Transport of benzenoid amino acids by system T and four broad scope systems in preimplantation mouse conceptuses

Consumption of a Branched-Chain Amino Acid (BCAA) during Days 2-10 of Pregnancy Causes Abnormal Fetal and Placental Growth: Implications for BCAA Supplementation in Humans

A relatively large branched-chain amino acid (BCAA) supplement, consumed for more than 10 days, appears to be especially effective at alleviating muscle damage and soreness during intense human training. However, perturbations in amino acid and protein consumption could have unwanted transgenerational effects on male and female reproduction. This paper hypothesizes that isoleucine consumption by female mice from days 2 to 10 of pregnancy will alter fetal and placental growth later in gestation. Mice that had received 118 mM isoleucine in their drinking water delivered pups on day 19 of pregnancy that were 9% larger than normal, whereas the reverse was true for pups born on day 20. Moreover, the inverse correlation between birth weight and litter size was lost in mice that previously consumed excess isoleucine. Similarly, the normal correlations between fetal and placental weights were lost by day 18 of pregnancy in mice that had consumed excess isoleucine. Mice that consumed excess isoleucine had placentas smaller than, and fetuses larger than normal on day 18 of pregnancy, but the reverse was true on day 15. Other unintended and unexpected effects of BCAA consumption should be studied more thoroughly due to the increasing use of BCAAs to alleviate muscle damage and soreness in athletes.

Amino acid transport system L activity in developing mouse ovarian follicles

BACKGROUND

Little is known about metabolic processes in the developing ovarian follicle. Using mouse ovarian follicles, we investigated uptake of L-leucine by follicles at varying stages of maturity in the presence of insulin-like growth factor (IGF)-1. METHODS Mouse ovarian follicles were cultured in vitro for 5 days in increasing concentrations of IGF-1, and follicle diameter and atresia measured as endpoints for growth. Uptake of (3)H-leucine was measured in follicles at different stages of development. In optimal IGF-1-mediated growth conditions, competitive inhibition of (3)H-leucine uptake by 2-aminobicyclo-(2,2,1)-heptane-2-carboxylic acid (BCH), a non-metabolizable substrate analogue of L-leucine, was performed to demonstrate specificity of influx, via system L transporters. To test whether uptake rates were dependent on intracellular amino acid availability, follicles from in vitro cultures were pre-treated with L-phenylalanine prior to (3)H-leucine uptake.

RESULTS

Follicle development (P< 0.001) and survival (P< 0.001) increased with IGF-1 treatment. As pre-antral follicles progressed to late antral stage, we observed an increase in L-leucine uptake, which was reduced in pre-ovulatory follicles. BCH decreased L-leucine uptake rates in early antral (P< 0.05), antral (P< 0.001) and pre-ovulatory follicles (P< 0.01). L-leucine influx increased in follicles preloaded with phenylalanine (trans-stimulation). In follicles lacking free intracellular amino acids (zero-trans suppression), uptake rate was reduced (P< 0.05).

CONCLUSIONS

These results demonstrate, for the first time, evidence of specific system L amino acid transport in intact, mouse ovarian follicles and profile L-leucine uptake during folliculogenesis. A better understanding of ovarian follicle metabolic pathways is necessary for improved in vitro maturation as well as determining the impact of altered metabolism on fertility.

Amino acid depletion and appearance during porcine preimplantation embryo development in vitro

Preimplantation embryos can consume and produce amino acids in a manner dependent upon the stage of development that may be predictive of subsequent viability. In order to examine these relationships in the pig, patterns of net depletion and appearance of amino acids by in vitro produced porcine preimplantation embryos were examined. Cumulus oocyte complexes derived from slaughterhouse pre-pubertal pig ovaries were matured for 40 h in defined TCM-199 medium (containing PVA) before being fertilised (Day 0) with frozen-thawed semen in Tris-based medium. After 6 h, presumptive zygotes were denuded and cultured in groups of 20, in NCSU-23 medium modified to contain 0.1 mM glutamine plus a mixture of 19 amino acids (aa) at low concentrations (0.02-0.11 mM) (NCSU-23(aa)). Groups of 2-20 embryos were removed (dependent on stage) on Day 0 (1 cell), Day 1 (two- and four-cells), Day 4 (compact morulae) and Day 6 (blastocysts) and placed in 4 mul NCSU-23aa for 24 h. After incubation, the embryos were removed and the spent media was analysed by HPLC. The net rate of amino acid depletion or appearance varied according to amino acid (P < 0.001) and, apart from serine and histidine, stage of development (P < 0.014). Glycine, isoleucine, valine, phenylalanine, tryptophan, methionine, asparagine, lysine, glutamate and aspartate consistently appeared, whereas threonine, glutamine and arginine were consistently depleted. Five types of stage-dependent trends could be observed: Type I: amino acids having high rates of net appearance on Day 0 that reached a nadir on Day 1 or 4 but subsequently increased by Day 6 (glycine, glutamate); Type II: those that exhibited lower rates of net appearance on Days 0 and 6 compared with the intermediate Days 1 and 4 (isoleucine, valine, phenylalanine, methionine, arginine); Type III: amino acids which showed a continuous fall in net appearance (asparagine, aspartate); Type IV: those that exhibited a steady fall in net depletion from Day 0 to Day 6 (glutamine, threonine); Type V: those following no discernable trend. Analysis of further embryo types indicated that presumptive polyspermic embryos on Day 0 had increased (P < 0.05) net rates of leucine, isoleucine, valine and glutamate appearance, and reduced (P < 0.05) net rates of threonine and glutamine depletion compared with normally inseminated oocytes. These data suggest that the net rates of depletion and uptake of amino acids by pig embryos vary between a) amino acids, b) the day of embryo development and, c) the type of embryos present at a given stage of development. The results also suggested that the net depletion and appearance rates of amino acids by early pig embryos might be more similar to those of the human than those of the mouse and cow.

System B0,+ amino acid transport regulates the penetration stage of blastocyst implantation with possible long-term developmental consequences through adulthood

Amino acid transport system B(0,+) was first characterized in detail in mouse blastocysts over two decades ago. Since then, this system has been shown to be involved in a wide array of developmental processes from blastocyst implantation in the uterus to adult obesity. Leucine uptake through system B(0,+) in blastocysts triggers mammalian target of rapamycin (mTOR) signalling. This signalling pathway selectively regulates development of trophoblast motility and the onset of the penetration stage of blastocyst implantation about 20 h later. Meanwhile, system B(0,+) becomes inactive in blastocysts a few hours before implantation in vivo. System B(0,+) can, however, be activated in preimplantation blastocysts by physical stimuli. The onset of trophoblast motility should provide the physiological physical stimulus activating system B(0,+) in blastocysts in vivo. Activation of system B(0,+) when trophoblast cells begin to penetrate the uterine epithelium would cause it to accumulate its preferred substrates, which include tryptophan, from uterine secretions. A low tryptophan concentration in external secretions next to trophoblast cells inhibits T-cell proliferation and rejection of the conceptus. Suboptimal system B(0,+) regulation of these developmental processes likely influences placentation and subsequent embryo nutrition, birth weight and risk of developing metabolic syndrome and obesity.

Reabsorption of neutral amino acids mediated by amino acid transporter LAT2 and TAT1 in the basolateral membrane of proximal tubule

In order to understand the renal reabsorption mechanism of neutral amino acids via amino acid transporters, we have isolated human L-type amino acid transporter 2 (hLAT2) and human T-type amino acid transporter 1 (hTAT1) in human, then, we have examined and compared the gene structures, the functional characterizations and the localization in human kidney. Northern blot analysis showed that hLAT2 mRNA was expressed at high levels in the heart, brain, placenta, kidney, spleen, prostate, testis, ovary, lymph node and the fetal liver. The hTAT1 mRNA was detected at high levels in the heart, placenta, liver, skeletal muscle, kidney, pancreas, spleen, thymus and prostate. Immunohistochemical analysis on the human kidney revealed that the hLAT2 and hTAT1 proteins coexist in the basolateral membrane of the renal proximal tubules. The hLAT2 transports all neutral amino acids and hTAT1 transports aromatic amino acids. The basolateral location of the hLAT2 and hTAT1 proteins in the renal proximal tubule as well as the amino acid transport activity of hLAT2 and hTAT1 suggests that these transporters contribute to the renal reabsorption of neutral and aromatic amino acids in the basolateral domain of epithelial proximal tubule cells, respectively. Therefore, LAT2 and TAT1 play essential roles in the reabsorption of neutral amino acids from the epithelial cells to the blood stream in the kidney. Because LAT2 and TAT1 are essential to the efficient absorption of neutral amino acids from the kidney, their defects might be involved in the pathogenesis of disorders caused by a disruption in amino acid absorption such as blue diaper syndrome.

Exogenous gamma-glutamyl cycle compounds supplemented to in vitro maturation medium influence in vitro fertilization, culture, and viability parameters of porcine oocytes and embryos

High concentrations of intracellular glutathione (GSH) enhance in vitro production of porcine embryos. Objectives were: (1) to determine the effects of gamma-glutamyl cycle compound supplements to the IVM medium on IVF and IVC; and (2) to evaluate embryo viability. Porcine oocytes were matured in NCSU 23 medium supplemented with either l-cysteine (3.3 mM), l-cysteamine (150 P < 0.05microM), l-cysteine and l-cystemaine, l-glycine (1, 2.5, or 5 mM), l-glutamate (1, 2.5, or 5 mM), l-alpha-aminobutyrate (3.3mM), beta-mercaptoethanol (BME) (25 microM), l-cysteine and BME, or l-alpha-aminobutyrate and BME. Increases (P < 0.05) in GSH concentrations were observed using l-cysteine, 1.0 mM l-glutamate, l-alpha-aminobutyrate, and l-alpha-aminobutyrate with BME. Oocytes matured with l-alpha-aminobutyrate and BME had a lower (P < 0.05) occurrence of polyspermy during IVF compared to controls and a greater percentage (P < 0.05) of embryos reaching the blastocyst stage compared to other treatment groups. For Objective 2, oocytes were matured in NCSU 23 or NCSU 23 supplemented with l-alpha-aminobutyrate with BME. Embryo cell death was determined using an Annexin V-FITC assay. Supplementation had no effect on the time of cell death. Embryo mortality was increased (P < 0.05) from 24 to 42 h post-IVF, with the greatest occurrence around 36 h. In conclusion, supplementing l-alpha-aminobutyrate and BME into the IVM medium increased intracellular GSH concentrations, decreased the occurrence of polyspermy during IVF, and increased embryo development parameters during IVC, but did not affect cell death during embryo development. The onset of cell death occurred from 24 to 42 h post-IVF, with the greatest occurrence around 36 h post-IVF.

New mechanism for amino acid influx into human epidermal Langerhans cells: L-dopa/proton counter-transport system

We have characterized a stereospecific transport mechanism for L-dopa into human epidermal Langerhans cells (LCs). It is different from any other amino acid transport system. It is highly concentrative, largely pH-independent, and independent of exogenous Na+, glucose and oxygen, and fuelled by a renewable intracellular energy source inhibited by iodoacetate but not by arsenate. We propose that the mechanism is a unidirectional L-dopa/proton counter-transport system. We have recently demonstrated anaerobic glycolysis in human epidermis, which substantiates the need of proton pumps for resident LCs. The findings prompt a re-evaluation of the profound changes LCs undergo when exposed to oxygen in aerobic culture. L-dopa is not metabolized by LCs but can rapidly be dislocated to the intercellular space by certain extracellular amino acids, i.e. LCs can profit by L-dopa in a dualistic way, altogether a remarkable biological phenomenon.

The glycine neurotransmitter transporter GLYT1 is an organic osmolyte transporter regulating cell volume in cleavage-stage embryos

Cells subjected to sustained high osmolarity almost universally respond by accumulating compatible organic osmolytes that, in contrast to inorganic ions, are not deleterious even at high intracellular concentrations. Their accumulation from the external environment by known organic osmolyte transporters, such as the four identified in mammals, occurs only slowly in response to sustained high osmolarity, by synthesis of new transporter proteins. Most cells, however, are not subject to high or varying osmolarity, and it is not clear whether organic osmolytes are generally required at normal osmolarities or how they are regulated. The fertilized egg of the mouse is protected in the oviduct from perturbations in osmolarity. However, deleterious effects of osmotic stress were evident in vitro even at normal oviductal osmolarity. Glycine was found to protect development, indicating that early mouse embryos may use glycine as an organic osmolyte at physiological osmolarity. We have now found that GLYT1, a glycine transporter of the neurotransmitter transporter gene family, functions as the organic osmolyte transporter that mediates the osmotically regulated accumulation of glycine and regulates cell volume in early embryos. Furthermore, osmotic stimulation of GLYT1 transport was immediate, without a requirement for protein synthesis, implying regulation different from known organic osmolyte transporters. Thus, GLYT1 appears to have a previously unidentified role as an organic osmolyte transporter that functions in acute organic osmolyte and volume homeostasis near normal osmolarity.

The human T-type amino acid transporter-1: characterization, gene organization, and chromosomal location

System T is a Na+-independent transport system that selectively transports aromatic amino acids. Here, we determined the structure of the human T-type amino-acid transporter-1 (TAT1) cDNA and gene (SLC16A10). The human TAT1 cDNA encoded a 515-amino-acid protein with 12 putative membrane-spanning domains. Human SLC16A10 was localized on human chromosome 6, mapped to 6q21-q22. SLC16A10 contains six exons spanning 136 kb. In contrast to rat TAT1, which is mainly present in the intestine, human TAT1 was strongly expressed in human kidney as well as in human intestine. Expression of human TAT1 in Xenopus laevis oocytes demonstrated the Na+-independent transport of tryptophan, tyrosine, phenylalanine, and L-dopa, indicating that human TAT1 is a transporter subserving system T. Because human TAT1 is proposed to be crucial to the efficient absorption of aromatic amino acids from intestine and kidney, its defect could be involved in the disruption of aromatic amino-acid transport, such as in blue diaper syndrome.

Expression cloning of a Na+-independent aromatic amino acid transporter with structural similarity to H+/monocarboxylate transporters

A cDNA was isolated from rat small intestine by expression cloning which encodes a novel Na+-independent transporter for aromatic amino acids. When expressed in Xenopus oocytes, the encoded protein designated as TAT1 (T-type amino acid transporter 1) exhibited Na+-independent and low-affinity transport of aromatic amino acids such as tryptophan, tyrosine, and phenylalanine (Km values: approximately 5 mm), consistent with the properties of classical amino acid transport system T. TAT1 accepted some variations of aromatic side chains because it interacted with amino acid-related compounds such as l-DOPA and 3-O-methyl-DOPA. Because TAT1 accepted N-methyl- and N-acetyl-derivatives of aromatic amino acids but did not accept their methylesters, it is proposed that TAT1 recognizes amino acid substrates as anions. Consistent with this, TAT1 exhibited sequence similarity (approximately 30% identity at the amino acid level) to H+/monocarboxylate transporters. Distinct from H+/monocarboxylate transporters, however, TAT1 was not coupled with the H+ transport but it mediated an electroneutral facilitated diffusion. TAT1 mRNA was strongly expressed in intestine, placenta, and liver. In rat small intestine TAT1 immunoreactivity was detected in the basolateral membrane of the epithelial cells suggesting its role in the transepithelial transport of aromatic amino acids. The identification of the amino acid transporter with distinct structural and functional characteristics will not only facilitate the expansion of amino acid transporter families but also provide new insights into the mechanisms of substrate recognition of organic solute transporters.

Amino acid transport regulation and early embryo development

Amino acids are essential components of media utilized to culture fertilized human eggs to the blastocyst stage in vitro. Use of such media has led to a significant increase in the proportion of embryos that implant upon transfer to the uterus and to a decrease in the number that need to be transferred to achieve pregnancy. Little is known about the mechanisms by which amino acids foster development of healthy human blastocysts. Indications are, however, that many of these mechanisms are the same in human and mouse embryos. Both essential and nonessential amino acid transport benefit preimplantation mouse embryo development, albeit at different stages. Nonessential amino acid transport improves development primarily during cleavage, whereas essential amino acid transport supports development of more viable embryos, especially subsequent to the eight-cell stage. This review discusses likely mechanisms for these beneficial effects.

Induction of tryptophan 2,3-dioxygenase in the mouse endometrium during implantation

Indoleamine 2,3-dioxygenase (IDO) is expressed in trophoblasts and defends the conceptus against rejection by reducing the tryptophan level and suppressing the T cell activity. We isolated a cDNA for tryptophan 2,3-dioxygenase (TDO), another key catabolizing enzyme of tryptophan, from a mouse uterus cDNA library enriched with pregnancy-induced genes. Northern blot and in situ hybridization analyses demonstrated that the TDO mRNA was induced in the decidualized stromal cells around the implanted embryo at the time of implantation. The expression was then upregulated and primarily localized at the mesometrial decidua. TDO mRNA was induced by deciduoma formation as well as embryo transfer but not by ovarian steroid hormones. These findings demonstrated that TDO is induced in the endometrial stromal cells concomitant with decidualization and suggested its involvement in the implantation process by regulating the tryptophan level at the implantation site.

Multiple components of arginine and phenylalanine transport induced in neutral and basic amino acid transporter-cRNA-injected Xenopus oocytes

The induced uptakes of L-[3H]phenylalanine and L-[3H]arginine in oocytes injected with clonal NBAT (neutral and basic amino acid transporter) cRNA show differential inactivation by pretreatment with N-ethylmaleimide (NEM), revealing at least two distinct transport processes. NEM-resistant arginine transport is inhibited by leucine and phenylalanine but not by alanine or valine; mutual competitive inhibition of NEM-resistant uptake of arginine and phenylalanine indicates that the two amino acids share a single transporter. NEM-sensitive arginine transport is inhibited by leucine, phenylalanine, alanine and valine. At least two NEM-sensitive transporters may be expressed because we have been unable to confirm mutual competitive inhibition between arginine and phenylalanine transport. The NEM-resistant transport mechanism appears to involve distinct but overlapping binding sites for cationic and zwitterionic substrates. NBAT is known to form oligomeric protein complexes in cell membranes, and its functional roles when expressed in Xenopus oocytes may include interaction with oocyte proteins, leading to increased native amino acid transport activities; these resemble NBAT-expressed activities in terms of NEM-sensitivity and apparent substrate range (including an unusual inhibition by beta-phenylalanine.

Endogenous amino acid transport systems and expression of mammalian amino acid transport proteins in Xenopus oocytes

Oocyte amino acid transport has physiological significance to oocytes and practical importance to molecular biologists and transport physiologists. Expression of heterologous mRNA in Xenopus oocytes is currently being used to help clone cDNAs for amino acid transporters and their effectors. A major question to be resolved in many of these studies is whether the injected mRNA codes for a transporter or an activator of an endogenous system. Nevertheless, the cDNAs of several families of amino acid transporters or their activators appear already to have been cloned. One such transporter is the anion exchanger, band 3, which may also transport glycine and taurine under some important physiological conditions such as hypoosmotic stress. Site-directed mutagenesis of band 3 has already shown that an amino acid residue believed to be at or near the active site nevertheless does not appear to influence Cl- transport in Xenopus oocytes expressing the modified band 3 protein. Continuation of such studies along with examination of transport of all possible substrates of band 3 should yield insight into the relationship between the structure and function of this transporter. Each of three other families not only contains amino acid transporters, but also appears to contain members that serve as transporters of neurotransmitters or their metabolites. Because of the distinct structural differences in the preferred substrates of different transporters within some of these families, elucidation of the tertiary and possibly quaternary structural relationships among the members of such families may reveal transport mechanisms. In addition, the grouping of neurotransmitters or their metabolites according to the family to which their transport systems and transporters belong could yield insight into mechanisms of brain development, function and evolution. Another family of transporters for cationic amino acids also serves, at least in one case, as a viral receptor. Hence, these or other transporters also could conceivably function in eggs as receptors for sperm and, more broadly, in cell-cell interactions as well as in amino acid transport. Moreover, a family of apparent amino acid transport activators are homologous to a family of glycosidases, so these activators could also serve to recognize carbohydrate structures on other cells or the extracellular matrix. Some of these activators appear to increase more than one amino acid transport activity in Xenopus oocytes. In other studies, expression of heterologous mRNA in oocytes has led apparently to detection of inhibitors as well as activators of amino acid transport. Some amino acid transport systems also could conceivably contain nucleic acid as well as glycoprotein components.(ABSTRACT TRUNCATED AT 400 WORDS)

Aspartate and glutamate transport in unfertilized pig oocytes and blastocysts

Amino acid transport is facilitated by specific transporters within the plasma membrane of the cell. Mediated Na(+)-independent transport of L-glutamate can be easily detected in mouse oocytes, but it is nearly undetectable in blastocyst-stage embryos. In contrast, the Na(+)-dependent transport of L-aspartate is not detectable in oocytes, but it is detectable in eight-cell embryos and reaches relatively high levels by the blastocyst stage. It is believed that the amino acid transporters responsible are systems x-c and X-AG, respectively. Here we report the detection of Na(+)-dependent L-aspartate transport, which increased as pig blastocysts developed, although Na(+)-dependent aspartate transport was not detected in pig oocytes. Mediated Na(+)-independent L-glutamate transport was not detected in pig oocytes, in contrast to the mouse, nor in early or hatched pig blastocysts. Thus, while the developmental regulation of system X-AG is similar in both the pig and the mouse, system x-c was not detectable in pig oocytes or blastocysts. Elucidation of the molecular mechanisms controlling amino acid transport and other gene expression in early embryos should contribute to an understanding of whether and even why some aspects of developmental regulation of gene expression may need to differ among species.

Alanine and leucine transport in unfertilized pig oocytes and early blastocysts

Amino acid transport is facilitated by specific transporters within the plasma membrane of the cell. In mouse oocytes and cleavage-stage conceptus Na(+)-dependent L-alanine and L-leucine transport are nearly undetectable. Sodium-dependent transport via system B0,+ in the mouse conceptus increases greatly between the 8-cell and blastocyst stages. By contrast, data presented here for the pig show that L-alanine and L-leucine transport is mainly Na(+)-dependent in the oocyte; this Na(+)-dependent component of transport becomes undetectable by the blastocyst stage. The Na(+)-dependent component of transport in oocytes is inhibited by BCH (2-aminoendo-bicyclo[2.2.1] hexane-2-carboxylic acid) and L-lysine and thus could be a form of system B0,+. In both oocytes and blastocysts Na(+)-independent L-leucine transport is inhibited by BCH, which is consistent with the presence of system L. The dramatic decrease in Na(+)-dependent amino acid transport activity could occur in pig conceptuses in association with the onset of RNA synthesis during the 4-cell stage. Regardless of the precise time during development at which it occurs, however, this dramatic, developmentally regulated decrease in Na(+)-dependent alanine and leucine transport activity contrasts sharply with the large increase in Na(+)-dependent system B0,+ activity that occurs during preimplantation development of murine conceptuses. Elucidation of the molecular mechanisms by which these changes occur should contribute to an understanding of regulation of gene expression during early development.

Thyroid hormone concentrative uptake in rat erythrocytes. Involvement of the tryptophan transport system T in countertransport of tri-iodothyronine and aromatic amino acids

The kinetic properties of transport system T, which is specific for uptake of aromatic amino acids, were studied in rat erythrocytes in the presence of leucine in order to block the neutral amino acid transport system L. Since the triiodothyronine (T3) transport system and system T are closely related, the trans effect of T3 and tryptophan on [3H]tryptophan transport and the trans effects of aromatic amino acids on [125I]T3 transport were studied. Equilibrium-exchange, zero-trans and infinite-trans studies of [3H]tryptophan transport indicated that system T in rat erythrocytes is a simple carrier with exchanging properties resulting in trans-acceleration of influx and trans-inhibition of efflux when tryptophan was present at the trans side of the membrane. In erythrocytes preloaded with unlabelled tryptophan, countertransport resulted in a 7-fold accumulation of labelled substrate inside the cells. T3 on the trans side of the membrane inhibited both influx and efflux of tryptophan, with Ki values similar to the Km values of the T3 transport system. Extracellular tryptophan trans-inhibited [125I]T3 efflux in a manner similar to [3H]tryptophan efflux. Preloading erythrocytes with tryptophan resulted in trans-acceleration of T3 uptake and a transient 5-fold accumulation of free T3 into erythrocytes. Phenylalanine and tyrosine (but not the D-isomer of tryptophan or non-aromatic amino acids) also produced trans-acceleration for T3 uptake and T3 countertransport. These results are compatible with a kinetic model assuming a common simple carrier of T3 and tryptophan transport and point to a countertransport pathway driving the uphill uptake of T3 by hetero-exchange with intracellular aromatic amino acids.

Na(+)-dependent transport of anionic amino acids by preimplantation mouse blastocysts

Negatively charged amino acids, such as aspartate and glutamate, were selected as substrates by low- and high-Km components of mediated Na(+)-dependent transport in preimplantation mouse blastocysts. These and other relatively small anionic amino acids with two carbon atoms between the negatively charged groups (or up to three carbon atoms when the groups were both carboxyl groups) interacted strongly with the low-Km component of transport, whereas larger anionic amino acids interacted weakly or not at all. The low-Km system was also stereoselective except in the case of aspartate. Moreover, transport was Cl(-)-dependent and slower at pH values outside the range 5.6-7.4. L-Aspartate, D-aspartate and L-glutamate each interacted strongly with the low-Km component of transport with Km values for transport nearly equal to their Ki values for inhibition of transport of one of the other amino acids. By these criteria, the low-Km component of transport of anionic amino acids in blastocysts appears to be the same as the familiar system X-AG that is present in other types of mammalian cells. In contrast, the high-Km component of transport in blastocysts preferred L-aspartate to L-glutamate, whereas the reverse is true for fibroblasts. Therefore, transport of anionic amino acids in blastocysts may occur via at least one process that has not been described in other types of cells. Roughly half of mediated glutamate and aspartate transport in blastocysts may occur via the high-Km component of transport at the concentrations of these amino acids that may be present in uterine secretions.

Ouabain-sensitive Rb+ uptake in mouse eggs and preimplantation conceptuses

The results of histochemical and immunocytochemical studies have been used elsewhere to support the hypothesis that Na+/K(+)-ATPase expression is initiated or increases dramatically in preimplantation mouse conceptuses just before they begin to cavitate. Moreover, localization of the enzyme in the inner membrane of the mural trophoblast is thought to be involved directly in formation and maintenance of the blastocyst cavity. Presumably, Na+/K(+)-ATPase extrudes the cation, Na+, and therefore water into the cavity. The cation transporting activity of the enzyme can be determined by measuring ouabain-sensitive Rb+ uptake by cells. Therefore, we measured Rb+ uptake in mouse eggs and preimplantation conceptuses at various stages of development. 86Rb+ uptake by conceptuses increased linearly with time for at least 60 min in medium containing 0.7 mM total Rb+ plus K+ in the absence or presence of 1.0 mM ouabain, and ouabain inhibited more than 70% of 86Rb+ uptake. The ouabain concentration at 1/2 of maximum inhibition of the ouabain-sensitive component of 86Rb+ uptake was about 10-20 microM in eggs and conceptuses at all stages of preimplantation development. Moreover, ouabain-sensitive Rb+ uptake had a twofold higher Vmax value in blastocysts than in eggs or conceptuses at earlier stages of development (i.e., approximately 173 vs 70-100 fmole.conceptus-1.min-1), although the total cell surface area also was probably about two times greater in blastocysts than in eggs or other conceptuses. Ouabain-sensitive Rb+ transport in eggs and conceptuses may have occurred via a single ouabain-sensitive Rb+ transporter with a Hill coefficient of 1.5-1.8 (Hill plots). When it was assumed that the Hill coefficient had a value of 2.0, however, eggs and conceptuses appeared to contain at least two forms of Na+/K(+)-ATPase activity. These studies are the first to show that the cation transporting activity of Na+/K(+)-ATPase can be measured quantitatively in mammalian eggs and preimplantation conceptuses. Inclusion of this assay in experiments designed to determine how Na+/K(+)-ATPase activity is controlled in oocytes and conceptuses should yield further insight into the role of this enzyme in oogenesis and preimplantation development.