UNLABELLED: Transport results from in vitro studies may not be applicable to in vivo situations. In this study, we extended our previous in vitro observations regarding the intestinal transport of 5-methyltetrahydrofolate to in vivo studies in the unanesthetized rat and examined the effect of the unstirred water layer on the absorption process. We used a well defined intestinal perfusion technique. Absorption of 0.5 and 5 microM 5-methyltetrahydrofolate proceeded in a linear manner for 40 min of perfusion at 0.31 and 1.74 nmol/100 cm per min, respectively. Absorption of 0.5 microM 5-methyltetrahydrofolate increased with increasing perfusate flow-rate from 0.5 to 2 to 4 ml/min, indicating an unstirred water layer influence on the absorption rate. Absorption of the substrate was saturable with an apparent Kt of 5.7 microM and Vmax of 3.45 nmol/100 cm per min. Absorption was pH-dependent, and was inhibited by structural analogues. In contrast to the in vitro data, addition of glucose (20 mM) to the perfusate was unnecessary for in vivo absorption to proceed. Unconjugated cholic (5 mM) and deoxycholic (1 mM) acids and the organic anion rose bengal (0.1 mM) inhibited the absorption of 0.5 microM 5-methyltetrahydrofolate when added to the perfusate. CONCLUSIONS: the results of previous in vitro studies of 5-methyltetrahydrofolate intestinal transport are applicable to in vivo situations, except that luminal glucose was found to be unnecessary in the latter. The unstirred water layer modulated the absorption of 5-methyltetrahydrofolate, while unconjugated bile acids and rose bengal inhibited it.
UNLABELLED: Transport results from in vitro studies may not be applicable to in vivo situations. In this study, we extended our previous in vitro observations regarding the intestinal transport of 5-methyltetrahydrofolate to in vivo studies in the unanesthetized rat and examined the effect of the unstirred water layer on the absorption process. We used a well defined intestinal perfusion technique. Absorption of 0.5 and 5 microM 5-methyltetrahydrofolate proceeded in a linear manner for 40 min of perfusion at 0.31 and 1.74 nmol/100 cm per min, respectively. Absorption of 0.5 microM 5-methyltetrahydrofolate increased with increasing perfusate flow-rate from 0.5 to 2 to 4 ml/min, indicating an unstirred water layer influence on the absorption rate. Absorption of the substrate was saturable with an apparent Kt of 5.7 microM and Vmax of 3.45 nmol/100 cm per min. Absorption was pH-dependent, and was inhibited by structural analogues. In contrast to the in vitro data, addition of glucose (20 mM) to the perfusate was unnecessary for in vivo absorption to proceed. Unconjugated cholic (5 mM) and deoxycholic (1 mM) acids and the organic anion rose bengal (0.1 mM) inhibited the absorption of 0.5 microM 5-methyltetrahydrofolate when added to the perfusate. CONCLUSIONS: the results of previous in vitro studies of 5-methyltetrahydrofolate intestinal transport are applicable to in vivo situations, except that luminal glucose was found to be unnecessary in the latter. The unstirred water layer modulated the absorption of 5-methyltetrahydrofolate, while unconjugated bile acids and rose bengal inhibited it.