The Uptake of Amino Acids by Mouse Pancreas in Vitro

Nutritional Evaluation of Milk-, Plant-, and Insect-Based Protein Materials by Protein Digestibility Using the INFOGEST Digestion Method

The INFOGEST method is a valuable tool for understanding and monitoring food digestion as an alternative to in vivo assays. However, few studies have compared animal and alternative protein sources in terms of protein quality using the INFOGEST method. This study aimed to evaluate the protein quality of milk-, plant-, and insect-based protein materials by in vitro protein digestibility and in vitro digestible indispensable amino acid score (DIAAS), following the INFOGEST method. Milk-based protein materials had the highest protein digestibility (86.1-90.8%), followed by soy (85.1%) and wheat (82.3%). These materials had significantly higher protein digestibility compared with zein (65.1%), cricket (63.6%), and mealworm (69.5%). Additionally, the mean values of in vitro DIAAS of milk-based protein materials (105.0-137.5) were higher than those of plant- and insect-based protein materials (1.9-91.0). Milk-based protein materials have higher protein quality than plant- and insect-based protein materials by the nutritional evaluation following the INFOGEST digestion method.

Is the plasma amino acid consumption test an accurate test of exocrine pancreatic insufficiency?

BACKGROUND/AIMS

The amino acid consumption test has been proposed as an accurate test of exocrine pancreatic function. The diagnostic accuracy of this test was determined by simultaneously measuring plasma amino acids and enzyme secretion during stimulation of the pancreas with cholecystokinin octapeptide (CCK-OP) in 60 consecutive patients suspected of having pancreatic insufficiency.

METHODS

All patients underwent duodenal intubation and intravenous infusion of CCK-OP (40 ng.kg-1.h-1). Pancreatic enzyme (lipase and trypsin) outputs and plasma amino acids were measured for a period of 1 hour. Total and individual plasma amino acids were quantitated by ion-exchange chromatography. The severity of pancreatic insufficiency was graded on the basis of enzyme output during CCK-OP infusion.

RESULTS

There was no relationship between pancreatic enzyme output and plasma concentrations of individual or total amino acids before or during CCK-OP stimulation. Using a total amino acid decrease of 12% as the cutoff, the amino acid consumption test was 91% sensitive, but very nonspecific (21% specificity) for detection of pancreatic insufficiency.

CONCLUSIONS

The amino acid consumption test with CCK-OP stimulation does not discriminate between patients with normal and impaired exocrine pancreatic secretion.

Effect of pancreatic polypeptide, thyrotropin-releasing hormone, and glucagon on plasma amino acid uptake by human pancreas

The effects of pancreatic polypeptide, thyrotropin-releasing hormone, and glucagon on plasma amino acid uptake by the exocrine pancreas were studied in 12 healthy volunteers aged 22-31 years. Pancreatic amino acid uptake was determined by measuring free plasma amino acid concentration before and during pancreatic stimulation with cerulein (50 ng/kg.h). The administration of this peptide caused a significant decrease (by 14%-20%) in plasma amino acid concentration. Pancreatic polypeptide and thyrotropin-releasing hormone, given at respective doses of 195 pmol/kg.h and 2 micrograms/kg.h, significantly prevented this decrease by 79.3% and 55.8%, respectively. Glucagon, administered at a dose of 7.5 micrograms/kg.h, significantly augmented (by 68.8%) the decreasing effect of cerulein on plasma amino acid concentration. In 2 patients with severe exocrine pancreatic insufficiency, cerulein had no effect on the concentration of plasma amino acids, whereas the addition of glucagon caused a marked decrease. The results indicate that pancreatic polypeptide and thyrotropin-releasing hormone are able to inhibit plasma amino acid uptake by pancreatic acinar cells; this inhibitory effect could be a mechanism by which these peptides decrease pancreatic enzyme secretion. Glucagon does not seem to affect pancreatic amino acid uptake, at least not under the experimental conditions of this study. The decrease in plasma amino acid concentration following glucagon administration was likely the result of the stimulation of amino acid uptake by extrapancreatic tissues by this peptide.

Effect of somatostatin on plasma amino acid uptake by human pancreas

We studied the effect of somatostatin on amino acid uptake by pancreatic acinar cells in 12 healthy male volunteers, aged 20-48 yr. Pancreatic amino acid uptake was assessed by measuring free plasma amino acid concentration before and during pancreatic stimulation with secretin (1 CU/kg body wt.h) and cerulein (50 ng/kg body wt.h). Pancreatic stimulation with these peptides caused a significant decrease in plasma amino acid concentrations. Somatostatin, given at the dosages of 0.15 and 1.35 micrograms/kg body wt.h, significantly diminished this decrease. The effect of the higher dose of somatostatin was more marked than that produced by the lower dose, compatible with dose dependence. The results suggest that somatostatin is able to inhibit the amino acid uptake by the pancreatic acinar cells. This inhibitory effect could be an important mechanism by which the peptide decreases pancreatic enzyme secretion.

Kinetics of the Na+/alanine cotransporter in pancreatic acinar cells

Electric currents associated with Na+-coupled alanine transport in pancreatic acinar cells were investigated by the technique of tight-seal whole-cell recordings. In a previous study the observed concentration dependence of alanine-dependent currents was found to be consistent with a 'simultaneous' transport mechanism with 1:1 stoichiometry. In the present work the sidedness of the cotransporter was investigated by comparing inward (I") and outward currents (I') measured under mirror-symmetrical conditions. I' and I" were found to be nearly equal (within a factor of approx. 2) in a wide range of Na+ and alanine concentrations. The transport model was further tested by 'infinite-cis' experiments with fixed, saturating concentrations of Na+ and L-alanine on one side of the membrane and variable concentrations on the other. By measuring transmembrane currents as a function of Na+ and alanine concentrations, numerical values of the equilibrium dissociation constants of both substrates could be estimated.

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.

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.

Electrogenic properties of the sodium-alanine cotransporter in pancreatic acinar cells: I. Tight-seal whole-cell recordings

Electrical currents associated with sodium-coupled alanine transport in mouse pancreatic acinar cells were studied using the method of whole-cell recording with patch pipettes. Single cells or small clusters of (electrically coupled) cells were isolated by collagenase treatment. The composition of the intracellular solution could be controlled by internal perfusion of the patch pipette. In this way both inward and outward currents could be measured under "zero-trans" conditions, i.e., with finite concentrations of sodium and L-alanine on one side and zero concentrations on the other. Inward and outward currents for equal but opposite concentration gradients were found to be of similar magnitude, meaning that the cotransporter is functionally nearly symmetric. The dependence of current on the concentrations of sodium and L-alanine exhibited a Michaelis-Menten behavior. From the sodium-concentration dependence of current as well as from the reversal potential of the current in the presence of an alanine-concentration gradient, a sodium/alanine stoichiometric ratio of 1:1 can be inferred. The finding that N-methylated amino acids may substitute for L-alanine, as well as the observed pH dependence of currents indicate that the pancreatic alanine transport system is similar to (or identical with) the "A-system" which is widespread in animal cells. The transport system is tightly coupled with respect to Na+; alanine-coupled inward flow of Na+ is at least 30 times higher than uncoupled Na+ flow mediated by the cotransporter. The current-voltage characteristic of the cotransporter could be (approximately) determined from the difference of transmembrane current in the presence and in the absence of L-alanine. The sodium-concentration dependence of the current-voltage characteristic indicates that a Na+ ion approaching the binding site from the extracellular medium has to cross part of the transmembrane electric field.

Decrease in plasma amino acid level after secretin and pancreozymin as an indicator of exocrine pancreatic function

Total plasma amino acids were determined by the ninhydrin method in 37 controls and 30 patients with chronic pancreatitis and normal (n = 7) pancreatic enzyme output or mildly (n = 6), moderately (n = 8), and severely (n = 9) reduced pancreatic enzyme output. Intravenous injection of synthetic secretin did not change plasma amino acid levels. During a combined intravenous infusion of secretin (1 CU/kg X h) and pancreozymin (1 Ivy dog unit/kg X h), amino acid concentrations decreased maximally by 31% +/- 19% (mean +/- SD) in controls, but only by 6.3% +/- 4.7% in patients with exocrine pancreatic insufficiency (p less than 0.001 vs. controls). At a cutoff limit of less than or equal to 12% for the decrease in total amino acids, mild exocrine insufficiency (20%-40% of mean normal chymotrypsin output) was identified with a sensitivity of 67%, whereas moderately to severely impaired function was detected in every case (overall sensitivity 91%). Pancreatic function, as assessed by duodenal intubation and the tubeless amino acid test, was significantly correlated (e.g., rs = 0.73 for chymotrypsin output, p much less than 0.001). In 15 controls and 13 patients with mildly (n = 5) to severely impaired pancreatic function, individual amino acids were estimated. Plasma serine kinetics completely distinguished both groups. Kinetics of serine, valine, isoleucine, and histidine correlated even better with pancreatic function than those of total amino acids.

Studies on isolated subcellular components of cat pancreas. III. Alanine-sodium cotransport in isolated plasma membrane vesicles

Transport of alanine was studied in isolated plasma membrane vesicles from cat pancreas using a rapid filtration technique. The uptake is osmotically sensitive and the kinetics of L-alanine transport are biphasic showing a saturable and a nonsaturable component. The saturable component is seen only when a sodium gradient directed from the medium to the vesicular space is present. Under this condition an overshooting uptake of L-but not of D-alanine occurs. The Na+ gradient stimulated uptake of L-alanine is inhibited by L-serine and L-leucine and stimulated when the membrane vesicles had been preloaded with L-alanine, L-serine or L-leucine. The ionophore monensin inhibits stimulation of uptake caused by a sodium gradient. In the presence of valinomycin or carbonyl cyanide p-trifluoromethoxyphenylhydrazone (CFCCP), the sodium-dependent transport is augmented in vesicles preloaded with K2SO4 or H+ ions (intravesicular pH 5.5), respectively. In the presence of different anions, the Na+-dependent transport is stimulated according to increasing anionic penetration through membrane (lipid solubility). We conclude that a sodium dedpendent electrogenic amino acid transport system is present in pancreatic plasma membranes.

Amino acid transport in the rat exocrine pancreas. I. Transport of neutral amino acids and their utilization in protein synthesis

The transport and utilization of three neutral amino acids in protein synthesis was studied using isolated pancreatic lobules in vitro. The significance of the extracellular and intracellular amino acid pool in this process was analyzed removing the extracellular pool (labeled by inulin) by a cold-wash procedure. This was especially useful in short-term experiments. Double-labeling experiments indicated a predominant utilization of the intracellular amino acid pool during protein synthesis. The advantage of isolated pancreatic lobule preparations compared to tissue fragments or slices was emphasized by fine structural studies. Using freeze-etching techniques on the same preparations, differences in the distribution of membrane particles between luminal and lateral plasma membranes described earlier were confirmed, as well as the abundant occurrence of gap junctions on both membrane faces.

Changes in amino acid transport in the rat pancreas in response to fasting and feeding

Transport of amino acids (in vitro) in the rat pancreas is affected by the nutritional state of the animal. A fast of 24 h (young animals) or 48 h (adult animals) reduces the rate of amino acid uptake in the isolated rat pancreas in vitro. In contrast, refeeding of animals after a fast shows an increase in transport in young as well as adult animals. The effects of refeeding after a fast are mimicked to a significant extent by injection of mixtures of pancreozymin and carbamylcholine. Addition of these agents in vitro has no effect. The incorporation of amino acids into the total proteins of the rat pancreas follows the pattern of amino acid uptake. Even at high external levels of glycine (5 mM), incorporation increases although the glycine level in the cell is in excess of 25 mM. Reduction of glycine uptake by ouabain by 75% results in a substantial (44%) diminution of amino acid incorporation into proteins. The data suggest that inhibition of amino acid incorporation under the various metabolic conditions examined is due largely to a decreased availability of amino acids.