Hypothesis: Control of hepatic utilization of alanine by membrane transport or by cellular metabolism?

The rate of alanine transport into the liver limits its utilization even under the high alanine load resulting from a 90% casein diet, given that the rat has been adapted to that diet. A coordinated acceleration of alanine catabolism allows transport to remain ratelimiting. which in turn allows the adaptive regulation of transport to remain effective at high alanine loads. Accelerated degradation of alanine may change the hepatic amino acid content in a way that derepresses the activity of the alanine carrier system(s).Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/44189/1/10540_2005_Article_BF01140659.pd

Adaptive regulation of amino acid transport in cultured avian fibroblasts. Influence of the amino acid composition of the culture media

The regulation of amino acid transport across the cell membrane by adaptive mechanisms has been studied in cultured chick embryo fibroblasts. Time-dependent changes of transport activity by the A system (a Na+-dependent agency with affinity for a discrete group of neutral amino acids), as a function of the composition in amino acids of the culture medium, have been evaluated by measurements of 14C-labelled l-proline uptake under conditions approaching initial entry rates. Reults and conclusions based on the adopted experimental procedures include the following: 1.(1) Transport of l-proline in cultured avian fibroblasts is an inverse function of the concentration of amino acid substrates of system A in the medium used for cell culturing before uptake assay. 2.(2) Cells grown in media containing amino acids that are substrates for system A (repressive conditions) exhibit a marked increase of l-proline uptake upon incubation in media devoid of these amino acids (derepressive conditions). 3.(3) Cells grown in media containing amino acids which are not typical substrates of system A (derepresive conditions) undergo a definite decrease of l-proline uptake upon incubation in media supplemented with amino acid substrates of this agency (repressive conditions). 4.(4) The adaptive increases in the transport of l-proline observed when ‘repressed’ cells are incubated under depressive conditions, are abolished in the presence of cycloheximide. 5.(5) The results presented suggest that adaptive regulation of amino acid transport by system A is an intrinsic property of the cells and has a normal function in vivo. Its occurrence under culture conditions demands that one must consider the actual composition of amino acids in the growth medium when investigating amino acid transport in cultured cells

The transport of neutral amino acid s in cultured human fibroblasts

The transport of neutral amino acids and their interactions for uptake have been studied in skin-derived-diploid human fibroblasts cultured at confluency. Properly timed preincubations in a medium defined in composition and requirements have been adopted to control size of internal amino acid pools and regulatory interferences affecting amino acid transport. L-Proline, L-alanine, and L-leucine were used as natural amino acid substrates. Amino acid uptake and exchange with preaccumulated molecules have been measured under conditions approaching initial entry rates in the absence and in the presence of Na+ and of transport-specific model substrates. Cultured human fibroblasts were found to contain three neutral amino acid transport systems: A, ASC, and L. L-Proline was taken up preferentially by System A. System ASC appeared to mediate the largest fraction of L-Leucine entered the cells mainly by the Na+-independent System L. Preaccumulation of the cells with Site A-reactive amino acids caused trans-inhibition of the activity of System A. The activity of System L was strongly trans-stimulated in cells preloaded with Site L-reactive amino acids. The inward transport of L-alanine was trans-inhibited by internal Site A-reactive amino acids and trans-stimulated by preaccumulated L-alanine exchanging with the external amino acid through the operation of System ASC

Energization of amino acid uptake by system A in cultured human fibroblasts.

The energization of System A in cultured human fibroblasts has been studied by measuring the energy transfer from the electrochemical gradient of Na+ to the chemical gradient of the site A-specific substrate amino acid 2-methylaminoisobutyric acid. The co-transport Na+/amino acid, studied by kinetic analysis and radiochemical measurements, showed a coupling ratio of 1:1. The assessment of the Na+ electrochemical gradient in cultured adherent cells relied on the development of noninvasive procedures as follows: the membrane electrical potential was estimated from the accumulation of L-arginine at equilibrium (Bussolati, O., Laris, P. C., Nucci, F. A., Dall'Asta, V., Longo, N., Guidotti, G. G., and Gazzola, G. C. (1987) Am. J. Physiol. 253, C391-C397); the chemical gradient of Na+ was determined from spectrometric measurements of Na+. The accumulation of 2-methylaminoisobutyric acid was strongly sensitive to changes of Na+ gradient and of membrane electrical potential, indicating that the electrochemical gradient of Na+ contributed energy for the uphill transport of the amino acid through System A. Changes in the Na+ electrochemical gradient were obtained by: (i) alterations of extracellular concentration of Na+; (ii) changes of membrane electrical potential obtained by variation of extracellular [K+]; and (iii) changes of [Na+]in and membrane electrical potential upon incubation of the cells in serum-free saline solutions (Dall'Asta, V., Gazzola, G. C., Longo, N., Bussolati, O., Franchi-Gazzola, R., and Guidotti, G. G. (1986) Biochim. Biophys. Acta 860, 1-8). The correlation between the chemical gradient of 2-methylaminoisobutyric acid and the Na+ electrochemical potential followed a straight line with a yield close to the thermodynamic equilibrium, thus suggesting that the energy stored in the gradient of Na+ electrochemical potential is fully adequate to energize the intracellular accumulation of site A-reactive amino acids in human fibroblasts

Using land inequality to inform restoration strategies for the Brazilian dry forest

Forest and landscape restoration aims not only to restore ecosystems but to improve people's livelihoods. However, mapping of restoration often neglects key socioeconomic aspects such as land-access inequalities. In this article, we quantify and describe the distribution of vegetation deficits (areas that demand mandatory restoration) across small and large rural properties across 1,204 municipalities of the Brazilian Caatinga biome. We mapped 313,537 ha of vegetation deficit, i.e. areas previously cleared that will need to be restored for legal compliance. This vegetation deficit is almost equally shared between 141,144 smallholders and only 2,986 in large properties. Around half of this vegetation deficit is spatially clumped mainly in the East and along the main river basin of the region dominated by rich large-holders. On the other hand, spatially scattered and poor small landowners hold around another half of the vegetation deficit across several municipalities. Because of such an unequal distribution of native vegetation deficit, we propose three different restoration strategies to account for land-access inequalities and socioeconomic differences at the municipality level. This is one of the first studies to clearly address land inequality and socioeconomic profile of landowners to inform adapted restoration strategies. Restoration planning that ignores land concentration and socioeconomic contexts may reproduce inequalities known to be one of the main causes of ecosystem degradation across Global South countries

The transport of alanine, serine, and cysteine in cultured human fibroblasts.

The transport of L-alanine, L-serine, and L-cysteine has been studied in skin-derived diploid human fibroblasts in culture. Competition analysis, mathematical discrimination by nonlinear regression, and conditions varying the relative contribution of the various mediations have been used to characterize the systems engaged in the inward transport of these amino acids. All the adopted criteria yielded results showing that L-alanine, L-serine, and L-cysteine enter the cell by two Na+-dependent systems, System A and System ASC, and by a Na+-independent route, whose major component has been identified as System L. The apparent affinity of L-alanine, L-serine, and L-cysteine for the putative carrier was higher for System ASC than for System A. The transport Vmax for System A increased in response to cell starvation; after 12 h, its values were similar or higher than those exhibited by System ASC. At amino acid concentrations approaching those present in human plasma, System ASC appeared to be the primary mediation for the inward transport of L-alanine, L-serine, and L-cysteine in human fibroblasts. The contribution of System A was negligible in nonstarved cells and became appreciable under conditions of cell starvation. The Na+-independent System L made no substantial contribution to the uptake of L-alanine and L-serine and accounted for approximately one-fourth of the total uptake of L-cysteine

The transport of L-glutamine into cultured human fibroblasts.

The transport of L-glutamine has been studied in diploid human fibroblasts in culture. Mathematical discrimination by nonlinear regression, competition analysis, and conditions varying the relative contribution of the various mediations have been used to characterize the systems engaged in the inward transport of this amino acid. The adopted criteria showed that L-glutamine enters the fibroblast by the Na(+)-dependent systems ASC and A and by a Na(+)-independent route identified as system L. The relative contribution of these agencies to the total saturable uptake of glutamine varied with the concentration of the amino acid and with the nutritional state of the cell. At amino acid concentrations approaching those encountered in human plasma: (1) system ASC represented the primary mediation for entry of L-glutamine in human fibroblasts; (2) the contribution of system A was lower, though significant, in unstarved repressed cells and became predominant in starved derepressed cells; (3) the Na(+)-dependent system L accounted for less than one-fifth of glutamine uptake in either nutritional condition. The changes in the relative contribution of the various systems to the uptake of glutamine as a function of its concentration may have implications in pathophysiology under conditions associated with enhanced glutamine concentrations in the extracellular fluids

Insulin regulation of amino acid transport in mesenchymal cells from avian and mammalian tissues.

Insulin regulation of amino acid transport across the cell membrane was studied in a variety of mesenchymal cell directly isolated from avian and mammalian tissues or collected from confluent cultures. Transport activity of the principal systems of mediation in the presence and absence of insulin was evaluated by measuring the uptake of representative amino acids under conditions approaching initial entry rates. Insulin enhanced the transport rate of substrate amino acids from the A system(alpha-aminoisobutyric acid, L-proline, glycine, L-alanine and L-serine) in fibroblasts and osteoblasts from chick-embryo tissues, in mesenchymal cells (fibroblasts and smooth muscle cells) from immature rat uterus, in thymic lymphocytes from young rats and in chick-embryo fibroblasts from confluent secondary cultures. In these tissues, the uptake of amino acid substrates of transport systems L and Ly+ (L-leucine, L-phenylalanine, L-lysine) was not affected by the presence of the hormone. No insulin control of amino acid transport was detected in chick-embryo chondroblasts and rat peritoneal macrophages. These observations identify the occurrence of hormonal regulatory patterns of amino acid transport for different mesenchymal cells types and indicate that these properties emerge early during cell differentiation