Electrogenic Absorption of Sugars and Amino Acids in the Small Intestine of the Human Fetus

Some characteristics and the degree of intestinal absorption in the developing human fetus were examined by measuring solute evoked potentials and ¹⁴C-D-glucose uptake into the everted jejunal segments.

In all segments, the Michaelis-Menten relationship was observed between D-glucose concentrations and the evoked potentials or D-glucose uptake. Increase of Na-ion concentrations enhanced both D-glucose evoked potentials and uptake.

Both D-glucose and L-α-alanine evoked potentials increased in a significant correlation to the fetal age; however, the apparent Michaelis constants did not show any significant change. The structural specificity of sugar for generating evoked potentials was the same as that reported for adult mammals. Among amino acids, only the L-form of neutral and acidic amino acids generated markedly high evoked potentials, but the basic ones hardly at all. Oligopeptides such as glycyl-glycine and glycyl-glycyl-glycine also generated high evoked potentials.     

Stimulus-secretion coupling of arginine-induced insulin release. Functional response of islets to L-arginine and L-ornithine

L-Arginine and L-ornithine stimulate insulin release from pancreatic islets exposed to D-glucose. This coincides with an increased outflow of 86Rb and 45Ca from prelabelled islets and an increased net uptake of 45Ca by the islets. In the presence of D-glucose, L-lysine stimulates insulin secretion to the same extent as L-arginine or L-ornithine, but the hormonal release is not further enhanced by combinations of these cationic amino acids. L-Arginine or L-ornithine failed to enhance insulin release evoked by either L-leucine or 2-ketoisocaproate. The inhibitor of ornithine decarboxylase D,L-alpha-difluoromethyl ornithine failed to affect the metabolism and insulinotropic action of D-glucose in pancreatic islets, and only caused a partial inhibition of the secretory response to either L-arginine or L-ornithine. The latter amino acids inhibited modestly but significantly D-glucose utilization and oxidation by pancreatic islets. These and complementary findings suggest that the secretory response to L-arginine and L-ornithine is not attributable to any major change in the overall oxidative catabolism of nutrients, but involves mainly a biophysical component, such as the depolarization of the plasma membrane by these cationic amino acids.     

Stimulus-secretion coupling of arginine-induced insulin release: significance of changes in extracellular and intracellular pH.

The possible relevance of changes in extracellular and/or intracellular pH to the insulinotropic action of L-arginine and L-homoarginine was investigated in rat pancreatic islets. A rise in extracellular pH from 7.0 to 7.4 and 7.8 augmented the secretory response to these cationic amino acids whilst failing to affect the uptake of L-arginine by islet cells and whilst decreasing the release of insulin evoked by D-glucose. Under these conditions, a qualified dissociation was also observed between secretory data and 45Ca net uptake. Moreover, at high extracellular pH, the homoarginine-induced increase in 86Rb outflow from prelabelled islets rapidly faded out, despite sustained stimulation of insulin release. The cationic amino acids failed to affect the intracellular pH of islet cells, whether in the absence or presence of D-glucose and whether at normal or abnormal extracellular pH. These findings argue against the view that the secretory response to L-arginine would be related to either a change in cytosolic pH or the accumulation of this positively charged amino acid in the beta-cell. Nevertheless, they suggest that the yet unidentified target for L-arginine and its non-metabolized analogue in islet cells displays pH-dependency with optimal responsiveness at alkaline pH.     

Uterine uptake of amino acids and placental glutamine--glutamate balance in the pregnant ewe

The uterine uptake of amino acids was studied in 10 pregnant sheep with gestational ages of 114-146 days. After recovery from surgery, arterial and uterine venous samples were drawn simultaneously via indwelling catheters and analysed for amino acid and oxygen content. In seven ewes, amino acid concentrations were measured by a chromatographic technique. In four ewes, glutamate and glutamine arterio-venous differences across the uterine and umbilical circulations were measured by an enzymatic method. The uptake of neutral and basic amino acids was 66 mumol/mmol O2 and 17.3 mumol/mmol O2, respectively. Comparison of uterine and umbilical uptake shows that the bulk of the neutral and basic amino acids taken up by the pregnant uterus are transferred to the fetus. there was no significant uptake of acidic amino acids (i.e. glutamate, aspartate and taurine). glutamate was delivered from the fetus to the placenta but excretion of glutamate into the uterine circulation was negligible. Glutamine and asparagine were delivered to the fetus in amount which were two to three times larger than the placental uptake of glutamate and aspartate. Therefore placental conversion of exogenous glutamate and aspartate to glutamine and asparagine cannot account entirely for the fetal uptake of these amino acids.     

1-O-n-octyl-beta-D-glucopyranoside as a competitive inhibitor of Na+-dependent D-glucose cotransporter in the small intestine brush-border membrane

1-O-n-Octyl-beta-D-glucopyranoside is a competitive inhibitor of the Na+-dependent D-glucose uptake into rabbit, rat and human intestinal brush-border membrane vesicles. The lack of effect on the equilibrium uptake demonstrates that the detergent does not act by rupturing the vesicles; no membrane leakiness was apparent at the concentrations of octylglucopyranoside used, since D-glucose uptake is not inhibited even in the absence of the Na+ gradient (in K+ solution). There is a competitive interaction between octylglucopyranoside and D-glucose, as shown by Dixon and by Hunter and Down plots. The selectivity of the detergent effect is confirmed by its modest influence on amino acid uptake.     

Towards the mechanism of altered nutrients uptake in renal brush border membrane vesicles from pyelonephritic rats

Affinity constant (Km) of D-glucose, L-alanine, L-aspartate, L-lysine, L-proline and nutrients coupled Na+ were determined in renal brush border membrane vesicles prepared from control and pyelonephritic rats. The Km of D-glucose, amino acids and nutrients coupled Na+ was noted to be significantly increased (p less than 0.001) in experimental animals. The Vmax of D-glucose and amino acids was determined at different concentrations of nutrients keeping extravesicular Na+ constant or at different concentrations of extravesicular Na+ keeping nutrient concentration constant. In the experimental rats the Vmax decreased significantly (p less than 0.01) when compared to control. The increased Km and decreased Vmax may be one of the underlying mechanism leading to decrease in the uptake of D-glucose and amino acids.     

Root uptake of cationic amino acids by Arabidopsis depends on functional expression of amino acid permease 5

* Specific transporters mediate uptake of amino acids by plant roots. Earlier studies have indicated that the lysine histidine transporter 1 and amino acid permease 1 participate in this process, but although plant roots have been shown to absorb cationic amino acids with high affinity, neither of these transporters seems to mediate transport of L-arginine (L-Arg) or L-lysine (L-Lys). * Here, a collection of T-DNA knockout mutants were screened for alterations in Arabidopsis root uptake rates of L-Arg and it was found that only the AAP5 mutant displayed clear phenotypic divergence on high concentrations of L-Arg. A second screen using low concentrations of (15)N-labelled L-Arg in the growth media also identified AAP5 as being involved in L-Arg acquisition. * Momentaneous root uptake of basic amino acids was strongly affected in AAP5 mutant lines, but their uptake of other types of amino acids was only marginally affected. Comparisons of the root uptake characteristics of AAP5 and LHT1 mutants corroborated the hypothesis that the two transporters have distinct affinity spectra in planta. * Root uptake of all tested amino acids, except L-aspartic acid (L-Asp), was significantly affected in double AAP5*LHT1 mutants, suggesting that these two transporters account for a major proportion of roots' uptake of amino acids at low concentrations.     

Effect of natural or synthetic detergents on the transport of D-glucose in the membranes of vesicles of the brush border of the intestine of the rabbit

We describe here the effects of natural and synthetic detergents on the D-glucose transport into brush-border membranes of vesicles of rabbit's intestine. Two synthetic detergents: Triton X-100 and dodecyltrimethylammonium bromide have been found very strong inhibitors (more than 50 p. 100 of inhibition of maximal D-glucose uptake). Kinetic studies showed that these detergents behaved as mixed type inhibitors. The Na+-dependent transport of amino acids (aspartic acid, lysine, phenylalanine) is only poorly affected by dodecyltrimethylammonium bromide, while Triton X-100 inhibits unspecifically all the transport studied.     

Insulin restores glucose inhibition of adenosine transport by increasing the expression and activity of the equilibrative nucleoside transporter 2 in human umbilical vein endothelium

L-Arginine transport and nitric oxide (NO) synthesis (L-arginine/NO pathway) are stimulated by insulin, adenosine or elevated extracellular D-glucose in human umbilical vein endothelial cells (HUVEC). Adenosine uptake via the human equilibrative nucleoside transporters 1 (hENT1) and 2 (hENT2) has been proposed as a mechanism regulating adenosine plasma concentration, and therefore its vascular effects in human umbilical veins. Thus, altered expression and/or activity of hENT1 or hENT2 could lead to abnormal physiological plasma adenosine level. We have characterized insulin effect on adenosine transport in HUVEC cultured in normal (5 mM) or high (25 mM) D-glucose. Insulin (1 nM) increased overall adenosine transport associated with higher hENT2-, but lower hENT1-mediated transport in normal D-glucose. Insulin increased hENT2 protein abundance in normal or high D-glucose, but reduced hENT1 protein abundance in normal D-glucose. Insulin did not alter the reduced hENT1 protein abundance, but blocked the reduced hENT1 and hENT2 mRNA expression induced by high D-glucose. Insulin effect on hENT1 mRNA expression in normal D-glucose was blocked by N(G)-nitro-L-arginine methyl ester (L-NAME, NO synthase inhibitor) and mimicked by S-nitroso-N-acetyl-L,D-penicillamine (SNAP, NO donor). L-NAME did not block insulin effect on hENT2 expression. In conclusion, insulin stimulation of overall adenosine transport results from increased hENT2 expression and activity via a NO-independent mechanism. These findings could be important in hyperglycemia-associated pathological pregnancies, such as gestational diabetes, where plasma adenosine removal by the endothelium is reduced, a condition that could alter the blood flow from the placenta to the fetus affecting fetus growth and development.     

Na+ -D-glucose cotransporter in the kidney of Leucoraja erinacea: molecular identification and intrarenal distribution

Studies on membrane vesicles from the kidney of Leucoraja erinacea suggested the sole presence of a sodium-D-glucose cotransporter type 1 involved in renal D-glucose reabsorption. For molecular characterization of this transport system, an mRNA library was screened with primers directed against conserved regions of human sglt1. A cDNA was cloned whose nucleotide and derived amino acid sequence revealed high homology to sodium glucose cotransporter 1 (SGLT1). Xenopus laevis oocytes injected with the respective cRNA showed sodium-dependent high-affinity uptake of D-glucose. Many positions considered functionally essential for sodium glucose cotransporter 1 (SGLT1) are also found in the skate protein. High conservation preferentially in transmembrane helices and small linking loops suggests early appearance and continued preservation of these regions. Larger loops, especially loop 13, which is associated with phlorizin binding, were more variable, as is the interaction with the specific inhibitor in various species. To study the intrarenal distribution of the transporter, a skate SGLT1-specific antibody was generated. In cryosections of skate kidney, various nephron segments could be differentiated by lectin staining. Immunoreaction with the antibody was observed in the proximal tubule segments PIa and PIIa, the early distal tubule, and the collecting tubule. Thus Leucoraja, in contrast to the mammalian kidney, employs only SGLT1 to reabsorb d-glucose in the early, as well as in the late segments of the proximal tubule and probably also in the late distal tubule (LDT). Thereby, it differs also partly from the kidney of the close relative Squalus acanthias, which uses SGLT2 in more distal proximal tubule segments but shows also expression in the later nephron parts.     

Transintegumental uptake of metabolic substrates in male and female Schistosoma mansoni.

A new method for measuring transintegumental uptake in living schistosomes in vitro has been applied to the study of individual males and females. Uptake of a 14-C labeled test metabolite was compared to that of tritiated water (a highly diffusible reference substance). Use of the short half-life (T 1/2 = 100 min) isotope 113m-Indium, bound to EDTA (ethylene diamine tetra-acetic acid, a nondiffusible reference substance) permitted quantification of the relative amount of 14-C test substance passively adhering to the schistosoma surface. Substraction of this amount provided an estimate of net uptake. D-glucose uptake, as measured by this method, increased with time, approaching equilibrium by two min; a positive correlation between temperature and glucose uptake was also observed. Nondialyzable components in rat, human, horse and fetal calf sera did not enhance glucose uptake. In both male and female schistosomes, minimal uptakes were seen for the nonmetabolizable sugar alcohol mannitol (MW = 182). L-glucose uptake was similarly low, but high uptakes were observed in both sexes for D-glucose. In addition to confirming the stereospecificity of hexose uptake, these studies suggested our technique provides a sensitive method for measurement of both high and low uptake compounds. The uptakes of D-glucose and the L-amino acids--arginine, ornithine, lysine, histidine, phenylalanine and serine--were comparatively higher in female than male schistosomes. Slight elevations in uptake by females were observed for threonine, valine and glycine, but aspartate uptake was slightly higher in males. No dramatic male-female differences were immediately apparent for the uptakes of proline, leucine, isoleucine, tyrosine and glutamate. Schistosomal uptake of L-amino acids that are essential for vertebrates was generally higher than uptake of the nonessential amino acids.     

Chloroquine,a novel inhibitor of amino acid transport by rat renal brush border membrane vesicles

Summary Chloroquine is an antimalarial and antirheumatic lysosomotropic drug which inhibits taurine uptake into and increases efflux from cultured human lymphoblastoid cells. It inhibits taurine uptake by rat lung slices and affects the uptake and release of cystine from cystinotic fibroblasts. Speculations on its mode of action include a proton gradient effect, a non-specific alteration in membrane integrity, and membrane stabilization. In this study, the effect of chloroquine on the uptake of several amino acids by rat renal brush border membrane vesicles (BBMV) was examined. Chloroquine significantly inhibited the secondary active, NaCl-dependent component of 10µM taurine uptake at all concentrations tested, but did not change equilibrium values. Analysis of these data indicated that the inhibition was non-competitive. Taurine uptake was reduced at all osmolarities tested, but inhibition was greatest at the lowest osmolarity. Taurine efflux was not affected by chloroquine, nor was the NaCl-independent diffusional component of taurine transport. Chloroquine (1 mM) inhibited uptake of the imino acids L-proline and glycine, and the dibasic amino acid L-lysine. It inhibited the uptake of D-glucose, but not the neutral-amino acids L-alanine or L-methionine. Uptake of the dicarboxylic amino acids, L-glutamic acid and L-aspartic acid, was slightly enhanced. With regard to amino acid uptake by BBMV, these findings may support some of the currently proposed mechanisms of the action of chloroquine but further studies are indicated to determine why it affects the initial rate of active amino acid transport.     

Insulin-induced alterations in amino acid metabolism in the fetal lamb

To investigate the role of insulin in modulation of fetal amino acid metabolism, insulin infusions were performed in 10 chronically-catheterized fetal lambs. Fetal insulin infusion caused a dose related fall in the arterial blood concentrations of 13 of 15 amino acids studied as well as a 15-25% decrease in total amino acid concentration. Fetal lambs exhibited a biphasic response of umbilical total amino acid uptake when compared to fetal blood insulin concentration, i.e., at achieved fetal insulin concentrations less than 100 microU/ml, umbilical uptake of 9 specific amino acids as well as summed amino acid uptake from the umbilical circulation were depressed, but at insulin concentrations of 100-350 microU/ml, amino acid uptakes were similar to or above control values. Insulin infusion also caused a drastic diminution in the rate of fetal urea excretion. These findings suggest that insulin acts in the fetus to depress amino acid catabolism, thus altering amino acid extraction and uptake. Depressed protein catabolism with or without enhanced amino acid uptake would have the theoretical effect of stimulation of net protein synthesis with a shift toward use of nonprotein substrates for energy purposes.     

Sonolysis of concentrated aqueous solutions of nonvolatile solutes: spin-trapping evidence for free radicals formed by pyrolysis

The sonolysis of argon-saturated aqueous solutions of sodium acetate, sodium propionate, amino acids, and sugars was investigated by ESR and spin trapping over a large range of concentrations. The spin trap 3,5-dibromo-2,6-dideutero-4-nitrosobenzene sulfonate was used to examine the possibility of detecting new radicals specifically generated in the high temperature zones surrounding the collapsing cavitation bubbles. At lower concentrations of these solutes, no evidence for specific new radicals formed in the high temperature regions induced by cavitation could be found, and only radicals formed by hydroxyl radical and hydrogen atom abstraction reactions could be detected. These were identified by comparison with the radicals produced by uv photolysis in the presence of hydrogen peroxide. However, at higher concentrations, new radicals (typically methyl radicals) formed in the high temperature interfacial regions induced by cavitation were found for sodium acetate, sodium propionate, amino acids, and sugars (D-mannose, D-glucose, 2-deoxy-D-ribose). These results indicate that pyrolysis radicals can be detected when the nonvolatile solutes are present at high concentrations in the interfacial regions of the cavitation bubbles.     

Uptake of l-[14C]-alanine by glutaraldehyde-treated and untreated spores of Bacillus subtilis

The effect of glutaraldehyde on the uptake of L-alanine, and subsequent germination, in spores of Bacillus subtilis NCTC 8236 was examined. Germination was induced by single amino acids, D-glucose and phosphate buffer at 37 degrees C. L-alanine was the best germinant of all amino acids tested. Pretreatment of spores with low concentrations of acid and alkaline glutaraldehyde inhibited subsequent germination, complete inhibition being observed at concentrations of 0.1% (w/v). This concentration also prevented the loss of heat resistance of spores placed in germination medium and exposed to 75 degrees C. Radioactive studies indicated that maximum uptake of L-alanine occurred after ca 30 min at 37 degrees C. Only 1.2% of available L-alanine was taken up during germination. Pretreatment of spores with glutaraldehyde did not interfere with L-alanine uptake at aldehyde concentrations up to 0.5% (w/v). However, this was significantly reduced at a glutaraldehyde concentration of 1.0% (w/v). Minimal differences were observed between acid and alkaline forms of the aldehyde. The results are discussed in terms of the mode of action of glutaraldehyde.     

L-glutamine and palmitate catabolism in pancreatic islets from rats depleted in long-chain polyunsaturated omega 3 fatty acids

The catabolism of D-glucose was recently found to be impaired in pancreatic islets from rats depleted in long-chain polyunsaturated omega3 fatty acids. The specificity of this alteration was now investigated by characterizing the oxidative fate of endogenous nutrients in islets preincubated with either L-[U-14C]glutamine or [U-14C]palmitate and then incubated variously in the absence of D-glucose, presence of the hexose or presence of metabolic poisons. Relative to their radioactive content after preincubation, the production of 14CO2 by islets prelabelled with [U-14C]glutamine was higher in omega3-depleted rats than control animals. The enhancing action of D-glucose upon such production was impaired, however, in the omega3-depleted rats. The net uptake of 14C-palmitate and absolute value for 14CO2 output were both increased in omega3-depleted rats, whilst the ratio between 14CO2 output and islet radioactive content was decreased in the same animals. The inhibition of 14CO2 production by metabolic poisons was comparable in all cases. These results are consistent with recent findings on such items as the availability of endogenous amino acids and uptake of unesterified fatty acids in extrapancreatic sites of the omega3-depleted rats. They also support the view that the alteration of D-glucose metabolism in the islets of the latter animals may be attributable, in part at least, to alteration of glucokinase kinetics by high intracellular acyl-CoA levels.     

Uptake of intact amino acids by plants depends on soil amino acid concentrations

Studies in different ecosystems have shown that plants take up intact amino acids directly but little is known about the influence of free amino acid concentrations in the soil on this process. We investigated the effect of three different soil amino acid N concentrations (0.025, 0.13 and 2.5 μg N g^?1 soil) on direct uptake of four dual labelled (¹⁵N, ¹³C) amino acids (glycine, tyrosine, lysine, valine) in a greenhouse experiment using Anthoxantum odoratum as a model plant.Our results revealed that 8–45% of applied ¹⁵N was incorporated into plant root and shoot tissue 48 h after labelling. Additional ¹³C enrichment showed that 2–70% of this incorporated ¹⁵N was taken up as intact amino acid. Total ¹⁵N uptake and ¹⁵N uptake as intact amino acids were significantly affected by soil amino acid N concentrations and significantly differed between the four amino acids tested.We found a positive effect of soil amino acid concentrations on uptake of mineralized ¹⁵N relative to amino acid concentrations for all amino acids which was presumably due to higher diffusion rates of mineralized tracer to the root surface. However, intact amino acid uptake relative to amino acid concentrations as well as the proportion of total ¹⁵N taken up directly decreased with increasing soil amino acid N concentrations for all amino acids, irrespective of their microbial degradability. This effect is most likely controlled by the mineral N concentration in soil and perhaps in plants which inhibits direct amino acids uptake.Overall, we conclude that plant internal regulation of amino acid uptake controlled by mineral N is the main mechanism determining direct uptake of amino acids and thus a lower contribution of intact amino acid uptake to the plants N nutrition has to be expected for higher amino acid concentrations accompanied by mineralization in soil.     

Primary structure, genomic organization and heterologous expression of a glucose transporter from Arabidopsis thaliana.

Both genomic and full length cDNA clones of an Arabidopsis thaliana sugar carrier, STP1, have been obtained using a cDNA clone of the H+/hexose cotransporter from the green alga Chlorella kessleri as hybridization probe. The peptide predicted from these sequences in 522 amino acids long and has a molecular weight of 57,518 kd. This higher plant sugar carrier contains 12 putative transmembrane segments and is highly homologous to the H+/hexose cotransporter from Chlorella, with an overall identity in the amino acid sequence of 47.1%. It is also homologous to the human HepG2 glucose transporter (28.4%), and other sugar carriers from man, rat, yeast and Escherichia coli. The definite proof for the function of the STP1 protein as a hexose transporter and data on its substrate specificity were obtained by heterologous expression in the fission yeast Schizosaccharomyces pombe. Transformed yeast cells transport D-glucose with a 100-fold lower KM value than control cells. Moreover only the transformed cells were able to accumulate the non-metabolizable D-glucose analogue 3-O-methyl-D-glucose, indicating that the Arabidopsis carrier catalyses an energy dependent, active uptake of hexoses. Expression of STP1 mRNA is low in heterotrophic tissues like roots or flowers. High levels of expression are found in leaves.     

Development of rat embryos in culture media containing different concentrations of normal and diabetic rat serum.

In vitro culture of rodent embryos has been extensively used in the search for teratologic agents, with possible relevance to diabetic pregnancy. However, the high concentrations of rat serum added to the culture medium (approximately 75%) have raised concern that the teratogenic effects of some compounds may be attenuated or masked in this culture system and thereby forced the addition of pharmacological concentrations of the compounds (e.g., D-glucose and beta-hydroxybutyrate) to the medium. This issue has been examined in the present study where the effects of different concentrations of rat serum on growth and differentiation of rat embryos were recorded in cultures supplemented with increased concentrations of D-glucose and beta-hydroxybutyrate. The embryonic development was also evaluated after culture in medium supplied with serum from diabetic rats. Compared with normal rat serum, the diabetic serum had an elevated glucose concentration as well as markedly increased levels of triglycerides and branched amino acids, indicating a potentially rich supply of major nutrients for the cultured embryos. Lowering the serum concentration in the culture medium from 80% to 50% yielded progressively retarded embryonic growth but no increased rate of other morphological malformations. At 40% serum concentration, however, there was a sharp rise in the incidence of somatic malformations, in addition to the prevailing growth retardation. When the embryonic growth and development were compared at 50% and 80% serum concentrations, increased D-glucose or beta-hydroxybutyrate concentrations caused similar degrees of embryonic dysmorphogenesis. Also, the uptake of each compound by the embryos exposed to elevated levels of the two agents were similar in 50% and 80% serum cultures. There was, therefore, no protection against the teratogenic and growth-retarding effects of increased D-glucose or beta-hydroxybutyrate offered by high serum concentrations in the culture medium (i.e., 80% vs. 50%). Embryos cultured in 50% or 80% diabetic rat serum at 30 mmol/L or 50 mmol/L D-glucose concentration showed similar rates of somatic malformations as did embryos exposed to the same proportion of normal rat serum at similar glucose concentrations. By contrast, the diabetic rat serum amplified the general retarding effects of high D-glucose levels, yielding lower protein levels and somite numbers in embryos from diabetic serum culture than in embryos cultured in normal rat serum.(ABSTRACT TRUNCATED AT 400 WORDS)     

The placenta releases branched-chain keto acids into the umbilical and uterine circulations in the pregnant sheep

There was net uptake of branched-chain keto acids by the fetus from the umbilical circulation. Mean fetal uptake of the 3 keto acids 2-keto isovalerate, 2-keto isocaproate and 2-keto methylvalerate was 1.8 mumol/min per kg of fetus. The concentrations in the umbilical vein for these keto acids were 10.9 +/- 3.8 microM (mean +/- SD: 2-keto isovalerate), 19.7 +/- 6.1 microM (2-keto isocaproate) and 14.8 +/- 5.3 microM (2-keto methylvalerate) respectively. The coefficients of extraction for the same keto acids were 17.2%, 16.8% and 11.9% respectively. Fetal uptakes (both mumol/min and mumol/min per kg fetus) were positively correlated with umbilical supply. There were concentration gradients across the placenta, with fetal concentration: maternal concentration ratios of 3.3 +/- 1.5 for 2-keto isovalerate, 2.1 +/- 0.8 for 2-keto isocaproate and 1.3 +/- 0.6 for 2-keto methylvalerate. The net release of 2-keto acids into the umbilical circulation may conserve the carbon skeleton of branched-chain amino acids for fetal metabolism and growth. In the uterine circulation there was not a consistent pattern of release from or uptake by the uteroplacental tissues. It is suggested that branched-chain keto acids may contribute to fetal growth or energy metabolism.