Epithelia of the ovine and bovine forestomach express basolateral maxi-anion channels permeable to the anions of short-chain fatty acids

Abstract

It has long been established that the absorption of short-chain fatty acids (SCFA) across epithelia stimulates sodium proton exchange. The apically released protons are not available as countercations for the basolateral efflux of SCFA anions and a suitable transport model is lacking. Patch clamp and microelectrode techniques were used to characterize an anion conductance expressed by cultured cells of the sheep and bovine rumen and the sheep omasum and to localize the conductance in the intact tissue. Cells were filled with a Na-gluconate solution and superfused with sodium salts of acetate, propionate, butyrate, or lactate. Reversal potential rose and whole cell current at +100 mV decreased with the size of the anion. Anion-induced currents could be blocked by diisothiocyanato-stilbene-2,2'-disulfonic acid (DIDS), NPPB (200 μmol l(-1)), or pCMB (1 mmol l(-1)). In patches of bovine ruminal cells, single channels were observed with a conductance for chloride (327 ± 11 pS), acetate (115 ± 8 pS), propionate (102 ± 10 pS), butyrate (81 ± 2 pS), and gluconate (44 ± 3 pS). Channels expressed by sheep rumen and omasum were similar. Microelectrode experiments suggest basolateral localization. In conclusion, forestomach epithelia express basolateral maxi-anion channels with a permeability sequence of chloride > acetate > propionate > butyrate. SCFA absorption may resemble functionally coupled transport of NaCl, with the Na(+)/K(+)-ATPase driving the basolateral efflux of the anion through a channel. Since protons are apically extruded, the model accurately predicts that influx of buffers with saliva is essential for the pH homeostasis of the ruminant forestomach.

Evidence for the functional involvement of members of the TRP channel family in the uptake of Na+ and NH4 + by the ruminal epithelium

“…Thus, bumetanide, which blocks Na + -K + -2Cl − transport, had no effect in this study, while past attempts to stimulate CFTR by elevating cAMP have failed [16,67]. Current models suggest that chloride is absorbed by the rumen via an electrically silent process [35] that involves apical Cl − /HCO 3 − exchange coupled to Na + /H + exchange [3,20] and basolateral efflux via an anion channel [20,45,55], driven by the Na + /K + -ATPase. A central clue toward identifying the apical uptake pathway for NH 4 + comes from the observation that both in bovine tissues and in isolated cells in patch clamp experiments, the NH 4 + -induced current could be strongly enhanced by removal of Ca 2+ and Mg 2+ (Figs.…”

“…Capacitance was 34.45 ± 5.01 pF (n = 8). Since all ions except Ca 2+ were in equilibrium across the membrane, the negative reversal potential reflects a significant contribution of chloride to total current [20,55]. Accordingly, I 100 was significantly reduced by the subsequent replacement of Cl − by gluconate (p < 0.05).…”

“…Removal of tissue occurred some 15 min after death, after which it was immediately stripped and transported to the laboratory (∼1 h) in a warm (37°C) and gassed (95 % O 2 /5 % CO 2 ) transport buffer containing (in mmol·l −1 ) the following: 115 NaCl, 25 NaHCO 3 , 0.4 NaH 2 PO 4 , 2.4 Na 2 HPO 4 , 5 KCl, 5 Glucose, 1.2 CaCl 2 , and 1.2 MgCl 2 (pH∼7.4) [33]. For cell culture, tissues were transported in ice-cold buffer, isolated via trypsination and cultured as in previous studies [17,20].…”

“…All whole cell patch clamp experiments were performed with an EPC9 amplifier using TIDA or Patchmaster software with automatic offset potential, capacitance, and series resistance correction (HEKA Elektronik, Lamprecht, Germany) [20,55]. A DMZ-Universal-Puller (Zeitz-Instruments, Munich, Germany) was used to pull pipettes to a resistance of ∼4 to 5 MΩ.…”

From microbe to man: the role of microbial short chain fatty acid metabolites in host cell biology

“…There are several reports in the literature of other potential candidates to mediate colonic SCFA transport: for example, one recent report described a basolateral maxi-anion channel in the bovine GI tract that is permeable to SCFAs (26), and an organic anion transporter (mOat2) found in the kidney and liver has also been identified as an SCFA transporter (39). The most well-described candidate aside from SLC5A8 is likely monocarboxylate transporter 1 (MCT1).…”