Water absorption in relation to fermentation in the colon of the ostrich (Struthio camelus).

The colon is a major site for fermentation and water absorption in the ostrich. Water absorption along the colon was evaluated and its relationship to osmolality, Na+ concentration, short chain fatty acid (SCFA) concentration and carbohydrate content of digesta analysed. Mean water content decreased from 5.30 +/- 0.99 to 2.51 +/- 0.13 mf/g dry mass in the first 5 m of the colon. Correspondingly, mean carbohydrate content fell from 529.85 +/- 46.61 to 434.99 +/- 29.89 mg/g dry mass. A significant correlation was shown between the decreases in mean carbohydrate and water content along the colon (r2 = 0.997, P < 0.05). Changes in mean osmolality (+/- 10 mOsm/kg) and SCFA concentration (+/- 7 mmol/l) were minimal in comparison to the change in Na+ concentration (-54 mmol/l). These findings reflect a close coupling between SCFA production and absorption on the one hand and water absorption on the other.

Insight into the mechanism of short chain fatty acid absorption in the ostrich (Struthio camelus) proximal colon.

1. Occurrence of short chain fatty acids (SCFA) in anionic form limits their diffusion across the absorptive membrane. The present study sought to establish the mechanism of SCFA absorption in the ostrich. 2. Epithelial tissues were taken from the sacculated part of the colon and mounted in Ussing chambers in a bathing solution. The tissues were voltage-clamped and allowed to equilibrate to obtain a baseline short circuit current (Isc). 3. Propionate (23 mM) on the mucosal side increased the Isc. The SCFA-induced Isc was completely inhibited by anoxia, ouabain (1 to 2 mM), acetazolamide (0.5 mM) and 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (0.1 mM) on the mucosal side. 4. These findings indicate that SCFA stimulate hydrogen ion secretion through an electrogenic H(+)-K(+)-ATPase, the source of hydrogen ions being carbonic anhydrase catalysed hydration of CO2. 5. Simultaneous activation of Cl(-)/HCO3(-) exchange prevents intracellular accumulation of bicarbonate ions. This system may provide hydrogen ions for protonation of SCFA anions and subsequent absorption by non-ionic diffusion.