A single mild episode of subacute ruminal acidosis does not affect ruminal barrier function in the short term

Twenty-four German Merino sheep (72.3±10.1kg of body weight) were fed an all-hay diet and assigned to either the subacute ruminal acidosis (SARA) treatment (n=17) or sham treatment (n=7). The SARA sheep were orally dosed with a 2.2 M glucose solution to supply 5g of glucose/kg of body weight, whereas sham sheep received an equal volume of water. Ruminal pH was measured for 48h before and 3h after the oral dose. Sheep were then killed and ruminal epithelia from the ventral sac were mounted in Ussing chambers. The serosal-to-mucosal flux rate of partially 3H-labeled mannitol (Jmannitol-SM), an indicator of barrier function, was measured while epithelia were exposed to 3 sequential in vitro measurement periods lasting 1h each. The measurement periods consisted of baseline, challenge, and recovery periods and were interspersed by 30-min periods for treatment equilibration. Baseline conditions were pH 6.1 (mucosal solution) and pH 7.4 (serosal solution) with a bilateral osmolarity of 293mOsm/L. During the challenge period, the mucosal side of the epithelia was exposed to either an acidotic challenge (pH 5.2, osmolarity 293mOsm/L) or an osmotic challenge (pH 6.1, osmolarity 450mOsm/L); a third group served as control (pH 6.1, osmolarity 293mOsm/L). The mucosal buffer solution was replaced for the recovery period. In vivo, sheep on the SARA treatment had lower mean (5.77 vs. 6.67) and nadir (5.48 vs. 6.47) ruminal pH for the 3h following the oral drench compared with sham sheep, indicating the successful induction of SARA with the oral glucose dose. Despite the marked reduction in pH in vivo, induction of SARA had no detectable effects on the baseline measurements of Jmannitol-SM, tissue conductance (Gt), and short-circuit current (Isc) in vitro. However, reducing mucosal pH to 5.2 in vitro had negative effects on epithelial barrier function in the recovery period, including increased Jmannitol-SM, increased Gt, and decreased Isc. The osmotic challenge increased Jmannitol-SM and Gt and decreased Isc during the challenge period, which was reversible in the recovery period except for slight reduction in Isc. Interactions between the in vitro treatment and measurement period were detected for Jmannitol-SM, Gt, and Isc. These data indicate that a mild episode of SARA (nadir pH, 5.48; duration ruminal pH