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F V Sepúlveda

Publications and source records attributed to F V Sepúlveda.

At least 91 records · Page 5Linked to original sources
PubMed1980

Stoichiometry versus coupling ratio in the cotransport of Na and different neutral amino acids.

The stoichiometry of Na coupling to amino acid movement across the brush border membrane of the rabbit distal ileum has been determined under initial rate conditions. The coupling ratio, defined as the amino acid-dependent Na influx/the Na-dependent amino acid influx, was equal to unity for alanine, measured over a 10-fold range of Na and alanine concentrations. Coupling ratio values determined under a single set of conditions for a number of amino acids varied from 1 for serine to 4.6 for methionine. Reducing the methionine concentration from 12.5 to 1.5 mM caused the coupling ratio value to fall from 4.6 to 1.2. These results are explained by assuming a fixed stoichiometry of 1 : 1 under all conditions, with initial binding of the amino acids (A) to the Na-dependent carrier (E) but with some amino acids being able to cross on the Na-dependent carrier in the absence of Na. The variation in coupling ratio values can be used to calculate KA, the apparent dissociation constant of amino acid from the Na-dependent carrier in the absence of Na, and the ratio kappa 1/kappa 2, where kappa 1 and kappa 2 are first-order rate constants for translocation of the complexes EA and EANa, respectively. This method of processing results has been defined as delta analysis. The value of KA for methionine is 3.6 +/- 1.1 mM and the kappa 1/kappa 2 ratio is 1.01 +/- 0.07. The constant coupling ratio value for 1 for alanine indicates that the value for KA is extremely high or that the kappa 1 value is extremely low.

Alanine↗
PubMed1980

Animal variation in alanine uptake by rabbit ileal mucosa.

Alanine uptake by rabbit ileal mucosa has been measured in the presence and absence of Na+ to establish the characteristics of biological variation. Common Km values, calculated for two systems of alanine entry, were used for estimation of two Jmax values for individual rabbits. The distribution of Jmax1 and Jmax2 within the population was normal and there was minimal interaction between these two parameters, judged by a cross-correlation analysis. Ways of processing data to account for this type of animal variation are discussed.

Alanine↗
PubMed1980

A sodium-indpendent low affinity transport system for neutral amino acids in rabbit ileal mucosa.

1. The kinetic parameters for serine, alanine and methionine uptake by rabbit ileal mucosa have been determined in the absence of Na. 2. Uptake of all three amino acids took place through a single mediated system. The apparent Km values of serine, alanine and methionine for this system were equal to their respective apparent K1 values (approximately 89, 75 and 23 mM respectively). 3. Autoradiography was used to measure the cellular location of alanine uptake by rabbit ileum. Approximately 80% of the total uptake took place in the upper third of each villus. This uptake was reduced by 75% either by removal of Na or addition of serine. The proportional distribution of Na-dependent and Na-independent alanine uptakes along the villus was found to be equal. 4. The kinetic properties of the low affinity uptake mechanism for neutral amino acids, seen in the absence of Na, were virtually identical with those of one of the uptake mechanisms seen previously in the presence of Na. 5. The low affinity uptake mechanism appears to be Na-independent. It is suggested that the Na-coupled uptake of amino acid takes place through the high affinity system.

Alanine↗
PubMed1979

A comparison of amino acid transport and ouabain binding in brain endothelium and salivary epithelium studied in vivo by rapid paired-tracer dilution.

Amino acid tmransport at the luminal side of brain capillary endothelium and the basal side of salivary epithelium were compared using a nondestructive, first-circulation, paired-tracer dilution method. In the brain, the reference molecule was an intravascular marker (Crone's method) whereas inthe salivary gland the reference was an extracellular marker of similar size to the test molecule. The unidirectional flux was related to a maximal uptake, U: U = 1 --test concentration/reference concentration Uptake and cross-inhibition experiments in brain suggest the presence of only a long-chain neutral L-amino acid transport system (leucine, phenyl-alanine, tryptophan, tyrosine, valine, methionine). In contrast, results in the salivary gland suggest 4 transport systems: 1. large neutral, 2. small neutral (alanine, serine), 3. basic (lysine) and 4. acidic (aspartic, glutamic). The same method was applied to localize binding sites. Ouabain bound very significantly to the salivary epithelium but not at all to the brain endothelium. The method described is of interest since it can be extended to any organ and possibly to man.

Amino Acids↗
PubMed1979

Different mechanisms for neutral amino acid uptake by new-born pig colon.

1. Mucosal amino acid uptake by pig proximal colon, measured independently for fourteen different amino acids each used at a concentration of 1 mM, ranged from 0.6 to 8.6 n-mole. cm(-2). min(-1) in the new-born to 0 to 0.3 n-mole. cm(-2). min(-1) in the 2-day-old animal. Long chain amino acids entered the mucosa of new-born pig proximal colon much more readily than did short chain amino acids.2. Glycine was used extensively to inhibit the uptake of other neutral amino acids. The degree of maximal inhibition produced depended on the amino acid used. The relative inability of glycine to inhibit the uptake of long chain amino acids suggested that these compounds could cross the brush border on a carrier inaccessible to glycine. The glycine-sensitive uptake remained more or less constant for all amino acids tested (1-2 n-mole.cm(-2).min(-1)); the glycine-insensitive uptake varied from 0 to 7 n-mole.cm(-2).min(-1) (glycine and methionine respectively).3. It is suggested that at least two mechanisms exist for the entry of neutral amino acids into pig proximal colon, one showing specificity for hydrophobic amino acids and the other having broad specificity. The mechanism responsible for the uptake of long chain essential amino acids predominates over the less specific mechanism.4. These results are discussed in relation to previous work carried out on the rabbit ileum where two similar systems for neutral amino acid entry have been shown to be present. Both tissues transport hydrophobic amino acids on their own specific carrier at approximately the same rate; the ability of the pig colon to transport amino acids on the broad specificity carrier is eight times less than in the rabbit ileum. The possibility is raised that this system is subject to regulation.

Amino Acids↗
PubMed1979

Two-carrier influx of neutral amino acids into rabbit ileal mucosa.

1. The influx of serine, alanine and methionine across the brush border membrane of the rabbit ileal mucosa has been measured during short periods of incubation. 2. A kinetic analysis of the uptake data, assuming one mediated entry mechanism or one mediated entry mechanism plus a diffusion component to be present, does not provide an adequate explanation for the results obtained. Methionine inhibition of serine uptake provided direct evidence that the diffusive entry of serine into the rabbit ileum was small or non-existent. 3. Data taken from amino acid inhibition and substrate-uptake experiments, fitted simultaneously to a double hyperbolic model of amino acid uptake, give good agreement between predicted and experimental results. There is also good quantitative agreement between computer-derived kinetic constants in the present work and similar constants obtained previously using a different method of analysis. 4. Present work supports the general hypothesis that neutral amino acids use two mediated pathways to enter the rabbit ileal mucosa. The possible physiological significance of these results and their probable effect on currently held concepts of how amino acids cross the brush border membrane of the rabbit intestinal mucosa is discussed.

Alanine↗
PubMed1978

Discrimination between different entry mechanisms for neutral amino acids in rabbit ileal mucosa.

1. Influx of neutral and basic amino acids into the mucosal epithelium of the rabbit ileum was measured in the presence of high and low concentrations of Na. Uptake measured at an amino acid concentration of 1 mM varied from 10 to 16 nmole . cm-2 . min-1. Removal of Na inhibited the uptake of short-chain amino acids more than long-chain amino acids. 2. Inhibition of uptake of glycine, alanine, leucine and phenylalanine by a selection of non-radioactive amino acids was found to follow a particular pattern. Long-chain amino acids inhibited the uptake of all test amino acids; short-chain amino acids inhibited preferentially the uptake of glycine. 3. The maximum inhibition serine could cause to the uptake of other amino acids was found to vary. Serine inhibited completely the uptake of glycine but a portion of uptake of long-chain amino acids was found to persist, even in the presence of high concentrations of serine. This is taken as evidence for the presence of an amino acid uptake mechanism having no affinity for serine. 4. The apparent affinities of neutral amino acids for the serino-inhibitable system (system 1) varied from about 0.5 mM (for long-chain amino acids) to about 3 mM (for short-chain amino acids). The total uptake of individual amino acids by system 1 was essentially similar when compared at an amino acid concentration of 1 mM. 5. The serine-resistant uptake of neutral amino acids (system 2) constituted up to two fifths of total uptake for long-chain amino acids, measured at amino acid concentrations of 1mM. The affinities of long-chain amino acids for system 2 is thought to be less than for system 1. Serine appears not to interact with system 2. 6. A second component to serine uptake was found to be related linearly to the concentration of serine in the medium. A similar component may contribute to the total uptake of phenylalanine. The possibility that such a component could arise as a space marker artifact is discussed. 7. An independent kinetic analysis of phenylalanine uptake by rabbit ileal mucosa showed that it could not be accounted for on the basis of a single entry system. However uptake could be described kinetically, assuming two systems of mediated entry to be present. The possible presence of a third non-saturable component to uptake does not affect these conclusions. 8. It is concluded that least two systems exist for the mediated entry of neutral amino acids into rabbit ileal mucosa. This fact should be taken into account in any future mechanistic interpretation of carrier-mediated amino acid transport in the small intestine.

Amino Acids↗
PubMed1978

Kinetics of the co-transport of phenylalanine and sodium ions in the guinea-pig small intestine. 1. Phenylalanine fluxes.

The kinetics of phenylalanine influx into guinea-pig small intestinal rings at different external sodium-ion concentrations have been examined. A change in the sodium concentration alters the value of Kt for phenylalanine influx without affecting the Jmax for the process. By studying the relationship between the value of Kt and the sodium-ion concentration, the applicability of various models to the overall process of sodium-coupled phenylalanine influx has been considered. The results are most consistent with a general non-compulsory model for the formation of a ternary complex between carrier, phenylalanine molecule and a sodium ion, which can be formed from either binary complex, i.e., either species can combine first to the carrier. The consistency of the model was tested by determining phenylalanine influx as a function of the sodium-ion concentraion at different amino-acid concentrations, and a good fit was obtained with the predictions of the non-compulsory model.

Animals↗
PubMed1978

Kinetics of the co-transport of phenylalanine and sodium ions in the guinea-pig small intestine. II. Sodium fluxes and flux ratios.

The validity of a general non-compulsory model for the description of the co-transport of sodium and phenylalanine in the small intestinal mucosa of the guinea-pig has been examined by measuring the influx of sodium and the flux ratios. The simultaneous influxes of sodium and phenylalanine have been determined by incubating intestinal tissues at a fixed sodium and variable phenylalanine concentrations. The experiment was repeated at a number of sodium concentrations, and a straight-line relationship between the fluxes was always obtained. The slope of this line, the flux ratio, is dependent on the sodium concentration; the function is biphasic inasmuch as it rises with sodium concentration at low external sodium, but decreases when the sodium concentration is raised above a level of about 70 mM. This response is compatible with the model under examination. No saturable component of sodium influx could be detected in the absence of phenylalanine, but the values of this influx corresponded with those predicted from the flux ratio experiment. In the presence of phenylalanine, sodium influx could be resolved into saturable and non-saturable components, and the Kt for sodium influx via the saturable mechanism agreed with that predicted from the constants derived from measurements of phenylalanine influx. The maximal velocity for sodium influx was similar to the maximal velocity for phenylalaline influx. Equations were derived to examine the behaviour of sodium influx as a function of the external phenylalanine concentration, and a Kt for phenylalanine was deduced which agrees closely with that obtained by studying phenylalanine fluxes directly. These results provide evidence in favour the applicability, in this species, of a model involving no compulsory pathway for the formation of a ternary complex between the carrier, a sodium ion and the phenylalanine molecule. Examination of the literature suggests that in other species different models may describe more accurately this co-transport mechanism.

Animals↗
PubMed1977

The reversibility of the inhibition of intestinal amino-acid transport by harmaline.

The inhibitory aciton of harmaline on L-phenylalanine uptake by guinea-pig intestinal rings is fully reversible provided only low concentrations of the inhibitor are used; if the concentration is raised to a sufficient extent to enable the drug to interfere with sodium pumping or cellular metabolic reactions, as witnessed by its effect on tissue oxygen consumption, then the inhibition of L-phenylalanine uptake is only reversible if short contact times are employed.

Alkaloids↗
PubMed1977

Functional and structural characteristics of the jejunum and ileum in the dog and the rat.

The dog jejunum is a much denser tissue than the ileum, with a greater weight per unit length and higher proportion of mucosal tissue. Morphometric analysis reveals longer and wider villi, deeper crypts and larger enterocytes in the jejunal mucosa. Uptake of phenylalanine or beta-methyl-glucoside by tissue slices in vitro is slightly greater in jejunal than in ileal tissue. The levels of acid phosphatase and alkaline phosphatase in the individual enterocytes are significantly greater in the jejunum, according to quantitative histochemical analysis. The absorption of water, sodium, potassium, chloride and glucose in vivo is significantly smaller in the jejunal than in ileal loops, particularly when expressed in terms of unit mucosal weight. Sodium and water absorptions are stimulated by glucose at both sites, but the stimulation is significantly greater in the ileum. Opposite results have been obtained in rats where the transport of phenylalannie in vitro is greater in the ileum, but water, electrolyte and glucose absorption in vivo is greater in the jejunum.

Acid Phosphatase↗
PubMed1977

Enhanced non-electrolyte uptake in vitro by the dog jejunal mucosa in the cholera-toxin-induced secretory phase.

A characteristic secretory response in the dog jejunumis observed following three hours' perfusion with cholera toxin. When this mucosa in the secretory phase is examined in vitro, the steady-state accumulation of phenylalanine and beta-methyl-glucoside is considerably enhanced. Since the initial rates of influx of these substrates are unchanged and the tissue ion contents are the same as normal, it is concluded that the increase in steady-state uptake may be attributed to a change in the rate of efflux of the substrates across the baso-lateral membrane of the cell. This change might be related to the reported collapse of the intercellular channels when the intestine is in a secretory state.

Animals↗
PubMed1976

Effect of (+)-catechin on renal and intestinal transport.

The ability of dog renal cortex slices to accumulate beta-methyl-glucoside or glycine is enhanced by the flavonoid (+)-catechin at a concentration of 3.5 mM. This stimulatory effect is apparently due to a decreased rate of efflux of either substrate. On the other hand, the uptake of p-amino-hippuric acid and N1-methyl-nicotinamide is inhibited by (+)-catechin. The drug at the same concentration is without action on amino-acid transport by guinea-pig intestine in vitro.

Aminohippuric Acids↗