Transport Of L-lysine Research Articles

The effect of drug charge type and charge density on corneal transport

In order to understand the effect of charge type on corneal transport of peptides, it is useful to first investigate the mechanism of transport of amino acids across corneal tissue. The transport characteristics of l-[ 14C]lysine and l-[ 14C]glutamic acid were based on in vitro flux measurements. Net flux of l-lysine occurs via a sodium-dependent cotransport mechanism (with a coupling ratio of 1:1). The transport of l-lysine involves the Na +-K +-ATPase pump and requires a stereospecific carrier-mediated transport system. The permeability coefficient ( P) of l-lysine was decreased from 10.1 ± 0.80 to 2.34 ± 0.75 × 10 −6 (cm/s by blocking the active pump and carrier-mediated system. However, l-glutamic acid was apparently absorbed by a passive aqueous transport mechanism which was oubain-insensitive, [Na +]-independent, temperature-insensitive and non-stereospecific. Overall, the cornea was 2–3-fold more permeable to a cation ( l-lysine) than an anion ( l-glutamic acid). In addition, the negatively charged drug, salicylic acid, was 2.3 times less permeable than the cationic drug, benzylamine.

Identification of a new transport system (y+L) in human erythrocytes that recognizes lysine and leucine with high affinity.

1. The effect of neutral amino acids on the transport of L-lysine across the human erythrocyte membrane was studied. 2. All neutral amino acids tested (range 0.3-5 mM) inhibit the influx of L-[14C]lysine (1 microM). The inhibition pattern is biphasic, and tends to reach a maximum at approximately 50% of the original flux. The concentrations that give 25% inhibition are (mM): L-cysteine (2.7), L-alanine (1.3), L-serine (0.9), L-isoleucine (0.6), L-phenylalanine (0.35), L-methionine (< 0.3), L-leucine (< 0.3). L-lysine and L-arginine completely inhibit the rate at the highest concentration. 3. These results can be explained by assuming that L-lysine transport occurs through two independent transporters that differ in their affinity for neutral amino acids. A detailed kinetic analysis of the effect of L-leucine on L-lysine entry is consistent with this hypothesis. 4. Using a new experimental strategy, the substrate and inhibitor transport parameters for the two systems were determined. The half-saturation constants for lysine (+/- S.E.M.) are found to be: KmA, 0.014 +/- 0.002 mM and KmB, 0.112 +/- 0.017 mM. The maximum rates differ by a factor of 8.2 (VmaxB/VmaxA). The leucine inhibition constants are: KiA, 0.022 +/- 0.003 mM and KiB, 30.36 +/- 7.9 mM. If the sodium in the incubation medium is replaced by potassium, the apparent affinity for leucine (1/KiA) is reduced approximately 30-fold. 5. The maximum inhibition caused by leucine decreases as the lysine concentration is raised, showing that leucine acts upon the higher affinity system. 6. When added to the trans side, L-leucine, L-phenylalanine and L-isoleucine do not cause inhibition, but stimulate the flux by approximately 30%. This indicates that these analogues are also transported. 7. In conclusion, in the concentration range 1-100 microM, lysine crosses the red cell membrane through two distinct transport systems, one of which recognizes both neutral and cationic amino acids with high affinity.

Use of membrane vesicles to estimate the numbers of system y+ and system L amino acid transporters in human erythrocytes

We have used equilibrium values for L-leucine and L-lysine uptake by right-side-out vesicles to estimate the membrane abundance (sites/cell) of Na(+)-dependent amino acid transport systems L and y+ in human erythrocytes. All of the intravesicular space was accessible to L-leucine, as judged by comparisons with uridine uptake via the equilibrative nucleoside transporter (10(4) sites/cell). In contrast, only 28% of the total intravesicular space was accessible to L-lysine uptake via system y+. Since human erythrocyte membranes generate an average of approximately 1000 vesicles/cell, these data provide evidence that system L is a relatively high-abundance membrane transport protein in human erythrocytes, while system y+ is present in smaller amounts (approximately 300 copies/cell). Calculated turnover numbers for L-lysine transport by system y+ at 37 degrees C are 24 s-1 for zero-trans influx and 150 s-1 for equilibrium-exchange influx.

Cationic and anionic amino acid transport studies in rat red blood cells

The transport of L-proline, L-lysine and L-glutamate in rat red blood cells has been studied. L-proline and L-lysine uptake were Na(+)-independent. When the concentration dependence was studied both showed a non-saturable uptake assimilable to a difussion-like process, with high Kd values (0.718 and 0.191 min-1 for L-proline and L-lysine respectively). Rat red blood cells showed high impermeability to L-glutamate. No sodium dependence was observed and the Kd value was low (0.067 min-1). Our results show firstly, that rat red blood cells do not have amino acid transport systems for anionic and cationic amino acids and secondly that erythrocytes show no sodium-dependent L-proline transport, and that these cells are very permeable to this amino acid.

Expression of mammalian renal transporters in Xenopus laevis oocytes

We have injected mRNA from rabbit renal cortex into Xenopus oocytes and demonstrated the expression of renal carriers for Na +-independent transport of l-phenylalanine and l-lysine and Na +-dependent transport of l-alanine and succinate. Maximal expression of renal amino acid transporters occurred 6–8 days following mRNA injection. The proteins responsible for transport of these four substrates were translated from mRNAs which are between 1.5 and 3.0 kb. This information serves as a starting point for expression cloning of these transport proteins.

Alteration of renal amino acid transport system in cadmium-intoxicated rats

Effects of cadmium intoxication on renal transport systems for various amino acids were studied. Subcutaneous injections of CdCl 2, at a dose of 2 mg Cd/kg · day for 2 weeks, resulted in polyuria, proteinuria, glycosuria, phosphaturia, and aminoaciduria, as observed in chronic cadmium-intoxicated humans and experimental animals. The nature of aminoaciduria was nonspecific, including iminoacid as well as almost all species of neutral, acidic, and basic amino acids. In renal cortical brush border membrane vesicles isolated from cadmium-intoxicated rats. Na +-dependent transport of l-proline, l-alanine, and l-lysine was markedly attenuated, whereas the amino acid transport in the basolateral membrane vesicle was not significantly affected. Similar results were obtained in the normal membrane vesicles directly exposed to inorganic cadmium. These results indicate that cadmium intoxication impairs various Na +-amino acid cotransport systems in the renal brush border membrane, which leads to panaminoaciduria.

Inhibition of transport system b 0,+ in blastocysts by inorganic and organic cations yields insight into the structure of its amino acid receptor site

The most conspicuous, Na +-independent amino acid transport process in preimplantation mouse blastocysts was provisionally designated system b 0,+ because it accepts some cationic and zwitterionic amino acids about equally well as substrates. Although system b 0,+ is not Na +-stimulated, it has not been determined if it is inhibited by Na +, or if its activity is affected by most other ions. Therefore, we measured uptake of amino acids by blastocysts in isotonic solutions of different ionic and nonionic osmolites. Na +-independent l-leucine uptake was unaffected by the ion concentration, but l-lysine transport was several-fold faster in isotonic solutions of non-electrolytes than in similar solutions of inorganic and organic salts or zwitterions. The K m value for ‘Na +-independent’ l-lysine transport was about 10-fold higher in isotonic salt solutions than in solutions of nonionic osmolites, whereas the K m value for l-leucine transport was about the same in either type of solution. Therefore, inorganic and organic cations and the cationic portions of zwitterions appear to compete with cationic but not zwitterionic amino acids for system b 0,+ receptor sites. The cation, harmaline, was a particularly strong competitive inhibitor of ‘Na +-independent’ l-lysine uptake by system b 0,+, even in isotonic salt solutions, whereas it inhibited l-leucine uptake noncompetitively. Moreover, harmaline appeared to compete with inorganic cations for the lysine receptor sites of system b 0,+. Harmaline also has been found by other investigators to competitively inhibit l-lysine uptake by the Na +-independent system asc 1 in horse erythrocytes, whereas it noncompetitively inhibits alanine uptake by the same system. Similarly, harmaline noncompetitively inhibits l-alanine uptake by the Na +-dependent system ASC in human erythrocytes, but it appears to compete for binding with l-alanine's cosubstrate, Na +. In addition, others have found that the positively-charged side chains of cationic amino acids seem to take the place of Na + needed near side chains in order for zwitterionic amino acids to be transported by systems ASC and y +. We conclude that system b 0,+ may be similar to systems asc 1, ASC and y +, and that each of these systems may be a variant of the same ancestral transport process. We speculate that since it appears to accept a broader scope of substrates and to interact with a wider variety of cations than do systems asc 1, ASC or y +, system b 0,+ may more closely resemble the proposed ancestral transport process than any of the other contemporary systems.

Phenylmethylsulfonyl fluoride protects l-lysine transport in Schizosaccharomyces pombe against inactivation by ammonium ions

Ammonium ions inactivate the basic amino acid transport system in Schizosaccharomyces pombe in an irreversible manner. The inactivation is accompanied by a 4-fold decrease of K T of l-lysine transport, leaving its J max unchanged; phenylmethylsulfonyl fluoride protects the system against inactivation. In contrast, two basic amino acid transport systems in a gap1 mutant of Saccharomyces cerevisiae are influenced by NH 4 + ions in such a way that only the J max decreases while the K T of l-lysine transport is unchanged. Phenylmethylsulfonyl fluoride does not act here as a protective agent.

Characterization of amino acid transport systems in human placental basal membrane vesicles

The amino acid transport systems have been characterized in basal membrane vesicles prepared from human full-term placental syncytiotrophoblasts. Transport of amino acids across basal membranes occurred via passive diffusion and Na +-independent and Na +-dependent carrier-mediated systems. Passive diffusion was responsible for a substantial fraction of transport. l-Glutamate and α-(methylamino)isobutyrate were transported only Na +-independently, while the transport of l-alanine was dependent solely on an Na + gradient from the outside to the inside of the vesicles. l-Methionine, l-leucine, glycine and l-proline transport were supported by both Na +-independent and Na +-dependent systems. l-Lysine transport was decreased in the presence of cations, an inwardly directed Na + gradient was much more effective than a K + gradient at slowing l-lysine transport. A cross-inhibition analysis of these amino acids indicates that at least three Na +-independent and five Na +-dependent carrier-mediated systems exist in the human placental syncytiotrophoblast basal membranes. One Na +-independent system interacts with all substrates tested. Another Na +-independent system carries glycine, l-methionine, l-leucine and l-lysine; it is sensitive to l-glutamate, but not to l-proline or α-(methylamino)isobutyrate. The third system is selective for l-lysine, which is inhibited by l-methionine, glycine and l-leucine, but inaccessible to l-glutamate, l-proline and α-(methylamino)isobutyrate. One Na +-dependent system carries l-alanine, glycine, l-methionine and l-leucine, and it is sensitive to l-proline. The second system mediates transport of l-alanine, glycine, l-methionine and l-proline, but is not sensitive to l-leucine. The third system carries l-alanine, glycine and l-proline, and is inaccessible to l-methionine and l-leucine. The fourth system is responsible for l-methionine and l-leucine; it is sensitive to l-alanine and glycine, but not to l-proline. The fifth system is selective for l-proline.

Characterization of amino-acid transport systems in guinea-pig intestinal brush-border membrane

The amino-acid transport systems have been characterized in brush-border membrane vesicles prepared from guinea-pig small intestine. Uptake of all amino acids tested was measured at the initial velocity for 5 s. l-Proline, α-(methylamino)isobutyrate, glycine, l-alanine and l-methionine were transported dependent solely on an Na + gradient from the outside to the inside of the vesicles, and l-cysteine, l-phenylalanine and l-leucine were transported dependent largely on the Na + gradient with a small fraction of Na +-independent transport. The transport of l-aspartic acid and l-lysine was independent of the Na + gradient and l-lysine transport was somewhat inhibited by the presence of cations, including Na +, K + and Li +. A cross-inhibition study of the uptake of these amino acids in the brush border of guinea-pig intestine revealed the presence of at least three Na +-dependent and three Na +-independent carrier-mediated systems. One Na +-dependent system interacted mainly with amino acid. Another Na +-independent system interacted with neutral amino acids, while a third system was selective for glycine. One Na +-independent system is for acidic amino acids, another is responsible for neutral amino acids and a third for cationic amino acids. These transport systems of amino acids in guinea-pig small intestine are compared with those in rabbit and mouse intestine.

Adaptation of pancreatic amino acid transport in rats after treatment with the synthetic protease inhibitor camostat mesilate.

Exocrine pancreatic amino acid transport was studied following oral treatment of rats with the synthetic protease inhibitor camostat mesilate. Camostat (200 mg/kg/day) was administered via an orogastric tube for 10 days, and kinetics of amino acid transport were then examined in the isolated perfused pancreas using a dual-isotope dilution technique. Administration of camostat doubled the rate constant (Vmax/Kt) for L-phenylalanine transport via System L by increasing the Vmax from 12.3 +/- 0.4 (n = 12) to 19.6 +/- 2.2 mumol/min/g (n = 4). Although the Kt (35 +/- 2 mM, n = 4) and Vmax (47.7 +/- 1.6 mumol/min/g) for L-serine transport via System asc increased, the Vmax/Kt ratio decreased from a control value of 1.85 to 1.36. Increases in Kt (5 +/- 2 mM, n = 4) and Vmax (5.18 +/- 1.01 mumol/min/g) for L-lysine transport via System y+ were also detected but Vmax/Kt remained unchanged. In the presence of a low Na+ perfusate, influx of L-serine, L-phenylalanine, and L-lysine were inhibited between 36 and 50%. Treatment with camostat results in an increase in the rate and Na+-dependence of large neutral amino acid transport in the rat exocrine pancreas. Pancreatic growth induced by repeated oral administration of camostat appears to be accompanied by an enhanced uptake of precursor amino acids.

Transport of l-lysine in the fission yeast Schizosaccharomyces pombe

Systems of l-lysine transport in Schizosaccharomyces pombe are not constitutive, as at no phase of growth in a rich medium is lysine taken up. Transport activity appears only after preincubation of harvested cells with glucose or another suitable source of energy. If cycloheximide is added during this preincubation no transport systems are synthesized. After removal of glucose, the activity of the transport system decays with a half-time of 13 min. The transport of l-lysine into S. pombe cells from thetionary phase of growth preincubated for 60 min with 1% d-glucose is mediated by at least two systems, the high-affinity one with a K t of 26 μmol/l and J max of 4.95 nmol/min per mg dry wt., the low-affinity one with a K T of 1.1 mmol/l and J max of 11.8 nmol/min per mg dry wt. The transport of lysine mediated by these two systems proceeds uphill. The high-affinity system has a pH optimum at 4.0–4.2, the accumulation ratio is highest at a cell density 2–5 mg dry wt. per ml and decreases with increasing lysine concentrations. Lysine accumulated by this system does not exit from cells. The only potent competitive inhibitors are l-arginine, l-histidine and d-lysine. The other amino acids tested do not behave as competitive inhibitors. Of the various metabolic inhibitors tested, the most potent were proton conductors and antimycin A.

Lysine and alanine transport in the perfused guinea-pig placenta

The characteristics of l-lysine transport were investigated at brush-border (maternal) and basal (fetal) sides of the syncytiotrophoblast in the term guinea-pig placenta artificially perfused either through the umbilical vessels in situ or through both circulations simultaneously. Cellular uptake, efflux and transplacental transfer were determined using a single-circulation paired-tracer dilution technique. Unidirectional l-[ 3H]lysine uptake (%) (perfusate lysine 50 μM) was high on maternal ( M = 87 ± 1) and fetal ( F = 73 ± 2) sides. l-[ 3H]Lysine efflux back into the ipsilateral circulation was asymmetrical (F/M ratio = 2.3) and transplacental flux occurred in favour of the fetal circulation. Unidirectional lysine influx kinetics (0.05–8.00 mM) gave K m values of 1.75 ± 0.70 mM and 0.90 ± 0.25 mM at maternal and fetal sides, respectively; corresponding V max values were 1.95 ± 0.38 and 0.87 ± 0.10 μmol·min −1·g −1. At both sides, lysine influx (50 μM) could be inhibited (about 60–80%) by 4 mM l-lysine and l-ornithine and less effectively (about 10–40%) by l-citrulline, l-arginine, d-lysine and l-histidine. At the basal side: (i) lysine influx kinetics were greatly modified in the presence of 10 mM l-alanine ( K m = 6.25 ± 3.27 mM; V max = 2.62 ± 0.94 μmol·min −1 · g −1), but unchanged by equimolar l-phenylalanine or l-tryptophan; (ii) in the converse experiments, lysine (10 mM) did not affect the kinetic characteristics for either l-alanine or l-phenylalanine; (iii) l-lysine and l-alaninie influx kinetics were not dependent on the sodium gradient; (iv) the inhibition of l-[ 3H]lysine uptake by 4 mM l-homoserine was partially (60%) Na +-dependent. At the maternal side the kinetic characteristics for alanine influx were highly Na +-dependent, while lysine influx was partially Na +-dependent only at low concentrations (0.05–0.5 mM). Bilateral perfusion with 2,4-dinitrophenol (1 mM) reduced l-[ 3H]lysine uptake into the trophoblast and abolished transplacental transfer. It is suggested that lysine transport in the guinea-pig placenta is mediated by a specific transport system (y +) for cationic amino-acids. The asymmetry in the degree of sodium-dependency at both trophoblast membranes may in part explain the maternal-to-foetal polarity of placental amino-acid transfer in vivo.

Membrane transport in an osmotically fragile mutant of Saccharomyces cerevisiae.

Transport properties of the osmotically fragile strain VY1160 of Saccharomyces cerevisiae were compared with those of the parent S288c strain. Mediated diffusion of 6-deoxy-D-glucose was practically unaffected; membrane-potential-dependent transport of D-glucosamine was very much depressed in the fragile strain. The H+-driven transport of L-lysine and L-proline, as well as that of the hitherto uninvestigated D-glucose-6-phosphate, were also very depressed. 2-Deoxy-D-glucose transport displayed slightly different kinetic parameters. Primary H+ extrusion by the plasma membrane H-ATPase was not diminished although the ATP-splitting activity was depressed by about 50%. The overall proton-motive force (pmf) of the fragile mutant at pH 5.5 was only 20 mV while in the parent strain it was 108 mV. In parallel with this, spontaneous acidification of the external medium (a CO2-associated event) was only about 2% of that in the parent strain. The defect in this, together with the inability to stimulate transport protein synthesis by glucose, may account for the generally poorer transport performance of the fragile mutant.

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 K t = 9.4 ± 1.1 mM and V max = 14.8 ± 0.9 μmol/min per g pancreas ( n = 6). Saturation of lysine transport occurred at lower concentrations (0.05–10 mM) with an apparent K t = 2.40 ± 0.09 mM and V max = 2.44 ± 0.18 μmol/min per g ( n = 6). The characteristics of phenylalanine transport were modified after adaptation to either high ( K t = 3.6 ± 1 mM, V max = 8.2 ± 0.9 μmol/min per g, n = 3) or low ( K t = 4.2 ± 0.9 mM, V max = 6.8 ± 0.5 μmol/min per g, n = 3) carbohydrate diets. Increasing the dietary protein content (0–45% casein) led to a linear increase in the K t for phenylalanine transport whereas V max values remained unchanged. Unlike phenylalanine, adaptation to a 0% casein diet significantly elevated the V max for lysine transport (4.82 ± 0.21 μmol/min per g, n = 4) without altering the K t (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.

Is leucine an allosteric modulator of the lysine transporter in the intestinal basolateral membrane?

The transport of the dibasic amino acid L-lysine was investigated using basolateral membrane vesicles prepared from rat jejunal mucosal scrapings. The majority of the carrier-mediated transport was unaffected by the presence of sodium in the incubation medium, but voltage clamping of the vesicles did increase lysine uptake, indicating an associated movement of charge. Kinetic analysis of lysine influx and efflux showed the system to be symmetrical, but although the Vmax was comparable to other amino acid transport systems in this membrane, the dissociation constant for the overall reaction (KT) was an order of magnitude larger. This low affinity for lysine would explain the relatively slow rate of transport of this amino acid across the basolateral membrane. Competition experiments indicated that this system has a relatively narrow specificity carrying only lysine, arginine, ornithine, and histidine. In contrast the presence of L-leucine caused a marked stimulation of lysine efflux and influx across the vesicles. This effect was observed with leucine concentrations as low as 0.1 microM. It is concluded that although the lysine transport system in the basolateral membrane is slow in its basal state it can be rapidly turned on by the presence of L-leucine. The remarkably low concentrations required to do this suggest a possible allosteric interaction between the transporter and this neutral amino acid.

Further studies on amino acid transport in murine P388 leukemia cells in vitro. Presence of system y +

The transport of glycine and l-lysine into murine P388 leukemia cells has been examined. Glycine transport appears to be shared by both systems A and ASC in P388 cells. Glycine transport is Na +-dependent and is effectively blocked by α-(methylamino)isobutyric acid, threonine and alanine but only a marginal reduction in transport is seen with 100-fold excess cold 2-aminobicyclo[2,2,1]heptane-2-carboxylic acid. System gly is not expressed in P388 cells. Lysine is largely transported by a Na +-independent, pH-insensitive system with a K m of 0.079 mM. Lysine transport is relatively unafected by the addition of 100-fold excess cold α-(methylamino)isobutyric acid, 2-aminobicyclo[2,2,1]heptane-2-carboxylic acid and the anionic amino acids, l-glutamate and l-aspartate. A partial inhibition of lysine transport was observed with l-threonine and l-leucine while l-arginine and l-histidine radically decreased lysine transport. Lysine appears to be transported by a system similar to the system y + seen in cultured human fibroblasts, Ehrlich ascites cells, and hepatoma cell lines.

通電透析発酵法のＬ‐リジン発酵への応用

Aspartate kinase of Brevibacterium flavum QL-5 is inhibited by L-lysine plus L-threonine. Ele-ctrodialysis fermentation was applied to L-lysine production to reduce this inhibition. After 72 hr, the amount of L-lysine produced by electrodialysis fermentation was 57.2 g, which was 1.2 times greater than that by non-electrodialysis fermenta-tion (46.9 g), and similar to that by diffusion dialysis fermentation (57.0g). In electrodialysis fermentation, the L-lysine concentration in dialyz-ing solution was only 0.2%, while that in the fermentation broth was about 2%. It was sug-gested that production of L-lysine was limited by transport of L-lysine across the hollow fiber wall. L-Lysine monohydrochloride was added to the cul-ture broth at various culture stage, but the produc-tion of L-lysine was not influenced by the L-lysine added. Therefore, it was considered that the inhibition by L-lysine plus L-threonine in L-lysine production by Brevibacterium flavum QL-5 was not alleviated even when the L-lysine concentration in the fermentation broth was maintained at low by electrodialysis fermentation or diffusion dialysis fermentation.

Effect of different Giardia lamblia inocula on glucose and amino acids transport in the intestinal brush border membrane vesicles of infected mice.

The transport of D-glucose, L-phenyl-alanine, L-lysine and L-aspartic acid was measured in the brush border membrane vesicles prepared from intestines of Swiss albino mice infected with three different inocula of Giardia lamblia cysts (100, 1,000 and 10,000 cysts). A significantly low uptake of nutrients was observed in the infected animals as compared with the uninfected groups (P less than 0.05). However, the uptake of nutrients did not alter significantly with varying inoculum size. With D-glucose the respective peak uptakes at 30 seconds were 59.3 +/- 5.5, 60.4 +/- 3.8 and 60.5 +/- 4.02 pmoles mg-1 protein, while with L-phenyl-alanine they were 158.1 +/- 8.3, 151.7 +/- 17.3 and 140.7 +/- 6.0 pmoles mg-1 protein. The same trend was observed for L-lysine and L-aspartic acid. Therefore we conclude that inoculum size does not influence the uptake of nutrients.

Proabsorptive properties of forskolin: Disposition of glycine, leucine and lysine in rat jejunum

The effects of forskolin on mucosal cyclic AMP levels and active transport of glycine, L-lysine and L-leucine were studied in rat jejunum in vitro. Furthermore, the effects on lysine and glycine incorporation into mucosal protein and on mucosal cell volume were investigated. Elevation of intestinal mucosal cyclic AMP to threefold control levels by 10 mumol 1(-1) forskolin was accompanied by increased absorption of glycine (+33%), L-leucine (+72%) and L-lysine (+188%), as determined in a three compartment model suitable to measure active transport. Increased intracellular accumulation could be demonstrated for lysine as a transport substrate. Accordingly, using a dual label method, calculated values for uphill transport of lysine at the site of the brush border membrane were markedly enhanced. Forskolin up to 10 mumol 1(-1) had no effects on the fraction of lysine or glycine incorporated into TCA-precipitable proteins of jejunal absorptive cells. Serosal to mucosal transfer, as well as basolateral entry into mucosal cells remained unchanged for all three amino acids. Likewise, intracellular fluid space, calculated from distribution spaces for 14C-inulin and 3H2O as well as the response of cellular volume to an osmotic gradient were not affected by forskolin. As comparable stimulatory effects of forskolin on active hexose transport were reported earlier, it is suggested that forskolin - known to inhibit sodium-coupled fluid absorption - may stimulate active transport by enhancing sodium availability for sodium dependent intestinal cotransporters in general.