LAUSR

Abstract The present study investigated the effects of choline on growth, intestinal histology, intestinal amino acid (AA) absorption capacity and the underlying signalling in juvenile grass carp (Ctenopharyngodon idella). Fish were fed six-graded levels of choline (142.2, 407.4, 821.6, 1215.8, 1589.3, 1996.6 mg kg−1) for 70 days. The results showed that choline insufficiency deteriorated the growth performance and injured the fish intestinal health. Strikingly, choline influenced the absorptive capacity for various sorts of AAs in different intestinal segments by non-uniform mechanisms. Firstly, choline deficiency reduced the contents of free neutral AAs including Met, Trp, Thr, Ile, Leu, Val, Gly, Pro, Ser, Cys and Tau in all intestinal segments partly attributed to decrease the mRNA levels of the corresponding transporters including SLC7A7, SLC7A6, SLC1A5, SLC7A9 (except in MI), SLC7A8, SLC6A6, SLC6A19b, SLC38A2 (except in PI) and peptide transporter 1 (SLC15A1) as well as the protein level of SLC1A5 in the intestine. Secondly, choline deficiency decreased the contents of free basic AAs including Lys, Arg, and His (except in DI) in three segments partly associated with downregulating cationic AAT SLC7A1 and SLC15A1 mRNA levels as well as the protein level of SLC7A1 in gut segments. Thirdly, choline deficiency decreased the contents of free acidic AAs including Glu and Asp in three gut segments partly associated with the decreasing mRNA levels of the corresponding transporters including SLC1A2a and SLC15A1 in the intestine. Choline insufficiency depressed the TOR (target of rapamycin) signalling molecule mRNA levels and TOR protein abundance in the fish gut. In addition, choline requirement of juvenile grass carp was calculated to be 1330.7 and 1283.4 mg kg−1 diet based on the PWG (percent weight gain) and FE (feed efficiency), respectively. In general, choline deficiency attenuated the absorptive abilities of intestinal AAs in juvenile grass carp, which might be connected with the aggravation of intestinal health, the decrease of amino acid transporters might be related to restraint the TOR signalling pathway.