Biochemical reactions involved in pancreatic enzyme secretion. 4. Effects of cytochalasin B on functions of the exocrine pancreas.

These experiments were performed to evaluate the effects of cytochalasin B on pancreatic enzyme secretion and thus perhaps establish a role for microfilaments in the exocytosis process. The alkaloid at a concentration of 1 microgram/ml (2micron) inhibits amylase secretion induced by urecholine or cholecystokinin-pancreozymin (CCK-PZ) but does not modify that induced by dibutyryl cyclic AMP. The inhibitory effect of the drug is reversible after a 30-min washing out period. It does not affect O2 consumption, basal calcium efflux, or efflux caused by CCK-PZ. Amino acid accumulation in the tissue and their incorporation into proteins are not modified. It is suggested that cytochalasin B inhibits pancreatic enzyme secretion, probably through an effect on the microfilament system.     

Localization of pancreatic enzyme secretion-stimulating activity and trypsin inhibitory activity in zymogen granule of the rat pancreas

The intracellular localization of pancreatic enzyme secretion-stimulating activity in rat pancreas was investigated. We found and purified a pancreatic enzyme secretion-stimulating peptide from rat bile/pancreatic juice. The peptide is trypsin-sensitive (showing temporary trypsin inhibitory activity), and it is hypothesized that it acts as a trypsin-sensitive mediator in the feedback regulation of diet-induced pancreatic enzyme secretion. The zymogen granule fraction was purified 5-fold by ultracentrifugation by the Percoll density gradient method. The purity of the zymogen granule fraction was determined from the specific amylase activity and electron microscopic morphology. The specific enzyme activities of amylase and trypsin and the trypsin inhibitory activity increased in parallel during the purification, and the pancreatic enzyme secretion-stimulating activity was also localized in the zymogen granule fraction. These results suggest that the pancreatic enzyme secretion-stimulating peptide originates from the acinar cells, and that it is secreted through exocytosis of zymogen granules into the small intestine, its ratio to trypsin thus remaining constant. This idea supports our hypothesis that the stimulating peptide acts as a mediator for the feedback regulation of pancreatic enzyme secretion by trypsin.     

Glutamate inhibits protein phosphatases and promotes insulin exocytosis in pancreatic beta-cells

In human type 2 diabetes mellitus, loss of glucose-sensitive insulin secretion from the pancreatic beta-cell is an early pathogenetic event, but the mechanisms involved in glucose sensing are poorly understood. A messenger role has been postulated for L-glutamate in linking glucose stimulation to sustained insulin exocytosis in the beta-cell, but the precise nature by which L-glutamate controls insulin secretion remains elusive. Effects of L-glutamate on the activities of ser/thr protein phosphatases (PPase) and Ca(2+)-regulated insulin exocytosis in INS-1E cells were investigated. Glucose increases L-glutamate contents and promotes insulin secretion from INS-1E cells. L-glutamate also dose-dependently inhibits PPase enzyme activities analogous to the specific PPase inhibitor, okadaic acid. L-glutamate and okadaic acid directly and non-additively promote insulin exocytosis from permeabilized INS-1E cells in a Ca(2+)-independent manner. Thus, an increase in phosphorylation state, through inhibition of protein dephosphorylation by glucose-derived L-glutamate, may be a novel regulatory mechanism linking glucose sensing to sustained insulin exocytosis.     

Secretory phospholipase A2 is released from pancreatic beta-cells and stimulates insulin secretion via inhibition of ATP-dependent K+ channels

The release of sPLA(2) from single mouse pancreatic beta-cells was monitored using a fluorescent substrate of the enzyme incorporated in the outer leaflet of the plasma membrane. Stimulation of beta-cells with agents that increased cytosolic free Ca(2+) concentration ([Ca(2+)](i)) induced a rapid release of sPLA(2) to the extracellular medium. Exogenous sPLA(2) strongly stimulated insulin secretion in mouse pancreatic islets at both basal and elevated glucose concentrations. The stimulation of insulin secretion by sPLA(2) was mediated via inhibition of ATP-dependent K(+) channels and an increase in [Ca(2+)](i). Measurements of cell capacitance in single beta-cells revealed that sPLA(2) did not modify depolarisation-induced exocytosis. Our data suggest that a positive feedback regulation of insulin secretion by co-released sPLA(2) is operational in pancreatic beta-cells and point to this enzyme as an autocrine regulator of insulin secretion.     

Evidence against cyclic GMP as a mediator of the actions of secretagogues on amylase release from guinea-pig pancreas

In dispersed acini from guinea-pig pancrease several pancreatic secretagogues increased calcium outflux, cyclic GMP and amylase secretion, whereas nitroprusside and hydroxylamide increased cyclic GMP but did not increase calcium outflux or amylase secretion and did not alter the action of secretagogues on calcium outflux or amylase secretion. Secretin and vasoactive intestinal peptide increased cyclic AMP and increased secretion but did not alter cyclic GMP. Nitroprusside and hydroxylamine did not alter cyclic AMP or the action of secretin or vasoactive intestinal peptide on cyclic AMP and enzyme secretion. Agents that increased cyclic GMP also caused release of the nucleotide into the extracellular medium; however, this release did not correlate with secretion of amylase into the extracellular medium. 8-Bromo cyclic AMP as well as 8-bromo cyclic GMP increased enzyme secretion and potentiated the increase in enzyme secretion caused by cholecystokinin or carbachol. The increase in amylase secretion caused by vasoactive intestinal peptide or secretin plus either of the cyclic nucleotide derivatives was the same as that caused by the peptide alone. These results indicate that cyclic GMP does not mediate the action of secretagogues on pancreatic enzyme secretion, that the release of cyclic GMP into the extracellular medium does not occur by exocytosis and that the increase in enzyme secretion caused by 8-bromo cyclic GMP results from its stability to mimic the action of endogenous cyclic AMP.     

Dose-response for inhibition by amylin of cholecystokinin-stimulated secretion of amylase and lipase in rats

BACKGROUND AND AIMS: The neuroendocrine hormone amylin, cosecreted with insulin from pancreatic beta-cells in response to nutrient ingestion, has several physiologic actions to limit the rate of nutrient uptake, including the slowing of gastric emptying. METHODS: To investigate whether amylin might modulate digestive enzyme secretion from the exocrine pancreas, anesthetized Sprague Dawley rats were cannulated via the pancreatic duct and the secretory response (flow, amylase and lipase) to cholecystokinin (1 microg s.c.) was measured in the absence and in the presence of 0.1, 0.3 and 1 microg s.c. doses of amylin. RESULTS: Amylin alone did not affect pancreatic secretion, but it dose-dependently inhibited cholecystokinin-stimulated amylase secretion by up to 58% and lipase secretion by up to 67%. The ED50's for these responses were 0.21 microg+/-0.18 log and 0.11 microg+/-0.05 log, respectively, doses that result in excursions of plasma amylin concentration that are within the reported physiological range. Amylin did not evoke cell signalling in the Ar42j model of pancreatic acinar cells, and responses to amylin were not observed in either Ar42j cells or isolated pancreatic acini in a microphysiometer indicating that the effect of amylin was indirect. CONCLUSIONS: Inhibition of stimulated pancreatic enzyme secretion is likely to be a physiological, extrapancreatic, action of amylin. Amylinergic mechanisms modulating both gastric emptying and pancreatic enzyme secretion may thus match, respectively, the appearance of substrate and enzymes in the gut lumen.     

Regulation of human eIF4E by 4E-BP1: binding analysis using surface plasmon resonance

The mitochondria play a pivotal role in regulating glucose-induced insulin secretion in the pancreatic beta cell. We have recently demonstrated that glutamate derived from mitochondria participates directly in the stimulation of insulin exocytosis. In the present study, mitochondria isolated from the beta cell line INS-1E generated glutamate when incubated with the tricarboxylic acid cycle intermediate succinate. The generation of glutamate correlated with stimulated mitochondrial activity monitored as oxygen consumption and was inhibited by the mitochondrial uncoupler carbonyl cyanide p-trifluoromethoxyphenylhydrazone. Glutamate is formed by the mitochondrial enzyme glutamate dehydrogenase from alpha-ketoglutarate. Transient overexpression of glutamate dehydrogenase in INS-1E cells resulted in potentiation of glucose-stimulated hormone secretion without affecting basal release. These results further point to glutamate as an intracellular messenger playing a key role in the control of insulin exocytosis.     

Dominant negative Rab3D inhibits amylase release from mouse pancreatic acini

Rab3 proteins are believed to play an important role in regulated exocytosis and previous work has demonstrated the presence of Rab3D on pancreatic zymogen granules. To further understand the function of Rab3D in acinar cell exocytosis, adenoviral constructs were prepared encoding hemagglutinin-tagged wild type Rab3D and three mutant forms, N135I and T36N (both deficient in guanine nucleotide binding) and Q81L (deficient in GTP hydrolysis), which also expressed enhanced green fluorescent protein driven by a separate promoter. When isolated mouse pancreatic acini were cultured with 5 x 10(6) pfu/ml adenovirus, nearly 100% of acini were infected as visualized by expression of green fluorescent protein. Cultured acini showed a biphasic dose-response to cholecystokinin (CCK); basal amylase secretion was 1.8 +/- 0.3%/30 min, peak release was 7.3 +/- 0.2%/30 min at 30 pm CCK and reduced secretion was observed at higher CCK concentrations. Control beta-galactosidase virus infection had no effect on either basal or CCK-induced secretion in the titer range from 0.5 to 10 x 10(6) pfu/ml. While the expression of Rab3D and Rab3D Q81L had no effect on amylase secretion, Rab3D N135I and T36N functioned as dominant negative mutants and inhibited CCK-induced amylase release by 40-50% at all points on the CCK dose-response curve from 3 to 300 pm. Inhibition was stronger during the first 5 min (71 +/- 5%) than over 30 min (36%+/-5%). Similar inhibition was found using other agonists including bombesin, carbachol, and cAMP. Localization of adenoviral expressed Rab protein showed wild type Rab3D localized to zymogen granules. The two dominant negative mutants did not localize to granules and were primarily in the basolateral region of the cell. Since both dominant negative Rab3D mutants had no effect on intracellular calcium increase induced by CCK, it is unlikely that they acted at receptors or transmembrane signaling. These results suggest that Rab3D plays an important role in regulating the terminal steps of acinar exocytosis and that this effect is greatest on the early phase of amylase release.     

Intestinal fat suppresses protein-induced exocrine pancreatic secretion in chronically bile-pancreatic juice-diverted rats

Previously, we showed that the increase in pancreatic enzyme secretion was lower after feeding a casein diet containing fat than that after feeding a fat-free casein diet in chronically bile-pancreatic juice (BPJ)-diverted rats. In the present study, we determined whether the suppressive effects of fats on flow volume of BPJ and pancreatic enzyme secretion depend on delaying gastric emptying and examined the characteristics of the suppression with intraduodenal instillation of soybean oil or lecithin in BPJ-diverted rats. The study was conducted as three separate experiments using conscious rats with chronic BPJ diversion by means of a common bile-pancreatic duct catheter. The flow volume of BPJ and the secretion of pancreatic amylase and trypsin were determined after intraduodenal instillation of the test solution. Exocrine pancreatic secretion was strongly stimulated by administration of guanidinated casein hydrolysate (HGC, 150 mg/ml) in chronic BPJ-diverted rats. However, pancreatic secretion after administration of an emulsion containing HGC with either soybean oil (100 mg/ml) or mixed fat (50 mg/ml soybean oil + 50 mg/ml lecithin) was much lower than that after administration of HGC alone. In contrast, administration of the soybean oil emulsion without HGC resulted in a small, but significant increase in the volume of BPJ. The suppressive effects of soybean oil (100 mg/ml) on the increases in the BPJ flow and enzyme secretion were similar to those of sodium taurocholate (10 mg/ml), and there was no additive effect of soybean oil on taurocholate suppression. In conclusion, duodenally instilled soybean oil suppressed increases in flow volume of BPJ and pancreatic enzyme secretion induced by HGC in chronic BPJ-diverted rats, showing that the suppressive effect of the fat does not depend on delaying gastric emptying.     

Insulin-stimulated insulin secretion in single pancreatic beta cells

Functional insulin receptors are known to occur in pancreatic beta cells; however, except for a positive feedback on insulin synthesis, their physiological effects are unknown. Amperometric measurements at single, primary pancreatic beta cells reveal that application of exogenous insulin in the presence or absence of nonstimulatory concentrations of glucose evokes exocytosis mediated by the beta cell insulin receptor. Insulin also elicits increases in intracellular Ca2+ concentration in beta cells but has minimal effects on membrane potential. Conditions where the insulin receptor is blocked or cell surface concentration of free insulin is reduced during exocytosis diminishes secretion induced by other secretagogues, providing evidence for direct autocrine action of insulin upon secretion from the same cell. These results indicate that the beta cell insulin receptor can mediate positive feedback for insulin secretion. The presence of a positive feedback mechanism for insulin secretion mediated by the insulin receptor provides a potential link between impaired insulin secretion and insulin resistance.     

A tetanus toxin sensitive protein other than VAMP 2 is required for exocytosis in the pancreatic acinar cell

The neurotoxin sensitivity of regulated exocytosis in the pancreatic acinar cell was investigated using streptolysin-O permeabilized pancreatic acini. Treatment of permeabilized acini with botulinum toxin B (BoNT/B) or botulinum toxin D (BoNT/D) had no detectable effect on Ca(2+)-dependent amylase secretion but did result in the complete cleavage of VAMP 2. In comparison, tetanus toxin (TeTx) treatment both significantly inhibited Ca(2+)-dependent amylase secretion and cleaved VAMP 2. These results indicate that regulated exocytosis in the pancreatic acinar cell requires a tetanus toxin sensitive protein(s) other than VAMP 2.     

3H-serotonin as a marker of oscillatory insulin secretion in clonal beta-cells (INS-1)

Serotonin release from preloaded pancreatic beta-cells has been used as a marker for insulin release in studying exocytosis from single cells using the amperometric technique. We found that single pancreatic beta-cells exhibited oscillations in exocytosis with a period of 1-1.5 min as measured amperometrically by serotonin release. We also show that 3H-serotonin can be used to monitor exocytosis from intact and streptolysin-O permeabilized clonal insulin-secreting cells preloaded with labeled serotonin and that serotonin release correlated with insulin secretion in the same cells. The use of 3H-serotonin provides a real-time indicator of exocytosis from populations of clonal insulin-secreting cells.     

Diverse effects of phytohaemagglutinin on gastrointestinal secretions in rats.

Kidney bean lectin phytohaemagglutinin (PHA) is known for its binding capacity to the small intestinal surface inducing marked hyperplasia and hypertrophy and an increased pancreatic function. Recent observations indicate that PHA is able to attach to gastric mucosal and parietal cells. Therefore, we compared the effects of PHA on gastric acid secretion, and pancreatic amylase secretion in rats. To study gastric secretion in conscious animals, rats were surgically prepared with chronic stainless steel gastric cannula and with indwelling polyethylene jugular vein catheter. Acid secretion was determined by titration of the collected gastric juice to pH 7.0. Similar studies were performed to investigate the effect of PHA on pancreatic enzyme secretion in conscious rats supplied with pancreatic cannula. Pancreatic enzyme secretion was also studied in rats anesthetized with either halothane or urethane. In conscious rats PHA significantly inhibited basal acid secretion when compared to vehicle-treated controls. The effect was dose-dependent and reversible. On the other hand, given in the same doses as in the acid-secretory studies, PHA stimulated pancreatic amylase secretion in rats prepared with chronic pancreatic cannula. This effect was blocked by devazepide, a CCK-A receptor antagonist. In halothane-anesthetized rats PHA administration increased pancreatic amylase secretion, too. During urethane anesthesia, however, the stimulatory effect of PHA was not observed. These results provide evidence that intragastric PHA treatment induces opposite effects on gastric acid secretion and pancreatic enzyme secretion: it is a potent inhibitor of acid output, and a stimulator of pancreatic enzyme discharge. Our data also show that the stimulatory effect of PHA on pancreatic enzyme secretion can be blocked by urethane, an anaesthetic that is known to turn off the negative pancreatic feedback control of pancreatic function in rats.     

The RalA GTPase is a central regulator of insulin exocytosis from pancreatic islet beta cells

RalA is a small GTPase that is thought to facilitate exocytosis through its direct interaction with the mammalian exocyst complex. In this study, we report an essential role for RalA in regulated insulin secretion from pancreatic beta cells. We employed lentiviral-mediated delivery of RalA short hairpin RNAs to deplete endogenous RalA protein in mouse pancreatic islets and INS-1 beta cells. Perifusion of mouse islets depleted of RalA protein exhibited inhibition of both first and second phases of glucose-stimulated insulin secretion. Consistently, INS-1 cells depleted of RalA caused a severe inhibition of depolarization-induced insulin exocytosis determined by membrane capacitance, including a reduction in the size of the ready-releasable pool of insulin granules and a reduction in the subsequent mobilization and exocytosis of the reserve pool of granules. Collectively, these data suggest that RalA is a critical component in biphasic insulin release from pancreatic beta cells.     

The number of secretory vesicles remains unchanged following exocytosis.

Earlier studies using electron microscopy demonstrate that there is no loss of secretory vesicles following exocytosis. Depletion however, of vesicular contents resulting in the formation of empty or partially empty vesicles is seen in electron micrographs, post exocytosis, in a variety of cells. Our studies using atomic force microscopy (AFM) reveal that following stimulation of secretion, live pancreatic acinar cells having 100-180 nm in diameter fusion pores located at the apical plasma membrane, dilate only 25-35% during exocytosis. Since secretory vesicles in pancreatic acinar cells range in size from 200 nm to 1200 nm in diameter, their total incorporation at the fusion pore, would distend the structure much more then what is observed. These earlier results prompted the current study to determine secretory vesicle dynamics in live pancreatic acinar cells following exocytosis. AFM studies on live acinar cells reveal no loss of secretory vesicle number following exocytosis. Parallel studies using electron microscopy, further confirmed our AFM results. These studies demonstrate that following stimulation of secretion, membrane-bound secretory vesicles transiently dock and fuse to release vesicular contents.     

Ionic currents across pancreatic acinar cell membranes and their role in fluid secretion

Fluid and enzyme secretion from a number of mammalian exocrine glands is controlled by the action of neurotransmitters and hormones on acinar cell membranes. Sustained stimulation evoking sustained fluid and enzyme secretion also evokes sustained membrane depolarization and increase in conductance. Mouse and rat pancreatic fluid and enzyme secretion, as well as membrane depolarization and conductance increase evoked by sustained stimulation with acetylcholine or cholecystokinin-gastrin peptides, are acutely dependent on extracellular calcium. However, the initial stimulant-evoked conductance increase and secretion appear to be triggered by calcium released from inside the cells. Direct measurement of membrane current during sustained stimulation in voltage-clamp experiments with resolution of the total current into its Na, Cl and K components has allowed calculations of stimulant-evoked Na and Cl uptake into the acinar cells. The NaCl uptake is quantitatively sufficient to account for the stimulant-evoked fluid secretion. The role of the stimulant-evoked transmembrane ionic current appears to be the supply of salt for the fluid secretion. Calcium derived from intracellular sources in the initial phase of secretion, and from the extracellular fluid in the sustained phase, couples fluid and enzyme secretion to hormone-receptor interaction.     

GAD65-mediated glutamate decarboxylation reduces glucose-stimulated insulin secretion in pancreatic beta cells

Mitochondrial metabolism plays a pivotal role in the pancreatic beta cell by generating signals that couple glucose sensing to insulin secretion. We have demonstrated previously that mitochondrially derived glutamate participates directly in the stimulation of insulin exocytosis. The aim of the present study was to impose altered cellular glutamate levels by overexpression of glutamate decarboxylase (GAD) to repress elevation of cytosolic glutamate. INS-1E cells infected with a recombinant adenovirus vector encoding GAD65 showed efficient overexpression of the GAD protein with a parallel increase in enzyme activity. In control cells glutamate levels were slightly increased by 7.5 mm glucose (1.4-fold) compared with the effect at 15 mm (2.3-fold) versus basal 2.5 mm glucose. Upon GAD overexpression, glutamate concentrations were no longer elevated by 15 mm glucose as compared with controls (-40%). Insulin secretion was stimulated in control cells by glucose at 7.5 mm (2.5-fold) and more efficiently at 15 mm (5.2-fold). INS-1E cells overexpressing GAD exhibited impaired insulin secretion on stimulation with 15 mm glucose (-37%). The secretory response to 30 mm KCl, used to raise cytosolic Ca(2+) levels, was unaffected. Similar results were obtained in perifused rat pancreatic islets following adenovirus transduction. This GAD65-mediated glutamate decarboxylation correlating with impaired glucose-induced insulin secretion is compatible with a role for glutamate as a glucose-derived factor participating in insulin exocytosis.     

The Rab3-interacting molecule RIM is expressed in pancreatic beta-cells and is implicated in insulin exocytosis

The putative Rab3 effector RIM (Rab3-interacting molecule) was detected by Northern blotting, RT-PCR and Western blotting in native pancreatic beta-cells as well as in the derived cell lines INS-1E and HIT-T15. RIM was localized on the plasma membrane of INS-1E cells and beta-cells. An involvement of RIM in insulin exocytosis was indicated by transfection experiments of INS-1E cells with the Rab3 binding domain of RIM. This domain enhanced glucose-stimulated secretion in intact cells and Ca(2+)-stimulated exocytosis in permeabilized cells. Co-expression of Rab3A reversed the effect of RIM on exocytosis. These results suggest an implication of RIM in the control of insulin secretion.     

Phospholipase D1 regulates secretagogue-stimulated insulin release in pancreatic beta-cells

Phospholipase D (PLD) has been strongly implicated in the regulation of Golgi trafficking as well as endocytosis and exocytosis. Our aim was to investigate the role of PLD in regulating the biphasic exocytosis of insulin from pancreatic beta-cells that is essential for mammalian glucose homeostasis. We observed that PLD activity in MIN6 pancreatic beta-cells is closely coupled to secretion. Cellular PLD activity was increased in response to a variety of secretagogues including the nutrient glucose and the cholinergic receptor agonist carbamoylcholine. Conversely, pharmacological or hormonal inhibition of stimulated secretion reduced PLD activity. Most importantly, blockade of PLD-catalyzed phosphatidic acid formation using butan-1-ol inhibited insulin secretion in both MIN6 cells and isolated pancreatic islets. It was further established that PLD activity was required for both the first and the second phase of glucose-stimulated insulin release, suggesting a role in the very distal steps of exocytosis, beyond granule recruitment into a readily releasable pool. Visualization of granules using green fluorescent protein-phogrin confirmed a requirement for PLD prior to granule fusion with the plasma membrane. PLD1 was shown to be the predominant isoform in MIN6 cells, and it was located at least partially on insulin granules. Overexpression of wild-type or a dominant negative catalytically inactive mutant of PLD1 augmented or inhibited secretagogue-stimulated secretion, respectively. The results suggest that phosphatidic acid formation on the granule membrane by PLD1 is essential for the regulated secretion of insulin from pancreatic beta-cells.     

Effect of dopamine on amylase secretion from guinea pig pancreatic acinar cells in vitro

Dopamine has been shown to effect pancreatic flow, protein output and amylase secretion in a variety of species. However, there is conflicting evidence regarding the role of dopamine on amylase release in vitro. Specific studies were conducted to evaluate the effect of dopamine and to compare its effects with other substances on basal- and secretagogue-stimulated amylase secretion in a guinea pig dispersed pancreatic acinar cells preparation. Dopamine (10(-6) M) induced a small, but significant (P less than 0.05) increase of amylase secretion. Established secretagogues (10(-6) M) including bombesin, cholecystokinin-octapeptide (CCK-8) and carbachol as anticipated induced significantly larger responses. Other substances tested (10(-6) M) including thyrotropin-releasing hormone (TRH) and muscimol were without effect. Complete dose-response studies (10(-11)-10(-3) M) in the presence of bombesin, CCK-8 and carbachol revealed that dopamine does not affect amylase release in response to these secretagogues. These findings suggest that dopamine is a weak stimulant of amylase secretion in vitro, and that it may therefore play a minor role in regulation of pancreatic enzyme secretion. Several factors including vascular, hormonal and neural have been implicated in regulation of pancreatic exocrine secretion. In particular, autonomic nervous system activity, notably cholinergic, has been shown to affect the secretory status of the pancreatic acinar cell. In addition, several biologically active peptides including bombesin, cholecystokinin (CCK), secretin, vasoactive intestinal peptide (VIP), substance P, gastrin and stimulation of cholinergic (muscarinic) receptors with carbachol have been shown to stimulate pancreatic enzyme secretion both in vivo and in vitro. Certain controversy regarding the role of the sympathetic nervous system in regulation of pancreatic exocrine secretion does exist. For example, several studies with agonists and antagonists of noradrenergic and dopaminergic receptor subtypes suggest a stimulatory effect on pancreatic fluid, electrolyte and enzyme secretion. However, these responses are species-specific and variations inherent to the model have been described. Dopamine administration has been shown to stimulate pancreatic bicarbonate and enzyme secretion in a variety of species including mice, dogs, and man. Radioligand binding studies with 3H-dopamine have revealed the presence of high- and low-affinity dopamine binding sites in dog pancreatic acinar cells. Stimulation of these receptors has been correlated with dose-dependent increases in intracellular cAMP levels.(ABSTRACT TRUNCATED AT 400 WORDS)