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Department of Physiology, School of Veterinary Medicine, Hannover, Germany.

1. Effects of H+ secreting mechanisms on unidirectional passage of the short-chain fatty acids (SCFA) acetate, propionate and n-butyrate across isolated guinea-pig caecum, proximal and distal colon were studied under short-circuit current conditions in Ussing chamber isotope flux experiments. 2. In the caecum and the proximal colon the serosal-to-mucosal fluxes (Jsm) were higher than the mucosal-to-serosal fluxes (Jms). Thus a net secretion of SCFA was present in the caecum and proximal colon. The higher Jsm appears to be coupled to the Na+ gradient established by the basolateral membrane Na(+)-K(+)-ATPase, whereas Jms is related to the operation of an apical membrane Na(+)-H+ exchanger. Inhibition of Na(+)-H+ exchange by amiloride (1 mM) added to the mucosal solution decreased Jms of SCFA in caecum and in proximal colon, but had no major effect in distal colon. 3. In distal colon Jms exceeds Jsm and thus a net absorption of SCFA was observed. Jms is Na+ independent and coupled to the activity of the apical membrane K(+)-H(+)-ATPase. Inhibition of the K(+)-H(+)-ATPase by addition of ouabain (0.1 mM) to the mucosal solution diminished Jms in the distal colon; in the caecum and proximal colon ouabain had no effect on Jms. 4. Neither amiloride nor ouabain caused major changes in Jsm in any of the large intestinal segments. 5. In conclusion, absorption of SCFA, e.g. Jms, in all large intestinal segments is related to the presence and activity of H+ secreting systems located in the apical membrane of colonocytes. In the caecum and proximal colon the predominant system appears to be Na(+)-H+ exchange, and in the distal colon K(+)-H(+)-ATPase. Supply of H+ ions allows protonation of SCFA anions and subsequent permeation by non-ionic diffusion across the apical membrane. These mechanisms account for 35, 40-50 and 60-80% of SCFA transport in the caecum, the proximal and the distal colon of guinea-pig, respectively. The nature of the Na(+)-dependent secretory pathway in the caecum and proximal colon remains to be determined.

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