1324 Fig. 3 Expression of taurine transporter (TauT) in dissected S. sen- egalensis larvae reared under standard procedures. DAH, days after hatching. Results for expression of taurine transporter in head (open circles), digestive tract (ﬁlled triangles) and body (ﬁlled circles) fractions are shown as TauT copy:eEF1a1 copy and given as mean ± standard deviation (n = 3 pooled samples). Different letters within the same fraction represent signiﬁcant differences throughout larval development replacement of S322 by an alanine increased TauT activity by threefold in Xenopus laevis oocytes. Since alanine and cysteine phosphorylation by PKC cannot occur, these ﬁndings suggest that the replacement of S322 by C322 also result in a higher TauT activity. The biological explanation and consequences of this replacement on TauT activity in mammals compared with ﬁsh, reptiles and birds remain unknown and should be clariﬁed. In Senegalese sole larvae, TauT mRNA was found at the onset of exogenous feeding, and was found to signiﬁcantly increase during the larval stage and during metamorphosis. A similar trend was observed in all fractions of dissected Fig. 4 Tissue expression of taurine transporter (TauT) in juvenile S. Senegalensis. Br brain, Ey eye, Gi gills, He heart, Hk headkidney, SP spleen, Gb gallbladder, Li liver, St stomach, Mg midgut, Hg hindgut, Sec sections from anterior to posterior. Results are shown as TauT copy:eEF1a1 copy and given as mean ± standard deviation (n = 4). Different letters represent signiﬁcant differences among fractions 123 W. Pinto et al. larvae (head, digestive tract and body), with TauT expression signiﬁcantly increasing throughout metamor- phosis. These results indicate that metamorphosis is an important developmental trigger to promote taurine trans- port in Senegalese sole larval tissues. These ﬁndings also show that although TauT mRNA can be found in Sene- galese sole at the onset of exogenous feeding, it is during metamorphosis that the capacity to transport taurine increases. Nevertheless, it should be kept in mind that factors that inﬂuence taurine accumulation include ionic environment, electrochemical charge, post-translational and transcriptional factors (Han et al. 2006). However, Han et al. (2006), and Han and Chesney (1994) suggested that the long term adaptive response to diet is regulated at the mRNA level. Compared to the remaining fractions, TauT expression was very high in Senegalese sole body during the larval stage, indicating a high capacity to transport taurine in muscle during this period. In a developmental stage where growth potential is extremely high (Kamler 1992), this capacity may be important for larval growth, since taurine has been indicated to mediate the uptake of calcium in muscle sarcoplasmatic reticulum, playing a role on excitation–contraction mechanisms of muscle ﬁbres (Huxtable 1992; De Luca et al. 2000), and to induce the uptake of AA into skeletal muscle cells (Huxtable et al. 1987). Since AA are the building blocks for growth (Carter and Houlihan 2001), a high capacity to transport taurine in muscle may be related to an increase of AA retention, leading to a high growth potential, as observed for Sene- galese sole larvae reared under dietary taurine supple- mentation (Pinto et al. 2010). In addition, dietary taurine supplementation has also been reported to simultaneously increase taurine body levels and larval growth performance in other ﬁsh species, such as cod (Matsunari et al. 2005), red seabream (Chen et al. 2004) and Japanese ﬂounder (Chen et al. 2005).