Hepatic cumulative net appearance of amino acids and related gene expression response to different protein diets in pigs

The liver is an important organ responsible for amino acids (AA) conversion, and catabolism. To investigate the metabolic characteristic of AA in liver, eighteen castrated male pigs (20±1 kg), implanted with arterial-portal-hepatic-mesenteric catheters, were fed three crude protein (CP) diets (14%, 17%, 20%) with six pigs per group. Blood was sampled for 7.5 h post-feeding. Hepatic tissue was sampled from additional fifteen pigs (three CP diets groups, 5 per group) on 3 h post-feeding. Portal vein plasma flow (PVPF), hepatic vein plasma flow (HVPF), hepatic cumulative net appearance and related gene expression were analyzed. There were no difference (P>0.05) for PVPF and HVPF among the three CP diets. Compared with the 20% CP diet, the 14% CP diet increased (P<0.05) the hepatic metabolism of lysine (59.50% vs. 43.00%) and threonine (44.30% vs. 30.22%). Likewise, the mRNA levels of α-aminoadipate δ-semialdehyde synthase (AASS) and l-threonine 3-dehydrogenase (TDH) were highest (P<0.05) in 14% CP group. The branched-chain AA (BCAA) had a low metabolic rate (3.4-13.4%) in liver. Morever, both the branched-chain amino-transferase 2 (BCAT2) and branched-chain α-keto acid dehydrogenase (BCKDHA) genes had a low expression. In contrast, aromatic AA (AAA) were almost entirely metabolized (63-93%) and the genes of phenylalanine hydroxylase (PAH) and tyrosine aminotransferase (TAT) had a high expression in liver. Notably, compared with the 20% CP diet, the 14% CP diet increased (P<0.05) hepatic metabolism of BCAA, AAA and gene expressions of BCAT, BCKDHA, PAH and TAT. These results revealed pigs fed on a low CP diets, only supplemented with lysine, methionine, threonine and tryptophan, had a numerical increasing EAA metabolism and EAA related metabolic gene expression in liver.