Thiamine transport in thiamine-deficient rats role of the unstirred water layer

As part or a systematic study of alcoholism and thiamine absorption, the effect of diet-induced thiamine deficiency and the role of the unstirred water layer on thiamine transport were investigated. Using ³H-labeled dextran as a marker of adherent mucosal volume, jejunal uptake of ¹⁴C-labeled thiamine hydrochloride was measured, in vitro, in thiamine-deficient rats and pair-fed controls. Uptake of low thiamine concentrations (0.2 and 0.5 μM) was greater in the thiamine-deficient rats thatn in the controls. In contrast, uptake rates for high thiamine concentrations (20 and 50 μM) were similar in both groups. While *J_max was unaltered, *K_m was decreased in thiamine deficiency, suggesting a decrease in unstirred water layer thickness. Accordingly, the thickness of the water layer was measured in both groups of animals and correlated with *J_max and *K_m under unstirred and st irred conditions. Without stirring, there was no difference in *J_max between the two groups. In contrast, both *K_m and the water layer were reduced in the thiamine-deficient rats. With stirring, *J_max was not affected, but both *K_m and the water layer thickness were reduced to similar values in both groups. Reversal of thiamine deficiency resulted in the return of thiamine uptake and the unstirred water layer thickness to control values. These data support the concept of a dual system of thiamine transport and emphasize the role of the unstirred water layer as an important determinant of transport kinetics not only under physiologic situations but also in diet-induced rat thiamine deficiency, a model for a clinical pathological state. The decrease in the unstirred water layer thickness in thiamine deficiency may be also viewed as a possible adaptive mechanism to facilitate absorption of meager supplies of thiamine.