Kinetic relations of the Na-amino acid interaction at the mucosal border of intestine

Mathematical model of the multi-amino acid multi-transporter system predicts uptake flux in CHO cells

Supply and uptake of amino acids is of great importance to mammalian cell culture processes. Mammalian cells such as Chinese hamster ovary (CHO) cells express several amino acid (AA) transporters including uniporters and exchangers. Each transporter transports multiple AAs, making prediction of the effect of changed medium composition or transporter levels on individual AA transport rate challenging. A general kinetic model for such combinatorial amino acid transport, and a simplified analytical expression for the uptake rate as a function of amino acid concentrations and transporter levels is presented. From this general model, a CHO cell-specific AA transport model, to our knowledge the first such network model for any cell type, is constructed. The model is validated by its prediction of reported uptake flux and dependencies from experiments that were not used in model construction or parameter estimation. The model defines theoretical conditions for synergistic/repressive effect on the uptake rates of other AAs upon external addition of one AA. The ability of the CHO-specific model to predict amino acid interdependencies experimentally observed in other mammalian cell types suggests its robustness. This model will help formulate testable hypotheses of the effect of process changes on AA initial uptake, and serve as the AA transport component of kinetic models for cellular metabolism.

Team-based, problem-solving exercises using studies of diarrhea and oral rehydration encourage students to integrate knowledge of systems physiology

We describe an inquiry activity that aims to develop students' ability to interpret findings that span whole body systems and so encourage the integration of knowledge. The scenario we choose was the physiological challenge posed by diarrhea and the physiological mechanisms that underpin oral rehydration therapy. Before the staff-facilitated inquiry activity, students engage with an online information resource and complete a formative, but mandatory, prelaboratory quiz. These tasks encourage students to develop some mastery of the relevant physiology before the timetabled inquiry activity. The 3-h inquiry activity is driven by a paper workbook containing data from published studies, mainly from veterinary physiology, of the various consequences of diarrhea. Figures from published data are arranged so that, initially, the impact of dehydration on a single system (the cardiovascular system) could be appreciated. Integration with other systems (respiratory and renal systems) is then introduced progressively through the activity. The exercise is designed as a team-based inquiry activity that emphasizes the value of discussion to identify appropriate features for interpretation of the data. Students are obliged to complete a postlaboratory quiz within 5 days of the inquiry activity, serving to consolidate the students' learning and provide staff with feedback on the attainment of intended learning outcomes. Marks from formative pre- and postlaboratory quizzes typically have a median mark in excess of 80% (pass mark is 50%), and qualitative feedback suggests that the majority of students recognized the value of the activity, despite simultaneously reporting that it was intellectually demanding.

A randomized trial to assess beverage hydration index in healthy older adults

BACKGROUND

The beverage hydration index (BHI) is a composite measure of fluid balance after consuming a test beverage relative to water. BHI is a relatively new measure that has been explored in young, but not yet older, adults.

OBJECTIVE

The aim of this study was to investigate potential differences in BHI between euhydrated younger and older adults after drinking 4 different commercial beverages. We hypothesized that 1) older subjects would remain in positive fluid balance longer than young subjects after ingestion of each test beverage due to decreased urinary excretion rates, 2) glucose (glu)- and amino acid (AA)-based hydration beverages with sodium would have a BHI greater than water in both groups, and 3) the traditional 2-h postingestion BHI may be inappropriate for older adults.

METHODS

On 5 separate visits, 12 young (23 ± 3 yr, 7 M/5F) and 12 older (67 ± 6 yr, 5 M/7F) subjects consumed 1 L of distilled water, G-20 (6% CHO, 20 mmol/L Na+), G-45 (2.5% CHO, 45 mmol/L Na+), AA-30 (5 AAs, 30 mmol/L Na+), or AA-60 (8 AAs, 60 mmol/L Na+) over 30 min. Blood and urine samples were collected before ingestion and at 0, 60, 120, 180, and 240 min postingestion with additional venous blood sampling at 5, 10, 15, and 30 min postingestion.

RESULTS

In young subjects, BHI increased with increasing beverage Na+ concentration, and AA-60 had the highest BHI (AA-60 = 1.24 ± 0.10 compared with water = 1.00, P = 0.01). For older subjects, BHI was highest in AA-30 (AA-30; 1.20 ± 0.13 compared with water, P < 0.01) and was still in flux beyond 2 h in AA-60 (P < 0.05).

CONCLUSIONS

Beverage Na+ content progressively increased BHI in young adults independent of glucose or AA content. For older adults, the AA-30 beverage had the highest BHI. A 4-h BHI may be more appropriate for older adults due to attenuated urine excretion rates. This trial was registered at clinicaltrials.gov as NCT03559101.

Amino Acid Transport Across the Mammalian Intestine

The small intestine mediates the absorption of amino acids after ingestion of protein and sustains the supply of amino acids to all tissues. The small intestine is an important contributor to plasma amino acid homeostasis, while amino acid transport in the large intestine is more relevant for bacterial metabolites and fluid secretion. A number of rare inherited disorders have contributed to the identification of amino acid transporters in epithelial cells of the small intestine, in particular cystinuria, lysinuric protein intolerance, Hartnup disorder, iminoglycinuria, and dicarboxylic aminoaciduria. These are most readily detected by analysis of urine amino acids, but typically also affect intestinal transport. The genes underlying these disorders have all been identified. The remaining transporters were identified through molecular cloning techniques to the extent that a comprehensive portrait of functional cooperation among transporters of intestinal epithelial cells is now available for both the basolateral and apical membranes. Mouse models of most intestinal transporters illustrate their contribution to amino acid homeostasis and systemic physiology. Intestinal amino acid transport activities can vary between species, but these can now be explained as differences of amino acid transporter distribution along the intestine. © 2019 American Physiological Society. Compr Physiol 9:343-373, 2019.

Role of membrane transport in the regulation of skeletal muscle glutamine turnover

This paper reviews present understanding of the role played by the sarcolemmal glutamine transporter, system N(m), in control of intramuscular glutamine concentration. Glutamine transport in skeletal muscle is a saturable, stereospecific, Na dependent and insulin sensitive process. The activity of system N(m) is subject to modification during muscle denervation, diabetes and exposure to bacterial products in a manner consistent with the observed negative glutamine balance exhibited by muscle during such circumstances. The modification in transporter activity appears to be dependent on factors influencing the distribution of Na across the sarcolemma, the resting membrane potential and the active carrier population in the sarcolemma (possibly through up or down regulation of the number of transporter molecules). Derangements in net membrane glutamine transport during pathophysiological conditions may help, partly, to account for the loss in muscle glutamine which in turn may influence control of protein and carbohydrate metabolism in muscle. The free intramuscular glutamine concentration appears to act as a positive signal in the control of muscle protein turnover and glycogen synthesis, a finding that may have important therapeutic implications for limiting muscle wasting. The kinetic properties of the glutamine transporter and the dipeptidase activity in the muscle vascular bed allow the intramuscular glutamine pool to be repleted following administration of glutamine dipeptides (such as Ala-Gln) with the result that a net anabolic shift in protein balance and an amelioration in muscle glutamine efflux takes place.

Dextran sulfate sodium inhibits alanine synthesis in Caco-2 cells

To understand and characterize the pathogenic mechanisms of inflammatory bowel disease, dextran sulfate sodium (DSS) has been used to induce acute and chronic colitis in animal models by causing intestinal epithelium damage. The mechanism of action of DSS in producing this outcome is not well understood. In an effort to understand how DSS might impact epithelial cell metabolism, we studied the intestinal epithelial cell line Caco-2 incubated with 1% DSS over 56 hours using (1)H NMR spectroscopy. We observed no difference in cell viability as compared to control cultures, and an approximately 1.5-fold increase in IL-6 production upon incubation with 1% DSS. The effect on Caco-2 cell metabolism as measured through changes in the concentration of metabolites in the cell supernatant included a three-fold decrease in the concentration of alanine. Given that the concentrations of other amino acids in the cell culture supernatant were not different between treated and control cultures over 56 hours suggest that DSS inhibits alanine synthesis, specifically alanine aminotransferase, without affecting other key metabolic pathways. The importance of alanine aminotransferase in inflammatory bowel disease is discussed.

Amino acid transport across mammalian intestinal and renal epithelia

The transport of amino acids in kidney and intestine is critical for the supply of amino acids to all tissues and the homeostasis of plasma amino acid levels. This is illustrated by a number of inherited disorders affecting amino acid transport in epithelial cells, such as cystinuria, lysinuric protein intolerance, Hartnup disorder, iminoglycinuria, dicarboxylic aminoaciduria, and some other less well-described disturbances of amino acid transport. The identification of most epithelial amino acid transporters over the past 15 years allows the definition of these disorders at the molecular level and provides a clear picture of the functional cooperation between transporters in the apical and basolateral membranes of mammalian epithelial cells. Transport of amino acids across the apical membrane not only makes use of sodium-dependent symporters, but also uses the proton-motive force and the gradient of other amino acids to efficiently absorb amino acids from the lumen. In the basolateral membrane, antiporters cooperate with facilitators to release amino acids without depleting cells of valuable nutrients. With very few exceptions, individual amino acids are transported by more than one transporter, providing backup capacity for absorption in the case of mutational inactivation of a transport system.

Mechanism and putative structure of B(0)-like neutral amino acid transporters

The Na(+)-dependent transport of neutral amino acids in epithelial cells and neurons is mediated by B(0)-type neutral amino acid transporters. Two B(0)-type amino acid transporters have been identified in the neurotransmitter transporter family SLC6, namely B(0)AT1 (SLC6A19) and B(0)AT2 (SLC6A15). In contrast to other members of this family, B(0)-like transporters are chloride-independent. B(0)AT1 and B(0)AT2 preferentially bind the substrate prior to the Na(+)-ion. The Na(+)-concentration affects the K ( m ) of the substrate and vice versa. A kinetic scheme is proposed that is consistent with the experimental data. An overlapping binding site of substrate and cosubstrate has been demonstrated in the bacterial orthologue LeuT( Aa ) from Aquifex aeolicus, which elegantly explains the mutual effect of substrate and cosubstrate on each other's K ( m )-value. LeuT( Aa ) is sequence-related to transporters of the SLC6 family, allowing homology modeling of B(0)-like transporters along its structure.

Deciphering the mechanisms of intestinal imino (and amino) acid transport: The redemption of SLC36A1

The absorption of zwitterionic imino and amino acids, and related drugs, is an essential function of the small intestinal epithelium. This review focuses on the physiological roles of transporters recently identified at the molecular level, in particular SLC36A1, by identifying how they relate to the classical epithelial imino and amino acid transporters characterised in mammalian small intestine in the 1960s-1990s. SLC36A1 transports a number of D- and L-imino and amino acids, beta- and gamma-amino acids and orally-active neuromodulatory and antibacterial agents. SLC36A1 (or PAT1) functions as a proton-coupled imino and amino acid symporter in cooperation with the Na+/H+ exchanger NHE3 (SLC9A3) to produce the imino acid carrier identified in rat small intestine in the 1960s but subsequently ignored because of confusion with the IMINO transporter. However, it is the sodium/imino and amino acid cotransporter SLC6A20 which corresponds to the betaine carrier (identified in hamster, 1960s) and IMINO transporter (identified in rabbit and guinea pig, 1980s). This review summarises evidence for expression of SLC36A1 and SLC6A20 in human small intestine, highlights the differences in functional characteristics of the imino acid carrier and IMINO transporter, and explains the confusion surrounding these two distinct transport systems.

Physical characterization of high-affinity gastrointestinal Cu transport in vitro in freshwater rainbow trout Oncorhynchus mykiss

This study investigated the transport of copper (Cu) in the gut of trout. Examination of the spatial distribution of Cu along the digestive tract and a physical characterization of the uptake process was carried out using an in vitro gut sac technique and (64)Cu as a tracer. Unidirectional Cu uptake was highest in the anterior intestine followed in decreasing order by the posterior intestine, mid intestine and the stomach. Cu uptake was resistant to hypoxia and appeared to be fueled equally well by Cu(II) or Cu (I) at Cu concentrations typically found in the fluid phase of the chyme in vivo in the trout intestine. Transport demonstrated saturation kinetics (e.g. K (m) = 31.6 microM, J (max) = 17 pmol cm(-2) h(-1), in mid intestine) at low Cu levels representative of those measured in the chyme in vivo, with a diffusive component at higher Cu concentrations. Q (10) analysis indicated Cu uptake is via diffusion across the apical membrane and biologically mediated across the basolateral membranes of enterocytes. The presence of L-histidine but not D-histidine stimulated both Cu and Na uptake suggesting a common pathway for the transport of Cu/Na with L-histidine.

Characterization of mouse amino acid transporter B0AT1 (slc6a19)

The mechanism of the mouse (m)B0AT1 (slc6a19) transporter was studied in detail using two electrode voltage-clamp techniques and tracer studies in the Xenopus oocyte expression system. All neutral amino acids induced inward currents at physiological potentials, but large neutral non-aromatic amino acids were the preferred substrates of mB0AT1. Substrates were transported with K0.5 values ranging from approx. 1 mM to approx. 10 mM. The transporter mediates Na+-amino acid co-transport with a stoichiometry of 1:1. No other ions were involved in the transport mechanism. An increase in the extracellular Na+ concentration reduced the K0.5 for leucine, and vice versa. Moreover, the K0.5 values and Vmax values of both substrates varied with the membrane potential. As a result, K0.5 and Vmax values are a complex function of the concentration of substrate and co-substrate and the membrane potential. A model is presented assuming random binding order and a positive charge associated with the ternary [Na+-substrate-transporter] complex, which is consistent with the experimental data.

The molecular basis of neutral aminoacidurias

Recent success in the molecular cloning and identification of apical neutral amino acid transporters has shed a new light on inherited neutral amino acidurias, such as Hartnup disorder and Iminoglycinuria. Hartnup disorder is caused by mutations in the neutral amino acid transporter B(0) AT1 (SLC6A19). The transporter is found in kidney and intestine, where it is involved in the resorption of all neutral amino acids. The molecular defect underlying Iminoglycinuria has not yet been identified. However, two transporters, the proton amino acid transporter PAT1 (SLC36A1) and the IMINO transporter (SLC6A20) appear to play key roles in the resorption of glycine and proline. A model is presented, involving all three transporters that can explain the phenotypic variability of iminoglycinuria.

Amino acid transport by intestinal brush border vesicles of a marine fish, Boops salpa

The transport of glycine, alanine, methionine and alpha amino-isobutyric acid (AIB) was studied on brush border membrane vesicles of Boops salpa, a marine fish. This transport was Na(+)-, Cl(-)- and pH-dependent. In the presence of NaCl, the uptake decreased as the pH increased from 5.5 to 8.5. With Na2SO4, the transport of the four amino acids was strongly reduced and the pH optimum was 7-8. In the presence of NaCl, amino acid transport was described by high and low affinity kinetics. The K(t) of the high-affinity component was comparable for glycine, alanine and methionine (0.1 mM), and was significantly enhanced for AIB (0.6 mM). The J(max) of the low affinity component was significantly lower for methionine and AIB than for glycine and alanine. Lowering the sodium concentration from 80 to 20 mM significantly increased K(t) and J(max) of the high-affinity component of glycine transport. Moreover, the kinetics of AIB transport under 100 mM Na(+) were similar to glycine kinetics under 40 mM Na(+) and the two amino acids competed for the same carrier(s). These results suggest that chloride ions are essential in neutral amino acid transport in Boops, that multiple saturable components are involved in this process, and that sodium plays an important role in the differences between the transport kinetics of amino acids.

Transport of neutral, cationic and anionic amino acids by systems B, b(o,+), X(AG), and ASC in swine small intestine

Amino acid influx across the brush border membrane of the intact pig ileal epithelium was studied. It was examine whether in addition to system B, systems ASC and b(o,+) were involved in transport of bipolar amino acids. The kinetics of interactions between lysine and leucine demonstrates that system b(o,+) is present and accessible also to L-glutamine. D-aspartate (K(1/2) 0.3 mM) and L-glutamate (K(i) 0.5 mM) share a high affinity transporter with a maximum rate of 1.3 micromol cm(-2) h(-1), while only L-glutamate with a K(1/2) of 14.4 mM uses a low affinity transporter with a maximum rate of 2. 7 micromol cm(-2) h(-1), system ASC, against which serine has a K(i) of 1.6 mM. In the presence of 100 mM lysine, L-glutamine (A), leucine (B), and methionine (C) fulfilled the criteria of the ABC test for transport by one and the same transporter. However, serine inhibits not only transport of L-glutamate but also of glutamine (K(i) 0.5 mM), and L-glutamate inhibits part of the transport of glutamine. The test does, therefore, only indicate that the three bipolar amino acids have similar affinities for transport by systems B and ASC. Further study of the function of system B must be carried out under full inhibition by lysine and glutamate.

Role of caloric content on gastric emptying in humans

1. This study examined the effects of caloric content (caloric density and the nature of calories) on the rate of gastric emptying using the double-sampling gastric aspiration technique. Four test meals of 600 ml (glucose, 0.1 kcal ml-1; pea and whey peptide hydrolysates, both 0.2 kcal ml-1; milk protein, 0.7 kcal ml-1) were tested in six healthy subjects in random order on four separate occasions. 2. The glucose solution was emptied the fastest with a half-time of 9.4 +/- 1.2 min (P < 0.05) and the milk protein the slowest with a half-time of 26.4 +/- 10.0 min (P < 0.05); the pea peptide hydrolysate and whey peptide hydrolysate solutions had half-times of emptying of 16.3 +/- 5.4 and 17.2 +/- 6.1 min, respectively. The rates of gastric emptying for the peptide hydrolysate solutions derived from different protein sources were not different. 3. Despite the lower rate of gastric emptying for the milk protein solution, the rate of caloric delivery to the duodenum during the early phase of the gastric emptying process was higher than that for the other three solutions (46.3 +/- 6, 63.5 +/- 22, 62.5 +/- 19 and 113.8 +/- 25 cal min-1 kg-1 for the glucose, pea peptide hydrolysate, whey peptide hydrolysate and milk protein meals, respectively; P < 0.05). The caloric density of the test solutions was linearly related to the half-time of gastric emptying (r = 0.96, P < 0.05) as well as to the rate at which calories were delivered to the duodenum (r = 0.99, P < 0.001). 4. This study demonstrates that the rate of gastric emptying is a function of the caloric density of the ingested meal and that a linear relationship exists between these variables. Furthermore, the nature of the calories seems to play a minor role in determining the rate of gastric emptying in humans.

Chloride-dependent amino acid transport in the small intestine: occurrence and significance

The unidirectional influx of amino acids, D-glucose and ions across the brush-border membrane of the small intestine of different species has been measured in vitro with emphasis on characterization of topographic and species differences and on chloride dependence. The regional differences in transport along the small intestine are outlined and shown to be caused by variation in transport capacity, while the apparent affinity constants are unchanged. Rabbit small intestine is unique by exhibiting maximal rates of transport in the distal ileum and a very steep decline in the oral direction from where tissues are normally harvested for preparation of brush-border membrane vesicles. Transport in the guinea pig and rat is much more constant throughout the small intestine. Since the capacity of nutrient carriers is regulated by their substrates it is possible that bacterial breakdown of peptides and proteins in rabbit distal ileum increases the concentration of amino acids leading to an upregulation of the carriers. Chloride dependence is a characteristics of the carrier rather than the transported amino acid, and is used to improve the classification of amino acid carriers in rabbit small intestine. In this species the imino acid carrier, the beta-amino acid carrier, and the beta-alanine carrier, which should be renamed the B0,+ carrier, are chloride-dependent. The steady-state mucosal uptake of classical substrates for these carriers in biopsies from the human duodenum is also chloride-dependent. The carrier of beta-amino acids emerges as ubiquitous and chloride-dependent, and evidence of cotransport with both sodium and chloride is reviewed. A sodium:chloride:2-methyl-aminoisobutyric acid coupling stoichiometry of approx. 2:1:1 is suggested by ion activation studies. Direct measurements of coupled ion fluxes in rabbit distal ileum confirm that sodium, chloride and 2-methyl-aminoisobutyric acid are cotransported on the imino acid carrier with an identical influx stoichiometry. Control experiments and reference to the literature on the electrophysiology of the small intestine exclude alterations of the membrane potential as a feasible explanation of the chloride dependence. Thus, it is concluded that chloride is cotransported with both sodium and 2-methyl-aminoisobutyric acid across the brush-border membrane of rabbit distal ileum.

Na(+)-coupled alanine transport in LLC-PK1 cells

Transport of alanine (Ala) was characterized in LLC-PK1 renal epithelial cells. Transport capability for Ala falls by 75% in postconfluent cultures, while Na(+)-coupled alpha-methylglucoside (AMG) transport rises more than fourfold during the same interval. The kinetics of Ala transport were characterized in ATP-depleted cells to allow experimental imposition of changes in Na+ gradient and control of membrane potential across the plasma membrane. At 100 microM Ala and 135 mM Na+, > 98% of the unidirectional Ala influx is dependent on the presence of Na+ in cells from postconfluent cultures. Li+ is only 1% as effective as Na+, and other monovalent cations are ineffective in supporting Ala uptake. alpha-(Methylamino)isobutyric acid (MeAIB; 5 mM) causes only a small inhibition (approximately 10%) of 100 microM Ala influx. The low selectivity for Li+; low sensitivity to competition by MeAIB or aminoisobutyric acid; pronounced inhibition by serine, homoserine, cysteine, homocysteine and threonine; moderate inhibition by valine, isoleucine, proline and histidine; and lack of inhibition by lysine, arginine, and aspartate are more consistent with those characteristics reported for entry via the ASC amino acid transport system rather than those associated with the A system. Alanine influx exhibits a hyperbolic relationship with increasing Ala or Na+ concentration. Kinetic analysis suggests a single transport pathway with a Michaelis constant (Km) for alanine of 380 microM (when Na+ is 135 mM), apparent Km for Na+ of 32 mM (with 100 microM Ala), and a maximum velocity of 7 nmol.min-1.mg cell protein-1. An interior-negative diffusion potential induces a similar enhancement of [14C]alanine or [14C]tetraphenylphosphonium influx (approximately 40%). In contrast, AMG influx is enhanced by a factor of 2.2 under the same conditions. AMG uptake also shows a sigmoidal relationship with Na+ concentration. Hill coefficients are 1.56 for AMG and 1.0 for alanine. Direct measurement of Na(+)-Ala coupling stoichiometry yields a value of 1.01 +/- 0.07. Under the same conditions, Na(+)-AMG coupling stoichiometry is 2.1 +/- 0.25. The difference in coupling stoichiometries provides an explanation for differences in intensity of interaction between Na(+)-coupled transport systems for sugars and amino acids.

Variation in amino acid transport along the rabbit small intestine. Mutual jejunal carriers of leucine and lysine

The jejuno-ileal variation of amino and imino acid transport across the brush-border membrane of intact rabbit small intestine was studied. For the amino acids tested--beta-alanine, leucine, lysine, MeAIB, proline--and for D-glucose, the rates of transport at constant concentrations increase from very low values in the proximal jejunum to maximum values in the most distal 30 cm of the ileum. The apparent affinity constant for jejunal taurine transport is identical to that of the distal ileum, while the jejunal transport capacity is less than half. In the jejunum, as in the distal ileum, leucine and lysine share both sodium-dependent and sodium-independent carriers. Approx. 50% of the quantitative difference in transport capacity is accounted for by the absence of the beta-alanine carrier in the jejunum. These data indicate that the gradients of transport along the small intestine reflect gradients of transport capacities rather than affinities. In comparison with hamster, man and rat, the rabbit seems unique with respect to the location of transport maximum and the steepness of the gradient along the intestine.

Sodium-dependent succinate transport by isolated chick intestinal cells

Isolated chick intestinal epithelial cells take up succinate by a Na(+)-coupled transport system similar in some characteristics to those described for renal epithelium. The transport system exhibits a hyperbolic dependence on succinate concentration but a sigmoidal dependence on Na+ concentration. Best nonlinear fit of the Na+ dependence data to the Hill equation indicates a Michaelis constant for half-maximal transport rate (Km) for Na+ of approximately 20 mM, a maximal transport rate (Vmax) of 1.1 nmol succinate.min-1.mg protein-1, and a Hill coefficient of 2.5. Nearly equivalent fit was obtained with trial Hill coefficients down to 2.0. The data for succinate dependence indicated a Km of 25 microM and Vmax of 1.05 nmol.min-1.mg protein-1. The kinetic parameters indicate a higher affinity, lower capacity system than for succinate transport in the renal brush-border system. Thiocyanate-induced diffusion potentials cause no change in Na(+)-dependent succinate influx despite pronounced effects on the influx of tetraphenylphosphonium and on Na(+)-dependent alpha-methylglucoside (AMG) and alanine uptake. Several other dicarboxylic and tricarboxylic metabolic intermediates (but not the dicarboxylic amino acids) compete with succinate for uptake via the transport system. The data are consistent with the likelihood that these cells have a succinate transport system with a 2Na+:1succinate stoichiometry per transport cycle. The system catalyzes no net charge transfer and is therefore different from the potential-responsive succinate transporter described for renal tissue.

Glycine transport by intestinal brush border vesicles of the amphibian Discoglossus pictus

1. In order to determine the different components of glycine uptake by the intestine of the frog, Discoglossus pictus, we have used brush border membrane vesicles isolated by a classical precipitation technique. 2. Enzymatic tests showed that a good purification was obtained. The concentration ratio of alkaline phosphatase was 14.8. 3. Glycine entry in vesicles as a function of time, in presence or absence of sodium, indicated an overshoot which decreased when incubation time was prolonged. The overshoot was dependent on the presence of sodium. 4. The nature of the anion associated to sodium had little effect on glycine uptake. Nevertheless, chloride and thiocyanate appeared more efficient than glutarate. 5. The effect of transmembrane potential was studied by using valinomycin associated with a potassium gradient. The addition of this substance stimulated glycine transport by 43%. 6. The transport at different glycine concentrations showed two components: one non-saturable with weak affinity and the other saturable with strong affinity (Kt = 0.338 mM). 7. In conclusion, glycine transport by the brush border of D. pictus intestine presents a saturable component depending on sodium and on transmembrane electrical potential.

Electrophysiological investigation of the amino acid carrier selectivity in epithelial cells from Xenopus embryo

The electrical responses induced by external applications of neutral amino acids were used to determine whether different carriers are expressed in the membrane of embryonic epithelial cells of Xenopus laevis. Competition experiments were performed under voltage-clamp conditions at constant membrane potential. Gly, L-Ala, L-Pro, L-Ser, L-Asn and L-Gln generate electrical responses with similar apparent kinetic constants and compete for the same carrier.They are [Na]o and voltage-dependent, insensitive to variations in [Cl]o and [HCO3]o, inhibited by pHo changes, by amiloride and, for a large fraction of the current, by MeAIB. The increase in [K]o at constant and negative membrane potential reduces the response, whereas lowering [K]o augments it. L-Leu, L-Phe and L-Pro appear to compete for another carrier. They generate electrogenic responses insensitive to amiloride and MeAIB, as well as to alterations of membrane potential, [Na]o and [K]o. Lowering [Cl]o decreases their size, whereas increasing [HCO3]o at neutral pHo increases it. It is concluded that at least two and possibly three transport systems (A, ASC and L) are expressed in the membrane of the embryonic cells studied. An unexpected electrogenic character of the L system is revealed by the present study and seems to be indirectly linked to the transport function. L-Pro seems to be transported by system A or ASC in the presence of Na and by system L in the absence of Na. MeAIB induces an inward current.

The absorption of free and "peptide" amino acids in the small intestine of chicks

1. The accumulation of free L-tryptophan and glycine in the small intestine of chicks, as well as of amino acids released at the hydrolysis of glycyl-L-tryptophan, glycyl-L-leucine and glycyl-L-valine, was studied under experiments in vitro with accumulating preparations of chicks intestinal mucosa. 2. At the incubation of accumulating mucosal preparations (AMP) in a medium containing the investigated compounds in the presence of sodium ions (140 mM) or an equal amount of potassium ions, the existence of 2 ways of their transport was found: sodium-independent and sodium-dependent. The latter is decisive for the transport of free L-tryptophan. 3. In the case of peptide L-tryptophan the power of both mechanisms is about the same, but in case of free glycine the sodium-independent mechanism is dominant. 4. The transport of peptide glycine is realized only through the sodium-independent mechanism. 5. The existence of amino acid transport against the gradient is noted by replacing sodium by potassium ions in the incubation medium. Its absence was observed at the incubation of AMP under anaerobic conditions. 6. The study of kinetics of L-tryptophan accumulation showed that the replacement of sodium ions by potassium ones in the incubation medium is accompanied by a decrease of Kt from 2.23 to 0.84 mM as well as of V from 4.54 to 0.84 mM/min per cm. 7. Modifiers (L-valine, L-threonine, L-alpha-alanine) in concentrations of 100 mM inhibit drastically the accumulation of free and peptide L-tryptophan in the mucosa of chick small intestine, and this effect is mainly related to the action of sodium-dependent transport mechanisms.

Characteristics of L-glutamine transport in perfused rat skeletal muscle

1. We have investigated glutamine transport in the perfused rat hindlimb using the paired-tracer isotope dilution technique. 2. Uptake of L-glutamine was stereospecific, saturable, sodium dependent, insulin sensitive and pH insensitive in the physiological range. The maximum capacity of transport (Vmax) under normal perfusate conditions at 37 degrees C, 145 mM-Na+ and in the absence of insulin was 1156 +/- 193 nmol min-1 g-1 with transport being half-maximal at a perfusate glutamine concentration of 9.25 +/- 1.15 mM. 3. The kinetics of Na+ dependence strongly suggested co-transport of Na+ and glutamine with a stoichiometry of 1:1; furthermore, Na+ activated the carrier without any change in the concentration of glutamine at which transport was half-maximal, i.e. a 'Vmax effect' rather than a 'Km effect'. 4. The characteristics of glutamine transport, especially its substrate specificity and the pattern of competitive and non-competitive inhibition of glutamine transport by other amino acids, suggest that it is mediated by a carrier or carriers for which asparagine and histidine are also suitable substrates. 5. The characteristics of muscle glutamine transport are related but distinct from those of system N identified in hepatocytes; we suggest that they are sufficiently distinct to justify the identification of a new variant of mammalian amino acid transport systems which may be identified by the symbol Nm. 6. The kinetic characteristics of system Nm are such that glutamine is likely to be the most rapidly exchanging amino acid across the muscle membrane at physiological intra- and extracellular glutamine concentrations. Its hormone and ion sensitivities are likely to be important in the physiological modulation of whole-body glutamine metabolism and also during derangements observed in disease and after injury.

Acidic amino acid transport in animal cells and tissues

1. The occurrence and characterization of acidic amino acid transport in the plasma membrane of a variety of cells and tissues of a number of organisms is reviewed. 2. Several cell types, especially in brain, possess both high- and low-affinity transport systems for acidic amino acids. 3. High-affinity systems in brain may function to remove neurotransmitter amino acid from the extracellular environment. 4. Many cell systems for acidic amino acid transport are energized by an inwardly directed Na+ gradient. Moreover, certain cell types, such as rat brain neurons, human placental trophoblast and rabbit and rat kidney cortex epithelium, respond to an outwardly directed K+ gradient as an additional source of energization. This simultaneous action may account for the high accumulation ratios seen with acidic amino acids. 5. Rabbit kidney has been found to have a glutamate-H+ co-transport system which is subject to stimulation by protons in the medium. 6. Acidic amino acid transport in rat brain neurons occurs with a stoichiometric coupling of 1 mol of amino acid to 2 mol of Na+. For rabbit intestine, one Na+ is predicted to migrate for each mol of amino acid. 7. Uptake in rat kidney cortex and in high-K+ dog erythrocytes is electrogenic. However, uptake in rabbit and newt kidney and in rat and rabbit intestine is electroneutral. 8. Na+-independent acidic amino acid transport systems have been described in the mouse lymphocyte, the human fibroblast, the mouse Ehrlich cell and in rat hepatoma cells. 9. In a number of cell systems, D-acidic amino acids have substantial affinity for transport; D-glutamate, in a number of systems, however, appears to have little reactivity. 10. Acidic amino acid transport in some cell systems appears to occur via the "classical" routes (Christensen, Adv. Enzymol. Relat. Areas Mol. Biol. 49, 41-101, 1979). For example, uptake in the Ehrlich cell is partitioned between the Na+-dependent A system (which transports a wide spectrum of neutral amino acids), the Na+-dependent ASC system (which transports alanine, serine, threonine, homoserine, etc.), and the Na+-independent L system (which shows reactivity centering around neutral amino acids such as leucine and phenylalanine). Also, a minor component of uptake in mouse lymphocytes occurs by a route resembling the A system. 11. Human fibroblasts possess a Na+-independent adaptive transport system for cystine and glutamate that is enhanced in activity by cystine starvation.(ABSTRACT TRUNCATED AT 400 WORDS)

Electrical properties of a Na+-dependent phenylalanine transport in lizard (Lacerta galloti) duodenum

The unidirectional transepithelial fluxes of L-phenylalanine across lizard duodenum were determined in flux chambers. Phenylalanine was preferentially transferred from the mucosal to the serosal fluid. This transport was accompanied by an accumulation of substrate from the mucosal medium into the tissue to a similar level and against a concentration gradient. There was no net movement of phenylalanine when the sodium was substituted by choline. The influx of L-phenylalanine into the epithelial cells of lizard duodenum was examined by incubating slices of intestine in radioactively-labelled solutions of the substrate for 2 min. The steady-state uptake was assessed after similar incubations lasting 45 min. Phenylalanine influx obeys the Michaelis-Menten equation with a Km of 5.1 and is dependent on the presence of sodium ions in the incubation medium. Phenylalanine has been used to induce changes in short-circuit current (delta Isc) across intestine. delta Isc was a hyperbolic function of amino acid concentration characterized by the parameters Jm (maximum change in delta Isc) and Km (concentration needed to attain an delta Isc equal to half the Jm). delta Isc determined Km constants showed good agreement with values obtained from direct measurements of phenylalanine uptake into tissue.

Origin and voltage dependence of asparagine-induced depolarization in intestinal cells of Xenopus embryo

The kinetics and voltage dependence of asparagine (Asn)-induced depolarization in endoderm cells from Xenopus laevis embryos were analysed using current-clamp techniques. The depolarization is assumed to reflect the activation of an amino acid membrane carrier; it is accompanied by a slight increase in membrane resistance and cannot be explained by only the electrogenic character of the Asn carrier. It is proposed that the Asn depolarization arises, at least in part, from the decrease of the permeability ratio PK/PNa indirectly associated with the Na-coupled amino acid uptake. At room temperature (20-23 degrees C) the Asn response develops according to a single exponential function whose time constant is correlated with the final level of depolarization. Both amplitude and rise time of the depolarization are sensitive to variations of membrane potential and changes in Asn or Na external concentrations. Lowering the temperature decreases the amplitude of the Asn depolarization and increases its rise time with a Q10 factor of two; the kinetics remain of the Michaelis-Menten type, with a marked decrease in delta Emax and no change in Km. When the holding potential is altered by depolarizing and hyperpolarizing currents, the Asn response varies according to a bell-shaped characteristic presenting an optimum near the normal resting level. Membrane depolarizations induced by Na/K-pump inhibitors or high external K concentrations reduce the size of the Asn response; repolarizing the cell by current injection does not reverse the inhibitory effect of external K ions. Hyperpolarizing the membrane with a K-free Ringer solution increases the amplitude of the Asn response. In all these cases a decrease in delta Emax accounts for the apparent voltage sensitivity of the carrier mechanism. When induced by alterations of [K]o, an additional change in Km is observed, suggesting a K/Na-competitive inhibition of the Asn carrier. The results are discussed in terms of the amino acid carrier and passive membrane properties. It is suggested that the outward K-electrochemical gradient contributes an additional source of energy to the Na-dependent Asn uptake.

Role of solute drag in intestinal transport

This study presents experiments related to the role of solvent drag and solute drag in the transmembrane movement of nonelectrolytes in a perfused rat intestine preparation. Conditions were chosen to simulate the effects of luminal hyperosmolarity on the permeability of tracer solutes. Data are presented on net water flux, transepithelial potentials, and lumen-to-blood and blood-to-lumen tracer solute movements during control electrolyte perfusion and after making the perfusate hyperosmotic. The results indicate that both solvent drag and solute drag can play significant roles in the transepithelial movement of solute and solute permeabilities in the rat ileum preparation. It is suggested that the potential roles of solvent drag and solute drag should be accounted for or considered during the characterization of the mechanisms of biological membrane function.

In vitro inhibition of rat small intestinal absorption by lipophilic organic cations

Cationic, lipid-soluble organic compounds may interfere with cation-mediated membrane transport processes. Thus, small intestinal absorption may be influenced by lipophilic organic cations. Therefore a series of arylalkylamines was studied in the concentration range from 0.5 to 20 mmol/l for their effect on the transport of various monosaccharides and leucine in the rat small intestine in vitro by means of the tissue accumulation technique. Whereas the monophenyl substituted monoamines (e.g. benzylamine, 2-phenylethylamine, 3-phenylpropylamine) did not show a significant effect on the active transport, the corresponding omega,omega-diphenyl derivatives exhibited a strong inhibition of the active transport of the sugars and the amino acid. These monoamines and drugs of similar structure (e.g. benzoctamine, diphenydramine) exhibited a mixed or non-competitive type of inhibition which correlated quite well with their octanol-water partition coefficients. In contrast, di- or triamines (e.g. harmaline, imipramine, pyrilamine) revealed a rather pure competitive type of inhibition. These findings tentatively suggest a different mode of action on the active transport by lipid-soluble organic amines according to the molecular charge distribution. In addition, membrane vesicles were used to examine the effect of the different amines on the sucrase activity. Regarding the cation-dependent hydrolysis of sucrose, however, no distinct pattern developed.

Transport of neutral and cationic amino acids across the brush-border membrane of the rabbit ileum

The transport of sugars and amino acids across the brush-border membrane of the distal rabbit ileum has been studied. The kinetics of the transport of glucose demonstrated that the data obtained with the present technique are less distorted by unstirred layers than those obtained with the same technique adapted to the use of magnetic stirring. The role of depolarization of the electrical potential difference across the brush-border membrane in mutual inhibition between different classes of amino acids was estimated by measurements of the effects of high concentrations of alanine and lysine on the transport of galactose. It was found that this role would be insignificant in the present study. By measurements of the transport of alanine, leucine and lysine and the inhibitory interactions between these amino acids the function of three transport systems has been delineated. The transport of lysine is resolved in a high- and a low-affinity contribution. At 140 mM sodium these transport systems may also function as respectively high- and low-affinity contributors to the transport of neutral amino acids. At 0 mM sodium the high-affinity system remains a high-affinity system for cationic and neutral amino acids with reduced capacity especially for the neutral amino acids. At 0 mM sodium the low-affinity system's affinity for lysine is reduced and it is inaccessible to neutral amino acids. In addition to the two systems for lysine transport the existence of a lysine-resistant, sodium-dependent, high-affinity system for the transport of neutral amino acids has been confirmed. It seems unlikely that the distal ileum is equipped with a low-affinity, sodium-independent system for the transport of neutral amino acids.

Partial purification and reconstitution of the aspartate transport system from Halobacterium halobium

Membrane vesicles of Halobacterium halobium R1Wrm bind to an aspartic acid-agarose affinity column. After disruption of the bound vesicles by low ionic strength, a protein fraction is eluted from the column with 2.5% cholate in 3 M NaCl. When this fraction is reconstituted with soybean lipids to form proteoliposomes, the proteoliposomes exhibit active aspartate accumulation. Aspartate transport in the reconstituted system is driven by a chemical sodium gradient (out greater than in), exhibits sensitivity to an electrical potential, and is specific for L-aspartate. These characteristics are consistent with observations on aspartate transport in intact membrane vesicles of H. halobium. Initial aspartate transport rates in the reconstituted system are about ninefold enhanced over the native system. The system developed should be useful in future purification schemes and studies of the molecular details of membrane transport.

Kinetics of the sodium/beta-methyl-D-glucoside co-transport system in the guinea-pig small intestine

The kinetics of sodium-dependent beta-methyl-D-glucoside transport were examined in guinea-pig intestinal rings. Large-scale experiments were performed in which both sodium and monosaccharide concentrations were varied within the same animal. The results were evaluated by non-linear regression analysis, and an attempt was made to distinguish between the applicability of different models to describe the data set. Only two of the tested models provided a good fit to the data. These both involved the random formation of a ternary complex from either intermediate binary complex; in one, the constraint that only the ternary complex was able to cross the membrane was included, whereas in the other the ratio of the permeability coefficients for the binary complex between sugar and carrier and for the ternary complex was estimated. But this ratio did not differ significantly from zero, so the two acceptable models were equivalent. In addition, it was necessary to introduce into both these models the concept of a sodium reservoir at the surface of the brush-border membrane, such that the local sodium concentration in the vicinity of the carriers could never fall below a certain critical value (which was estimated at 4.8 mM), even in complete absence of this ion from the bulk medium.

Generalized kinetic analysis of ion-driven cotransport systems: a unified interpretation of selective ionic effects on Michaelis parameters

A major obstacle to the understanding of gradient-driven transport systems has been their apparently wide kinetic diversity, which has seemed to require a variety of ad hoc mechanisms. Ordinary kinetic analysis, however, has been hampered by one mathematically powerful but physically dubious assumption: that rate limitation occurs in transmembrane transit, so that ligand-binding reactions are at equilibrium. Simple models lacking that assumption turn out to be highly flexible and are able to describe most of the observed kinetic diversity in co- and counter-transport systems. Our "minimal" model of cotransport consists of a single transport loop linking six discrete states of a carrier-type molecule. The state transitions include one transmembrane charge-transport step, and one step each for binding of substrate and cosubstrate (driver ion) at each side of the membrane. The properties of this model are developed by sequential use of realistic experimental simplifications and generalized numerical computations, focussed to create known effects of substrate, driver ion, and membrane potential upon the apparent Michaelis parameters (Jmax, Km) of isotopic substrate influx. Specific behavior of the minimal model depends upon the arrangement of magnitudes of individual reaction constants among the whole set (12) in the loop. Well defined arrangements have been found which permit either increasing membrane potential or increasing external driver-ion selectively to reduce the substrate Km, elevate Jmax, jointly raise both Km and Jmax, or lower Km while raising Jmax. Other arrangements allow rising internal driver ion to act like either a competitive or a noncompetitive inhibitor of entry, or allow internal substrate to shut down ("transinhibit") influx despite large inward driving forces. These findings obviate most postulates of special mechanisms in cotransport: e.g., stoichiometry changes, ion wells, carrier-mediated leakage, and gating - at least as explanations for existing transport kinetic data. They also provide a simple interpretation of certain kinds of homeostatic regulation, and lead to speculation that the observed diversity in cotransport kinetics reflects control-related selection of reaction rate constants, rather than fundamental differences of mechanism.

Coupling ratio of electrogenic Na+-alanine cotransport in isolated rat hepatocytes

Na+-alanine cotransport across the cell membrane in isolated rat hepatocytes was studied. Changes in the cell membrane potential associated with the transport of alanine interfere with determination of the Na+-alanine coupling ratio of the cotransport. With valinomycin present to 'clamp' the cell membrane potential, a coupling ratio of 1:1 for the Na+-alanine influx was obtained.

A new class of inhibitors for in vitro small intestinal transport of sugars and amino acids in the rat

Polycationic compounds like polylysine, protamine or polyethylenimine may interfere with a cation-related membrane transport system depending on superficially accessible binding sites for particular cations. In vitro experiments were performed using either everted segments of rat small intestine to measure tissue accumulation or everted sacs to determine mucosal-to-serosal transport. The effect of polycations was also tested using brush-border membrane vesicles of rat jejunum. Polycations inhibited the tissue accumulation of methyl alpha-D-glucoside as well as binding of phlorizin. Inhibition of accumulation was increased by raising the polycation concentration and by preincubation of the tissue with the polycations. Kinetic experiments revealed a competitive type of inhibition for the uptake of neutral amino acids and actively transported sugars. Using everted sacs to compare the monomeric cations with their corresponding polymeric forms for their inhibitory effect, it was found that only polymers applied to the mucosal compartment impaired active transport. The passive diffusion of solutes, e.g. 2-deoxy-D-glucose or mannitol, was slightly increased by polycations. With some intermediate oligomers of lysine it could be shown that more than 20 cationic groups are required for approximate complete inhibition. That membrane-related events are responsible for the observed inhibition is suggested by the reduced uptake of D-glucose by brush-border membrane vesicles in the presence of polycations. Therefore an interaction with transport-related cation binding sites, i.e. anionic residues, at the mucosal surface may be assumed.

The characteristic changes of amino acid transport during development in brush border membrane vesicles of the guinea pig ileum

The characteristic changes of transport systems for amino acids during development were studied with brush border membrane vesicles from the guinea pig ileum. There was a difference of Km value for L-proline transport kinetics between the vesicles on the 10th day after birth and those on the 60th day after birth. Differences of fatty acid composition of phospholipids and unsaturated percentage of fatty acids were observed between these two vesicles. Fluorescence polarization of the vesicles on the 10th day after birth was larger than that of the vesicles on the 60th day after birth at each temperature examined. L-Proline transport at a substrate concentration below Km was larger in the vesicles on the 60th day after birth than in the vesicles on the 10th day after birth at each corresponding temperature. Differences of L-leucine transport characteristics were not observed between these two vesicles. These results suggest that the characteristic changes of L-proline and L-leucine transports during development are different and the characteristics of L-proline transport are regulated at least in part by membrane fluidity which is controlled by membrane lipid composition.

Vasopressin release in response to intracerebroventricular L-alanine and L-arginine, and its dependence upon CSF NaCl concentration

Influences on renal water, electrolyte, and arginine vasopressin (AVP) excretions of 1 h infusions (20 microliters/min) of a neutral (L-alanine) and two basic (L-lysine and L-arginine) amino acids into the lateral cerebral ventricle were studied in hydrated goats, and were compared to effects of control infusions of hypertonic (0.25 M) NaCl. L-alanine (0.11 M) dissolved in hypotonic NaCl caused more pronounced inhibition of the water diuresis and greater increase in AVP excretion than did the control infusions, but, in comparison to the latter, the responses developed very slowly. The effects were further delayed and were much attenuated when L-alanine was administered in isotonic glucose, but became considerably accentuated when isotonic NaCl was used as the solvent. L-lysine (0.09 M) in hypotonic NaCl did not inhibit the water diuresis or cause any apparent AVP release, whereas the corresponding L-arginine infusions caused inhibition of the water diuresis and increase in AVP excretion of approximately the same magnitudes and time courses as the control infusions. Like for L-alanine, these effects became accentuated when L-arginine was dissolved in isotonic NaCl, and became delayed and much attenuated when isotonic glucose was used as the solvent. L-arginine induced a more pronounced increase in renal Na excretion than did L-alanine and 0.25 M NaCl. Since transport together with Na (increasing the Na influx) generally is much more important for cellular uptake of neutral than of basic amino acids, the possibility is discussed that L-alanine here might have caused AVP release by increasing transmembrane Na transport of juxtacerebroventricular Na sensors regulating the AVP secretion--a suggestion supported by the lack of response to the basic L-lysine. The antidiuretic effect of the other basic amino acid, L-arginine, can not be explained along this line. However, with regard to the characteristic differences observed between the responses to L-alanine and L-arginine, the possibility is discussed that the latter might not have acted at a sensory level, but on the final neuronal link in the release of neurohypophyseal hormones, the hypothalamic neurosecretory cells. In contrast to L-alanine and L-arginine, L-lysine appeared to stimulate the appetite of the goats.

Relation between sodium-coupled amino acid and sugar transport and sodium/potassium pump activity in isolated intestinal epithelial cells

Cells isolated from the epithelium of the small intestine are used to study the relationship between amino acid or sugar-coupled sodium transport and potassium uptake through the sodium/potassium pump. Potassium influx is a saturable function of the external potassium concentration. Uptake in the presence of ouabain, a specific pump inhibitor, is greatly reduced. This remaining influx is linearly related to the concentration up to 6 mM potassium. Sugars and amino acids are actively accumulated by the intestinal cells. Their transport is accompanied by an initial extra influx of sodium. Although cells seem to regulate their internal sodium concentrations, this is not accompanied with a concomitant increase in potassium uptake through the pump. Thus L-alanine, 3-0-methyl-D-glucoside, and alpha-methyl-D-glucoside all fail to increase the rate of ouabain-sensitive potassium uptake. A very high coupling ratio of sodium efflux to potassium influx through the pump would be a likely explanation of the present results though they cannot be regarded as conclusive.

Single-site uptake of neutral amino acids into guinea-pig intestinal rings

1. A kinetic analysis of amino acid influx into guinea-pig small intestinal rings has been performed in an attempt to ascertain whether one or more transport sites for these substrates exists in the luminal membrane of the enterocyte. 2. No indirect correction for uptake into the extracellular space was applied, but it was assumed in the analysis that the total uptake included a diffusion term. This procedure was vindicated by the results obtained. 3. All analyses were performed by non-linear regression techniques. In many experiments, both substrate and inhibitor concentrations were varied within the same experiment, thus giving rise to three-dimensional diagrams describing transport processes. 4. All results pointed to the existence of a single transport agency shared by all amino acids tested. The kinetic constants, Km and Vmax, were independent of the concentration range used for their estimation; such behaviour would not be expected if several sites with different kinetic constants were available for transport. The value of KD, the constant describing the diffusive component of uptake, was the same when estimated from the uptake of an amino acid alone or when determined as the asymptote of the curve describing the inhibition of this uptake by an analogue. Finally, the Ki for an amino acid when used as an inhibitor was identical to its Km when used as substrate. This property was maintained even when the most disparate pair of amino acids, threonine and isoleucine, was examined.

Proton-sulfate co-transport: mechanism of H+ and sulfate addition to the chloride transporter of human red blood cells

Proton and sulfate inhibition of the obligatory chloride-chloride exchange of human erythrocytes was measured at 0 degrees C to determine their mechanism of reaction with the anion transporter. The proton and sulfate that are co-transported by this mechanism at higher temperatures behaved as nontransported inhibitors at 0 degrees C. We analyzed the data in terms of four molecular mechanisms: (1) HSO4- addition to the transporter; (2) ordered addition with the proton first; (3) ordered addition with the sulfate first; (4) random addition to the transporter. The Dixon plots of 1/MCl vs. [SO4] at different proton concentrations were not parallel. Thus protons and sulfate ions were not mutually exclusive inhibitors. The slope of these Dixon plots was independent of pH above 7.0, which indicates that sulfate could bind to the unprotonated carrier and excludes the first two mechanisms. Protons were inhibitors of chloride flux in the absence of sulfate, which indicates that protons could bind to the unloaded carrier and excludes mechanism 3. The KI for sulfate was 4.35 +/0 0.36 mM. The pK for the protonatable group was 5.03 +/- 0.02. The binding of either a proton or sulfate to the carrier decreased the KI of the other by ninefold. The only simple mechanism consistent with the data is a random-ordered mechanism with more transporters loaded with a sulfate than loaded with a proton at the pH and sulfate concentrations of plasma.

Amino acid influx across the mucosal border of the rat intestine in vivo

The lack of an in vivo method for measuring influx of amino acid into the mucosa has prevented a systematic comparison of characteristics of amino acid influx in vivo with prior in vitro studies. We developed and validated a technique for measuring amino acid influx in vivo. The mucosa is exposed briefly to labelled amino acid perfused luminally at a rapid rate and tissue uptake is measured. The brief exposure period insures that amino acid is confined to the segment. The rapid perfusion rate minimizes concentration of endogenous Na+ in the lumen and permits Na-dependency for alpha-aminoisobutyric acid influx to be demonstrated in vivo for the first time. We also demonstrated the inhibitory effect of K+ and competition by glycine on alpha-aminoisobutyric acid influx in vivo. The saturation kinetics for L-leucine in vivo and in vitro were compared under varying perfusion rates and with and without stirring with air. Under optimal conditions of agitation (rapid perfusion and bubbling with air), the apparent Michaelis constant (Kt) is decreased to be almost equal to that determined under comparable influx conditions in vitro. These studies demonstrate no major difference between characteristics of amino acid transport under more physiologic in vivo conditions as compared with prior in vitro studies.