Absorption of bicarbonate in sheep omasum.

Transport of bicarbonate across the isolated epithelium of sheep omasum was studied in vitro in Ussing chambers in combination with the pH-Stat method. The transport of HCO occurred in both directions, but J HCO was significant larger than J. Reducing the activity of the apical Na/H exchanger by a low mucosal Na concentration caused a significant reduction of J HCO. Mucosal amiloride or short chain fatty acids (25 mmol l SCFA) numerically decreased J HCO, but their combination (amiloride + SCFA) caused a significant reduction, which was also observed after addition of the carboanhydrase inhibitor ethoxyzolamide. Concentrations of 5 or 15 mmol·l mucosal ammonia did not change transport rates. The obtained results indicate the importance of an undisturbed cytosolic pH for transcellular HCO transport, which is probably mediated by an anion exchanger in both the apical and basolateral membranes. Possible impairment of HCO transport appears to be an overlooked factor in the pathogenesis of displacement of the abomasum.