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Transport of Tri- and Dicarboxylic Acids Across the Intestinal Brush Border Membrane of Calves^1,2,

Institute of Veterinary Physiology, CH-8057 Zürich, Switzerland

Transport of tri- and dicarboxylic acids across the intestinal brush border membrane was investigated using citrate and fumarate as transport substrates. The experiments were performed with brush border membrane vesicles isolated from calf proximal jejunum. Citrate and fumarate uptake by the brush border membrane vesicles occurred by a common Na⁺-dependent transport mechanism that appears to be specific for tri- and dicarboxylates. The protonated forms of citrate (citrate^-1 and citrate^-2) seem to be much better transported than the trivalent form, as indicated by the strong stimulation of citrate uptake at an extravesicular pH of 5.6 compared to that at pH 7.8. Furthermore, citrate transport across the intestinal brush border membrane appears to be mediated by an electroneutral process.

KEY WORDS: • citrate • fumarate • intestinal brush border membrane • intestinal transport • calves

¹ Some of the results were presented at the Ninth meeting of the European Intestinal Transport Group in Szeged, Hungary, 1989 (ref. 1).

² This work was supported by the Schweizerische Nationalfonds (grant no. 3.808-0.87).

Manuscript received 18 September 1989. Revision accepted 17 January 1990.