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Uptake of Riboflavin by Rat Intestinal Mucosa In Vitro^1,2,

Gastrointestinal Research Laboratory, Medical Research Service, Veterans Affairs Medical Center (151) and the Medical College of Georgia, Augusta, GA 30910

The uptake of riboflavin was studied in everted sacs of rat intestine using [¹⁴C]riboflavin and [³H]polyethylene glycol to define the mechanism of mucosal membrane transport. Three-minute incubations were used throughout. Initial studies indicated the presence of saturable uptake in duodenum, jejunum and ileum. Studies in jejunum at low riboflavin concentrations demonstrated saturable uptake [K_m = 0.154-0.177 µmol/L, V_max = 19.6-25.8 pmol/(100 mg dry tissue·min)]. In contrast, uptake was linear with respect to higher concentrations of vitamin (10–50 µmol/L). Uptake at low (0.1 µmol/L) but not high (20 µmol/L) riboflavin concentrations was inhibited by 50 µmol/L lumiflavin, anoxia, 5 mmol/L indoacetamide, Na⁺-free buffer and low temperature (Q₁₀ = 1.64). Conclusions are as follows: 1) Saturable uptake of riboflavin occurs throughout the rat small intestine; 2) uptake by the jejunal mucosa is competitively inhibited and is consistent with a transport carrier located in the brush border membrane; 3) saturable uptake is energy-dependent and may be directly or indirectly driven by a Na⁺ gradient; and 4) riboflavin is also taken up by rat intestinal mucosa by a nonsaturable, energy-independent mechanism consistent with simple, passive diffusion.

KEY WORDS: • riboflavin • intestinal absorption • biological transport • active transport • rats

¹ Presented in part as a poster presentation at Digestive Disease Week, May 24, 1983, Washington, D.C. [MIDDLETON, H. M., III (1983) Uptake of riboflavin by the rat intestinal mucosa in vitro. Gastroenterology 84: 1249 (abs.)].

² This work was supported by the Department of Veterans Affairs.

Manuscript received 16 May 1989. Revision accepted 23 January 1990.

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