Amino acid efflux in the isolated perfused rat pancreas: trans-stimulation by extracellular amino acids

Amino acid transporters expression in acinar cells is changed during acute pancreatitis

Pancreatic acinar cells accumulate amino acids against a marked concentration gradient to synthesize digestive enzymes. Thus, the function of acinar cells depends on amino acid uptake mediated by active transport. Despite the importance of this process, pancreatic amino acid transporter expression and cellular localization is still unclear. We screened mouse pancreas for the expression of genes encoding amino acid transporters. We showed that the most highly expressed transporters, namely sodium dependent SNAT3 (Slc38a3) and SNAT5 (Slc38a5) and sodium independent neutral amino acids transporters LAT1 (Slc7a5) and LAT2 (Slc7a8), are expressed in the basolateral membrane of acinar cells. SNAT3 and SNAT5, LAT1 and LAT2 are expressed in acinar cells. Additional evidence that these transporters are expressed in mature acinar cells was gained using acinar cell culture and acute pancreatitis models. In the acute phase of pancreatic injury, when acinar cell loss occurs, and in an acinar cell culture model, which mimics changes occurring during pancreatitis, SNAT3 and SNAT5 are strongly down-regulated. LAT1 and LAT2 were down-regulated only in the in vitro model. At protein level, SNAT3 and SNAT5 expression was also reduced during pancreatitis. Expression of other amino acid transporters was also modified in both models of pancreatitis. The subset of transporters with differential expression patterns during acute pancreatitis might be involved in the injury/regeneration phases. Further expression, localization and functional studies will follow to better understand changes occurring during acute pancreatitis. These findings provide insight into pancreatic amino acid transport in healthy pancreas and during acute pancreatitis injury.

Regulation of L-arginine transport and nitric oxide release in superfused porcine aortic endothelial cells

1. We have investigated whether changes in extracellular ion composition and substrate deprivation modulate basal and/or bradykinin-stimulated L-arginine transport and release of nitric oxide (NO) and prostacyclin (PGI2) in porcine aortic endothelial cells cultured and superfused on microcarriers. 2. Saturable L-arginine transport (Km = 0.14 +/- 0.03 mM; Vmax = 2.08 +/- 0.54 nmol min-1 (5 x 10(6) cells)-1) was pH insensitive and unaffected following removal of extracellular Na+ or Ca2+. 3. Cationic arginine analogues, including L-lysine and L-ornithine, inhibited L-arginine transport, whilst 2-methylaminoisobutyric acid, beta-2-amino-bicyclo[2,2.1]-heptane-2-carboxylic acid, L-phenylalanine, 6-diazo-5-oxo-norleucine, L-glutamine, L-cysteine and L-glutamate were poor inhibitors. 4. Deprivation of L-arginine (30 min to 24 h) reduced intracellular free L-arginine levels from 0.87 +/- 0.07 to 0.40 +/- 0.05 mM (P < 0.05) and resulted in a 40% stimulation of L-arginine, L-lysine and L-ornithine transport. 5. L-arginine and NG-monomethyl-L-arginine (L-NMMA), but not N omega-nitro-L-arginine methyl ester (L-NAME), trans-stimulated efflux of L-[3H]arginine. 6. Depolarization of endothelial cells with 70 mM K+ reduced L-arginine influx and prevented the stimulation of transport by 100 nM bradykinin, but agonist-induced release of NO and PGI2 was still detectable. 7. Basal rates of L-arginine transport and NO release were unaffected during superfusion of cells with a nominally Ca(2+)-free solution. Bradykinin-stimulated L-arginine transport was insensitive to removal of Ca2+, whereas agonist-induced NO release was abolished. 8. Although bradykinin-stimulated NO release does not appear to be coupled directly to the transient increase in L-arginine transport, elevated rates of L-arginine influx via system y+ in response to agonist-induced membrane hyperpolarization or substrate deprivation provide a mechanism for enhanced L-arginine supply to sustain NO generation.

A role for gamma-glutamyl transpeptidase and the amino acid transport system xc- in cystine transport by a human pancreatic duct cell line

1. The roles of the gamma-glutamyl cycle and the anionic amino acid transport system xc- in mediating L-cystine uptake were investigated in cultured human pancreatic duct PaTu 8902 cells. This cell line exhibits morphological features of normal pancreatic duct cells and expresses gamma-glutamyl transpeptidase (gamma-GT, EC 2.3.2.2), an enzyme involved in the metabolism and regulation of intracellular glutathione (GSH). 2. Uptake of L-cystine (10 microM) was linear for up to 10 min, temperature dependent, Na+ independent, saturable (Michaelis-Menten constant (Km), 86 +/- 25 microM; maximal velocity (Vmax), 109 +/- 33 nmol (mg protein)-1 h-1) and reduced by 80-90% by a 50-fold excess concentration of L-glutamate and L-homocysteic acid, but not L-aspartate. These transport properties resemble those described for system xc-, which exchanges cystine for intracellular glutamate. 3. Acivicin, a known inhibitor of gamma-GT, decreased gamma-GT activity from 2.58 +/- 0.96 to 0.97 +/- 0.11 mU (mg protein)-1 and decreased the initial rates of L-cystine and L-glutamine uptake by 41-55%. Anthglutin (1-gamma-L-glutamyl-2-(2-carboxyphenylhyl)hydrazine), a structurally different inhibitor of gamma-GT, also caused a concentration-dependent (0.01-1 mM) decrease in gamma-GT activity and L-cystine uptake. 4. Neither acivicin nor anthglutin inhibited the uptake of L-glutamate, a poor substrate for gamma-GT. 5. In the presence of a 500-fold excess concentration of glutamate, which should abolish entry of cystine via system xc-, the remaining fraction of cystine transport was inhibited by 50% by acivicin, suggesting that transport is, in part, dependent on the activity of gamma-GT. 6. Cystine transport was also 60-80% inhibited by a series of gamma-glutamyl amino acids (5 mM) including gamma-glutamyl-glutamate, gamma-glutamyl-glutamine and gamma-glutamyl-glycine. alpha-Dipeptides inhibited cystine transport by only 6-22%. 7. These findings demonstrate that in human pancreatic duct PaTu 8902 cells, cystine uptake is mediated by system xc- (50-60%) and the gamma-glutamyl cycle. Our results provide the first evidence linking gamma-GT with cystine transport in human epithelial cells and are of relevance in view of the importance of cystine as a sulphur amino acid source for GSH synthesis in cells exposed to oxidative stress.

Antioxidant therapy for recurrent pancreatitis: biochemical profiles in a placebo-controlled trial

The usefulness of micronutrient antioxidant therapy for recurrent (non-gallstone) pancreatitis has recently been endorsed by a 20-week double-blind double-dummy cross-over trial in 20 patients. Treatment was delivered as two types of tablets, providing daily doses of 600 micrograms organic selenium, 9000 i.u. beta-carotene, 0.54 g vitamin C, 270 i.u. vitamin E and 2 g methionine. We report antioxidant profiles in blood samples collected before entry, at the cross-over stage and upon completion of trial. Baseline serum concentrations of selenium, beta-carotene and vitamin E in the patients were significantly lower than in healthy controls, were unaltered by placebo and normalized by active treatment, but reverted to basal values in the subgroup that received placebo subsequently. The baseline serum concentration of a free radical marker--the 9-cis, 11-trans isomer of linoleic acid--was significantly higher in the patients than in controls, fell inexplicably in the placebo phase and fell further upon active treatment. Discriminant analysis eliminated the overlap in free radical marker and selenium concentrations between control sera on the one hand and baseline or post-placebo samples from the patients on the other: antioxidant treatment normalized the relationship between these biochemical parameters. Subnormal baseline serum levels of S-adenosylmethionine drifted downwards upon active treatment whereas a sharp rise was noted when a relapse of pancreatitis occurred during the placebo phase. The results confirm that adequate exposure to antioxidants in the active treatment phase was associated with amelioration of oxidative stress, and that there was no residual effect 10 weeks after switching over to placebo treatment. Furthermore, the paradoxical behaviour of S-adenosylmethionine may imply that the beneficial effect of micronutrient antioxidants in recurrent pancreatitis is linked with preservation of the methionine trans-sulfuration pathway in pancreatic acinar cells.

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.

Bradykinin and ATP stimulate L-arginine uptake and nitric oxide release in vascular endothelial cells

The effects of bradykinin and ATP on L-arginine transport and nitric oxide (NO) production were studied in porcine aortic endothelial cells cultured and perfused on microcarriers and deprived of L-arginine for 24 h. Stimulation of cells with bradykinin (100 nM) or ATP (100 microM) resulted in a rapid increase in L-arginine uptake and NO release. In the presence of nitro-L-arginine (100 microM), an inhibitor of NO synthase, the stimulatory effect of bradykinin on L-arginine uptake was partially inhibited while NO release was completely abolished. Nitro-L-arginine alone was not an inhibitor of basal L-arginine transport, suggesting that its inhibitory action was not directly on the L-arginine transporter but a result of the inhibition of NO generation. These data indicate that during agonist-stimulated NO production there is a concomitant increase in the transport of L-arginine into endothelial cells providing a mechanism for the continual generation of NO.

Cis-inhibition and trans-stimulation of cationic amino acid transport in the perfused rat pancreas

Transport of cationic amino acids in the isolated perfused rat pancreas was studied using dual-isotope dilution techniques. At 50 microM substrate concentration, unidirectional tracer uptakes for L-arginine (56 +/- 1%), L-lysine (49 +/- 2%), and L-ornithine (44 +/- 3%) were followed by rapid tracer efflux. In the presence of Na+, influx of L-arginine [Michaelis constant (Km) = 1.74 +/- 0.15 mM, maximum velocity (Vmax) = 1.97 +/- 0.07 mumol.min-1.g-1] and L-lysine (Km = 2.48 +/- 0.17 mM, Vmax = 2.42 +/- 0.08 mumol.min-1.g-1) was mediated by a common transport system, sensitive to cis-inhibition by L-ornithine, 2,4-L-diaminobutyric acid, D-lysine, and D-arginine. Substrates for system A [alpha-(methylamino)isobutyric acid] and an anionic carrier (L-aspartate) were poor cis-inhibitors of L-arginine entry. Removal of Na+ resulted in a 40% reduction in cationic amino acid influx. After cell loading (20 min), L-[3H]-lysine cleared predominantly from a slowly exchanging pool with a rate constant of 5.97 +/- 0.67 min. An influx/efflux permeability ratio of 14.5 +/- 1.6 was determined, and efflux of L-lysine was trans-stimulated by vascular challenges with cationic or large neutral amino acids. The specificity, relative Na+ independence, and exchange properties of this saturable cationic amino acid transporter in the pancreatic epithelium resemble those reported for system y+ in cultured fibroblasts and hepatocytes.

Significance of gamma-glutamyltranspeptidase in exocrine pancreatic amino acid transport

The exocrine pancreas is rich in gamma-glutamyltranspeptidase (GGT, EC 2.3.2.2) and exhibits high rates of amino acid transport and protein synthesis. The role of the gamma-glutamyl cycle in mediating neutral amino acid transport in the isolated perfused rat pancreas was investigated using acivicin, an inhibitor of GGT, and a rapid dual isotope dilution technique. When treatment in vivo with acivicin (50 mg/kg) was followed 1 h later by continuous perfusion of the isolated pancreas with 10 microM acivicin, GGT levels decreased from 53 +/- 3 IU/g to 4.9 +/- 1.5 IU/g. This marked inhibition of GGT activity was not associated with decreased uptake for either L-alanine or L-glutamine, suggesting that the gamma-glutamyl cycle plays a negligible role in amino acid transport across the basolateral membrane of the pancreatic epithelium.