Contents of extracellular different Met sources of IPEC-J2 cells. The concentration of DL-HMTBA is shown in (a), the concentration of DL-MM is shown in (b), the concentration of Met is shown in (c), and the concentrations of total Met sources are shown in (d). Cells were seeded onto six-well Transwell system plates and cultured in DMEM/F-12 medium for 21 d, then cultured in serum-free medium for 18 h and then in Met-free medium for 6 h, and finally treated with different Met sources in the medium for 2 h. Shown are representative 3 independent experiments. The data are expressed as the mean ± SEM. A, B, C, D, E means without the same letter had extremely significant difference, P < 0.01.