Synthesis, intracellular transport and discharge of exportable proteins in the pancreatic acinar cell and other cells

Recent advances in cystic fibrosis

Cystic fibrosis, one of the most common lethal inherited disorders in N. European and N. American populations, is characterized by the production of abnormally viscous mucous secretions in the lungs and digestive tract. The pathophysiological basis of the disease is unknown. However, during the last few years, rapid advances in molecular genetics and biochemical and physiological studies on cystic fibrosis epithelial cells have led to optimism that the cystic fibrosis defect will soon be identified. Current evidence suggests that the basic disturbance lies in altered regulation of protein secretion and electrolyte transport leading to an imbalance in composition of epithelial secretions in cystic fibrosis patients. Increasing knowledge of the mechanisms regulating production and secretion of mucins and movement of electrolytes across the cell membrane should lead to development of pharmacological manipulation(s) to correct the cellular abnormality. Ultimately, it is hoped that this will lead to the development of a rational treatment for cystic fibrosis patients.

Changes in the size and number of secretion granules in the rat exocrine pancreas induced by feeding or stimulation in vitro. A morphometric study

The size, number and volume per cell of secretion granules in rat exocrine pancreas have been measured using stereological techniques. The changes which occur as a result of feeding starved animals (90 min) or stimulating lobular fragments in vitro with carbachol are documented. In fasted animals mean acinar cell volume was estimated as 1670 micron 3 and the cells contained an average of around 450 secretion granules with a corrected mean diameter of 0.70 micron. They occupied around 7% of cell volume. After feeding mean cell volume was about 1300 micron 3 and the cells contained an average of about 190 granules per cell with a mean diameter of 0.58 micron. They occupied 3% of cell volume. A shift in the size frequency distribution of granule diameters occurred as a result of feeding. In vitro experiments in which lobules were induced to secrete with carbachol (10 microM, 3 h, 37 degrees C) had a similar effect. Mean cell volume was reduced from around 1760 micron 3 to 1360 micron 3, mean granule number from around 420 per cell to 180 per cell and the volume density of granules was reduced from about 8% to 3% of cell volume. There was no significant change in mean granule diameter or shift in the size-frequency distribution of granule diameters. Incubation of tissues with cycloheximide (1 mM, 3 h, 37 degrees C) did not prevent secretion by carbachol but it prevented replacement of granules. As a consequence, depletion by carbachol was greater in the presence of cycloheximide, the granules being reduced to around 110 per cell and to only 2.5% of cell volume. We conclude that feeding causes a preferential loss of larger granules and that during secretion replacement of granules occurs. Some of these granules are smaller than those evident in the glands of starved animals.

Synthesis and secretion of amylase in the rat parotid gland following autonomic nerve stimulation in vivo

Parasympathetic and sympathetic nerve stimulation in vivo, individually and in combination, was used to study secretion and synthesis of amylase in the rat parotid gland. After 30 min with sympathetic nerve stimulation (3 Hz) a decrease in glandular amylase was seen, which corresponded approximately to the salivary output. On the other hand, after parasympathetic stimulation (10 Hz), chosen to obtain comparable amylase output, there was no decrease in glandular amylase, which points to synthesis during such activation. Experiments with incorporation of [3H]leucine, reflecting amylase synthesis, showed that both types of nerve stimulations increased such uptake in parotid protein. These results indicate that beside sympathetic activity, which is the main stimulus for granular amylase secretion, parasympathetic nerve impulses can evoke considerable amylase secretion because amylase synthesis is stimulated and amylase is rapidly available from a special, possibly non-granular pool. As expected from previous experiments an augmentation of amylase secretion was found, and the present experiments also indicated an augmentation at the level of synthesis when the two nerves were stimulated at the same time.

Ontogeny of secretory function and cholecystokinin binding capacity in immature rat pancreas

Isolated acini were prepared from the pancreas of immature rats (age less than 1 hr. - 48 hrs) in order to study the development of the secretory process. The ultrastructural integrity of the acinar cells was maintained after digestion and stimulation with secretagogues. Acini prepared from rats aged 24 - 48 hours responded to both CCK-8 and carbachol with significant increases in amylase release. Although typical biphasic dose response curves were obtained, the curves were shifted to the right by 1 - 2 log units, compared to the responses of adult acini. At ages younger than 24 hours, acini were insensitive to secretagogues but were sensitive to the calcium ionophore A23187. CCK receptors were virtually absent from membranes prepared from newborn pancreases, but binding of CCK, although small, was measurable at 12 hours and slowly increased up to 48 hours. A greater amount of binding was seen at 72 hours, which appeared constant up to 14 days. At 21 days, adult levels of binding were found. These results confirm previous studies that the rat pancreas is insensitive to secretagogues in the first 24 hours of life. After age 24 hours the secretory process is intact but less sensitive to secretory agents than the more mature pancreas. In the case of CCK, this may be due to lesser numbers of CCK receptors and/or affinity of CCK for its receptor.

Significance of Ca2+, Rb+ fluxes, of cAMP and cGMP for the CCK8-modulated insulin release

In rat pancreatic islets the effects of cholecystokinin-8 (CCK8) on glucose-mediated insulin release, 45Ca2+ net uptake, 45Ca2+ efflux, 86Rb+ efflux, cAMP- and cGMP levels were studied. In the presence of a substimulatory glucose concentration (3 mM) CCK8 concentrations of up to 1 microM had no effect on insulin release, but CCK8 at 10 nM potentiated the stimulatory effect of glucose (11.1 mM). 10 nM CCK8 enhanced glucose-stimulated 45Ca2+ net uptake but was ineffective at substimulatory glucose levels. CCK8 had no effect on cAMP and cGMP levels in the presence of 11.1 mM glucose, CCK8 increased 86Rb+ (a measure of K+) in the presence of both 3 and 11.1 mM glucose. This effect was abolished when Ca2+ was omitted from the perifusion medium. CCK8 did not alter glucose (11.1 mM)-stimulated 45Ca2+ efflux rate. These data indicate that (1) CCK8 potentiates glucose-stimulated insulin secretion possibly via an effect on Ca2+ uptake, 2) by affecting Ca2+ uptake, CCK8 enhances K+ efflux, and 3) CCK8 does not mediate its effect via cAMP or cGMP. With respect to 86Rb+ efflux the mechanism of CCK8 action appears to be different from that of glucose. When the mechanism of CCK action on islets is compared with that on exocrine pancreas (data from others) there are similarities (importance of Ca2+ uptake and non-importance of cAMP and cGMP).

Effects of a cytosolic protein on the interaction of rat pancreatic zymogen granules in vitro

Photon correlation spectroscopy has been used to study the kinetics of aggregation of isolated rat pancreatic zymogen granules in vitro by monitoring time-dependent changes in mean particle size derived from the photon count autocorrelation function, g2(tau). Isolated granules were stable in isotonic sucrose (pH 5.4-7.0). At pH 6.0 they maintained a mean diameter of 1225 +/- 18 nm with a polydispersity index of 0.199 +/- 0.007. The mean granule diameter showed a limited decrease (approx. 20%) with increasing pH within the range 5.4-7.0, but the polydispersity index was unaltered. At pH greater than 7.0 granule instability was indicated by a rapid reduction in total photon counts. In solutions of monovalent cations ([M+] greater than 10 mM) and divalent cations ([M2+] greater than 0.5 mM) zymogen granules aggregated at a rate dependent upon both ion and granule concentration. These effects were consistent with the bimolecular nature of the interaction mechanism and were clearly distinguishable from the limited size changes associated with osmolarity. At concentrations of Na+ or K+ salts greater than 50 mM granule aggregation was accompanied by anion-dependent solubilisation. A soluble protein fraction separated from the pancreatic acinar cell cytosol by gel filtration reduced the mean diameter and polydispersity index of zymogen granules suspended in isotonic sucrose, inhibited cation-induced aggregation and stabilised granules to solubilisation induced by raising pH greater than 7.0 or exposure to high ionic strength media. The inhibitory effects of this protein were apparent at concentrations less than or equal to 10 micrograms X ml-1 (i.e. at inhibitor: granule protein ratios less than 1:20) and could not be mimicked by bovine serum albumin, the Ca2+-binding proteins calmodulin and troponin C (less than or equal to 100 micrograms X ml-1), nor the highly negatively charged polymer polyglutamate (less than or equal to 10 micrograms X ml-1). Inhibitory activity was also absent from fractions of rat liver cytosol prepared identically to pancreatic acinar cytosol. These observations are consistent with the presence in pancreatic acinar cells of a specific cytosolic granule stabilisation factor (or factors) that normally restricts zymogen granule interaction and may therefore play an important role in the regulation of granule mobility and exocytosis.

The relationship between the morphology of cell organelles and kinetics of the secretory process in male sex accessory glands of mice

Two male sex accessory glands of the mouse, seminal vesicle and coagulating gland, were compared with the aim of relating differences in the morphology of organelles to the kinetics of the secretory process. The epithelial cells of the two glands were assessed by morphometric analysis, cytochemical staining, and electron-microscopic autoradiography after administration of a labeled amino acid. The rough endoplasmic reticulum of the seminal vesicle comprised narrow parallel cisternae, while that of the coagulating gland was greatly distended and occupied a much larger percentage of the cytoplasmic volume. Radioactively labeled products were secreted much more rapidly in the seminal vesicle than in the coagulating gland. The primary point of difference in kinetics of intracellular transport between the two glands was in exit of material from the rough endoplasmic reticulum. The more rapid drainage of the rough endoplasmic reticulum may be related to its relatively greater membrane surface density and lesser internal volume. In contrast, similarities in size and cytochemical staining in the Golgi apparatus of the two glands were accompanied by similar kinetics of intracellular transport of secretory protein through this organelle.

Calcium concentration and amylase secretion in guinea pig pancreatic acini: interactions between carbachol, cholecystokinin octapeptide and the phorbol ester, 12-O-tetradecanoylphorbol 13-acetate

The effects of the phorbol ester, 12-O-tetradecanoylphorbol 13-acetate (TPA) on amylase secretion and cytoplasmic free calcium concentration ([Ca2+]i) were investigated in dispersed guinea pig pancreatic acini. Carbachol evoked dose-dependent increases in amylase secretion and [Ca2+]i with half-maximal responses at 2.5 and 5 microM, respectively. Carbachol-induced calcium transients could be blocked by atropine. In the presence of a maximal effective dose of carbachol, cholecystokinin octapeptide caused no further increase in [Ca2+]i, suggesting that both agonists act on the same pool of trigger calcium. TPA (10(-9)-10(-6) M) stimulated amylase secretion with no change in [Ca2+]i. Maximum amylase secretion occurred at 0.5 microM TPA. Preincubation of acini in the presence of TPA resulted in a time- and dose-dependent inhibition (IC50 = 30 nM) of the carbachol-induced rise in [Ca2+]i, the maximal effect being observed within 3 min. The inactive phorbol ester, 4 alpha-phorbol 12,13-didecanoate was ineffective in inhibiting the carbachol-stimulated rise in [Ca2+]i. These findings suggest that, in addition to stimulating amylase secretion, probably through protein kinase C, TPA may also exert a negative feedback control over secretagogue-induced calcium transients.

Transport characteristics of system A in the rat exocrine pancreatic epithelium analyzed using the specific non-metabolized amino acid analogue alpha-methylaminoisobutyric acid

The selectivity and kinetics of system A amino acid transport in the rat exocrine pancreatic epithelium were characterized using the specific analogue alpha-methylaminoisobutyric acid. Unidirectional influx of alpha-methylaminoisobutyric acid was measured in isolated perfused pancreata by rapid dual tracer dilution. In cross-inhibition experiments DL-methylalanine, L-serine, L-cysteine, glycine, L-phenylalanine and L-glutamine were effective inhibitors of influx, whereas L-glutamate and L-lysine were less effective. In the presence of sodium alpha-methylaminoisobutyric acid influx was saturable with an apparent Kt = 1.7 +/- 0.2 mM and Vmax = 0.49 +/- 0.03 mumol/min per g (mean +/- S.E., n = 6). Influx of alpha-methylaminoisobutyric acid at 50 microM and 100 microM concentrations was significantly inhibited as the perfusate sodium concentration was gradually decreased from 156 mM to 26 mM by isoosmolar choline replacement. Estimated Kt values for sodium at these two methylaminoisobutyric acid concentrations approximated 200 mM. System A activity in the basolateral membrane of the exocrine pancreatic epithelium exhibits a high transport affinity, a wide tolerance for different amino acids and a dependency upon the extracellular sodium concentration.

The calcium ionophore A23187 stimulates glycoprotein secretion by the guinea pig gallbladder

The purpose of this study was to examine the role of calcium ions in gallbladder glycoprotein secretion in cultured guinea pig gallbladder explants. The calcium ionophore A23187 showed a threshold of 2 micrograms per ml medium for stimulation of secretion of [3H]glucosamine-labeled glycoproteins over a 30 min incubation period. The ionophore at 3 and 5 micrograms per ml medium resulted in a 3- to 4-fold increase in secretion of [3H]glucosamine-labeled glycoproteins. Ionophore-induced stimulation of glycoprotein secretion was abolished by the addition of 0.01 mM verapamil to the medium. To study the effect of changes in extracellular calcium on basal glycoprotein secretion, explants were cultured for 24 hr in media with 0.007, 0.5 or 2.0 mM calcium; no differences in basal glycoprotein secretion were observed. When cultured in medium with 1.0 mM EGTA, basal secretion decreased significantly vs. controls in 0.007 mM total calcium medium. Total [3H]glucosamine incorporation by explants in medium with EGTA was unaltered, however, suggesting that the low level of calcium in the medium was selectively impairing the secretory process. These findings indicate that calcium ions are important in the regulation of gallbladder glycoprotein secretion.

Influence of extracellular [Ca2+] on secretory and redox responses to CCK-8 in perfused rat pancreas

There is a distinct discrepancy between the dose-dependent secretory responses (pancreatic protein output and juice flow) and the dose-dependent redox responses of cytochromes aa3, b, and c+c1 to various concentrations of CCK-8: a bell-shaped relation for the secretory responses contrasts with a sigmoidal relation for the redox responses. Continuous stimulation with CCK-8 at physiological low concentrations (5-10 pM) induced an increase in pancreatic protein output with little if any reduction of cytochromes. Continuous stimulation with CCK-8 at a pharmacological intermediate concentration (50 pM), however, induced an increase in pancreatic protein output with delayed reduction of cytochromes. The secretory and redox responses were completely abolished when CaCl2 was removed from the perfusing and bathing solution. An almost linear relation was found between the magnitude of the protein output in response to 50 pM CCK-8 and extracellular [Ca2+] [( Ca2+]o) in the range of 0.5-2.5 mM. Similar relations were also found between the levels of reduction of cytochromes and [Ca2+]o. These results are compatible with the view that continuous stimulation with CCK-8 at a pharmacological or a pathological concentration may cause excess increase in intracellular [Ca2+] and intramitochondrial Ca2+ and in this way may accelerate the formation of reducing power for oxidative phosphorylation.

Basolateral amino acid transport systems in the perfused exocrine pancreas: sodium-dependency and kinetic interactions between influx and efflux mechanisms

Basolateral amino acid transport systems have been characterized in the perfused exocrine pancreas using a high-resolution paired-tracer dilution technique. Significant epithelial uptakes were measured for L-alanine, L-serine, alpha-methylaminoisobutyric acid, glycine, methionine, leucine, phenylalanine, tyrosine and L-arginine, whereas L-tryptophan and L-aspartate had low uptakes. alpha-Methylaminoisobutyric acid transport was highly sodium dependent (81 +/- 3%), while uptake of L-serine, L-leucine and L-phenylalanine was relatively insensitive to perfusion with a sodium-free solution. Cross-inhibition experiments of L-alanine and L-phenylalanine transport by twelve unlabelled amino acids indicated overlapping specificities. Unidirectional L-phenylalanine transport was saturable (Kt = 16 +/- 1 mM, Vmax = 12.3 +/- 0.4 mumol/min per g), and weighted non-linear regression analysis indicated that influx was best described by a single Michaelis-Menten equation. The Vmax/Kt ratio (0.75) for L-phenylalanine remained unchanged in the presence of 10 mM L-serine. Although extremely difficult to fit, L-serine transport appeared to be mediated by two saturable carriers (Kt1 = 5.2 mM, Vmax1 = 7.56 mumol/min per g; Kt2 = 32.8 mM, Vmax2 = 22.9 mumol/min per g). In the presence of 10 mM L-phenylalanine the Vmax/Kt ratio for the two L-serine carriers was reduced, respectively, by 79% and 50%. Efflux of transported L-[3H]phenylalanine or L-[3H]serine was accelerated by increasing perfusate concentrations of, respectively, L-phenylalanine and L-serine, and trans-stimulated by other amino acids. In the pancreas neutral amino acid transport appears to be mediated by Na+-dependent Systems A and ASC, the classical Na+-independent System L and another Na+-independent System asc recently identified in erythrocytes. The interactions in amino acid influx and efflux may provide one of the mechanisms by which the supply of extracellular amino acids for pancreatic protein synthesis is regulated.

Dissociation between pancreatic enzyme secretory and synthetic dose-responses to cholecystokinin in man

The effect of varying the intensity of pancreatic stimulation on the synthesis of human pancreatic enzymes has not previously been studied. We have measured the secretion and synthesis of pancreatic enzymes in response to either secretin alone (1 CU.kg-1.h-1) or secretion plus increasing doses of cholecystokinin (CCK) (0.25, 0.5 or 1.0 IDU.kg-1.h-1). Enzyme synthesis was measured using the incorporation of 75Se-methionine (0.15 mCi (5.6 kBq).kg-1.h-1) into the trichloracetic acid-insoluble fraction of the duodenal aspirate. Outputs of trypsin, chymotrypsin, lipase and protein showed a bell-shaped dose response to increasing doses of cholecystokinin, with maximal outputs occurring in response to secretin plus cholecystokinin 0.5 IDU.kg-1.h-1. The rate of incorporation of 75Se-methionine increased with increasing doses of cholecystokinin and was maximal in response to secretion plus cholecystokinin 1.0 IDU.kg-1.h-1. There was therefore dissociation between the secretory and synthetic responses to increasing doses of cholecystokinin.

Calcium binding properties of purified zymogen granule membrane of pig pancreas. Evidence for calcium binding proteins

Ca2+ binding properties of purified zymogen granule membranes of pig pancreas have been measured: Binding increased linearly with Ca2+ concentration in the medium up to the micromolar range; in the millimolar range a sharp rise in binding capacity was observed. Binding increased with pH both at low and high concentrations of Ca2+. It was insensitive to Na+ and K+ ions at concentrations up to 100 mM. Mg2+ was inhibitory in the millimolar range whereas La2+ and Tb3+ were inhibitory in the micromolar range. The Ca2+ binding components of zymogen granule membranes were identified by two methods: (1) by measuring 45Ca2+ binding after counter-ion electrophoresis and (2) by Stain's-all (forms a complex with Ca2+ binding proteins absorbing maximally at 600 nm), after SDS-polyacrylamide gel electrophoresis. The first method, counter-ion electrophoresis, indicated that most of the 45Ca2+ was associated with an acidic band which could be subsequently subfractionated by SDS-polyacrylamide gel electrophoresis in five bands: 66, 57, 30, 27 and 22.5 kDa. The second method, Stain's-all, revealed six positive polypeptides after SDS-polyacrylamide gel electrophoresis of native zymogen granule membranes' two were unreactive after neuraminidase treatment (130 and 92 kDa, respectively), whereas four other bands were still reactive (66, 57, 43, 30 kDa, respectively.) Ca2+ binding was also measured on intact zymogen granules: the binding capacity was higher than for zymogen granule membranes. Among the Ca2+ binding proteins of the zymogen granule membrane only one is apparently located on the granule external surface: the 30 kDa polypeptide. If Ca2+ directly facilitates fusion of zymogen granules with plasma membrane by a Ca2+-protein interaction, then this protein is a presumptive candidate to play such a key role.

Ultrastructure of the secretory response of male mouse submandibular gland granular tubules

The organization and fine structure of granular convoluted tubule cells (GCT) from male mouse submandibular glands have been examined in controls and in animals injected with adrenergic and cholinergic secretagogues. Control submandibular glands exhibited a single population of GCT cells with numerous homogeneous granules filling the apical two-thirds of the cytoplasm. A zone of transition cells, exhibiting characteristics of both GCT and striated duct cells, was found between the agranular intercalated duct and GCT segments. These transition cells possessed apical granules of variable size as well as prominent basal striations. Dramatic changes in the morphology of GCT cells followed administration of the alpha-adrenergic agent, phenylephrine. The extensive degranulation involved formation of "secretory pools" of fused granules and release of secretory material into the lumen. The appearance of numerous smooth vesicles near luminal membranes suggested extensive membrane retrieval. Intracellular membrane-limited aggregates of membrane fragments suggested that much of the retrieved membrane was destined for degradation. Rough endoplasmic reticulum was highly dilated but there was no indication of increased size or activity of the Golgi complex. Ultrastructural evidence indicated that the secretory responses to isoproterenol, a beta-adrenergic agent, and to pilocarpine, a cholinergic agent, were much more modest, but it is clear that some secretory response to these agents does occur. The other cell types of the duct and tubule system did not exhibit comparable morphological changes in response to the agents used.

Pathways of protein secretion in eukaryotes

Protein secretion from cells can take several forms. Secretion is constitutive if proteins are secreted as fast as they are synthesized. In regulated secretion newly synthesized proteins destined for secretion are stored at high concentration in secretory vesicles until the cell receives an appropriate stimulus. When both constitutive and regulated protein secretion can take place in the same cell a mechanism must exist for sorting the correct secretory protein into the correct secretory vesicle. The secretory vesicle must then be delivered to the appropriate region of plasma membrane. Transfection of DNA encoding foreign secretory proteins into regulated secretory cells has provided insight into the specificity of sorting into secretory vesicles.

Calcium-dependent secretory and redox response to CCK-8 in isolated perfused rat pancreas

Continuous stimulation with 8 pM cholecystokinin octapeptide (CCK-8) induced a gradual increase in pancreatic protein output and little if any change in redox state of cytochromes aa3, b, and c + c1. The protein output was completely abolished when CaCl2 was removed from the perfusing and bathing solution. Continuous stimulation with 200 pM CCK-8 induced the rapid and largest protein output and a distinct reduction of cytochromes and nicotinamide nucleotides. These responses were partially decreased in the Ca2+-deficient environment and enhanced immediately after the replacement with the standard solution. These and other results are compatible with the view that reduction of electron transfer system in the pancreatic acinar cell may be induced by stimulation with the secretagogue in pharmacological concentration and that the reduction may coincide with uptake and retention of cytoplasmic excess Ca2+ by mitochondria.

What's new in in vitro studies of exocrine pancreatic cell injury?

Exocrine pancreas in vitro models are useful for the study of pancreatic differentiation, secretion mechanisms, cell injury, and lysosomal processing of secretory product. Syrian hamster pancreas in explant organ culture undergoes a series of morphologic changes which parallel in vitro acinar cell injury, differentiation, and phenotypic alteration. Within 48 hours, the cultured acinar cells show morphologic evidence of sublethal cell injury. Autophagy and crinophagy are particularly striking. The autophagic processes can be inhibited by the addition of the protein synthesis inhibitor cycloheximide or by culture at lowered temperatures (20 degrees C). Acinar cells lethally damaged show pyknotic nuclei, high amplitude swelling, and necrosis. Approximately 25% of each explant is viable after 72 hr in culture and the viability remains constant at 25-35% for up to 60 days of culture. The morphological changes of the explants are consistent with many of the features of pancreatitis and carcinoma of the exocrine pancreas. There is an increase in the ductal elements and a decrease in acini over time in culture. This may be due to: (a) an increased replication of ductal epithelial cells concomitant with necrosis of acinar epithelial cells and/or (b) phenotypic alteration of acinar cells to ductal cells. Acinar cell necrosis and phenotypic alterations may in part be due to the activation of lysosomal degradation pathways. Processes which inhibit lysosomal activation proved protective against these alterations, while processes which promote zymogen activation were deleterious.

Electrophoretic and cytological evidence for heterogeneity of pancreatic acinar cell responsiveness to carbachol, caerulein and secretin

Incubation of rat pancreatic lobules for 90 min with optimal concentrations of caerulein, carbachol or secretin caused the release of about 30% of the amylase content. Combination of secretin with carbachol or caerulein increased the amylase output to about 40%. With secretin, as with carbachol or caerulein, heterogeneity of cellular responsiveness was observed, some acini being partially or completely depleted of their zymogen granules, whereas others appeared to be resting. When secretin was combined with carbachol or caerulein, granule depletion, originally confined to small groups of neighbouring acini, spread to form large areas of degranulated cells, sometimes comprising a whole section of a lobule. In dispersed acini, under the same conditions, carbachol caused the release of about 60% of the amylase content, and secretin 40%. When both secretagogues were combined, a significant increase to 78% was observed. Under these conditions, there was some important cellular damage, as indicated by the release of 20% of the amylase content and between 6 and 12% of lactate dehydrogenase into the media, in the absence of stimulus. These results were corroborated by cytological observations. On the basis of their secretory response two groups of acini can be distinguished, those that respond to carbachol, caerulein or secretin and those that respond to the combination of secretin with carbachol or caerulein. Electrophoretic patterns of secretory proteins released by lobules stimulated by these different types of secretagogues were essentially similar. The pattern was quite different, however, in the absence of a stimulus.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulatory effects of insulin and experimental diabetes on neutral amino acid transport in the perfused rat exocrine pancreas. Kinetics of unidirectional L-serine influx and efflux at the basolateral plasma membrane

Relatively little is known about the hormonal regulation of amino acid transport in the normal and diabetic exocrine pancreas. In this study unidirectional influx and tracer efflux of L-serine at the basolateral interface of the rat pancreatic epithelium was investigated in the perfused exocrine pancreas using a rapid (less than 30 s) paired-tracer dilution technique. In the non-diabetic pancreas L-serine influx was saturable and stimulated by perfusion with exogenous bovine insulin (100 microU/ml). Transport of L-serine and methylaminoisobutyric acid was markedly elevated in pancreata isolated from streptozotocin diabetic rats and insulin partially reversed the stimulation of L-serine transport induced by experimental diabetes. These results suggest that insulin and diabetes modulate the epithelial transport activity for small neutral amino acids in the intact exocrine pancreas.

Effects of acetylcholine and short-chain fatty acids on acinar cells of the exocrine pancreas in sheep

In order to investigate the actions of acetylcholine and short-chain fatty acids on acinar cells of the exocrine pancreas of sheep, measurements of amylase release and 45Ca efflux from superfused segments, as well as changes in membrane potential, input resistance and equilibrium potential in the acinar cells, were carried out in vitro. The application of acetylcholine or short-chain fatty acids caused a dose-dependent increase in amylase release from the superfused tissue segments. The amylase release evoked by 10(-3) M-short-chain fatty acids containing 2-8 carbon atoms increased with increasing carbon number, up to 5 (i.e. it was maximum with iso-valerate, which has 5 carbon atoms). The amylase release stimulated by acetylcholine (5.5 X 10(-6) M) or caprylate (10(-3) was accompanied by an increase in 45Ca efflux, and was significantly reduced by the removal of extracellular Ca2+. The stimulating effect of caprylate (10(-3) M) on amylase secretion was also observed in superfused segments of the guinea-pig, but not in those of the mouse, rabbit or hamster. The resting membrane potential and input resistance of acinar cells of sheep pancreas were -31.1 +/- 1.6 mV and 2.7 +/- 0.7 M omega (means +/- S.E. of means), respectively. The application of acetylcholine or short-chain fatty acids always depolarized the cell membrane and reduced the input resistance. The effect of short-chain fatty acids was observed in the presence of atropine (1.4 X 10(-6) M). The equilibrium potentials for acetylcholine and butyrate were -15.0 +/- 0.8 and -16.0 +/- 1.1 mV, respectively. It is concluded that the cellular secretory process evoked by acetylcholine is qualitatively similar to that of short-chain fatty acids, and that Ca2+ ions might be an important mediator for these secretagogues in the acinar cells of the pancreas of sheep.

Ca2+ and cyclic nucleotide dependence of amylase release from isolated rat pancreatic acinar cells rendered permeable by intense electric fields

Enzyme digestion of rat pancreatic tissue yielded a preparation of isolated acinar cells, over 90% of which excluded trypan blue. These isolated cells responded to a variety of secretagogues, the responses being sensitive to the removal of extracellular calcium, increasing extracellular magnesium, and by trifluoperazine, an antagonist of Ca-dependent processes. When exposed to intense electric fields, isolated acinar cells became permeable to CaEGTA and MgATP, these markers gaining access to over 60% of the intracellular milieu within minutes. The accessibility to these markers seemed independent of the ionised Ca2+ level. Less than 0.5% of the cellular amylase was released when cells were rendered leaky in a medium containing about 10(-9) M Ca2+, but typically 4% was released when the Ca2+ level was subsequently raised to 10(-5)M levels, the EC50 for Ca2+ being 2 microM. This amount of amylase released was comparable to the amounts secreted from intact cells in response to a variety of agonists. The cytosolic marker lactate dehydrogenase was also released from leaky cells, but the extent was independent of Ca2+ concentration. No amylase was released at 10(-7)M Ca2+ when permeable cells were exposed to cyclic 3',5'-AMP or cyclic 3',5'-GMP. The calcium activation curve for amylase release seemed to be independent of cyclic nucleotides, but was markedly increased in both the extent of release and apparent affinity for Ca2+ in the presence of the phorbol ester 12-0-tetradecanoyl phorbol 13 acetate. These results suggest that when "functionally normal" isolated acinar cells are rendered permeable, Ca2+-but not cyclic nucleotides-acts as a second messenger for amylase secretion, and furthermore that protein kinase C may be involved in the secretory process.

Influence of calcium and cyclic nucleotides on beta-adrenergic sweat secretion in equine sweat glands

The effects of Ca2+, the cyclic nucleotides adenosine 3',5'-cyclic monophosphate (cAMP) and guanosine 3',5'-cyclic monophosphate (cGMP), and other parameters of sweat secretion from single equine sweat glands were examined in vitro. Extracellular Ca2+, the Ca2+ ionophore A23187, and the Ca2+ channel antagonist verapamil were all without effect on sweat secretion. Prolonged rinsing of the glands in Ca2+-free Ringer solution with 5 mM ethylenediaminetetraacetic acid decreased the secretion to 30% of the control sweat rate in response to the beta-adrenergic agonist isoproterenol; the sweat response was restored upon adding Ca2+ to the Ringer. cAMP but not cGMP analogues were as effective in stimulating sweat rates as isoproterenol, which elicited maximal secretory rates in vitro. cAMP stimulation was not inhibited by the beta-adrenergic antagonist propranolol. Because the equine sweat gland is predominantly stimulated via the beta-adrenergic receptor, we conclude that cAMP is a principal intracellular messenger in coupling this type of stimulus to the fluid secretion response in this tissue.

Cyclic nucleotide-dependent enzyme secretion in the rat lacrimal gland

To characterize the role of cyclic nucleotides in secretion of enzymes by the lacrimal gland, pieces of rat exorbital glands were perfused with (1) 8-bromoadenosine-3',5'-cyclic monophosphate (8 Br cyclic AMP), (2) 8-bromoguanosine-3',5'-cyclic monophosphate (8 Br cyclic GMP), (3) forskolin, a stimulator of adenylate cyclase activity, (4) 3-isobutyl-1-methylxanthine (IBMX), an inhibitor of phosphodiesterase activity, or (5) carbachol, a cholinergic agonist. As a measure of enzyme secretion, timed collections of the perifusate effluent were analysed for peroxidase, an enzyme secreted by the lacrimal gland. Control peroxidase secretion was 0.3-0.9 (u./min per milligram protein). Peroxidase secretion was stimulated by 8 Br cyclic AMP (1 mM), but not by 8 Br cyclic GMP (1 mM). A 2-fold increase was detected. Peroxidase secretion was also stimulated by forskolin (60 microM), IBMX (1 mM), and the cholinergic agonist carbachol, which all stimulated peroxidase secretion 2-or 3-fold. The effect of maximally effective concentrations of IBMX (1 mM) and carbachol (0.1 mM) on secretion was additive. Finally, Ca2+ depletion in the presence of EGTA (1 mM) inhibited both IBMX-and carbachol-induced secretion by 45% and 60% respectively. We conclude that cyclic AMP, but not cyclic GMP, can stimulate lacrimal gland enzyme secretion. Cyclic AMP appears to utilize a pathway separate from but convergent with cholinergic agonists.

Calcium-ion-transporting activity in two microsomal subfractions from rat pancreatic acini. Modulation by carbamylcholine

Two microsomal subfractions from isolated rat pancreatic acini were produced by centrifugation through a discontinuous sucrose density gradient and characterized by biochemical markers. The denser fraction ( SF2 ) was a highly purified preparation of rough endoplasmic reticulum; the less-dense fraction ( SF1 ) was heterogeneous and contained Golgi, endoplasmic reticulum and plasma membranes. 45Ca2+ accumulation in the presence of ATP and its rapid release after treatment with the bivalent-cation ionophore A23187 were demonstrated in both fractions. The pH optimum for active 45Ca2+ uptake was approx. 6.8 for the rough endoplasmic reticulum ( SF2 ) and approx. 7.5 for SF1 . Initial rate measurements were used to determine the affinity of the rough-endoplasmic-reticulum uptake system for free Ca2+. An apparent Km of 0.16 +/- 0.06 microM and Vmax. of 21.5 +/- 5.6 nmol of Ca2+/min per mg of protein were obtained. 45Ca2+ uptake by SF1 was less sensitive to Ca2+, half-maximal uptake occurring at 1-2 microM-free Ca2+. When fractions were prepared from isolated acini stimulated with 3 microM-carbamylcholine, 45Ca2+ uptake was increased in the rough endoplasmic reticulum. The increased uptake was due to a higher Vmax. with no significant change in Km. No effect was observed on 45Ca2+ uptake by SF1 . In conclusion, two distinct non-mitochondrial, ATP-dependent calcium-uptake systems have been demonstrated in rat pancreatic acini. One of these is located in the rough endoplasmic reticulum, but the precise location of the other has not been determined. We have shown that the Ca2+-transporting activity in the rough endoplasmic reticulum may have an important role in maintaining the cytosolic free Ca2+ concentration in resting acinar cells and is involved in Ca2+ movements which occur during stimulation of enzyme secretion.

Introduction of calcium chelators into isolated rat pancreatic acini inhibits amylase release in response to carbamylcholine

The Ca2+ chelators, EGTA and BAPTA, have been introduced into intact, isolated rat pancreatic acini using a hypotonic swelling method. This resulted in complete inhibition of amylase release, stimulated by carbamylcholine at a submaximal concentration and 82 - 85% inhibition at maximal concentrations. Acini swollen in the absence of Ca2+ chelators showed similar secretory responses to those of unswollen acini. Treatment of unswollen acini with chelators inhibited the maximum response to carbamylcholine by only 23%. The inhibitory effect of intracellular chelators was not due to ATP depletion or a lowering of the total cell Ca2+ content. Thus, these results provide the first direct demonstration that an increase in intracellular Ca2+ concentration is necessary for the stimulation of enzyme release from pancreatic acinar cells.

Cytosolic calcium regulates epidermal growth factor endocytosis in rat pancreas and cultured fibroblasts

Cholecystokinin octapeptide (CCK8), the COOH-terminal moiety of cholecystokinin (CCK), exerted a rapid inhibitory effect on total cell-associated 125I-labeled epidermal growth factor (125I-EGF) binding by decreasing the rate of EGF internalization in isolated rat pancreatic acini. Removal of CCK8 from incubation medium followed by extensive washing of acini did not abolish its inhibitory effect, indicating that its action was not readily reversible. Proglumide, a competitive antagonist of CCK8, blocked the inhibitory action of the secretagogue. Addition of CCK8 to cells previously exposed to 125I-EGF did not enhance the release of cell-associated 125I activity. CCK8 did not inhibit the binding of 125I-labeled insulin to pancreatic acini. Other pancreatic secretagogues that enhance digestive-enzyme release through Ca2+, including caerulein, bombesin, carbachol, gastrin, and the Ca2+ ionophore A23187, also inhibited cell-associated 125I-EGF radioactivity. Further, at 37 degrees C the ionophore A23187 inhibited specific 125I-EGF binding in human A-431 carcinoma cells, Swiss 3T3 cells, and Rat-1 fibroblasts, and this effect was abolished when 125I-EGF internalization was reduced by incubating cells at 4 degrees C. It is concluded that alterations in cellular Ca2+ in the pancreas and other cells lead to inhibition of EGF endocytosis.

Ethanol and the pancreas

The acute and chronic effects of ethanol on pancreatic structure and function are discussed. Acute necrotizing, acute edematous, acute relapsing, chronic relapsing, and painless pancreatitis have an established association with ethanol abuse. The management of these disorders is outlined.

An overview of pancreatic exocrine secretion

Genes for all proteins have encoded in their DNA sequences, information that specifies where these proteins will localize within the cell. Nascent translation products of transcripts of these genes, that possess a specific NH2-terminal signal sequence, are able to translocate into a specialized membranous conducting system called the endoplasmic reticulum (ER), or can be incorporated directly into a target organelle (i.e. mitochondrion). Nascent polypeptides lacking this signal sequence remain in the cytosol. Once segregation into the ER has occurred each protein appears to migrate at a characteristic rate to a connecting organelle; the Golgi Complex. Here, enzymatic modifications of these proteins determines the organelle with which each will eventually become associated. In secretory tissues such as the exocrine pancreas, many different proteins are directed into specialized secretory structures called zymogen granules. These granules are maintained in a "ready-to-release" state by steroid hormones. In the absence of estrogens and glucocorticoids the zymogen granules disappear. Physiologically, secretion from the exocrine pancreas is brought about by parasympathetic nerve stimulation, or by the gut hormone cholecystokinin (CCK). Interaction of acetylcholine or CCK with specific receptors on pancreatic acinar cells initiates the process of exocytosis; that is, fusion of the zymogen granule membrane with the plasma membrane resulting in extracellular release of the contents of the secretory granule.

Cystic fibrosis serum stimulates mucin secretion but not calcium efflux from rat submandibular acini

Cystic fibrosis serum stimulated release of mucins from rat submandibular acini to a significantly greater degree than did control serum. This action was not due to non-specific cell lysis, as monitored by lactate dehydrogenase release and was not accompanied by increases in 45Ca2+-efflux. The actions of cystic fibrosis serum and control serum in stimulating amylase release from rat exocrine pancreatic acini could not be distinguished. Thus, a specific action of cystic fibrosis serum in increasing mucin secretion, but not calcium permeability, in rat exocrine acini has been demonstrated.

Intracellular free calcium concentrations in isolated pancreatic acini; effects of secretagogues

Isolated pancreatic acini were loaded with the calcium selective fluorescent indicator, quin-2. Measurements of cellular K+ content and lactic dehydrogenase release indicated that cell viability was not affected by quin-2 loading. The concentration of intracellular free calcium of unstimulated acinar cells was calculated to be 180 +/- 4 nM. When cells suspended in media containing millimolar calcium were exposed to the secretagogues carbachol and cholecystokinin a rapid increase in [Ca2+]i occurred. Both the amplitude and rate of rise of the concentration increase were dose dependent with [Ca2+]i reaching a maximum of 860 +/- 41 nM. The dose-response relationship coincides with the known concentration dependence of the stimulation of amylase release by these agents. In the absence of extracellular calcium, carbachol was still able to elicit a rise in [Ca2+]i. These studies indicate that pancreatic secretagogues induce an increase in [Ca2+]i of acinar cells, both in the presence or absence of extracellular calcium.

On the mechanism of stimulation of H+ transport in gastric mucosa by Ca++ ionophore A23187. I. Pathways for cytosolic calcium increase

The pathways for cytosolic Ca++ increase under A23187 stimulation of H+ secretion were studied in the isolated gastric mucosa of the toad Bufo marinus. A23187 produced a more potent stimulation of secretion when added to the mucosal side which did not contain calcium. Measurements of ionophore incorporation by fluorometric methods indicated that A23187 incorporates into oxyntic cells intracellularly. The presence of divalent cations inhibited incorporation. This may be the reason for a more potent action when A23187 was added from the mucosal side. Withdrawal of calcium from serosal solution largely inhibited the secretory response to A23187 added to the mucosal side. Reintroduction of calcium into the serosal side in the presence of ionophore elicited H+ secretion. The results are consistent with an uptake of A23187 from the mucosal side into cellular organelles and basolateral membranes. Calcium entry through the serosal side may be responsible for triggering secretion. Although A23187 likely releases calcium from intracellular stores, its rate of release may not be sufficient to bring about a full stimulation of secretion in serosal-Ca++-free conditions.

Increased phospholipid synthesis in the stimulated rat and human pancreas

Stimulation of the exocrine pancreas is associated with marked changes in pancreatic phospholipid metabolism. It has been previously established that de novo synthesis of phospholipids constitutes part of this "phospholipid effect". This study has demonstrated that in vitro stimulation of the rat pancreas utilising bethanecol and pancreozymin results in increased incorporation of labelled glucose into phosphatidyl inositol and, to a lesser extent, other phospholipids, suggesting increased de novo synthesis of these compounds. However, secretin which is believed to act via a different intracellular pathway, did not exert such an effect. The relevance of this animal model is indicated by the demonstration of increased incorporation of labelled glucose into phospholipids of human pancreas stimulated in vitro by bethanecol or sincalide (the active carboxy terminal octapeptide of pancreozymin).

Exocrine glands and protein secretion: a stereological viewpoint

The exocrine pancreas and the parotid salivary gland have been widely used as models for studying the synthesis, intracellular transport and discharge of exportable proteins. This article briefly reviews quantitative morphological (stereological) studies which have been made of these glands and assesses their contribution to our understanding of the secretory process. A general stereological profile of these glands is presented and the way in which their morphology changes during development is outlined. Detailed consideration is given to the granule population of the cells, particularly the way in which granules are formed and discharged as a result of secretory stimuli. The membrane content of secretory cells and membrane dynamics during the secretory cycle are also examined. Throughout, the emphasis is placed on the interpretation of stereological data rather than on the methods themselves.

Uptake, storage and secretion of 5-hydroxytryptamine and its amino acid precursor by dispersed rat pancreas acinar cells

Rat pancreas acinar cells contain 5-hydroxytryptamine (5-HT; 10.86 +/- 2.52 ng/i.u. amylase), all of which can be accounted for by the 5-HT recovered from the zymogen granule fraction of these cells (10.70 +/- 3.06 ng/i.u. amylase). When incubated with [14C]5-HT dispersed acinar cells take up the amine and concentrate it in zymogen granules. These cells will also take up [14C]5-HTP (5-hydroxytryptophan), decarboxylate it and store the [14C]5-HT so produced in zymogen granules. 5-HTP itself is not taken up by the granules. 5-HT is incorporated into zymogen granules early in their formation; no amine is accumulated by mature zymogen granules and the amine within the mature granule is not exchangeable with extragranular amine. When dispersed acinar cells pre-labelled with [14C]5-HT and [3H]leucine are stimulated with caerulein, there is a synchronous increase in amylase activity and secretion of [14C]5-HT and [3H]protein; the ratios of [3H]protein/[14C]5-HT in zymogen granules and in the secretory products are identical. Pancreas acinar cells take up L-DOPA, decarboxylate it and store the dopamine produced in zymogen granules but the dopamine is not retained by the granules (t1/2 approximately equal to 90 min) and dopamine secretion from cells exposed to caerulein could not be demonstrated. It is concluded that 5-HT is a normal component of rat pancreas acinar cell zymogen granule. The granular amine has a turnover rate similar to that of granular protein and is released when the cells are stimulated to secrete protein. All the 5-HT released from the cell originates in zymogen granules.

Hepatic parenchymal cells in cystic fibrosis: ultrastructural evidence for abnormal intracellular transport

Few ultrastructural observations of hepatic parenchymal cells in cystic fibrosis (CF) have appeared in the literature. Utilizing a unique opportunity to examine freshly fixed hepatic tissue by transmission electron microscopy, we studied 12 patients dying with CF at Duke Hospital from 1979 to 1981 in order to identify possible abnormalities of intracellular architecture. The major findings include (1) intracellular fatty vacuoles, (2) distended bile ductules and bile ducts containing increased cellular debris, (3) profiles of distended rough endoplasmic reticulum containing material of medium electron density, and (4) membrane-bound deposits of electron-lucent material containing electron-dense cores resembling mucus. We suggest that the material seen within the cytocavitary network reflects a derangement of intracellular transport.

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.

The pharmacological modulation of [3H]-disaturated phosphatidylcholine overflow from perifused lung slices of adult rats: a new method for the study of lung surfactant secretion

Lung slices from adult rats incubated in [methyl-3H]-choline chloride formed [3H]-disaturated phosphatidylcholine ( [3H]-DSPC) which was used as an index of lung surfactant. The slices were perifused after 3 h incubation in [methyl-3H]-choline chloride and the overflow of [3H]-DSPC, as a rate coefficient, was used as a measure of surfactant secretion. The basal overflow of [3H]-DSPC rapidly declined over the first 30 min of perifusion and then declined slowly. Salbutamol induced a prolonged, and sometimes delayed, increase in [3H]-DSPC overflow, which was reduced by (+/-)-propranolol. Potassium chloride produced an immediate, and usually transient, increase in [3H]-DSPC overflow which was not modified by atropine or (+/-)-propranolol. Adenosine 5'-triphosphate, but not phenylephrine, also increased [3H]-DSPC overflow. This method can measure the magnitude and time-course of lung surfactant secretion induced by drugs.

Pathogenesis of alcoholic pancreatitis

The pathogenesis of alcoholic pancreatitis continues to be a puzzle. Of the many theories as to how alcohol might cause pancreatic damage, none satisfactorily explains why only a minority of alcoholics develop clinical pancreatitis. Hypertriglyceridemia and inherited factors could be important antecedents in some individuals, and high fat and protein diets may favour the development of the disease. Disturbances of the sphincter of Oddi have been postulated, but there are experimental and theoretical objections to the view that alcoholic pancreatitis generally results from sphincter dysfunction (obstruction-hypersecretion, biliary-pancreatic reflux and duodeno-pancreatic reflux). Biochemical studies of the effect of alcohol on pancreatic tissue have so far been relatively unrewarding. The most widely held view is that alcohol causes the deposition of protein in peripheral ducts leading to obstruction, inflammation and degeneration. However, it remains to be shown that these deposits are the cause rather than a result of pancreatic inflammation. Research might be facilitated by the development of a suitable animal model of the disease.