68 M. Desforges and others J Physiol 587.1 the SNAT4 isoform of system A is functional in human placental MVM during ﬁrst trimester and at term. This study is the ﬁrst to investigate the activity of a single SNAT isoform in the placenta, or in any tissue, which expresses all three system A isoforms. Western blotting revealed higher SNAT4 protein expression in ﬁrst trimester MVM compared to term. SNAT4 protein expression was also considerably variable between term MVM samples. The emerging pattern of SNAT4 expression and activity in human placenta across gestation appears to be quite complex. We have pre- viously shown SNAT4 protein expression is lower in ﬁrst trimester human placental homogenates, comprising many cell types, compared to term (Desforges et al. 2006). However, our current Western blotting data suggest that during ﬁrst trimester a greater proportion of the SNAT4 protein pool is localized to the MVM. Consistent with this observation, immunohistochemical detection of SNAT4 shows strong staining in the cytoplasm of the syncytiotrophoblast of term placental tissue whereas in ﬁrst trimester syncytiotrophoblast there is relatively more MVM staining of the syncytiotrophoblast layer. This accords with evidence from other cell types that shows preformed SNAT proteins are stored in cyto- plasmic compartments before translocation to the plasma membrane takes place (Hundal et al. 1994; Ling et al. 2001; Hyde et al. 2002). As SNAT4 protein was also localized to fetal blood vessels, which are widespread at term but poorly developed during the ﬁrst trimester, this may contribute to the higher SNAT4 protein expression previously observed in term placental homogenates (Desforges et al. 2006). Figure 3. [14C]MeAIB uptake by MVM vesicles A, the effect of arginine on [14C]MeAIB uptake by ﬁrst trimester (FT) MVM and B, term MVM vesicles (n = 6 in each group). Data presented are mean ± S.E.M. and statistical analysis by 2-way ANOVA. Symbols shown are; , control; , + 10 mM arginine; , + 20 mM arginine; , + 30 mM arginine; •, + 30 mM MeAIB. ∗∗P < 0.01, ∗∗∗∗P < 0.0001 versus control. [14C]MeAIB uptake in the presence of 30 mM MeAIB was signiﬁcantly lower than [14C]MeAIB uptake in all other conditions (P < 0.0001) and was considered non-system A-mediated due to saturation of all isoforms by this high concentration of substrate. In all conditions [14C]MeAIB uptake signiﬁcantly increased with time (P < 0.0001). C, comparison of system A-mediated [14C]MeAIB uptake (calculated by sub- tracting [14C]MeAIB uptake in the presence of 30 mM MeAIB from total [14C]MeAIB uptake) by FT and term MVM after 30 s. Data presented are median and interquartile range, n = 6 in each group, ∗P < 0.05 (Mann–Whitney U-test). D, arginine-inhibitable component of system A activity (expressed as a percentage of system A-mediated [14C]MeAIB uptake) by FT and term MVM. Data presented are median and interquartile range, n = 6 in each group, ∗∗ P < 0.01 (Mann–Whitney U-test). C 2009 The Authors. Journal compilation C 2009 The Physiological Society