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

ATA2 is predominantly expressed as system A at the blood-brain barrier and acts as brain-to-blood efflux transport for L-proline

Although system A is present at the blood-brain barrier (BBB), the physiological roles of system A have not been clarified. The efflux transport of the substrates of system A, such as L-proline (L-Pro), glycine (Gly), and alpha-methylaminoisobutyric acid (MeAIB), across the BBB was investigated using the in vivo Brain Efflux Index method. Over a period of 40 min, L-[(3)H]Pro and [(3)H]Gly underwent efflux from the brain, whereas [(3)H]MeAIB did not. The efflux of L-[(3)H]Pro was inhibited by the presence of unlabeled L-Pro and MeAIB, suggesting that carrier-mediated efflux transport of L-Pro across the BBB is involved in system A. L-[(3)H]Pro uptake by TR-BBB cells, used as an in vitro BBB model, was Na(+)-dependent with high-affinity (K(m1) = 425 microM) and low-affinity (K(m2) = 10.8 mM) saturable processes. The manner of inhibition of L-[(3)H]Pro uptake for amino acids was consistent with system A. Although GlnT, ATA2, and ATA3 mRNA were all expressed in TR-BBB cells, ATA2 mRNA was predominant. Under hypertonic conditions, ATA2 mRNA in TR-BBB cells was induced by up to 373%, and it activated [(3)H]MeAIB uptake. In light of these observations, our results indicate that L-Pro and Gly are transported from the brain across the BBB, whereas MeAIB is retained in the brain. System A is involved in efflux transport for L-Pro at the BBB. The predominantly expressed ATA2 mRNA at the BBB may play a role in maintaining the concentration of small neutral amino acids and cerebral osmotic pressure in the brain under pathological conditions.

Conditionally immortalized cell lines as a new in vitro model for the study of barrier functions

Conditionally immortalized brain and retinal capillary endothelial and choroid plexus epithelial cell lines were established from a transgenic rat (Tg rat) and mouse (Tg mouse) harboring the temperature-sensitive simian virus 40 (ts SV 40) large T-antigen. These cell lines exhibit temperature-sensitive cell growth due to the expression of ts SV 40 large T-antigen. Mouse brain (TM-BBB) and rat brain (TR-BBB) and rat retinal (TR-iBRB) capillary endothelial cell lines appear to have a spindle-fiber shaped morphology and exhibit the typical endothelial markers, such as von Willebrand factor and acetylated low-density lipoprotein uptake. These cell lines express in vivo influx and efflux transporters, such as P-glycoprotein (P-gp) and GLUT1, which is capable of 3-O-methyl-D-glucose transport. TM-BBB cells are able to undergo efflux transport of cyclosporin A, which is a substrate for P-gp transport activity. They may also express oatp2 and exhibit dehydroepiandrosterone sulfate and digoxin uptake activity. TR-BBB cells express the mRNA of multidrug resistance associated protein 1 (MRP1) and a large neutral amino acid transporter, which consists of LAT1 and 4F2hc. TR-iBRB cells exhibit pH-dependent L-lactic acid transport activity and express the mRNA of monocarboxylate transporter (MCT) 1 and 2. The choroid plexus epithelial cell line (TR-CSFB) has polygonal cell morphology, expresses the typical choroid plexus epithelial cell marker, transthyretin, and has Na+, K+-ATPase located on the apical side. TR-CSFB cells also exhibit amino acid transport activity which has been observed in vivo. These barrier cell lines established from the Tg rat and Tg mouse have in vivo transport functions and are good in vitro models for drug transport to the brain and retina and as a screen for drugs which might be capable of delivery to the brain and retina.

Characterization of the amino acid transport of new immortalized choroid plexus epithelial cell lines: a novel in vitro system for investigating transport functions at the blood-cerebrospinal fluid barrier

PURPOSE

To establish and characterize a choroid plexus epithelial cell line (TR-CSFB) from a new type of transgenic rat harboring the temperature-sensitive simian virus 40 (ts SV 40) large T-antigen gene (Tg rat).

METHODS

Choroid plexus epithelial cells were isolated from the Tg rat and cultured on a collagen-coated dish at 37 degrees C during the first period of 3 days. Cells were subsequently cultured at 33 degrees C to activate large T-antigen. At the third passage, cells were cloned by colony formation and isolated from other cells using a penicillin cup.

RESULTS

Five immortalized cell lines of choroid plexus epithelial cells (TR-CSFB 1 approximately 5) were obtained from two Tg rats. These cell lines had a polygonal cell morphology, expressed the typical choroid plexus epithelial cell marker, transthyretin, and possessed Na+, K+-ATPase on their apical side. TR-CSFBs cells expressed a large T-antigen and grew well at 33 degrees C with a doubling-time of 35 approximately 40 hr. [3H]-L-Proline uptake by TR-CSFB cells took place in an Na+-dependent, ouabain-sensitive, energy-dependent, and concentration-dependent manner. It was also inhibited by alpha-methylaminoisobutylic acid, suggesting that system A for amino acids operates in TR-CSFB cells. When [3H]-L-proline uptake was measured using the Transwell device, the L-proline uptake rate following application to the apical side was five-fold greater than that following application to the basal side. In addition, both Na+-dependent and Na+-independent L-glutamic acid (L-Glu) uptake processes were present in TR-CSFB cells.

CONCLUSIONS

Immortalized choroid plexus epithelial cell lines were successfully established from Tg rats and have the properties of choroid plexus epithelial cells, and amino acid transport activity was observed in vivo.

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

1. Epithelial uptake and efflux of the non-metabolized system A analogue 2-methylaminoisobutyric acid (MeAIB) and L-serine were studied in the isolated perfused rat pancreas using a dual tracer loading and wash-out technique. Uptakes of 2-[14C]MeAIB and L-[3H]serine were measured relative to D-[3H or 14C]mannitol (extracellular tracer) during a 20 min cell loading period. Maximal uptake for MeAIB (34 +/- 2%, n = 6) occurred within 2-3 min and decreased to 14 +/- 2% after 20 min tracer loading. Uptake for L-serine reached a maximum (62 +/- 4%, n = 7) within 1 min and decreased to 19 +/- 2% after 20 min tracer loading. 2. When tracer wash-out was monitored during subsequent perfusion of the preloaded pancreas with an isotope-free solution, D-mannitol predominantly cleared from a fast exchanging compartment (0.54 +/- 0.05 ml g-1, n = 9) with a time constant (Tfast) of 0.68 +/- 0.04 min. Although MeAIB and L-serine exhibited similar fast phases of wash-out, a much larger efflux occurred from a slowly exchanging pool with respective time constants (Tslow) of 15.47 +/- 0.45 min (n = 6) and 5.98 +/- 0.46 min (n = 7). 3. A rapid vascular challenge of the pancreas with 100 mM-L-serine transiently accelerated cellular efflux of 2-[14C]MeAIB and L-[3H]serine without affecting wash-out of D-[14C]mannitol. Tracer efflux following cell loading with 2-[14C]MeAIB or L-[3H]serine was not stimulated by a challenge with 100 mM-MeAIB. 4. The time course of amino acid evoked 2-[14C]MeAIB and L-[3H]serine efflux paralleled the extracellular dilution profile of a vascular stimulus, suggesting that the acceleration of efflux was due to trans-stimulation. 5. Trans-stimulation of 2-[14C]MeAIB and L-[3H]serine efflux by a further twenty-two naturally occurring and three synthetic amino acids was then examined. L-Proline, N-methyl-DL-alanine, L-lysine and D-lysine selectively stimulated MeAIB efflux. Efflux of both tracer amino acids was accelerated by aminoisobutyric acid (AIB), L-serine, L-alanine, L-cysteine, L-threonine, glycine, branched-chain and large neutral amino acids, but affected negligibly by D-alanine, beta-alanine, MeAIB, L-arginine, L-aspartate, L-glutamate, taurine or D-mannitol. 6. In summary, we have characterized amino acid exchange mechanisms in the isolated pancreas and conclude that efflux of intracellular amino acids from pancreatic acinar cells may be mediated by at least two transporters with differing specificity for extracellular amino acids.(ABSTRACT TRUNCATED AT 400 WORDS)

Amino acid specificity of the Na+/alanine cotransporter in pancreatic acinar cells

The method of tight-seal whole-cell recording was used to study the amino-acid specificity of the Na+/alanine cotransporter in pancreatic acinar cells. Single cells or small clusters of electrically coupled cells were obtained by enzymatic dissociation of mouse pancreas. Inward currents were measured under 'zero-trans' conditions, i.e., at finite concentrations of Na+ and amino acid at the extracellular side and vanishing concentrations at the cytoplasmic side. The cotransporter, which corresponds to 'system A', as previously defined in the literature, was found to exhibit a wide tolerance to neutral amino acids (L-cysteine, L-serine, L-alanine, glycine, L-phenylalanine). Competition experiments with 2-methylaminoisobutyric acid (MeAIB) indicate that for glycine a second electrogenic transport system exists in pancreatic acinar cells.

Secretagogue-induced changes in system A amino acid transport in the rat exocrine pancreas: stimulation of 2-methylaminoisobutyric acid efflux by carbachol

Secretagogue-induced changes in exocrine pancreatic amino acid transport are poorly understood. In this study uptake of the specific non-metabolized System A amino acid analogue 2-methylaminoisobutyric acid (2-MeAIB) was measured in the isolated perfused rat pancreas during 60 min loading with D-[3H]mannitol (extracellular tracer) and 2-[14C]MeAIB. Tracer 2-MeAIB reached a maximal uptake of 37 +/- 4% (n = 4) after 3 min of loading and gradually decreased to a steady-state uptake of 13 +/- 1%. Infusion of carbachol (3.10(-7) M) during the tracer loading period abolished net tracer 2-MeAIB uptake, and reperfusion in the absence of carbachol restored net uptake to the prestimulus value. Less than 41% of the arterial 2-[14C]MeAIB or D-[3H]mannitol activity appeared in the basal pancreatic secretion. Carbachol evoked a 4.8-fold increase in pancreatic juice flow and appeared to reduce the activity of both tracers in the exocrine secretion. During washout of the pancreas with an isotope-free medium 2-[14C]MeAIB cleared from a rapidly exchanging pool with a time constant (tau 1) of 1.4 +/- 0.3 min (n = 4) and a more slowly exchanging pool with a time constant (tau 2) of 20.7 +/- 1.1 min. Carbachol accelerated efflux of 2-[14C]MeAIB from the epithelium but had no effect on the slow phase of D-[3H]mannitol washout. Our findings suggest that activation of cholinergic receptors modifies Na+-dependent System A amino acid transport in the basolateral membrane of the exocrine pancreatic epithelium.

Ethanol-induced alterations in exocrine pancreatic amino acid transport and secretion

The effects of ethanol on exocrine pancreatic amino acid transport and secretion were investigated during perfusion of the isolated rat pancreas with ethanol concentrations ranging from 0.06% to 4.1%. Amino acid transport was quantitated using a rapid dual isotope dilution technique in which unidirectional substrate uptake (15-20 sec) is assessed relative to an extracellular tracer. Pancreatic secretion evoked by 0.3 microM carbachol was abolished during perfusion with 0.32% ethanol. Influx of L-lysine, L-serine and methylaminoisobutyric acid (MeAIB) was marginally increased by 0.32% ethanol but significantly inhibited during subsequent perfusion with 1.28-4.1% ethanol. Pancreatic oxygen consumption and effluent PCO2 levels decreased with increasing ethanol concentration, and the control venous pH (7.21 +/- 0.01, n = 8) gradually approached arterial pH values (7.46 +/- 0.02, n = 9). These results indicate that low concentrations of ethanol readily inhibit secretagogue-induced pancreatic secretion. Amino acid transport at the basolateral membrane of the exocrine pancreatic epithelium appears only to be inhibited after acute exposure to high ethanol concentrations.

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.

Ionic dependence of amino-acid transport in the exocrine pancreatic epithelium: calcium dependence of insulin action

Rapid unidirectional transport (15 sec) of L-serine and 2-methylaminoisobutyric acid (MeAIB) was studied in the isolated perfused rat pancreas using a dual-tracer dilution technique. Time-course experiments in the presence of normal cation gradients revealed a time-dependent transstimulation of L-serine influx and transinhibition of MeAIB influx. Transport of the model nonmetabolized System A analog MeAIB was Na+ dependent and significantly inhibited during perfusion with 1 mM ouabain. Although transport of L-serine was largely Na+ independent, ouabain caused a time-dependent inhibition of transport. Influx of both amino acids appeared to be inhibited by the ionophore monensin but unaffected by a lowered extracellular potassium concentration. Removal of extracellular calcium had no effect on influx of the natural substrate L-serine, whereas stimulation of transport by exogenous insulin (100 microU/ml) was entirely dependent upon extracellular calcium and unaffected by ouabain. Paradoxically, exogenous insulin had no effect on the time-course of MeAIB influx. The characteristics of L-serine influx described in earlier studies together with our present findings suggest that insulin may modulate the activity of System asc in the exocrine pancreatic epithelium by a calcium-dependent mechanism.