Kinetics of zinc absorption by in situ ligated intestinal loops of broilers involved in zinc transporters.

Two experiments were conducted to study the kinetics of Zn absorption and Zn transporter mRNA levels in the duodenum, jejunum, and ileum of broilers to investigate the main site of Zn absorption in the small intestine, the absorption mechanisms, and whether those transporters were involved. In experiment 1, we compared Zn absorption in 3 segments and at different post-perfusion time points, and found that Zn absorption increased linearly within 30 min and was higher (P < 0.05) in the ileum than in the other 2 segments. In experiment 2, intestinal loops were perfused with solutions containing 0, 0.077, 0.154, 0.308, 0.616, 1.232, or 2.464 mmol/L of ZnSO(4) . 7H(2)O, and Zn concentrations in perfusates were determined at 30 min after perfusion. The mRNA levels of transporters in 3 intestinal loops from the control group and the 0.616 mmol of Zn/L group were analyzed. The kinetic curves showed that Zn transported to the duodenum and jejunum depended on a saturable carrier-mediated process. The Michaelis-Menten constant (K(m)) value was higher in the duodenum than in the jejunum (1.44 +/- 0.33 vs. 0.51 +/- 0.17 mM, P = 0.012). Similarly, the maximum absorption velocity (J(max)) value was greater in the duodenum than in the jejunum (5.32 +/- 1.46 vs. 2.57 +/- 0.39 nmol/min per cm, P = 0.069), whereas absorption in the ileum occurred with a nonsaturable diffusion process and had a diffusive constant (P) of 5.72 x 10(-3) +/- 1.1 x 10(-4) cm(2)/min. Moreover, the mRNA levels of metallothionein, zinc transporter 1, and Zn transporter 5 were lower in the ileum than in the duodenum or jejunum in the Zn-supplemented group, further indicating that Zn absorption in the ileum occurred mainly by a nonsaturable diffusive pathway. The Zn fluxes were significantly higher (P < 0.005) in the ileum than in the other 2 segments at different Zn concentrations. This research suggests that the ileum is the main site of Zn absorption and that the mechanism involved is nonsaturable diffusion, which is different from that in the other 2 segments, depending on the regulation of Zn transporter expression.