potentiates calcium-dependent amylase secretion from rat pancreatic acinar cells

The cellular and molecular effects of forskolin, a direct, nonhormonal activator of adenylate cyclase, were assessed on the enzyme secretory process in dispersed rat pancreatic acinar cells. Forskolin stimulated adenylate cyclase activity in the absence of guanyl nucleotide. It promoted a rapid and marked increase in cellular accumulation of cyclic AMP alone or in combination with vasoactive intestinal peptide (VIP) but was itself a weak pancreatic agonist and did not increase the secretory response to VIP or other cyclic AMP dependent agonists. Somatostatin was a partial antagonist of forskolin stimulated cyclic AMP synthesis and forskolin plus cholecystokinin-octapeptide (CCK-OP) induced amylase release. Forskolin potentiated amylase secretion in response to calcium-dependent agonists such as CCK-OP, carbachol and A-23187, but did not affect the ability of CCK-OP and (or) carbachol to mobilize 45Ca from isotope preloaded cells; forskolin alone did not stimulate 45Ca release. In calcium-poor media, the secretory response to forskolin and CCK-OP was reduced in a both absolute and relative manner. The data suggests that calcium plays the primary role as intracellular mediator of enzyme secretion and that the role of cyclic AMP may be to modulate the efficiency of calcium utilization.     

Bay K 8644, a voltage-sensitive calcium channel agonist, facilitates secretion of atrial natriuretic polypeptide from isolated perfused rat hearts

Effects of Bay K 8644, a voltage-sensitive calcium channel agonist, on atrial natriuretic polypeptide (ANP) secretion from isolated rat hearts perfused with Krebs-Henseleit solution were investigated. After a ninety-min period for stabilization, coronary sinus effluents were collected every two min and ANP levels were measured by radioimmunoassay. The basal secretory rate of ANP was 1.65 +/- 0.15 ng/min (mean +/- standard error). Bay K 8644 stimulated ANP secretion dose-dependently. This stimulatory action was blocked by simultaneous administration of nifedipine, its competitive antagonist. Heart rate was also increased by Bay K 8644 administration. In the gel filtration study, the major secretory form of ANP corresponded to alpha-rat ANP, a 28-amino acid peptide. These results suggest that voltage-sensitive calcium channels are involved in two principal biological properties, contraction and ANP secretion, of atrial cardiocytes.     

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

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

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. © 1994 Wiley-Liss, Inc.     

The phorbol ester induced atrial natriuretic peptide secretion is stimulated by forskolin and Bay K8644 and inhibited by 8-bromo-cyclic GMP

The role of intracellular signals in the regulation of atrial natriuretic peptide (ANP) release was studied using the isolated perfused rat heart. The phorbol ester, 12-O-tetradecanoyl-phorbol-13-acetate (TPA), known to activate the protein kinase C pathway, produced a dose-dependent increase in perfusate ANP immunoreactivity. Bay k8644, a putative calcium channel activator, and forskolin, which stimulates adenylate cyclase, induced a sustained increase in ANP secretory rate. TPA in combination with either Bay k8644 or forskolin induced higher ANP secretion than the calculated additive value for each agent. 8-bromo-cyclic GMP and sodium nitroprusside, when given alone, had no effect on ANP secretion, but delayed the TPA-stimulated increase in perfusate ANP. ANP secretion appears therefore to be mediated both by the phosphoinositide and the cAMP system, whereas the cGMP pathway may be inhibitory.     

Phospholipase A2 activation by melittin causes amylase release from exocrine pancreas

Phospholipase A2-induced deacylation of membrane phospholipids is associated with changes in membrane fluidity. The importance of this reaction in the pancreatic amylase secretory process was tested using melittin, a phospholipase A2 stimulating peptide. Phospholipase A2 activity (using [3H]arachidonic acid release as an index) and amylase secretion were both increased in a time- and concentration-dependent manner by melittin. Phospholipids prelabelled with [3H]oleic acid or [14C]linoleic acid also released radioactive free fatty acids in response to melittin. Prostaglandin synthesis was not involved in the melittin response, since inhibitors of arachidonic acid oxidation (indomethacin, 5,8,11,14-eicosatetraynoic acid) did not alter the ability of melittin to release [3H]arachidonic acid or amylase. When melittin was co-applied with carbachol, cholecystokinin octapeptide, or vasoactive intestinal peptide, amylase secretion was additive. The effect of melittin on both fatty acid and amylase release was dependent on extracellular calcium, though melittin's effects were not dependent on the intracellular accumulation of second messengers such as calcium or cAMP. The data suggest that activation of phospholipase A2 by melittin results in the triggering of the secretory process in exocrine pancreas by a different mechanism than that for other pancreatic secretagogues.     

Cyclic GMP does not inhibit protein kinase C-mediated enzyme secretion in rat pancreatic acini

In pancreatic acini, cGMP can be increased by secretagogues such as cholecystokinin (CCK), cholinergic agents, and bombesin, whose actions on enzyme secretion are believed to be mediated by protein kinase C. However, the role of cGMP in acinar cell function has been unclear. A recent paper by Rogers et al. (Rogers, J., Hughes, R.G., and Matthews, E. K. (1988) J. Biol. Chem. 263, 3713-3719) reported that two analogues of cGMP, N2,O2-dibutyl guanosine 3':5'-monophosphate (Bt2cGMP) and 8-bromoguanosine 3':5'-monophosphate (8Br-cGMP), at concentrations in the nanomolar range, inhibited the stimulation of amylase secretion caused by CCK-8, bethanechol, bombesin, and 12-O-tetradecanoylphorbol-13-acetate (TPA). Rogers et al. also reported that sodium nitroprusside inhibited the stimulation of enzyme secretion caused by CCK-8 or TPA. These authors concluded that cGMP inhibits protein kinase C-mediated secretion in pancreatic acini. In the present study we attempted to confirm the findings of Rogers et al., We found, however, that Bt2cGMP inhibited CCK-8-stimulated amylase release only at concentrations of the nucleotide above 10 microM. Moreover, there was a close correlation between the ability of Bt2cGMP to inhibit CCK-8-stimulated amylase release and its ability to inhibit binding of 125I-CCK-8. Bt2cGMP, at concentrations as high as 3 mM, did not alter the stimulation of amylase release caused by carbachol, bombesin, TPA, or A23187. 8Br-cGMP, at concentrations up to 1 mM, did not inhibit the stimulation of amylase release caused by CCK-8 or TPA. At concentrations above 0.1 mM, 8Br-cGMP augmented the stimulation of amylase release caused by CCK-8, carbachol, bombesin, or TPA. Sodium nitroprusside, at a concentration that causes a 60-fold increase in cGMP, did not inhibit the stimulation of amylase release caused by CCK-8, carbachol, bombesin, or TPA. Our results do not confirm the findings of Rogers et al. and indicate that cGMP does not inhibit protein kinase C-mediated secretion in pancreatic acini.     

Blockage of cell-to-cell communication within pancreatic acini is associated with increased basal release of amylase

To assess whether junctional coupling is involved in the secretory activity of pancreatic acinar cells, dispersed rat acini were incubated for 30 min in the presence of either heptanol (3.5 mM) or octanol (1.0 mM). Exposure to either alkanol caused a marked uncoupling of the acinar cells which, in control acini, were extensively coupled. Uncoupling was associated with an increased basal release of amylase that was at least twice that of controls. By contrast, carbamylcholine (10(-5) M)-induced maximal amylase secretion, cytosolic pH, and free Ca2+, as well as the structure of gap junctions joining the acinar cells, were unaffected. Both uncoupling and the alteration of basal secretion were already observed after only 5 min of exposure to heptanol, they both persisted throughout the 30-min exposure to the alkanols, and were reversible after removal of either heptanol or octanol. Since neither of the two uncouplers appeared to alter unspecifically the secretory machinery and the nonjunctional membrane of acinar cells, the data are consistent with the view that junctional coupling participates in the control of the basal secretion of acinar cells.     

Effects of n-alcohols on junctional coupling and amylase secretion of pancreatic acinar cells

We have tested the effects of alcohols differing by their alkyl chain length on the membrane channels and amylase secretion of rat pancreatic acinar cells. In intact acini, alcohols with a chain of seven, eight, or nine carbons (C-7, C-8, and C-9) induced dye uncoupling and increased basal amylase release. These effects were readily reversible after alcohol removal. By contrast, an alcohol with a chain of 15 carbons (C-15) and several alcohols with chains of fewer than six carbons (C-2, C-4, and C-6) did not uncouple acinar cells and had no effects of amylase secretion. Neither did alkanes and oxidized derivatives of C-7 and C-8 alcohols did not affect dye coupling. Double patch-clamp experiments on pairs of acinar cells, under conditions of strong cytosolic Ca2+ and pH buffering, showed that C-7, C-8, and C-9 alcohols blocked completely and reversibly the electrical conductance of junctional channels. Furthermore, studies of single voltage-clamped acinar cells revealed that the uncoupling alcohols did not affect the resting nonjunctional membrane conductances. Thus the alcohols that did not affect acinar cells coupling did not affect amylase secretion, whereas the alcohols that caused uncoupling increased secretion. The latter effect was not mediated by changes in the conductance of nonjunctional membrane, cytosolic Ca2+, and pH and, as revealed by an immunological hemolytic plaque assay for amylase, had a time course consistent with the rapid (within 1 min) inhibition of coupling. These data provide new support for the view that the regulation of cell-to-cell communications is correlated with that of digestive enzyme secretion.     

Heterogeneity of calcium channel agonist binding sites in the coronary artery

The binding properties (3H) BAY k 8644 a 1,4-dihydropyridine calcium channel agonist were studied in the subcellular membrane fraction isolated from the coronary artery by differential centrifugation. The specific binding of (3H) BAY k 8644 to microsomal membranes of the coronary smooth muscle was rapid, saturable, reversible and of both high and low affinity. The dissociation constants obtained from Scatchard analysis with (3H) BAY k 8644 and nitrendipine were 0.60 +/- 0.02 nmol.l-1 and 9.1 +/- 0.1 nmol.l-1 for the high and low affinity binding site respectively and the estimated maximal numbers of binding sites in the plasma membrane fraction were 0.76 +/- 0.02 and 3.15 +/- 0.18 pmol.mg-1 of protein respectively. The substituted dihydropyridine calcium channel antagonists nitrendipine and nifedipine competitively inhibited specific (3H)BAY k 8644 binding suggesting a common high affinity 1,4-dihydropyridine binding site in the coronary microsomal fraction for calcium channel activator and antagonists. The low affinity agonist binding sites were significantly inhibited by adding nucleoside carrier inhibitors, 2-deoxyadenosine and dipyridamole, and by -SH alkylating agent N-ethylmaleimide. The results suggests that the coronary artery contains both high and low affinity calcium channel binding sites (in a 1:5 ratio) with the low affinity calcium channel agonist binding sites being associated with nucleoside carrier and/or with-SH groups.     

Amylase secretion from isolated pure acinar cells

Isolation of pure acinar cells of the rat pancreas was achieved employing counterflow sedimentation filtration technique (CSFT). The preparation of purified acinar cells contained an occasional red blood cell (RBC, 200:1) with total absence of endocrine and duct cells. A significant stimulation of amylase secretion from isolated pure acinar cells was produced by octapeptide of cholecystokinin (CCK8) and insulin produced potentiation of the effect of CCK8. Synthetic glucagon inhibited basal and CCK8 stimulated amylase secretion. Non-synthetic purified glucagon stimulated amylase secretion and potentiated the effect of CCK8. Vasoactive intestinal polypeptide (VIP) did not stimulate amylase secretion but potentiated the effect of CCK8. No leakage of lactic dehydrogenase (LDH) was detected from the cells in any of the secretion studies. Thus a highly purified preparation of isolated pure acinar cells of rat pancreas could be obtained with excellent morphologic and functional integrity.     

Simulation of regulated exocytosis of amylase from salivary parotid acinar cells by a consecutive reaction model comprising two sequential first-order reactions

Amylase secretion from parotid acinar cells results from stimulus-regulated fusion of apical membrane and secretory granules that contain amylase. The time course of amylase secretion induced by various secretagogues has been reported. Calcium-mobilizing agonists such as carbamylcholine and substance P induce rapid and transient secretion while cAMP-mobilizing agonists such as isoproterenol cause long-term secretion. Combination of these two types of agonists results in a rapid and high rate of secretion. To explain the various time courses of these stimulations, it was assumed that amylase secretion is a consecutive reaction that consists of two first-order reactions. It was postulated that secretory granules were classified into three states: (A) pre-docked, (B) docked, and (C) fusion. The simple simulation could explain the time course of amylase secretion induced by various secretagogues by simply changing the rate constants for docking (reaction A to B) and fusion (reaction B to C) steps. It was also found that calcium mainly enhances the last fusion step and that cAMP activates the docking step. The amount of docked granules is estimated to be quite small, which accounts for why amylase secretion is regulated mainly by cAMP. The effects of the two types of secretagogues were synergistic, meaning that their intracellular signaling pathways are independent. At the same time, this also suggests that basal and enhanced secretion induced by two types of agonists have the same exocytotic process and that two stimuli independently activate the same machinery that mediates docking or fusion. This simulation is useful in analysis of the effects of secretion modulators and the molecular mechanism of amylase secretion.