Figure 2. Conceptual diagram of active Na⁺ and Cl⁻ transport in mammalian colonic epithelium.

Possible transporters involved are shown, but not all are present in the same cell (e.g. rabbit distal colon lacks Na⁺-H⁺ exchange and Cl⁻-HCO₃⁻ exchange, while rat distal colon lacks Na⁺ channel). Na⁺-H⁺ exchange may be coupled to Cl⁻-HCO₃⁻ exchange, or independently coupled to SCFA⁻-HCO₃⁻ and Cl⁻-SCFA⁻ exchanges (Rajendran & Binder, 1994). Butyrate absorption into the cell may occur via non-ionic diffusion, and dissociation of butyrate within the cell may increase intracellular H⁺ ([H⁺]_i) with stimulation of Na⁺-H⁺ exchange. Butyrate⁻-HCO₃⁻ exchange (Mascolo et al. 1991) across the apical membrane will also increase [H⁺]_i and stimulate Na⁺-H⁻ exchange across the apical membrane. Increased [H⁺]_i will also inhibit Na⁺ absorption via Na⁺ channels (Chalfant et al. 1999). Cl⁻ secretion involves Cl⁻ channels at the apical membrane, and Na⁺-K⁺-2Cl⁻ cotransporter activity and sodium pump activity in the basolateral mambrane. Inhibition of Cl⁻ secretion by butyrate may occur at any of these levels. Butyrate also inhibits cAMP generation by colonic epithelium (Krishnan et al. 1999). *Potential levels at which butyrate may potentiate (+) or inhibit (-) processes that eventually affect net Na⁺ transport.