Glucose transport across lagomorph jejunum epithelium is modulated by AMP-activated protein kinase (AMPK) under hypoxia

The gastrointestinal epithelium possesses adaptation mechanisms to cope with huge variations in blood flow and subsequently oxygenation. Since sufficient energy supply is crucial under hypoxic conditions, glucose uptake especially must be regulated by these adaptation mechanisms. Therefore, we investigated glucose transport under hypoxic conditions. Jejunal epithelia of rabbits were incubated in Ussing chambers under short- circuit current conditions. Hypoxia was simulated by gassing with 1% O₂ instead of 100% O₂. The activity of SGLT1 (sodium-coupled glucose transporter 1) was assessed by measuring the increase of short circuit current (Isc) after the addition of 2 mM glucose to the mucosal buffer solution. We observed decreased activity of SGLT1 after hypoxia compared to control conditions. To investigate underlying mechanisms, epithelia were exposed to agonists and antagonists of AMP-activated protein kinase (AMPK), before assessing SGLT1-mediated transport and the pAMPK/AMPK protein ratio. Preincubation with the antagonist restored SGLT1 activity under hypoxic conditions to the level of control conditions, indicating an involvement of AMPK in the (down-)regulation of SGLT1 activity under hypoxia which was confirmed in western blot analysis of pAMPK/AMPK. Transepithelial flux studies using radioactively labelled glucose, ortho-methyl-glucose, fructose and mannitol revealed no changes after hypoxic incubation. Therefore, we could exclude a decreased transepithelial glucose transport rate and increased paracellular conductance under hypoxia. In conclusion, our study hints at a decreased activity of SGLT1 under hypoxic conditions in an AMPK-dependent manner. However, transepithelial transport of glucose is maintained. Therefore, we suggest other transport mechanisms, especially glucose transporter 1 and/or 2 to substitute SGLT1 under hypoxia.

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