Studies on intracellular transport of secretory proteins in the rat exocrine pancreas. V. Kinetic studies on accelerated transport following caerulein infusion in vivo

Repetitive Cerulein-Induced Chronic Pancreatitis in Growing Pigs-A Pilot Study

Chronic pancreatitis (CP) is an irreversible and progressive inflammatory disease. Knowledge on the development and progression of CP is limited. The goal of the study was to define the serum profile of pro-inflammatory cytokines and the cell antioxidant defense system (superoxidase dismutase-SOD, and reduced glutathione-GSH) over time in a cerulein-induced CP model and explore the impact of these changes on selected cytokines in the intestinal mucosa and pancreatic tissue, as well as on selected serum biochemical parameters. The mRNA expression of CLDN1 and CDH1 genes, and levels of Claudin-1 and E-cadherin, proteins of gut barrier, in the intestinal mucosa were determined via western blot analysis. The study showed moderate pathomorphological changes in the pigs' pancreas 43 days after the last cerulein injection. Blood serum levels of interleukin (IL)-1-beta, IL-6, tumor necrosis factor alpha (TNF-alpha), C-reactive protein (CRP), lactate dehydrogenase (LDH), gamma-glutamyl transpeptidase (GGTP), SOD and GSH were increased following cerulein injections. IL-1-beta, IL-6, TNF-alpha and GSH were also increased in jejunal mucosa and pancreatic tissue. In duodenum, decreased mRNA expression of CDH1 and level of E-cadherin and increased D-lactate, an indicator of leaky gut, indicating an inflammatory state, were observed. Based on the current results, we can conclude that repetitive cerulein injections in growing pigs not only led to CP over time, but also induced inflammation in the intestine. As a result of the inflammation, the intestinal barrier was impaired.

Experimental Acute Pancreatitis Models: History, Current Status, and Role in Translational Research

Acute pancreatitis is a potentially severe inflammatory disease that may be associated with a substantial morbidity and mortality. Currently there is no specific treatment for the disease, which indicates an ongoing demand for research into its pathogenesis and development of new therapeutic strategies. Due to the unpredictable course of acute pancreatitis and relatively concealed anatomical site in the retro-peritoneum, research on the human pancreas remains challenging. As a result, for over the last 100 years studies on the pathogenesis of this disease have heavily relied on animal models. This review aims to summarize different animal models of acute pancreatitis from the past to present and discuss their main characteristics and applications. It identifies key studies that have enhanced our current understanding of the pathogenesis of acute pancreatitis and highlights the instrumental role of animal models in translational research for developing novel therapies.

Translational Insights Into Peroxisome Proliferator-Activated Receptors in Experimental Acute Pancreatitis

Acute pancreatitis (AP) is an inflammatory disorder of the exocrine pancreas frequently associated with metabolic causes, contributing factors, or consequences, including hypertriglyceridemia, obesity, and disorders of intermediary metabolism, respectively. To date, there is no specific therapy for this disease. Future optimal therapy should correct both inflammatory and metabolic components of the disease. Peroxisome proliferator-activated receptors (PPARs) are lipid-sensing nuclear receptors that control inflammatory and metabolic pathways via ligand-dependent and ligand-independent mechanisms. There are 3 known subtypes, PPAR-α, PPAR-β/δ, and PPAR-γ, which are differentially expressed in various tissues. The PPARs interact closely with other transcription factors such as nuclear factor κB and signal tranducers and activators of transcription that have pivotal roles in the pathobiology of AP. In this comprehensive review, we summarize the role of PPARs in AP, highlighting important in vitro and in vivo experimental findings. Finally, we propose future research directions as well as potential translational use of PPAR agonists in the treatment of AP.

Agonist-induced vesiculation of the Golgi apparatus in pancreatic acinar cells

BACKGROUND & AIMS

The pancreatic acinar cell is known to regulate exocytosis, total protein synthesis, and secretory protein transport in response to a secretory stimulus. Whether secretory vesicle formation also is regulated is unclear. In this study, we determined whether agonist stimulation induces morphologic alterations in the acinar cell Golgi apparatus, and we evaluated the role of the vesicle severing protein dynamin.

METHODS

Changes in Golgi structural integrity by examining the distribution of various Golgi and TGN lipid and protein markers in live and fixed cells on stimulation with cholecystokinin were noted in a primary pancreatic acinar cell model. Multiple dynamin reagents were used to examine the distribution and function of this molecular pinchase in resting and stimulated cells.

RESULTS

Regulated secretion in acinar cells induced (1) marked fragmentation of the trans-Golgi network (TGN) that corresponded temporally with an increase in cytoplasmic calcium whereas pre-TGN compartments of the Golgi and regions of the TGN involved in the generation of constitutively trafficking vesicles were unaffected by agonist, and (2) significant recruitment of dynamin to the acinar cell Golgi apparatus that appeared to potentiate fragmentation of the TGN.

CONCLUSIONS

These results suggest that the TGN is a dynamic organelle that fragments in response to cholecystokinin stimulation, a process that may contribute to zymogen granule formation.

Caerulein stimulates pancreatic secretory response in conscious newborn rats

The aim of our study was to measure age-dependent, caerulein-stimulated pancreatic enzyme secretion of conscious CFY suckling rats without pancreatic duct cannulation. Pancreatic secretory response was expressed as the decrease in specific enzyme (trypsin, amylase) activity compared to saline-injected control. The study was performed in three phases. In 10-d-old conscious newborn rats, single 1 and 3 micrograms/kg sc doses of caerulein induced significant decreases in specific trypsin (42 and 47%) and amylase (34 and 33%) activity 15 min after the caerulein injection; the same doses injected at 0 and 30 min evoked a similar decrease 90 min after the first injection. The 0.5 microgram/kg dose was ineffective. In 10-d-old anesthetized rats, the 90-min-decrease in total pancreatic trypsin activity, induced by graded doses (1,3,10, and 30 micrograms/kg) of caerulein, was compared to the 90-min output of trypsin in their bile-pancreatic juice. Each of the applied doses induced significant change in the total trypsin activity both in the pancreas (-33-57%) and juice (+21 +/- 49%) and its decrease in the gland corresponded quantitatively well (r = 0.52; p less than 0.01) to the increase in the simultaneous 90-min trypsin output. The age- and dose-dependent pancreatic response of 3-, 5-, 10-, and 20-d-old conscious rats was investigated under the effect of 1,3,10, and 30 micrograms/kg sc doses of caerulein injected at 0 and 30 min. In 3-d-old rats, the 10 and 30 micrograms/kg and in 20-d-old rats, the 1 and 3 micrograms/kg doses were effective, whereas in 5- and 10-d-old rats each caerulein dose applied evoked a significant decrease in pancreatic-specific trypsin activity.

CONCLUSION

The pancreas of newborn rats is in vivo less sensitive to caerulein between postnatal d 3 and 10 than in already weaned rats.

Exocytosis occurs at the lateral plasma membrane of the pancreatic acinar cell during supramaximal secretagogue stimulation

In vitro and in vivo studies indicate that the secretory response to both caerulein and carbamylcholine stimulation is biphasic. Over the range of submaximal to maximal concentrations of secretagogues, discharge of exocrine proteins in vitro into the incubation medium and in vivo into the pancreatic duct increased and morphologic analysis indicated that exocytosis of zymogen granules occurred exclusively at the luminal membrane. Under in vivo conditions, supramaximal stimulation with caerulein or carbamylcholine resulted in a dose-dependent decrease in amylase release into the pancreatic duct and increase in the appearance of amylase in the blood circulation. Under in vitro or in vivo conditions, supramaximal secretagogue stimulation resulted in marked inhibition of exocytotic activity at the luminal plasma membrane, the appearance of intergranule contacts and fusions within the cytoplasm, and the appearance of exocytotic activity at the lateral plasma membrane. Lateral exocytotic images were observed with individual and fused zymogen granules and autophagic vacuoles. This redirection in the final step of the secretory pathway provides in part the biological basis for the increased appearance of pancreatic (pro)enzymes in the interstitial fluid and serum during supramaximal secretagogue stimulation.

Phasic release of newly synthesized secretory proteins in the unstimulated rat exocrine pancreas

Pancreatic lobules from fasted rats secrete pulse-labeled proteins in two phases comprising 15 and 85% of basal output, respectively. The first (0-6.5 h) is initially (less than or equal to 0.5 h) unstimulated by secretagogues, probably represents vesicular traffic of Golgi and post-Golgi origin (including condensing vaculoles/immature granules), and notably contains two groups of polypeptides with distinct release rates: zymogens (t1/2 approximately 2.4 h) and minor nonzymogens plus one unique zymogen (t1/2 approximately 1 h). The second phase (peak at 9-10 h) is stimulable, probably represents basal granule exocytosis (t1/2 approximately 5 h), and contains zymogens exclusively. Newly synthesized proteins released in both phases appear asynchronously, reiterating their asynchronous transport through intracellular compartments. Zymogens in both phases are secreted apically. The sorting of first from second phase zymogen release does not appear to be carrier-mediated, although the sorting of zymogens from other secretory proteins may use this process. Finally, data are presented that suggest that both secretory phases are subject to physiologic regulation.

Stimulation of pancreatic secretory process in the rat by low-molecular weight proteinase inhibitor. I. Dose-response study on enzyme content and secretion, cholecystokinin release and pancreatic fine structure

Application of a single dose of a new type of proteinase inhibitor camostate (FOY-305) via orogastric tube was used in rats to study the dose-response relationship of resulting pancreatic stimulation. Doses up to 10 mg/kg failed to elicit any response, while significant decrease in enzyme content and increase in serum CCK-levels were observed with doses ranging from 25 to 400 mg/kg. A single dose of 100 mg/kg was selected for a time-sequence analysis, which revealed a 60 to 70% depletion of enzyme stores persisting over 6 h and reverting to control levels by 12 h. Peak increases in serum CCK-levels (15-fold above the elevation observed after regular food intake) were found after 30 min and persisted as an 8- to 10-fold elevation for at least 3 h, then declined to control levels by 9 h. This prolonged endogenous hormone release and resulting pancreatic stimulation were also verified in a separate group of animals in which volume, protein, and enzyme output were measured after cannulation of the pancreatic duct. While volume secretion was not altered by feeding a single dose of 100 mg/kg FOY-305, protein and enzyme output increased 2- to 3-fold over a period of 7 h. Fine-structural analysis of the pancreas demonstrated efficient depletion of zymogen granules from acinar cells with all doses between 50 and 400 mg/kg, accompanied by the appearance of membrane material in the acinar lumina at 3 and 6 h. The same transient increase in the number of lysosomal bodies predominantly containing mitochondria with all doses above 50 mg/kg was interpreted as increased organelle turnover due to persisting hormonal stimulation.

In-vivo stimulation of rat pancreatic acinar cells by infusion of secretin. I. Changes in enzyme content, pancreatic fine structure and total rate of protein synthesis

Infusion of synthetic secretin in conscious unrestricted rats for periods up to 24 h was used to study the structural and functional adaptation of pancreatic acinar cells to this secretagogue. Initial dose-response studies established 16 clinical units (CU) per kg and h (corresponding to 4.64 micrograms X kg-1 X h-1) as optimal dose for persistent stimulation of enzyme discharge. Infusion of this dose led to a slow but progressive depletion of enzyme stores with minimal content by 12 h stimulation. As a result of persistent stimulation total protein synthesis in the acinar cells increased after a lag period of 3 h and reached maximal values 90% above controls by 6 and 12 h secretin infusion. No structural equivalent for pronounced fluid and bicarbonate secretion was observed for either acinar or duct cells over the entire dose range (1 to 64 CU X kg-1 X h-1) and infusion period (1-24 h), except an increased number of coated vesicles in duct cells. Discharge of enzymes from acinar cells was paralleled by a high frequency of exocytotic images at the luminal plasma membrane and was accompanied by the occurrence of membrane fragments in the luminal space, especially after 3 and 6 h secretin infusion. An increased number of lysosomal bodies at these time points especially in the vicinity of the Golgi complex was interpreted in relation to membrane recycling following massive exocytosis. This pattern of structural and functional adaptation of acinar cells following secretin infusion corresponds to previously described changes following caerulein and carbamylcholine stimulation.

Multiple-level caerulein control of the gene expression of secretory proteins in the rat pancreas

Continuous intravenous infusion of caerulein (0.25 micrograms X kg-1 X h-1) has recently been reported [Schick, J., Kern, H. & Scheele, G. (1984) J. Cell Biol. 99, 1569-1574] to enhance significantly the synthesis of both trypsinogen and chymotrypsinogen and to decrease that of amylase in the rat pancreas. With a view to achieving a better understanding of the mechanisms underlying caerulein modulation of pancreatic gene expression, the relative levels of active mRNA corresponding to these proteins were determined in caerulein-stimulated animals and compared to those of controls infused with a 0.9% NaCl solution. For this purpose, the translation products synthesized in vitro in a rabbit reticulocyte lysate translation system were measured. Prolonged caerulein infusion had less pronounced effects on mRNA levels as determined by the relative synthesis of translation products than on individual secretory proteins. No changes in mRNA levels were observed during 6 h of hormonal stimulation, whereas a 7-fold increase in the ratio of trypsinogen to amylase synthesis was obtained previously. After 24 h of caerulein infusion, only a slight change in active mRNA coding for amylase (1.7-fold) and serine protease zymogens (1.4-fold) occurred as compared to 14-fold and 2-fold variations in the synthesis rates of the corresponding proteins. These findings indicate that caerulein exerts a predominantly translational control on the biosynthesis of pancreatic amylase, trypsinogen and chymotrypsinogen even after 24 h of hormonal stimulation. However, additional control at a transcriptional or post-transcriptional level (i.e. via messenger RNA stability) may well take place.

Structural and biochemical characterization of maximal and supramaximal hormonal stimulation of rat exocrine pancreas

To study the regulation of the successive steps along the secretory pathway in the rat exocrine pancreas the model of in vivo infusion of synthetic caerulein in conscious rats for periods up to 72 h was combined with electron microscopy and in vitro analysis of protein synthesis, intracellular protein transport and enzyme discharge using isolated pancreatic lobules. Prolonged and maximal hormonal stimulation was obtained with 0.25 microgram kg-1 h-1 caerulein and resulted in a 80-90% depletion of enzyme stores within 1 to 3 h, followed by coordinate and anticoordinate changes in individual rates of (pro-)enzyme synthesis after a lag period of 3 h. One group (two amylases) revealed a decrease in synthesis to levels about 10-fold lower than controls. A third group of proteins (one trypsinogen, lipase, proelastase) did not show changes in synthesis with hormone stimulation. The sum of such alterations led to an increase in total rate of synthesis after 6 h, which was combined with acceleration of intracellular transport, packaging, and granule discharge, thus enabling a sustained rate of secretion over the period of stimulation. In contrast, infusion of a supramaximal dose of caerulein (5.0 micrograms kg-1 h-1) induced acute edematous pancreatitis and led to an almost complete reduction of volume and protein output from the cannulated main pancreatic duct. Using freeze-fracture techniques and thin-section electron microscopy, earliest structural alterations were observed at membranes of zymogen granules and the plasma membrane. Fusion of zymogen granules among each other led to formation of large membrane-bound vacuoles within the cytoplasm. These and individual zymogen granules fused with the lateral instead of the apical plasma membrane, discharging their content into the interstitial space. Vacuole formation was associated with activation of lysosomes and with cytoplasmic destruction of acinar cells. The findings indicated severe changes in the specificity of the intracellular membrane fusion process induced by supramaximal doses of caerulein, which finally resulted in autodigestion of the pancreas.

Hormonal stimulation in the exocrine pancreas results in coordinate and anticoordinate regulation of protein synthesis

24-h intravenous caerulein infusion studies in the rat were combined with in vitro amino acid incorporation studies followed by high-resolution separation of proteins by two-dimensional isoelectric focusing and SDS gel electrophoresis to study the extent to which persistent changes in the biosynthesis of exocrine pancreatic proteins are regulated by cholecystokinin-like peptides. Beginning in the third hour of optimal hormone infusion at 0.25 microgram kg-1 h-1, changes were observed in the synthetic rates of 12 proteins, which progressed over the course of the 24-h study. Based on coordinate response patterns, exocrine proteins could be classified into four distinct groups. Group I (trypsinogen forms 1 and 2) showed progressive increases in synthetic rates reaching a combined 4.3-fold increase over control levels. Group II (amylase forms 1 and 2) showed progressive decreases in synthesis to levels 7.1- and 14.3-fold lower than control levels, respectively. Group III proteins (ribonuclease, chymotrypsinogen forms 1 and 2, procarboxypeptidase forms A and B, and proelastase 1) showed moderate increases in synthesis, 1.4-2.8-fold, and group IV proteins (trypsinogen 3, lipase, proelastase 2, and unidentified proteins 1-4) did not show changes in synthesis with hormone stimulation. Regulation of protein synthesis in response to caerulein infusion was specific for individual isoenzymic forms in the case of both trypsinogen and proelastase. The ratio of biosynthetic rates of trypsinogen forms 1 + 2 to amylase forms 1 + 2 increased from a control value of 0.56 to 24.4 after 24 h of hormonal stimulation (43.5-fold increase). Biosynthetic rates for an unidentified protein (P23) with an Mr = 23,000 and isoelectric point of 6.2 increased 14.2-fold, and the ratio of synthesis of P23 to amylase 2 increased 200-fold during caerulein infusion. During hormone stimulation the anticoordinate response in the synthesis of pancreatic glycosidases (decreased synthesis) and serine protease zymogens (increased synthesis) explain previous observations that showed little change in rates of total protein synthesis under similar conditions.

The Golgi apparatus and lysosomes of rat pancreatic acinar cells following refeeding

The short term effects of refeeding on the Golgi apparatus and lysosomes of the rat exocrine pancreas were evaluated by ultrastructural, morphometric and cytochemical methods. Ten minutes after refeeding, there was a significant enlargement of Golgi cisternae and a significant increase, compared with the controls, in the number of condensing vacuoles and lysosomes. These modifications were accompanied by the appearance of acid phosphatase activity in stacked Golgi cisternae (as well as GERL) of some cells. One hour after refeeding, there were about the same numbers of condensing vacuoles and lysosomes as in the controls; Golgi cisternae were still significantly enlarged, compared with the controls, but they were no longer reactive for acid phosphatase. In both fasting and refed animals, acid phosphatase activity was demonstrable in tubular lysosomes. The data are interpreted in terms both of membrane disposal and recycling, leading to enhanced formation of zymogen granules, during physiologically stimulated secretion.

Secretory process in Brunner's glands during recovery from stimulation with a single dose of pilocarpine

The secretory pathway and kinetics of the secretory process were studied in Brunner's glands of mice after stimulation of secretion with a parasympatho-mimetic drug. Adult male mice were injected with pilocarpine, while unstimulated controls received saline. The animals were subsequently administered an intravenous injection of 3H-threonine, and tissue was prepared for electron microscope autoradiography at intervals ranging from 5 minutes to 2 hours after injection of the radioactive precursor. Stimulation with pilocarpine resulted in discharge of secretory granules, which was reflected in a significantly lower percentage of the cell volume occupied by granules. In both control and stimulated mice, at 5 minutes after injection of 3H-threonine, the highest percentage of silver grains was found over the rough endoplasmic reticulum. The proportion of silver grains over the rough endoplasmic reticulum declined at later intervals, and a peak of labeling was observed over the Golgi apparatus at 1 hour. Labeling of the secretory granules increased in the 1- and 2-hour samples from both control and stimulated mice, although the relative concentration of radioactivity in both Golgi-associated and apical secretory granules was greater in stimulated than control glands at 1 hour. The results suggest that the secretory protein produced by Brunner's glands was synthesized by the rough endoplasmic reticulum and transported to the Golgi apparatus where secretory granules were formed in both stimulated and control glands. Depletion of secretory granules by prior stimulation resulted in no change in the kinetics of arrival of radioactivity in the cell organelles involved in the secretory process. However, the drainage of the radioactive label from the rough endoplasmic reticulum was significantly slower in the stimulated glands than in the controls. The existence of two subcompartments within the rough endoplasmic reticulum is among the possible explanations for the latter observation.

A morphometric study of 24-hour variations in subcellular structures of the rat pancreatic acinar cell

Subcellular structures of pancreatic acinar cells were examined at six evenly spaced time points in the 24-h period (light cycle: 06.00 h--18.00 h) in four Wistar male rats at each time point. At each sampling point, the area and circumference of acinar cell bodies and the area, number and circumference of their cytoplasmic organelles were measured using a semiautomatic computer system for morphometry and a point-counting method. The area, number and circumference-area ratio of the cytoplasmic organelles were subjected to strong circadian variations, and the cellular area and circumference exhibited weak circadian variations. Variation pattern of the cytoplasmic organelles suggested an intracellular route for secretory proteins during a 24-h span. From the results it was possible to divide the 24-h period into three stages. 1. The resting or protein synthetic stage (00.00 h to 08.00 h): the area of the rough surfaced endoplasmic reticulum (rER) was strongly increased, and that of zymogen granules was clearly decreased. 2. The granule accumulation stage (08.00 h to 16.00 h): the area of the rER was markedly decreased; that of zymogen granules was increased. 3. The secretion stage (16.00 h to 00.00): as a result of the release of zymogen granules from the acinar cell, the area of zymogen granules decreased, and that of the rER increased. The relationship between the area of the rER and zymogen granules varied in a reciprocal manner. Other cytoplasmic organelles, namely the Golgi complex, condensing vacuoles, mitochondria and lysosomes also varied prominently during the 24-h span, corresponding to variations in the rER and zymogen granules.

Course and spontaneous regression of acute pancreatitis in the rat

Rat exocrine pancreatic function was studied structurally and biochemically after the in vivo production of actue interstitial pancreatitis by supramaximal stimulation with caerulein. Two major phases in the reaction of the gland were observed: During the first two days after cessation of the supramaximal stimulation a progressive infiltration of the interstitium and the pancreatic tissue with polymorphonuclear leucocytes, lymphocytes and macrophages occurred which led to further destruction of the gland and to decreased functional response. From two days after the cessation of the treatment, hypertrophy of centro-acinar cells and an increased rate of mitotic activity indicated regeneration of the pancreas. This was combined with an accelerated in vitro discharge of newly synthesized proteins over a period of four days. Between days three and six after the initial treatment mitotic activity was also observed in fully differentiated exocrine cells. Total structural and functional recovery of the pancreas was achieved nine to tweleve days after the cessation of the supramaximal stimulation.

Amino acid transport in the rat exocrine pancreas. II. Inhibition by lanthanum and tetracaine

The two calcium antagonistic agents lanthanum and tetracaine cause severe disturbances in the secretory process of the exocrine pancreas, including inhibition of the rate of protein synthesis and exocytosis. The former effect resulted mainly from the inhibition of amino acid transport. Lanthanum in a concentration up to 1 mM inhibited transport of different species of amino acids in an unspecific way whereas tetracaine interfered specifically with the Na+ -dependent transport system for neutral amino acids (14C-alpha-amino-isobutyric acid). Na+ -dependent transport of neutral amino acids (3H-leucine) was not affected. Transport inhibition was correlated to the acitvity of the Na+, K+ -ATPase system which was measured in isolated plasma membrane fractions. At higher concentrations (5-10 mM) some uptake of lanthanum into the cells by limited endocytosis was observed. At lower concentrations lanthanum seemed to bind exclusively to certain components of the plasms membrane, mainly at the lateral and basal cell surface. Even at a concentration of 5-10 mM no binding to the apical surface occurred. Similarly no binding of lanthanum was observed to the limiting membrane of isolated zymogen granules, while mitochondria, contained in the same fraction showed considerable binding affinity. The action of lanthanum and tetracaine on membrane carrier systems did not affect the interior organization of the plasma membrane. Particle density and distribution in freeze-fracture replicas as well as the submembrane microfilamentous=microtubular system and the junctional elements remained unaffected.

Acute interstitial pancreatitis in the rat induced by excessive doses of a pancreatic secretagogue

Conscious rats were infused via a jugular vein catheter with 5 x 10-6 g/kg/h caerulein for periods up to 24 h. On macroscopic inspection a progressive interstitial oedema is seen to develop in the pancreas, from one hour of infusion on and is most marked at twelve hours. This oedema is largely reabsorbed after 24 h treatment, but the pancreas is considerably indurated by this time. Serum amylase levels increase consistently to reach a tenfold elevation above controls after three, six or twelve hours infusion. Premature fusion of condensing vacuoles and secretory granules leads to formation of large vacuoles in the cytoplasm of exocrine pancreatic cells. These vacuoles fuse with the lateral and basal plasma membrane and realease their content into the extracellular space. Regular discharge of zymogen granules at the cell apex into the duct system does not occur. Vacuole formation is associated with cytoplasmic destruction of the pancreatic cells. The rate of protein synthesis decreases consistently as a result of these structural alterations and this change corresponds largely to a reduction of cellular respiration. Release of amylase from isolated pancreatic lobules of caerulein infused animals shows a progressive increase of unstimulated discharge, while in vitro stimulation with 5 x 10-6M carbamylcholine gives secretion patterns of wash-out kinetics. Stimulated discharge of labeled secretory proteins indicates a progressive reduction in the in vitro sensitivity of the pancreatic cells to secretagogues. After 24 h infusion of 5 x 10-6 g/kg/h caerulein the pancreatic lobules are totally insensitive to the in vitro effect of carbamylcholine or caerulein.

Studies on secretory glycoproteins in the rat exocrine pancreas. I. Fine structure of the Golgi complex and release of fucose-labeled proteins after in vivo stimulation with caerulein

Using a double-label technique on isolated rat pancreatic lobules, the rate of synthesis and discharge of regular and fucosylated secretory proteins was studied under control conditions and after in vivo prestimulation with caerulein. Both labeled leucine and fucose were incorporated into pancreatic proteins at a linear rate, which was potentiated by in vivo stimulation. In pulse-chase experiments both regular and fucosylated secretory proteins were discharged into the medium in parallel. The in vivo pretreatment with caerulein caused an earlier discharge and increased the total amount released. Kinetic analysis of unstimulated (baseline) discharge of both classes of secretory proteins indicated in vitro sensitivity by the previous in vivo treatment with caerulein. The biochemical data were compared to the fine structure of the Golgi complex under both control and prestimulated ocnditions. The Golgi stacks were composed of four to six individual cisternae which in some cases were connected by intercisternal pores. Transporting vesicles were observed fusing along the total length of the outermost cisterna on both the cis- and trans-side and with the lateral ends of the intermediate cisternae. Under control conditions only the last trans-cisterna contained some electron opaque material; in vivo prestimulation led to distension and filling of all disternae in an individual Golgi-unit. Numerous stages of transformation of the last transcisterna into condensing vacuoles were observed, lending support to the hypothesis that during packaging of secretory products the membranes of the Golgi complex undergo a continuous turnover.