Regulation of protein synthesis and enzyme accumulation in the rat pancreas by amount and timing of dietary protein

Circadian regulation of metabolic homeostasis: causes and consequences

Robust circadian rhythms in metabolic processes have been described in both humans and animal models, at the whole body, individual organ, and even cellular level. Classically, these time-of-day-dependent rhythms have been considered secondary to fluctuations in energy/nutrient supply/demand associated with feeding/fasting and wake/sleep cycles. Renewed interest in this field has been fueled by studies revealing that these rhythms are driven, at least in part, by intrinsic mechanisms and that disruption of metabolic synchrony invariably increases the risk of cardiometabolic disease. The objectives of this paper are to provide a comprehensive review regarding rhythms in glucose, lipid, and protein/amino acid metabolism, the relative influence of extrinsic (eg, neurohumoral factors) versus intrinsic (eg, cell autonomous circadian clocks) mediators, the physiologic roles of these rhythms in terms of daily fluctuations in nutrient availability and activity status, as well as the pathologic consequences of dyssynchrony.

Translational control of protein synthesis in pancreatic acinar cells

Translational control of protein synthesis in the pancreas is important in regulating growth and the synthesis of digestive enzymes. Regulation of translation is primarily directed at the steps in initiation and involves reversible phosphorylation of initiation factors (eIFs) and ribosomal proteins. Major sites include the assembly of the eIF4F mRNA cap binding complex, the activity of guanine nucleotide exchange factor eIF2B, and the activity of ribosomal S6 kinase. All of these involve phosphorylation by different regulatory pathways. Stimulation of protein synthesis in acinar cells is primarily mediated by the phosphatidylinositol 3-kinase-mTOR pathway and involves both release of eIF4E (the limiting component of eIF4F) from its binding protein and phosphorylation of ribosomal S6 protein by S6K. eIF4E is itself phosphorylated by a distinct pathway. Inhibition of acinar protein synthesis can be mediated by inhibition of eIF2B following phosphorylation of eIF2alpha.

Tissue localization of threonine oxidation in pigs

Two experiments were designed to determine the tissue distribution of threonine oxidation through the threonine dehydrogenase (EC 1.1.1.103) pathway in pigs. The first experiment was conducted on eleven Piétrain x Large White piglets. The piglets were slaughtered at 5, 12 or 20 kg after 1 h of infusion with L-[U-14C]threonine (55 kBq/kg) mixed with unlabelled threonine (100 mg/kg). In the second experiment, four Piétrain x Large White and four large White piglets (10 kg body weight) were infused with L-[1-13C]threonine (50 mg/kg) mixed with 50 mg/kg unlabelled threonine for 1 h, then killed for tissue sampling. In the two experiments, threonine dehydrogenase specific activity and threonine and glycine specific radioactivities and enrichments were measured in several tissues and in plasma. The higher level of labelling of threonine in the pancreas than in the liver suggested either a lower protein degradation rate or a faster rate of threonine transport in the liver than in the pancreas. Threonine dehydrogenase activity was found only in the liver and the pancreas. Whereas liver and pancreas threonine dehydrogenase specific activities were similar, glycine specific radioactivity and enrichment were 12- to 14-fold higher in the pancreas than in the liver. This is probably the consequence of a higher production rate of glycine from sources other than threonine (protein degradation, de novo synthesis from serine) in the liver than in the pancreas. Our results showed that Large White pigs could oxidize more threonine than Piétrain x Large White pigs. This could be related to the difference in growth performance and dietary N efficiency for protein deposition between these two genotypes.

The effect of feeding on the secretion of pepsin, trypsin and chymotrypsin in the Atlantic salmon,Salmo salar L

Pepsin stored in the stomach mucosa of the Atlantic salmon (Salmo salar L.) increases within two days of the onset of starvation. Trypsin and chymotrypsin in the pyloric caeca/pancreas behave similarly, indicating that when no food is present in the gut, digestive enzymes accumulate in the secretory tissues. As a corollary, trypsin and chymotrypsin activities in the gut contents fell during starvation, indicating that secretion is greatly reduced when food is not present. At the onset of feeding, pepsin is rapidly synthesised in the mucosal tissues and then secreted. Twenty four hours after feeding, the pepsin levels of the mucosa are still low, suggesting that synthesis may be a rapid response to the presence of food in the stomach. Secretion of trypsin and chymotrypsin appears to take place as soon as digesta enters the intestine, between 4 and 14h after feeding, and resynthesis of enzyme precursors appears to be complete again within a further 11h. It is suggested that both synthesis and release may be under the control of cholecystokinin.

The gastro-ileal digestion of 15N-labelled pea nitrogen in adult humans

The aim of the present study was to determine the gastro-ileal behaviour of pea protein in humans. For this purpose, twelve healthy volunteers were intubated with an intestinal tube located either in the jejunum (n 5) or in the ileum (n 7). After fasting overnight, they ingested 195 mmol N of [15N]pea. Intestinal samples were collected for 6 h in the jejunum and for 8 h in the ileum. Before meal ingestion the basal liquid flow rate (ml/min) was 2.01 (SD 0.31) in the jejunum and 2.02 (SD 0.33) in the ileum. After meal ingestion the liquid phase of the meal peaked in the 40-60 min period in the jejunum and in the 150-180 min period in the ileum. The jejuno-ileal transit time of the liquid phase of the meal was 102 min. The basal flow rate of endogenous N (mmol N/min) was 0.22 (SD 0.15) in the jejunum and 0.16 (SD 0.10) in the ileum. The endogenous N flow rate peaked significantly (P < 0.05) in the jejunum in the 40-60 min period whereas no stimulation of endogenous N could be detected in the ileum after meal ingestion. A significantly increased (P < 0.05) concentration of exogenous N was detected in the jejunum during the 20-320 min period and during the 90-480 min period in the ileum. The overall true gastro-ileal absorption of pea N was 89.4 (SD 1.1)% with 69 (SD 14)% absorbed between the stomach and the proximal jejunum and 20.4% between the proximal jejunum and the terminal ileum. The percentage of ethanol-insoluble fraction (PN) in the exogenous N at the terminal ileum increased significantly (P < 0.05) to 75% after 360 min. These results suggest that heat-treated pea protein has a digestibility close to that of animal protein.

Exogenous and endogenous nitrogen flow rates and level of protein hydrolysis in the human jejunum after [15N]milk and [15N]yoghurt ingestion

Milk and yoghurt proteins were 15N-labelled in order to measure the flow rate of exogenous N during digestion in the human intestine. After fasting overnight, sixteen healthy volunteers, each with a naso-jejunal tube, ingested either [15N]milk (n 7) or [15N]yoghurt (n 9). Jejunal samples were collected every 20 min for 4 h. A significant stimulation of endogenous N secretion was observed during the 20-60 min period after yoghurt ingestion and the 20-40 min period after milk ingestion. The endogenous N flows over a 4 h period did not differ between the groups (44.3(SEM 6.5) mmol for milk and 63.5(SEM 5.9) mmol for yoghurt). The flow rates of exogenous N indicated a delayed gastric emptying of the yoghurt N compared with N from milk. The jejunal non-protein N (NPN) flow rate increased significantly after milk and yoghurt ingestion due to an increase in the exogenous NPN flow rate. The NPN fraction of exogenous N ranged between 40 and 80%. The net gastro-jejunal absorption of exogenous N did not differ significantly between milk (56.7(SEM 8.5)%) and yoghurt (50.9(SEM 7)%). The high level of exogenous N hydrolysis is in accordance with the good digestibility of milk products. Fermentation modifies only the gastric emptying rate of N and does not affect the level of diet hydrolysis, the endogenous N stimulation or the digestibility rate.

Nonparallel patterns of circadian pancreatic and biliary secretions in fasting rats

We compared the circadian patterns of pancreatic and biliary secretions in fasting rats. For this purpose, indwelling plastic catheters were placed in 10 male Wistar rats (300-320 g) for the collection of biliary and pancreatic secretions. After small samples were taken for analysis, pancreatic and biliary secretions were recirculated into the duodenum by an additional connecting system. The rats were adapted to an inverse night-day cycle by artificial light during the night (8 PM-8 AM) and by darkroom housing at daytime (8 AM-8 PM). During a 24-h fasting period, samples of bile (100 microL) and pancreatic juice (20 microL) were taken every hour for determination of the following parameters: pancreatic and biliary flow rate, protein, amylase, lipase, trypsin, and bile acid output. Peak pancreatic flow rate (1.96 +/- 0.05 mL/h.kg) was achieved toward the end of the dark period at 7 PM. A significant increase of pancreatic secretion could be achieved merely by turning the lights off, a significant decrease by turning the lights on. Similar circadian patterns were found for pancreatic protein, amylase, and lipase output with peak secretions at 7 PM. An increase of nearly 5x was found between minimal (15.64 +/- 0.65 mg/h.kg) and maximal (72.43 +/- 2.83 mg/h.kg) pancreatic protein output. The amplitude was highest for amylase; peak amylase output (13740 +/- 832 U/h.kg) was about 18-fold above minimal output (758 +/- 44.3 U/h.kg). Conversely, the peak of trypsin concentration in pancreatic juice (1095 +/- 17.8 U/mL) occurred during the light period when flow rates were lowest.(ABSTRACT TRUNCATED AT 250 WORDS)

Dietary-induced increases of disaccharidase activities in rat jejunum

A study was carried out to examine whether the responsiveness of small intestinal epithelial cells to dietary carbohydrate varied during the daily 24 h cycle. The effect of sucrose on disaccharidase activities was compared during a period of decreasing disaccharidase activities, i.e. between 22.00 and 10.00 hours, and increasing disaccharidase activities, i.e. between 10.00 and 22.00 hours, in the jejunum of 7-week-old-rats. Rats were fed on a low-starch, high-fat diet (Lst; starch 5 and fat 73% of gross energy), or a high-starch, low-fat diet (Hst; starch 70 and fat 7% of gross energy). Both dietary groups exhibited typical diurnal variations in jejunal sucrase (EC 3.2.1.48), maltase (EC 3.2.1.20) and lactase (EC 3.2.1.23) activities, exhibiting a peak around 22.00 hours and a trough at approximately 10.00 hours. When rats were fed on diet Lst for 7 d and then force-fed on an isoenergetic sucrose diet (S; sucrose 40 and fat 37% of gross energy) for 6 or 12 h they exhibited increased sucrase, maltase and lactase activities compared with rats fed on diet Lst. The absolute increase in disaccharidase activities was similar regardless of the time diet S was given or whether rats were killed at 10.00 hours or at 22.00 hours. Analyses of sucrase and lactase activities along the villus-crypt columns showed that the distribution of cell cohorts that responded to diet S was not influenced by the time of introduction of diet S. These findings suggest that small intestinal epithelial cells possess the ability to respond to dietary carbohydrate throughout the daily 24 h cycle.

Rat pancreatic colipase mRNA: nucleotide sequence of a cDNA clone and nutritional regulation by a lipidic diet

A cDNA clone encoding rat pancreatic colipase was isolated using as a probe a synthetic deoxyoligonucleotide corresponding to a highly conserved amino acid sequence region in colipases from other species. The cloned messenger codes for a protein of 95 amino acids plus a signal peptide of 17 amino acids. The structure of the full-length cDNA was also determined and the corresponding amino acid sequence showed a high degree of homology with those of other known colipases. Quantification of the homologous mRNA in the pancreas of animals fed a high-lipid diet was consistent with a specific though moderate induction of colipase messenger by the nutritional manipulation.

Kinetics of trypsin from rainbow trout are not influenced by level of feeding

1. Specific activity and kinetic constants of trypsin from the pyloric caeca of rainbow trout (Salmo gairdneri) were measured. 2. Although one group was fed more than twice as much as the other (1.8 compared to 0.7% body weight per day), there were no significant differences in the weight of the pyloric caeca, specific activity of trypsin, or kinetic constants (apparent Km or Vmax) between the two groups. 3. The caecum of trout contains enough trypsin to digest all of the protein in a typical meal in less than 5 hr.