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 HCO3 - occurred in both directions, but Jms HCO3 - was significant larger than Jsm . Reducing the activity of the apical Na/H exchanger by a low mucosal Na concentration caused a significant reduction of Jms HCO3 - . Mucosal amiloride or short chain fatty acids (25 mmol l-1 SCFA) numerically decreased Jms HCO3 - , 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-1 mucosal ammonia did not change transport rates. The obtained results indicate the importance of an undisturbed cytosolic pH for transcellular HCO3 - transport, which is probably mediated by an anion exchanger in both the apical and basolateral membranes. Possible impairment of HCO3 - transport appears to be an overlooked factor in the pathogenesis of displacement of the abomasum.