The results indicate that AQP7 is present in the small and large intestine. The higher expression of AQP7 protein at the apical pole of the superficial epithelial cells suggests its involvement in rapid fluid movement through the villus epithelium.

Mucosal biopsy specimens obtained by routine endoscopy from 108 human subjects, including one patient with thiamine deficiency, were incubated at 37 degrees C in oxygenated calcium-free Krebs-Ringer solution (pH 7.5) containing tritiated thiamine and [14C]dextran as a marker of adherent mucosal water. The amount of labeled thiamine taken up was measured radiometrically. In subjects with no clinical evidence of thiamine deficiency, 1) thiamine uptake by duodenal mucosa had a hyperbolic time course, reaching equilibrium at 10 min; 2) thiamine concentrations < 2.5 mumol/L were taken up predominantly by a saturable mechanism displaying Michaelis-Menten kinetics (K(m) 4.4 mumol/L and Jmax 2.3 pmol.mg wet tissue-1.6 min-1), whereas higher concentrations were taken up by passive diffusion; 3) thiamine transport had different capacities along the gastrointestinal tract (duodenum >> colon > stomach); and 4) thiamine uptake was competitively inhibited in the duodenum by thiamine analogs, albeit with a different order of potency compared with rats, and was blocked by 2,4-dinitrophenol. In the thiamine-deficient patient, the duodenal saturable uptake was increased, with higher K(m) and Jmax values. In conclusion, physiologic concentrations of thiamine were transported in human small intestine by a specific mechanism dependent on cellular metabolism, whose transporters appear to be down-regulated.

No abstract

The results provide strong evidence for the presence in renal BBMVs of a thiamin/H+ antiport having a 1:1 stoichiometric ratio.