Transport of sugars and amino acids across guinea pig small intestine.

Using guinea pig small intestine for measurements of influx across the brush border membrane (Jmc), unidirectional transmural fluxes (Jms,Jsm), and steady-state epithelial uptake [A]c, the characteristics of the mutual inhibition were examined. The mutual inhibition of Jmc was in the range of 9--16%, and appeared to be sodium dependent, Jms and [A]c were inhibited by at least 40%, and Jsm was unaffected. All the results can be explained in terms of therheogenic version of the sodium gradient hypothesis. Whereas only the mutual inhibition of Jmc can also be explained in terms of allosteric interactions between the binding sites for sugars and amino acids.

Lysine transport in the guinea-pig small intestine.

Interactions between cationic and neutral amino acids in transport across the brush-border membrane, Jmc, of the small intestine have been examined using preparations from the distal rabbit ileum and the rat and guinea-pig mid-small intestine. (1) In the guinea pig, the dependence of Jmc Lys on the concentration of lysine is best described in terms of two saturable transport mechanism in addition to free diffusion. (2) It is shown that the discrepancy between cis-effects of low concentrations of neutral amino acids on the Jmc of cationic amino acids, cis-stimulation in the guinea pig contra cis-inhibition in the rabbit and rat, represents species differences. In the guinea pig, imposing sodium-free conditions turns cis-stimulation into cis-inhibition. (3) It is demonstrated that in rat and guinea pig, leucine is transported both by the transport system(s) for cationic amino acids and by transport system(s) which cannot be inhibited by cationic amino acids.

Imino acid transport across the brush-border membrane of the guinea-pig small intestine.

The transport of imino and non-alpha-amino acids across the brush-border membrane of the guinea-pig small intestine has been examined. It was found that the guinea pig is without a transport system for non-alpha-amino acids. The transport of imino acids was characterized using methylaminoisobutyrate (MeAIB) as a substrate. This choice was validated by lack of kinetic evidence that more than one transport system was involved in the transport of MeAIB, by the identical values of the estimates of the passive permeability of MeAIB, the magnitude of its proline-resistant transport, and the permeability of mannitol. The transport system for MeAIB is moderately stereospecific. It does not accept cationic amino acids. It accepts alpha-amino-monocarboxylic acids but N-methylation increases the affinities of these amino acids by an order of magnitude. The length of the side-chain of the aliphatic imino acids seems of little importance for the affinity for the transport system, but the data on inhibition of the transport of MeAIB by proline and piperidine-2-carboxylic acid indicate that it is sharply increased by ring formation.

Transport of glycine and lysine on the chloride-dependent beta-alanine (B0,+) carrier in rabbit small intestine.

Transport of glycine, lysine and beta-alanine in rabbit, guinea pig and rat small intestine has been examined by measurements of the unidirectional influx across the brush border membrane of the intact epithelium. In rabbit distal ileum the chloride-dependent fraction of glycine transport, and all sodium- and chloride-dependent lysine transport is carried on the beta-alanine carrier. Lysine eliminates all saturable, sodium-independent transport of glycine. The saturable, sodium-dependent, and lysine resistant influx of glycine is characterized by a K1/2Gly of 60 mM. Glycine transport in the mid intestine of the guinea pig is chloride-independent and in the rat only a minute fraction may be chloride-dependent. These species do not possess an equivalent of the rabbit beta-alanine carrier. In conclusion, glycine transport in rabbit distal ileum is by the sodium-dependent carrier of neutral amino acids, by the sodium-independent lysine carrier, and by the sodium- and chloride-dependent beta-alanine carrier which closely resembles the B0,+ carrier described in mouse blastocysts. All sodium dependent lysine transport in rabbit distal ileum is by the chloride- and sodium-dependent beta-alanine carrier. It is proposed that the beta-alanine carrier in rabbit distal ileum be renamed the B0,+ carrier.

Characteristics of rat jejunal transport of tryptophan.

The parameters of rat jejunal transport of tryptophan have been examined. The interactions between tryptophan and lysine or methionine have been reexamined, and some aspects of the trans effects of cellularly accumulates amino acids have been studied. It has been demonstrated that: (1) The influx of tryptophan across the jejunal brush border (Jmc-Trp) can be accounted for by the carrier of alpha-aminomonocarboxylic acids alone. (2) Tryptophan competes with lysine for the carrier of basic amino acids across the brush border membrane without itself being transported by this carrier. (3) Lysine has neither cis nor trans effects on Jmc-Trp, whereas intracellular tryptophan is highly inhibitory to Jmd-Lys. (4) The intracellular concentration of lysine and of tryptophan, [Lys]c and [Trp]c, are unaffected by tryptophan and lysine, respectively, although the transmural fluxes, from the mucosal side to the serosal side, Jms, of lysine, Jms-Lys, and of tryptophan, Jms-Trp, are inhibited by tryptophan and lysine, respectively. The latter effects thus represent inhibitory interactions at the basolateral membrane. (5) Methionine is a potent cis and transinhibitor of Jmc-Trp, but stimulated Jms-Trp and reduces [Trp]c. (6) Methionine causes trans acceleration of the influx of lysine across the brush border membrane, Jmc-Lys, but has no effect on the influx of galactose, Jmc-Gal. (7) Leucine causes trans inhibition of Jmc-Leu. (8) Tryptophan does not cause cis inhibition of Jmc-Gal, but is a strongtransinhibitor of Jmc-Gal. (9) Cellularly accumulated tryptophan appears to accelerate the eventual decline in transepithelial potential difference and short-circuit current. These results are consistent with the conclusions that: (1) Tryptophan is transported across the brush border membrane by the carrier of neutral amino acids alone, but leaves the cell across the basolateral membrane by a mechanism used by lysine also. (2) Leucine, methionine and probably tryptophan have a transeffect on the transport of neutral amino acids across the brush border membrane which may represent a phenomenon which can appropriately be termed decelerating exchange diffusion. (3) Cellularly accumulated tryptophan has a strong and indiscriminate depressive effect on all transport functions of rat jejunal epithelium.

Chloride-dependence of amino acid transport in rabbit ileum.

Chloride-dependence of influx across the brush-border membrane of distal rabbit ileum was examined for beta-alanine, 2-methylaminoisobutyric acid (MeAIB), leucine, lysine, proline and D-glucose. Influx of leucine at 2 mM and of D-glucose at 0.5 mM was chloride-independent indicating that substitution of isethionate for chloride has no unspecific effect on sodium gradient driven transport processes. In contrast influx of beta-alanine and MeAIB was totally dependent on the presence of chloride ions. In the absence of chloride, proline transport was reduced to 20% of its control level. This remaining transport can be accounted for by the function of the carrier of alpha-amino-monocarboxylic acids. Transport of leucine at 0.1 mM was reduced by absence of chloride. This is in accordance with the observation of leucine transport by the beta-alanine carrier. The kinetics of chloride and sodium activation of transport of MeAIB were examined at 1 mM MeAIB. Chloride activation was characterized by a Hill coefficient of 1 and a K1/2 of 23.5 mM, and sodium activation by a Hill coefficient of 2 and a K1/2 of 51 mM. Thus cotransport of chloride with an imino acid would be compatible with the known rheogenic nature of this transport. This study adds the imino acid carrier and the beta-alanine carrier to the group of chloride-dependent, epithelial amino acid transport systems.

Transport of the alpha-amino-mono-carboxylic acid L-alanine by the beta-alanine carrier of the rabbit ileum.

The proposal that the beta-alanine carrier of the rabbit ileum is a high affinity carrier of the neutral amino acids was examined by means of measurements of influx across the brush border membrane of the intact epithelium using L-alanine as a representative of the neutral amino acids. Confirming the proposal, evidence was provided for mutual competitive inhibition between beta-alanine and L-alanine; and it was also demonstrated that a process contributes to the influx of L-alanine, which is characterized by a maximum rate of transport equal to that of beta-alanine and a Kt, which is equal to the Ki of L-alanine against the influx of beta-alanine. In the concentration range 0.01 to 0.125 mM the influx of L-alanine was found to be linearly related to the concentration indicating a significant unstirred layer influence on present and previous estimates of the Kt values for influx of amino acids across the brush-border membrane of intact intestinal epithelia.

The rabbit jejunal 'imino carrier' and the ileal 'imino acid carrier' describe the same epithelial function.

For rabbit jejunal brush border vesicles an 'imino carrier' has been defined as a sodium-dependent, alanine-resistant transporter of cyclic imino acids, while for intact rabbit jejunal and ileal epithelia an 'imino acid carrier' has been defined as a sodium-dependent transporter of both aliphatic and cyclic imino acids. This study on intact rabbit intestine examines whether these two terms describe the same epithelial function. The KPro1/2 and the KProi against JMeAIBmc are identical and so are KMeAIB1/2 and KMeAIBi against JPromc. Likewise, KLeui is the same against the transport of both proline and MeAIB. It is, therefore, concluded that the terms 'imino carrier' and 'imino acid carrier' describe the same epithelial function: A sodium-dependent, relatively high afinity, saturable transporter of both aliphatic and cyclic imino acids. Estimates of the apparent affinity and inhibitory constants for MeAIB, proline and leucine confirm that the jejuno-ileal variation of amino acid transport along the rabbit small intestine is a variation of maximal transport capacity.

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.

Baclofen (beta-p-chlorophenyl-gamma-aminobutyric acid) transport across rat jejunum.

Intestinal transport of baclofen (beta-(p-chlorophenyl)-gamma-aminobutyric acid) in the rat has been examined in vitro. Influx of baclofen across the brush-border membrane (JmcBacl) and steady-state accumulation by everted segments of the intestine were measured. JmcBacl could be accounted for as the sum of a saturable process with a maximum rate of approx. 10-20 nmol cm-2 h-1 and a K1/2Bacl of approx. 0.3 mM and a diffusive contribution with a permeability of 0.073 cm h-1. JmcBacl was Na(+)- and Cl(-)-independent. The steady state distribution ratio between the intracellular space of the everted segments and incubation fluid was 1.0 +/- 0.1 (n = 12). Inhibition tests demonstrated that the Na(+)- and Cl(-)-independent, passive, but saturable fraction of baclofen transport can not be mediated by any of the known amino acid carriers of the rat small intestine. Preliminary results suggests that qualitatively baclofen transport in guinea-pig and rabbit is also by facilitated transport.

Lysine transport across isolated rabbit ileum.

Lysine transport by in vitro distal rabbit ileum has been investigated by determining (a) transmural fluxes across short-circuited segments of the tissue; (b) accumulation by mucosal strips; and (c) influx from the mucosal solution across the brush border into the epithelium. Net transmural flux of lysine is considerably smaller than that of alanine. However, lysine influx across the brush border and lysine accumulation by mucosal strips are quantitatively comparable to alanine influx and accumulation. Evidence is presented that the "low transport capacity" of rabbit ileum for lysine is due to: (a) a carrier-mediated process responsible for efflux of lysine out of the cell across the serosal and/or lateral membranes that is characterized by a low maximal velocity; and (b) a high "backflux" of lysine out of the cell across the mucosal membrane. A possible explanation for the latter observation is discussed with reference to the relatively low Na dependence of lysine transport across the intestinal brush border.