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Glutamine transport by mouse inner cell masses

Mouse blastocysts take up glutamine by specific transport systems. Glutamine is an important precursor for macromolecular synthesis and a potential alternative fuel to glucose. This study compared glutamine uptake in blastocysts and isolated inner cell masses and characterized the major participating systems in the latter. Inner cell masses take up glutamine by facilitated transport systems. The identity of these was investigated using substrate competition and kinetic studies. Na⁺-dependent uptake of 13 µmol glutamine l^–1 was inhibited by 60% by 1 mmol tryptophan l^–1, 25% by 1 mmol 2-amino-2-norbornanecarboxylic acid l^–1 and 50% by 1 mmol lysine l^–1. Furthermore, 1 mmol 2-methyl(amino)isobutyric acid (MeAIB) l^–1 inhibited uptake by 29%. Kinetic analysis of MeAIB-resistant uptake revealed a predominant Na⁺-dependent facilitated uptake system with K_m and V_max values of 434 ± 72 µmol l^–1 and 237 ± 38 fmol per inner cell mass per 10 min, respectively. The inhibition of Na⁺-dependent uptake by tryptophan, lysine and the analogue 2-amino-2-norbornanecarboxylic acid suggests that most uptake of glutamine by inner cell masses occurs via the same system that predominates in whole blastocysts, B^{o, +}. The period of assay was so brief that significant participation of the inner cell mass in whole blastocyst uptake was precluded showing that system B^o,+ is expressed by both the trophectoderm and inner cell mass components of the blastocyst. However, MeAIB inhibited uptake by inner cell masses but not by blastocysts. This MeAIB-sensitive uptake had a K_m value of 4.3 ± 1.7 mmol l^–1 and a V_max value of 451 ± 119 fmol per inner cell mass per 10 min. These characteristics suggest the first embryonic appearance of system A, which is a common Na⁺-dependent transporter in many somatic cells. Significant inhibition of Na⁺-independent glutamine uptake by tryptophan suggests participation of the ubiquitous system L, while inhibition by lysine suggests that b^o,+ may also be expressed in inner cell masses. Thus glutamine uptake by inner cell masses occurs via several transport systems expressed in whole blastocysts, including Na⁺-dependent system B^{o, +}, and probably Na⁺-independent systems L and b^o,+, but is supplemented by the first expression of Na⁺-dependent system A in these cells from which the fetus later develops.