Gossypol reduced the intestinal amino acid absorption capacity of young grass carp (Ctenopharyngodon idella)

This study was conducted to evaluate the effects of gossypol on the growth, intestinal histology, intestinal amino acid (AA) absorption capacity and the safe upper limit of gossypol in the diets for young grass carp (Ctenopharyngodon idella) growth. The fish were fed six diets containing different levels of free gossypol (0, 125 (121.38), 250 (243.94), 375 (363.89), 775 (759.93) and 1175 (1162.06) mg kg(-1) diet) from gossypol-acetic acid for 60 days. Results indicated that gossypol could reduce the growth performance and damage the intestines of young grass carp. Interestingly, gossypol modulated the absorptive abilities of various AAs, and mechanisms are not unified in fish. Firstly, gossypol decreased the contents of tryptophan (Trp), threonine (Thr), valine (Val), phenylalanine (Phe), glycine (Gly), serine (Ser) and tyrosine (Tyr) in the three intestinal segments of fish associated with down-regulating the mRNA levels of solute carrier (SLC) 6A19b, SLC7A5, SLC7A8, SLC1A5, SLC38A2 and peptide transporter 1 (PepT1) in the intestine. Secondly, gossypol reduced the contents of methionine (Met), isoleucine (Ile), proline (Pro), cysteine (Cys) and arginine (Arg) in the proximal intestine (PI) and mid intestine (MI) of fish by down-regulating the mRNA levels of SLC7A7, SLC7A6, SLC7A1, SLC6A19b, SLC7A5, SLC7A8, SLC1A5, SLC38A2 and PepT1 in the PI and MI. Thirdly, gossypol inhibited the aspartic acid (Asp) and taurine (Tau) contents in the PI and MI of fish by down-regulating the mRNA levels of SLC1A2a, SLC1A3, PepT1 and SLC6A6 in the PI and MI. Fourthly, gossypol suppressed the lysine(Lys) and histidine (His) contents in the MI and distal intestine (DI) of fish due to down-regulating the mRNA levels of SLC7A7, SLC7A5 and SLC38A2 in the MI, and SLC7A6, SLC7A1, SLC7A8 and PepT1 in the MI and DI. Gossypol down-regulated the above AA and peptide transporters in the intestine of fish partly related to suppressing the target of rapamycin (TOR) signalling pathway. Based on percent weight gain (PWG) and feed efficiency (FE), the safe upper limit of gossypol in the diets for young grass carp growth were estimated to be 182 and 179 mg kg(-1) diet, respectively. In summary, gossypol reduced the intestinal AA absorption capacity of young grass carp partly relate to damaging the intestinal structure, down-regulating the mRNA levels of AA and peptide transporters, and inhibiting the TOR signalling pathway.