Non‐ionic diffusion and carrier‐mediated transport drive extracellullar pH regulation of mouse colonic crypts.

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Abstract
1. Extracellular pH (pHo) regulation within mouse colonic crypt lumens is stimulated by transepithelial gradients of short‐chain fatty acids (SCFAs). Current work assesses underlying mechanisms contributing to pHo regulation. 2. Crypt luminal alkalinization was saturable by apical SCFA (substrate concentration activating half‐maximal transport (KT) of isobutyrate = 45 mM). However, saturation was consistent with either carrier‐mediated SCFA flux or non‐ionic diffusion, because the non‐ionized form was titrated by luminal alkalinization. Direct acidification of apical perfusates increased the magnitude of SCFA‐induced luminal alkalinization, roughly in the same proportion to the increased concentration of non‐ionized SCFA in the crypt lumen. 3. Transepithelial gradients of an alternative weak acid (CO2) produce pHo changes similar to SCFA. In contrast, a weak base (NH3) changes pHo with reverse dependence on the orientation of the transepithelial gradient compared with SCFA. Results implicate non‐ionic diffusion in pHo regulation, and suggest that pHo changes may underly SCFA‐stimulated bicarbonate secretion and ammonium absorption. 4. SCFA metabolism plays a minor role in extracellular pH regulation. An avidly metabolized SCFA (N‐butyrate) augments crypt luminal alkalinization only slightly (0.08 pH units) versus a poorly metabolized SCFA (isobutyrate). 5. Apical addition of 1 mM 4,4'‐diisothiocyanatostilbene‐2,2'‐disulphonic acid (DIDS) partially inhibits luminal alkalinization caused by apical SCFA. DIDS has no effect on luminal alkalinization caused by transepithelial CO2 gradients. Probenecid (1 mM), alpha‐cyano‐4‐hydroxycinnamic acid (4 mM) or basolateral DIDS (1 mM) do not affect pHo regulation. Results suggest that DIDS‐sensitive, SCFA‐dependent transport in the colonocyte apical membrane contributes to pHo regulation.