Dietary regulation of pancreatic protein synthesis. I. Rapid and specific modulation of enzyme synthesis by changes in dietary composition

Determining the protocol requirements of in-home cat food digestibility testing

In-home cat food digestibility testing has the potential to yield data that are highly representative of the pet population for which the food is intended. However, no standardized and validated in-home digestibility test protocols are currently available. Such protocols for in-home testing should address key factors that explain variation in cat food digestibility values and here we investigated the required period of adaptation, fecal collection and sample sizes. Thirty privately-owned indoor housed cats of various breeds (20♀ 10♂, 5.9 ± 3.9 yr, 4.5 ± 1.3 kg) received a relatively low and high digestible complete dry extruded food with the marker titanium (Ti) dioxide. Foods were given in a cross-over design of 2 periods of 8 consecutive days each. Owners collected feces daily for the determination of daily fecal Ti concentrations and digestibility of dry matter, crude protein, crude fat, and gross energy. Data originating from 26 cats were analyzed as mixed models and broken line regressions to investigate the required adaptation and fecal collection period. Bootstrap sampling was used to assess the impact of increasing the number of fecal collection days and sample size on the precision of the digestibility estimates. Feces were collected on 347 out of 416 study days (16 days/cat; 26 cats), implying the necessity for multiple collection days to account for cats not defecating every day. Cats showed stable fecal marker concentrations from day 2 onwards when fed the low digestible food and from 3 onwards when fed the high digestible food. Digestibility values were stable from day 1, 2 or 3 onwards, depending on the test food and nutrient. Increasing the number of fecal collection days from 1 to 6 days did not result in more precise digestibility estimates, whereas increasing the number of animals from 5 to 25 cats did. For future in-home digestibility tests of cat food, the findings support a minimum of 2 adaptation days and 3 fecal collection days. Appropriate sample sizes depend on the test food, the nutrient of interest, and the acceptable margin of error. The findings of this study support the protocol development for future in-home digestibility testing of cat foods.

Determining the protocol requirements of in-home dog food digestibility testing

In-home pet food testing has the benefit of yielding data which is directly applicable to the pet population. Validated and standardised in-home test protocols need to be available, and here we investigated key protocol requirements for an in-home canine food digestibility protocol. Participants were recruited via an online survey. After meeting specific inclusion criteria, sixty dogs of various breeds and ages received, during 14 consecutive days, a relatively low and high digestible complete dry extruded food containing titanium (Ti) dioxide. Both foods were given for 7 d in a cross-over design. Owners collected faeces daily allowing daily faecal Ti concentrations and digestibility of nitrogen (N), dry matter (DM), crude ash, organic matter (OM), crude fat (Cfat), starch and gross energy (GE) to be determined. Faecal Ti and digestibility values for all nutrients were not different (P > 0·05) from the second day onwards after first consumption for both foods. One day of faecal collection yielded reliable digestibility values with additional collection days not reducing the confidence interval around the mean. Depending on the accepted margin of error, the food and the nutrient of interest, the minimal required sample size was between 9 and 43 dogs. Variation in digestibility values could in part be explained by a dog's neuter status (N, crude ash) and age (crude ash, Cfat) but not sex and body size. Future studies should focus on further identifying and controlling sources of variation to improve the in-home digestibility protocol and reduce the number of dogs required.

Insulin protects acinar cells during pancreatitis by preserving glycolytic ATP supply to calcium pumps

Acute pancreatitis (AP) is serious inflammatory disease of the pancreas. Accumulating evidence links diabetes with severity of AP, suggesting that endogenous insulin may be protective. We investigated this putative protective effect of insulin during cellular and in vivo models of AP in diabetic mice (Ins2^Akita) and Pancreatic Acinar cell-specific Conditional Insulin Receptor Knock Out mice (PACIRKO). Caerulein and palmitoleic acid (POA)/ethanol-induced pancreatitis was more severe in both Ins2^Akita and PACIRKO vs control mice, suggesting that endogenous insulin directly protects acinar cells in vivo. In isolated pancreatic acinar cells, insulin induced Akt-mediated phosphorylation of 6-phosphofructo-2-kinase/fructose-2,6-biphosphatase 2 (PFKFB2) which upregulated glycolysis thereby preventing POA-induced ATP depletion, inhibition of the ATP-dependent plasma membrane Ca²⁺ ATPase (PMCA) and cytotoxic Ca²⁺ overload. These data provide the first mechanistic link between diabetes and severity of AP and suggest that phosphorylation of PFKFB2 may represent a potential therapeutic strategy for treatment of AP.

Mitochondrial complex I structure reveals ordered water molecules for catalysis and proton translocation

Mitochondrial complex I powers ATP synthesis by oxidative phosphorylation, exploiting the energy from ubiquinone reduction by NADH to drive protons across the energy-transducing inner membrane. Recent cryo-EM analyses of mammalian and yeast complex I have revolutionized structural and mechanistic knowledge and defined structures in different functional states. Here, we describe a 2.7-Å-resolution structure of the 42-subunit complex I from the yeast Yarrowia lipolytica containing 275 structured water molecules. We identify a proton-relay pathway for ubiquinone reduction and water molecules that connect mechanistically crucial elements and constitute proton-translocation pathways through the membrane. By comparison with known structures, we deconvolute structural changes governing the mammalian 'deactive transition' (relevant to ischemia-reperfusion injury) and their effects on the ubiquinone-binding site and a connected cavity in ND1. Our structure thus provides important insights into catalysis by this enigmatic respiratory machine.

Influence of cholesterol and fat in food on indirect parameters of exocrine pancreatic function

Fecal elastase 1, chymotrypsin activity, and fat content in stool are clinical parameters of exocrine pancreatic function. The aim of this study was to clarify the possible impact of extreme changes in diet on fecal enzyme concentration/activity, since extreme diets may lead to wrong conclusions in the diagnosis of pancreatic insufficiency. Twelve healthy test persons followed 4 diet phases, each with a duration of 6 days. The 4 diet phases were (1) nearly fat-free with a low-cholesterol content; (2) high in fat and low in cholesterol; (3) high in cholesterol deriving from meat, and (4) high in cholesterol deriving from eggs. At the end of each diet phase, a 72-hour stool collection was carried out to measure fecal elastase 1, chymotrypsin and fecal fat content. The results showed no significant changes after each of the 4 diet phases. The clinical parameters of fecal elastase 1 and chymotrypsin activity in stool do not seem to be significantly influenced by fat and cholesterol deriving from food.

Differential increases in syntheses of newly identified trypsinogen 2 isoforms by dietary protein in rat pancreas

We have found that dietary protein markedly induced pancreatic serine protease activity via a mechanism independent of luminal trypsin activity in pancreaticobiliary-diverted (PBD) rats. The aim of this study was to examine the effects of dietary protein on the synthesis of trypsinogen isoforms by comparing in vivo incorporation of [35S] L-methionine into isoform proteins in PBD and sham-operated rats. A small duodenal segment including the ampulla of Vater was sectioned and transposed to the upper ileum with end-to-side anastomosis (PBD) or duodenal transection was followed by reanastomosis (sham) in male Sprague-Dawley rats. After recovery, PBD and sham rats were fed a 25% or 60% casein-sucrose-based diet (NC or HC) for 14 days. Rats were then intravenously injected with [35S] L-methionine (15 MBq/kg body weight) 30 mins before being sacrificed for analysis of pancreatic enzymes by two-dimensional SDS-polyacrylamide gel electrophoresis. By using electrophoresis with narrow range of isoelectric focusing (pI 4.5-5.5), five trypsinogen 2 (2-x) isoform spots were identified using both [35S] incorporation and Coomassie brilliant blue (CBB) staining in PBD rats, but not in sham rats. N-terminal sequences of these trypsinogen 2-x spots were identical to known rat trypsinogen 2 with the exception that the third valine was changed to isoleucine in one isoform. In PBD rats, feeding of HC specifically increased the [35S] and CBB intensities of these trypsinogen 2-x isoforms and trypsinogen 3. The degree of induction of the five trypsinogen 2-x molecules by HC varied greatly. Trypsinogen 1 and 4, which are the major trypsinogens in normal rats, showed no changes. We conclude that increases in synthesis of a few newly identified trypsinogen 2-x isoforms mainly contribute to the induction of trypsin activity in the pancreas by HC in PBD rats.

Luminal dietary protein, not amino acids, induces pancreatic protease via CCK in pancreaticobiliary-diverted rats

We determined whether pancreatic adaptation to a high-protein diet depends on ingested protein in the intestinal lumen and whether such adaptation depends on a CCK or capsaicin-sensitive vagal afferent pathway in pancreaticobiliary-diverted (PBD) rats. Feeding a high-casein (60%) diet but not a high-amino acid diet to PBD rats increased pancreatic trypsin and chymotrypsin activities compared with those after feeding a 25% casein diet. In contrast, feeding both the high-nitrogen diets induced pancreatic hypertrophy in PBD rats. These pancreatic changes by the diets were abolished by treatment with devazepide, a CCK-A receptor antagonist. Protease zymogen mRNA abundance in the PBD rat was not increased by feeding the high-casein diet and was decreased by devazepide. Perivagal capsaicin treatment did not influence the values of any pancreatic variables in PBD rats fed the normal or high-casein diet. We concluded that luminal protein or peptides were responsible for the bile pancreatic juice-independent induction of pancreatic proteases on feeding a high-protein diet. The induction was found to be dependent on the direct action of CCK on the pancreas. Pancreatic growth induced by high-protein feeding in PBD rats may depend at least partly on absorbed amino acids.

The effect of ethanol on pancreatic enzymes--a dietary artefact?

The effects of ethanol on pancreatic digestive and lysosomal enzymes may be relevant to the pathogenesis of alcoholic pancreatitis since pancreatic enzymes are thought to play an important role in the development of pancreatic injury. Previous studies, using the Lieber-DeCarli pair-feeding model of ethanol administration, have demonstrated that ethanol significantly increases the content and gene expression of pancreatic enzymes. However, these findings have been questioned because, in the Lieber-DeCarli model, ethanol-fed rats have a lower carbohydrate intake than their pair-fed controls, making it difficult to ascribe any observed changes to ethanol alone. This study was designed to distinguish between the effects of ethanol and those of reduced dietary carbohydrate on pancreatic enzymes, using a quartet-feeding model of ethanol administration. Rats were fed liquid diets containing low (11%) and high (47%) amounts of carbohydrate, with and without ethanol, for four weeks. The effects of ethanol on pancreatic content and messenger RNA levels for digestive enzymes (trypsinogen, chymotrypsinogen and lipase) and a lysosomal enzyme (cathepsin B) were assessed. Ethanol feeding resulted in a significant increase in glandular content with a corresponding increase in mRNA levels for all four enzymes studied. By contrast, a reduction in dietary carbohydrate intake alone did not alter pancreatic content or gene expression for the above enzymes. These results indicate that (i) ethanol significantly increases the capacity of the acinar cells to synthesise digestive enzymes and the lysosomal enzyme cathepsin B, and (ii) these changes are due to ethanol itself and are not due to variations in dietary carbohydrate intake.

Stability of the mRNA encoding some pancreatic hydrolases is modulated by dietary protein intake in the rat

Wistar rats fed on either a high-protein or a protein-free diet were examined to determine their pancreatic hydrolase mRNA stabilities in comparison with those of control animals receiving a standard diet. Actinomycin D was used to inhibit transcription and, after isolating the pancreatic RNA, the specific messengers were quantified by performing dot-blot hybridization with cDNA probes. In the rats fed on a high-protein diet, only the half-lives of anionic trypsinogen I and elastase I (EC 3.4.21.36) were affected. Interestingly, when rats were fed on the protein-free diet, most of the hydrolase mRNA half-lives showed changes, except that corresponding to lipase. In these rats, the half-life values of the mRNA coding for anionic trypsinogen I, chymotrypsinogen and procarboxypeptidase B increased, in sharp contrast with those of the amylase and elastase I mRNA, which decreased. These results strongly suggest that the mechanism whereby the biosynthesis of pancreatic hydrolases is regulated, depending on the presence or absence of proteins in the diet, is not unique and provide evidence that the stability of mRNA encoding most, if not all, the hydrolases in pancreatic cells is modulated by the dietary protein content.

Pancreatic enzyme secretion in response to test meals differing in the quality of dietary fat (olive and sunflowerseed oils) in human subjects

The aim of the present study was to investigate in human subjects whether or not the ingestion of two liquid meals that differed only in their fatty acid composition (due to the addition of olive oil (group O) or sunflowerseed oil (group S) as the source of dietary fat) would lead to differences in the pancreatic enzyme activities secreted into the duodenum. The experiments were performed in eighteen cholecystectomized subjects who, during the 30 d period immediately before surgery, modified their habitual diets in such a way that their fat composition would reflect, as far as possible, that of the experimental meals. Lipase (EC 3.1.1.3), colipase, amylase (EC 3.2.1.1), chymotrypsin (EC 3.4.21.1) and trypsin (EC 3.4.21.4) activities were measured in duodenal contents aspirated before and after the ingestion of the test meals. The plasma levels of secretin and cholecystokinin (CCK) were also examined. Duodenal enzyme activities were similar in resting conditions. No significant differences were revealed in postprandial enzyme activities, except for lipase activity, which was higher in group O, probably in relation to the greater plasma CCK concentrations observed in this group. In the absence of enzyme output data, we should not exclude the possibility that the type of dietary fat will affect human pancreatic enzyme secretion to a greater extent than is evident from the present study, for instance through a flow-mediated effect, as we previously observed in dogs.

Amylase and trypsin responses to intake of dietary carbohydrate and protein depend on the developmental stage in sea bass (Dicentrarchus labrax) larvae

Sea bass (Dicentrarchus labrax) larvae were fed from day 15 to day 35 with 4 isoenergetic formulated diets that varied in protein (30, 40, 50, 60%) and carbohydrate (37, 27, 17, 7%) content. The diets were designated as P30, P40, P50 and P60, respectively. Best growth and survival were noted for P50 larvae. Poor growth and survival were observed for P30 larvae. The specific activity of amylase increased in direct relation to the dietary carbohydrate level from day 18 onwards. This increase was the result of extensive amylase synthesis. Trypsin activity was positively correlated with dietary protein level only at day 35. This study shows that the mechanisms involved in amylase regulation are efficient in very young sea bass larvae, while those related to trypsin appear later in ontogeny and thus suggests an age-dependent regulation of enzyme synthesis.

Relationship between amino acid transport and protein synthesis in rat isolated pancreatic acini under stimulation with cholecystokinin

The effects of cholecystokinin octapeptide (CCK8) on amino acid transport and protein synthesis in the pancreatic acini were determined. The question of whether those effects differed between amino acid transport systems A and L was also investigated. The influx of alpha-[3H]aminoisobutyric acid (AIB, system A) and [14C]cycloleucine (system L) into isolated rat pancreatic acini and the incorporation of [14C]alanine (system A) and [3H]leucine (system L) into trichloroacetic acid (TCA)-precipitable protein in pancreatic acini were studied in the presence of various concentrations of CCK8. CCK inhibited the amino acid transport across the basolateral membrane of pancreatic acini. This effect was mediated by Ca2+, and was more responsive to amino acid transport system L than to A. CCK also inhibited protein synthesis in pancreatic acini in a dose-dependent manner. The inhibition of protein synthesis by CCK was thought to be mainly due to the inhibition of amino acid transport by CCK. Therefore, the predominant utilization of extracellular amino acids in protein synthesis was suggested.

Cholecystokinin-induced changes in polysome structure regulate protein synthesis in pancreas

Translational regulation of digestive enzyme synthesis during short-term stimulation by cholecystokinin-octapeptide (CCK-OP), was examined in minced rabbit pancreas by measuring protein synthesis and monitoring alterations in the size of polysomes attached to the rough endoplasmic reticulum (RER). The effect of CCK-OP on protein synthesis was determined by measuring [3H]leucine incorporation into trichloroacetic-acid-precipitable proteins. Concentrations of CCK-OP that caused maximal enzyme secretion (10 and 30 nM) decreased protein synthesis by approx. 50% compared to control. Protein synthesis returned to the control level 60 min after terminating the action of CCK-OP. Autoradiography of [35S]methionine-labeled proteins separated by one-dimensional SDS-polyacrylamide gel electrophoresis demonstrated that CCK-OP reversibly inhibited the synthesis of all of the major groups of digestive enzymes. Northern blot analysis revealed that CCK-OP did not alter the cellular content of amylase and elastase mRNA. Incubation with CCK-OP caused a decrease in the size distribution of RER-bound polysomes. Polysome profiles returned to the control pattern 60 min following termination of the stimulus. These results suggest that the inhibitory effects of CCK-OP on the synthesis of digestive enzymes is regulated at translation by decreasing the number of RER-bound ribosomes that are actively translating digestive enzyme mRNA.

Influx and incorporation into protein of L-phenylalanine in the perfused rat pancreas: effects of amino acid deprivation and carbachol

The rate of protein synthesis in the isolated perfused rat pancreas was measured from the rate of incorporation of L-[3H]phenylalanine into total protein, and was compared with the transport of this amino acid into the epithelium. Unidirectional (15 s) and net (15-30 min) uptake of L-[3H]phenylalanine was measured relative to D-[14C]mannitol (extracellular marker) using a cell loading technique. The fractional rate of protein synthesis in the pancreas was also measured in vivo using a flooding dose technique and found to be 118 +/- 10% day-1 (corresponding to an absolute rate of incorporation of L-Phe into protein of 36.1 +/- 3 nmol min-1 g-1) in overnight fasted rats. Compared with the in vivo rate, the perfused pancreas exhibited a markedly lower rate of protein synthesis which increased significantly when amino acids were added to the perfusate (15.6 +/- 1.9 vs. 22.5 +/- 0.9% day-1 or 4.7 +/- 0.6 vs. 6.9 +/- 0.3 nmol L-Phe min-1 g-1). Carbachol (3 x 10(-7) M) stimulated protein synthesis provided amino acids were also supplied in the perfusate. Protein synthesis rates measured under all conditions in vivo and in vitro were at least an order of magnitude lower than the unidirectional influx (121 +/- 14 nmol min-1 g-1) of L-phenylalanine into the pancreatic epithelium. These results demonstrate that amino acid transport across the basolateral membrane of the epithelium is not rate-limiting for pancreatic protein synthesis.

Intracellular mechanisms involved in short-term regulation of net protein synthesis in pancreatic acini

The mechanisms regulating the net synthesis of digestive enzymes during short-term stimulation by agonists were examined in pancreatic acini isolated from the rat. Dispersed pancreatic acini were stimulated for up to 60 min with various concentrations of cholecystokinin octapeptide (CCK-OP), carbachol, A23187, 4 beta-phorbol 12-myristate 13-acetate (PMA). The effects of these agonists on net protein synthesis was determined by measuring the incorporation of [3H]leucine or [35S]methionine into protein. Carbachol, PMA, A23187 and concentrations of CCK-OP of 100 pM and greater caused inhibition of protein synthesis. Fluorography of [35S]methionine labeled acinar cell proteins separated by one-dimensional SDS-polyacrylamide gel electrophoresis demonstrated that the agonists inhibited the synthesis of the digestive enzymes. Northern blot analysis using cDNA probes revealed that CCK-OP, carbachol and PMA did not alter the cellular content of amylase, lipase and elastase mRNA. The protein kinase C inhibitors 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7) and staurosporine failed to reverse the inhibitory effects of CCK-OP, carbachol and PMA on protein synthesis. CCK-OP and PMA activated phospholipase A (PLA) which liberated lysophosphatidylcholine (LPC) and free fatty acids from membrane phosphatidylcholine. Exogenously added PLA2 (Naja naja venom) inhibited protein synthesis and increased LPC to a similar extent as CCK and PMA. The results suggest that the inhibitory effects of CCK and carbachol on net protein synthesis are due to their effects on intracellular calcium and PLA-mediated breakdown of phosphatidylcholine rather than protein kinase C activation.

Changes in pancreatic trophism and gene expression during a prolonged fasting period in rats

To examine the effects of fasting on trophism and gene expression in pancreas, adult male rats were deprived of food from 0-6 d. Total DNA, RNA, and proteins, and specific mRNAs for rat amylase, chymotrypsinogen B, trypsinogen I, proinsulin I, and actin (assessed by employing cloned cDNAs and dot-blot hybridization) were quantitated in pancreas. Body and pancreatic wt diminished progressively to reach 65 and 75% of initial values at the 6th d of fasting. Protein/DNA and total RNA/DNA ratios decreased 2.04 and 2.31-fold, respectively, during 6 d of fasting. The concentration of amylase, chymotrypsinogen B, trypsinogen I, and actin mRNA, expressed as cpm/microgram RNA, decreased significantly throughout the study period, whereas the decrease observed in Proinsulin I mRNA concentration was not significantly different. When mRNA concentrations were refereed to the total content of DNA, however, the decrease was significant for all messengers tested. It is concluded that the prolonged absence of nutrients in the digestive tract exerts negative trophic influence on pancreas and triggers differential changes in pancreatic gene expression. These changes are gradual, asynchronic, and nonparallel.

Are diets associated with different rates of human interdigestive and postprandial pancreatic enzyme secretion?

To determine if diets are associated with different rates of interdigestive and postprandial enzyme secretion and how quickly enzyme secretion is modulated by nutrients, 27 healthy humans were randomly selected to follow one of five diets. The calorie proportions of carbohydrate, fat, and protein in each diet was assigned by a mixture design. After the subjects followed a diet for 2 weeks, they were intubated with an oroduodenal tube, and enzyme outputs were measured during the interdigestive period and after eating a meal identical to meals eaten during the previous 2 weeks. For the next 24 hours subjects either followed the same diet or a diet that contained the same amount of fat, but the percent of carbohydrate and protein was changed by 30%. Then interdigestive and postprandial pancreatic enzyme outputs were remeasured. After 2 weeks, diets containing the most carbohydrate (50%-80%) were associated with the lowest interdigestive and postprandial amylase and lipase (P less than 0.05) and trypsin outputs (P less than or equal to 0.05). In contrast, diets containing the most fat (40%) were associated with the highest interdigestive and postprandial outputs of amylase (P less than 0.05) and trypsin (P less than 0.05). Maintaining or altering diets for 24 hours did not change interdigestive pancreatic enzyme outputs, but postprandial amylase output was significantly increased (P less than 0.05) by increasing protein and decreasing carbohydrate content of the diets by 30% for 24 hours. We conclude that diets containing a high proportion of calories as carbohydrate for 2 weeks are associated with lower interdigestive and postprandial pancreatic secretion than diets that have a high fat content. In response to diets, changes in postprandial pancreatic enzyme secretion occur within 24 hours whereas changes in interdigestive secretion (no nutrients in the lumen) occur after 24 hours.

Dietary regulation of amino acid transport activity in the exocrine pancreatic epithelium

Dietary-induced alterations in exocrine pancreatic amino acid transport were investigated in rats adapted for 14 days to isocaloric diets of varying casein and carbohydrate content. The kinetics of unidirectional (15 s) L-phenylalanine and L-lysine transport were measured relative to D-mannitol (extracellular tracer) in the perfused pancreas isolated from dietary adapted animals. In rats adapted to a 20% casein diet a weighted non-linear regression analysis of phenylalanine transport (1-24 mM) indicated an apparent Kt = 9.4 +/- 1.1 mM and Vmax = 14.8 +/- 0.9 mumol/min per g pancreas (n = 6). Saturation of lysine transport occurred at lower concentrations (0.05-10 mM) with an apparent Kt = 2.40 +/- 0.09 mM and Vmax = 2.44 +/- 0.18 mumol/min per g (n = 6). The characteristics of phenylalanine transport were modified after adaptation to either high (Kt = 3.6 +/- 1 mM, Vmax = 8.2 +/- 0.9 mumol/min per g, n = 3) or low (Kt = 4.2 +/- 0.9 mM, Vmax = 6.8 +/- 0.5 mumol/min per g, n = 3) carbohydrate diets. Increasing the dietary protein content (0-45% casein) led to a linear increase in the Kt for phenylalanine transport whereas Vmax values remained unchanged. Unlike phenylalanine, adaptation to a 0% casein diet significantly elevated the Vmax for lysine transport (4.82 +/- 0.21 mumol/min per g, n = 4) without altering the Kt (2.54 +/- 0.23 mM). The present findings suggest that changes in dietary composition induce select adaptive responses in the transport activities of System L (phenylalanine) and System y+ (lysine) in the basolateral membrane of the exocrine pancreatic epithelium.

Fasting and refeeding modulate neutral amino acid transport activity in the basolateral membrane of the rat exocrine pancreatic epithelium: fasting-induced insulin insensitivity

The effects of fasting and refeeding on amino acid transport in the perfused rat exocrine pancreas were investigated using a rapid dual tracer dilution technique. Unidirectional amino acid influx (15 s) was quantified (relative to the extracellular tracer D-mannitol) over a wide range of perfusate concentrations in pancreata isolated frm fed and 24 h, 48 h, and 72 h fasted and 72 h fasted and refed (24 h) animals. In fed animals transport of phenylalanine (1-24 mM) and L-serine (1-50 mM) was saturable and weighted non-linear regression analyses of the overall transport indicated an apparent Kt = 10 +/- 3 mM and Vmax = 7.0 +/- 1.0 mumol/min per g (n = 7) for phenylalanine and Kt = 16 +/- 3 mM and Vmax = 20.6 +/- 2.1 mumol/min per g (n = 5) for serine. Fasting animals for 24 h or 48 h did not change the kinetics of either phenylalanine or serine transport. After a 72 h fast the rate of phenylalanine transport (Vmax = 15.9 +/- 2.9 mumol/min per g, n = 5) was enhanced whereas the transport affinity (Kt = 11 +/- 3 mM) remained unaltered. L-Serine transport was essentially unaltered. When 72 h fasted animals were refed for 24 h the Vmax for phenylalanine transport was reduced to values observed in fed animals. In parallel experiments refeeding had no significant effect on serine transport. Perfusion of pancreata isolated from 72 h fasted animals with bovine insulin (1 mU/ml or 1 microU/ml) did not stimulate either phenylalanine or serine transport.(ABSTRACT TRUNCATED AT 250 WORDS)

[Regulation of acinar cell receptors of the pancreas by peptides]

Peptides may act on the same receptor they regulate or on another receptor by causing regulations via receptor interactions. These receptor regulations include changes of receptor affinity and capacity. Receptor capacity is regulated by internalization, recycling, degradation, synthesis, and modification of bioavailability without migration of the receptor. Examples for those regulations, mostly based on experiments with isolated pancreatic acini from the rat, mouse, or guinea pig, are given. For the CCK receptor these examples include complex regulations of this receptor by CCK itself, bringing into discussion the hypothesis of negative cooperativity and the two-site receptor model, desensitization of the receptor by CCK, in vivo CCK influences on its receptor, and insulin receptor/CCK receptor interactions. For the insulin receptor the physiological significance of "up and down regulation" of this receptor by insulin itself is discussed. For the IGF receptors and the EGF receptor CCK-induced, Ca2+-mediated regulation of receptor internalization are another type of regulation with unknown physiological and pathophysiological significance. Finally CCK-induced, Ca2+-mediated regulation of somatostatin receptor capacity and affinity are mentioned. It is postulated that those regulations play an important role in influencing the biological effect of hormones and that knowledge about them may improve our understanding of pathophysiology.

Regulation of proteolytic enzyme activities and mRNA concentrations in rat pancreas by food content

Regulation by food content of the expression of genes encoding pancreatic proteases was studied in rats fed diets containing 15%, 25% or 70% protein (w/w) (diet I, II and III). Trypsin, chymotrypsin and elastase activities in pancreas were 1.4, 2.8 and 2 times higher in diet III than in diet I whereas carboxypeptidase A level was unchanged. As compared to diet I, the pancreatic concentration of mRNAs encoding trypsinogen I and chymotrypsinogen B, measured by filter hybridization to specific cDNA probes, were found respectively 3.6 and 3.9 times higher in diet III, and 1.9 and 2.6 times higher in diet II. Elastase I mRNA concentration was 1.8 times higher in diet III, but unchanged in diet II. Procarboxypeptidase A mRNA concentration was not affected. It is concluded to a coordinate pre-translational regulation of serine protease genes expression by the protein content of diet, differing however in amplitude and sensitivity among the three species of enzymes studied.

Lack of adaptive changes in human pancreatic amylase and lipase secretion in response to high-carbohydrate, low-fat diet applied by a 10-day continuous intraduodenal infusion

In order to investigate whether the human exocrine pancreas is capable of adapting to a diet with a high-carbohydrate, low-fat, and normal protein content, 10 healthy subjects were given a continuous intraduodenal infusion of such a dietary composition (8760 kJ in 2400 ml/day) via a portable infusion pump over a period of 10 days. The diet consisted of 76% of calories as carbohydrates (80% oligosaccharides, 20% mono- and disaccharides), 10% as fat (more than 90% C18 fatty acids) and 14% as protein (oligo- and polypeptides; 11.8 g nitrogen per day). A complete pancreozymin-secretin test was carried out before and after the experimental period. The results show that the above dietary regimen leads to a significant (P less than 0.05) increase in the stimulated secretion rates of trypsin and chymotrypsin, whereas, in contrast to the findings in animal experiments, no change could be measured in the secretion rates of amylase and lipase.

Pancreatic secretion after secretin and cholecystokinin stimulation in chronic alcoholics with and without cirrhosis

We have studied the volume, protein concentration, total protein, and chymotrypsin and trypsin outputs in pure pancreatic juice (PPJ) following endoscopic cannulation of the pancreatic duct in 11 male and 2 female patients with advanced alcoholic cirrhosis (AC). Results were compared to those obtained from 21 nonalcoholic volunteers (NAV) and 26 chronic alcoholic (CA) patients without cirrhosis. Intravenous stimulation with secretin followed 10 min later by intravenous cholecystokinin-pancreozymin (CCK-PZ) resulted in highly significant increases in volumes during both phases of pancreatic stimulation in AC compared to NAV and CA. Protein concentration and total output during secretin stimulation was not different among the three groups. During CCK-PZ stimulation, CA exhibited a significant elevation in protein concentration and total output compared to NAV and AC. Although total chymotrypsin output was lower in secretin-stimulated CA than other groups, no other differences between the groups were observed in either of the hormone-stimulation phases. Marked elevations in trypsin output were observed in secretin-stimulated AC and in CCK-PZ-stimulated AC and CA. The high PPJ volume and the relatively low protein concentration observed in AC may effect a washout phenomenon resulting in a decreased tendency for ductal protein precipitation in these patients.

Comparison of secretory protein profiles in developing rat pancreatic rudiments and rat acinar tumor cells

We have previously established that secretory proteins from a rat acinar cell tumor lack two forms of procarboxypeptidase B, are deficient in a major lipase species, and possess markedly reduced amounts of the basic proteins proelastase, basic chymotrypsinogen, basic trypsinogen and ribonuclease (Iwanij, V., and J.D. Jamieson, J. Cell Biol., 95:734-741). Because secretory proteins are markers for acinar cell differentiation, we sought to establish whether the secretory protein profile of the acinar cell tumor is unique to the transformed cell or whether it resembles that of a stage of normal pancreatic development. To this end, we compared the secretory protein pattern from acinar tumor cells with that of rat pancreatic rudiments at days 19-22 of gestation and through day 21 of the postnatal period. Two-dimensional IEF-SDS gel electrophoresis coupled with biosynthetic labeling and fluorography indicates a time-dependent appearance of individual secretory proteins with basic polypeptides, except for amylase, appearing in the terminal stages of differentiation. In comparison, the secretory protein pattern of the acinar tumor cells most closely resembles that of day-19 embryonic pancreatic rudiments. We propose that the cells of the acinar cell tumor may, in part, mirror a stage of normal pancreatic development.

Biochemical analysis of secretory proteins synthesized by normal rat pancreas and by pancreatic acinar tumor cells

We have examined the secretogogue responsiveness and the pattern of secretory proteins produced by a transplantable rat pancreatic acinar cell tumor. Dispersed tumor cells were found to discharge secretory proteins in vitro when incubated with hormones that act on four different classes of receptors: carbamylcholine, caerulein, secretin-vasoactive intestinal peptide, and bombesin. With all hormones tested, maximal discharge from tumor cells was only about one-half that of control pancreatic lobules, but occurred at the same dose optima except for secretin, whose dose optimum was 10-fold higher. Biochemical analysis of secretory proteins discharged by the tumor cells was carried out by crossed immunoelectrophoresis and by two-dimensional isoelectric focusing-SDS polyacrylamide gel electrophoresis. To establish a baseline for comparison, secretory proteins from normal rat pancreas were identified according to enzymatic activity and correlated with migration position on two-dimensional gels. Our results indicate that a group of basic polypeptides including proelastase, basic trypsinogen, basic chymotrypsinogen, and ribonuclease, two out of three forms of procarboxypeptidase B, and the major lipase species were greatly reduced or absent in tumor cell secretion. In contrast, the amount of acidic chymotrypsinogen was notably increased compared with normal acinar cells. Although the acinar tumor cells are highly differentiated cytologically and express functional receptors for several classes of pancreatic secretagogues, they show quantitative and qualitative differences when compared with normal pancreas with regard to their production of secretory proteins.

Dietary regulation of pancreatic protein synthesis. II. Kinetics of adaptation of protein synthesis and its effect on enzyme content

The kinetics of the adaptative changes in the relative rates of synthesis and pancreatic concentrations of amylase, chymotrypsinogen and trypsinogen were studied over 10 days of adaptation to a carbohydrate-rich (G), or a protein-rich (P) diet. During adaptation to the P diet, 60% of the adaptative decrease of the amylase to chymotrypsinogen ratio of incorporation was complete within 24 h of feeding and plateau values were obtained after five days. Adaptation to the G diet was only 20% complete after 24 h and plateau values were obtained later than with the P diet. The evolution of the ratio of concentrations of amylase and chymotrypsinogen followed those of incorporation in the adaptation to both diets. These results support the determinant role of adaptative changes in the rates of synthesis of individual enzymes on the dietary adaptation of enzyme proportions in the pancreas. The differences in the kinetics of adaptation to the two diets suggest that different mechanisms are involved in the adaptative regulation of protein synthesis to a carbohydrate-rich diet or a protein-rich diet.