Effects of dissolved carbon dioxide on the integrity of the rumen epithelium: An agent in the development of ruminal acidosis.

The carbon dioxide released and dissolved in rumen fluid may easily permeate across the epithelial cell membrane. Thus, we hypothesized that CO may act as proton carrier and induce epithelial damage under acidotic conditions. Ovine ruminal epithelia were mounted in Ussing chambers under short-circuit conditions. The serosal buffer solution had a constant pH of 7.4 and was gassed either with 100% oxygen or with carbogen (95% O /5% CO ). The mucosal solution was gassed with either 100% oxygen or 100% carbon dioxide. The mucosal pH was lowered stepwise from 6.6 to 5.0 in the presence or absence of short-chain fatty acids (SCFA). The transepithelial conductance (G ) as an indicator of epithelial integrity and the short-circuit current (I ) as an indicator of active electrogenic ion transfer were continuously monitored. At an initial mucosal pH of 6.6, there was no significant difference in G between the treatment groups. In the absence of both SCFA and CO , G remained constant when the mucosal solution was acidified to pH 5.0. In the presence of SCFA, mucosal acidification induced a significant rise in G when the solutions were gassed with oxygen. A small increase in G was observed in the mucosal presence of CO . However, no difference in final G was observed between SCFA-containing and SCFA-free conditions under carbon dioxide gassing during stepwise mucosal acidification. The SCFA+proton-induced increase in G could also be minimized by serosal gassing with carbogen. Because of the SCFA+proton-induced changes in G and their attenuation by CO , a protective role for mucosally available carbon dioxide may be assumed. We suggest that this effect may be due to the intraepithelial conversion of carbon dioxide to bicarbonate. However, the serosal presence of CO at a physiological concentration may be sufficient to protect the epithelia from SCFA+proton-induced damage for a certain period of time.