Role of Na+ in the Transport of Amino Acids in Rabbit Red Cells

Changes in alanine and glutamine transport during rat red blood cell maturation

Alanine and glutamine transport have been studied during red blood cell maturation in the rat. Kinetic parameters of Na(+)-dependent L-alanine transport were: Km 0.43 and 1.88 mM and Vmax 158 and 45 nmoles/ml ICW/min for reticulocytes and erythrocytes, respectively. During red cell maturation in the rat there is a loss of capacity and affinity of the system ASC for L-alanine transport. The values for Na(+)-dependent L-glutamine transport in reticulocytes were Km 0.51 mM and Vmax 157 nmoles/ml ICW/min. On the other hand, a total loss of L-glutamine transport mediated by both N and ASC systems is demonstrated in mature red cells. This seems to indicate that during rat red cell maturation the system N disappears. Furthermore, the system ASC specificity in mature cells changes, and glutamine enters the red cell by non-mediated diffusion processes.

Transport of neutral amino acids by human erythrocytes

The transport of several neutral amino acids by human erythrocytes in vitro was studied. The measurements made included steady-state distributions, kinetics of initial rates of uptake, effects of monovalent cations and anions, general mutual inhibitory interactions, kinetics of inhibitions, effluxes, ability to produce accelerative exchange diffusion, and the inhibitory action of the thiol reagent N-ethylmaleimide. The results are interpreted as showing that the human erythrocyte membrane possesses several distinct transport systems for these amino acids, including one Na+-dependent system and one dependent on both Na+ and a suitable anion, that are qualitatively similar to those systems previously described in pigeon erythrocytes and mammalian reticulocytes. Quantitatively, however, the systems differ among the different kinds of red cell and a major difference lies in their abilities to produce accelerative exchange diffusion.

The complete rate equation, including the explicit dependence on Na+ ions, for the influx of alpha-aminoisobutyric acid into mouse brain slices

The rate equation, including dependence on Na+-ion concentration for the influx of alpha-aminoisobutyric acid into mouse brain slices incubated in isotonic glucose medium at 37 degrees C, is v = 0.402 S/(1.02(1 + 788/[Na+]2)+S)+0.0477S, where v = influx in mu mol/min, g final wet wt of slices; [Na+] = concentbutyric acid in medium, in mM. This equation shows two kinetically independent, parallel pathways of concentrative uptake: one, saturable and dependent on Na+; the other, unsaturable and independent of Na+. Influx is independent of ionic strength, Cl- ion per se, and a moderate increase in tonicity. The binding of substrate to the saturable carrier depends on the Na+ concentration; the maximum capacity of this carrier does not. For transport, 2 Na+ ions must interact with each saturable transport site. This does not imply coupling between the flux of Na+ and the flux of alpha-aminoisobutyric acid.

Effects of essential fatty acid deficiency on various functions of the rat erythrocyte membrane

Various membrane transport functions have been studied in erythrocytes from essential fatty acid (EFA) deficient rats in order to determine whether or not functional abnormalities induced by documented EFA-deficiency of the membrane could be demonstrated. No differences were found between EFA deficient and control cells with respect to mean values for osmotic resistance or intracellular sodium and potassium concentrations. However, uptake of leucine by EFA deficient erythrocytes was significantly greater than that of control erythrocytes. Kinetic studies suggest that EFA deficiency enhances the passive diffusion component of this transport.

The basic asymmetry of Na+-dependent glycine transport in Ehrlich cells

Influx and efflux of glycine have been examined as a function of external and internal Na+ concentrations, respectively, when : formula: (see text) = O. With : formula: (see text) = O it was found that at comparable external and cellular Na+ levels, the Km for efflux was larger by an order of magnitude than the value for influx and the V for efflux was several times greater than the V for influx. For both fluxes the major effect of Na+ was to decrease the Km value. The observations are consistent with the conclusion that the Na+-dependent transport system is asymmetric per se. Influx and efflux of glycine were increased in a near linear manner by increasing the Na+ concentration from 13 to 100 mM, the half-time for glycine equilibration being a funcion of the Na+ concentration in absence of an electrochemical potential difference for Na+. In Na+-free media ([Na+] less than 5 mM) equilibration of glycine between cells and medium was not achieved after 60 min at 25 degrees C. With : formula: (see text) = O, efflux (or uptake) of glycine was not affected by internal (or external) K+ between 20 and 120 mM suggesting that K+ plays no direct role in Na+-dependent transport of glycine in Ehrlich cells.

Changes in amino acid transport during red cell maturation

We studied amino acid transport in sheep red blood cells (RBCs) as a function of cell maturation. Transport of amino acids is decreased strikingly in the mature mammalian RBC compared to the immature reticulocyte. Blood obtained 5-6 days after massive bleeding was fractionated on dextran gradients. In the mature erythrocyte amino acids are taken up only slowly, and in the normal experimental interval (60 min) the concentration in the cell does not reach that of the medium. In contrast, the reticulocyte-rich (top) fraction (50-90% reticulocyte) accumulates certain amino acids, particularly histidine, methionine, and leucine. The underlying process is ATP-independent and Na+-insensitive, and has properties consistent with exchange diffusion, i.e., accelerated uptake or efflux when unlabeled solute is present on the trans side. The process is apparent not only in intact cells but also in resealed ghosts. The decrease in activity of amino acid transport is a function of red cell maturation. Thus it can be shown that (a) separation of cells according to their density 1, 2, and 3 weeks after bleeding leads to progressively lower amino acid transport activity with increasing cell density; and (b) during in vitro long-term incubation at 37 degrees C of reticulocyte-rich, unfractionated blood (5-10% reticulocytes), amino acid transport decreases while red cell integrity is maintained, as evidenced by the retention of a normal K+ gradient and the absence of hemolysis. The progressive loss is seen with resealed ghosts as well as with intact cells. Not all the amino acids examined participate in this exchange process. The most actively exchanged are histidine, leucine, methionine, and phenylalanine. Glycine, proline, arginine, and a-amino isobutyric acid do not participate in the exchange process.

Maturation of membrane function: transport of amino acid by rat erythroid cells

The membrane changes which occur during cellular maturation of erythroid cells have been investigated. The transport of alpha-aminoisobutyric acid, alanine, and N-methylated-alpha-aminoisobutyric acid have been studied in the erythroblastic leukemic cell, the reticulocyte, and the erythrocyte of the Long-Evans rat. The dependence of amino acid transport on extracellular sodium concentration was investigated. Erythrocytes were found to transport these amino acids only by Na-independent systems. The steady state distribution ratio was less than 1. Reticulocytes were found to transport alpha-aminoisobutyric acid and alanine by Na-dependent systems, but only small amounts of N-methylated-alpha-aminoisobutyric acid. Small amounts of these amino acids were transported by Na-independent systems. The steady state distribution ratio was greater than one for Na-dependent transport. The erythroblastic leukemia cell, a model immature erythroid cell, showed marked Na-dependence (greater than 90%) for alpha-aminoisobutyric acid and alanine transport, and greater than 80% for the Na-dependent transport of N-methyl-alpha-aminoisobutyric acid. The steady state distribution ratio for the Na-dependent transport was greater than 4. In the erythroblastic leukemic cell, at least three Na-dependent systems are present: one includes alanine and alpha-aminoisobutyric acid, but excludes N-methyl-alpha-aminoisobutyric acid; one is for alpha-aminoisobutyric acid, alanine and also N-methyl-alpha-aminoisobutyric acid; and one is for N-methyl-alpha-aminoisobutyric acid alone. In the reticulocyte, the number of Na-dependent systems are reduced to two: one for alpha-aminoisobutyric acid and alanine; one for N-methyl-alpha-aminoisobutyric acid. In the erythrocytes, no Na-dependent transport was found. Therefore, maturation of the blast cell to the mature erythrocyte is characterized by a systematic loss in the specificity and number of transport system for amino acids.

Reversed transport of amino acids in Ehrlich cells

Gramicidin induces a marked Na+-dependent efflux of amino acids from Ehrlich cells. In absence of Na+, gramicidin does not alter the efflux. In presence gramicidin, glycine efflux is inhibited by methionine and less so by leucine. Glycine efflux caused by HgCl2 is neither Na+ dependent nor inhibitable by amino acids. Neither efflux of inositol which is transported by an Na+-dependent route, nor efflux of several other solutes which are transported by Na+-independent routes, is affected by gramicidin. The antibiotic appears to permit a reversal in the direction of of the operation of the Na+-dependent amino acid transport system. The increased efflux is partly, but not entirely, due to an increase in the cellular Na+ concentration and a reduction of the electrochemical potential difference for Na+.

Membrane transport during development in animals

This brief and necessarily incomplete survey of available evidence on the development of transport systems in animal cells reveals a primitive state of knowledge full of interesting possibilities for future development. The assembly of membrane-bound transport systems during embryonic development provides unique opportunities for approaching questions relating to gene expression, the synthesis and insertion of membrane proteins into phospholipid layers, the composition and structure of transport systems and the conditions required for their functioning. It seems plausible to assume that the growth and differentiation of animal cells is regulated, in part at least, by the rate of transport of metabolites and ions across the cell membranes. Therefore the sequence of the expression of transport systems is likely to have a profound effect on subsequent stages of growth and differentiation. Feedback regulation of the synthesis of transport proteins by changes in the intracellular or extracellular concentrations of the transported metabolites or ions [52, 53, 85-87] may be a key element in the regulation of the rate of transport processes during development.