Characteristics of the chicken proximal cecum hexose transport system

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Abstract
The properties of the sugar transport system present in chicken proximal cecum have been studied and compared to the jejunal transport system. Experiments were carried out in isolated enterocytes from 5- to 7-weak-old birds. Results show that: (1) Cecal cells are capable of high sugar transport rates by a phloridzin-sensitive mechanism. After 60 min incubation, the accumulation ratio (control/phloridzin-incubated cells) for 0.1 mmol/l α-methyl-d-glucoside (α-MG) was 43 and that of 3-oxy-methyl-d-glucose (3-OMG) was 25. In jejunal cells, ratios were 37 for α-MG and 13 for 3-OMG. The differences found in cumulative capacity of 3-OMG between cecal and jejunal cells suggest that the sodium-independent pathway offers a very small contribution to sugar efflux in the steady-state in the former cells. (2) Lowering external Na+ concentration reduces the steady-state α-MG accumulation in cecal cells (as in jejunal cells), indicating that the transport system is Na+-dependent. (3) The process depends on the electrochemical Na+ gradient across the cell membrane since both 2,4-dinitrophenol (0.2 mmol/l) and ouabain (0.25 mmol/l) abolish sugar accumulation. (4) Addition of 10 mmol/l 3-OMG to the incubation medium markedly reduces the uptake of α-MG (concentration: 0.1 mmol/l), indicating that the cecal transport system can be inhibited by analogues of the transported substrate. (5) The specific sugar transport process is a saturable function of α-MG concentration, the apparentK m being 1.02 mmol/l andV m 10.7 nmol/mg cell protein · min. Kinetic constants in jejunal enterocytes were 1.58 mmol/l (K m) and 24.7 nmol/mg cell protein · min (V m), respectively. In brief, the proximal cecal epithelium has a sugar transport system with properties similar to those of the jejunum which suggests a role of this epithelium in the absorption of hexoses of either ileal or cecal origin.