Subacute ruminal acidosis (SARA) is considered a major animal health and welfare issue in intensive ruminant production systems (Plaizier et al., 2008). While initially the focus was on low pH, it is now recognized that outcomes arising from SARA initiated by low digesta pH, increased SCFA concentration, and hyperosmolarity (Owens et al., 1998; Hernandez et al., 2014; Humer et al., 2018). The previously mentioned changes in the rumen milieu coupled with rapid fermentation leads to the release of microbial metabolites and cell wall fragments such as HIS and LPS (Pilachai et al., 2012; Dong et al., 2013; Liu et al., 2013; Mao et al., 2016). Low pH, hyperosmolality, and accumulation of microbial associated molecular patterns (MAMPS) are considered as the potential triggers of ruminal epithelial barrier damage and the subsequent activation of systemic inflammation (Aschenbach et al., 2019). However, the study of Schurmann et al. (2014) reported that although ruminal acidosis was not induced, tissue conductance and mannitol flux, as a measure of permeability, linearly increased as calves were fed a diet containing 50% concentrate for 3, 7, 14, and 21 d suggesting that the RE barrier function could be modulated even in the absence of ruminal acidosis. Similarly, experiments by Meissner et al. (2017) and Greco et al. (2018) demonstrated that low pH alone does not increase the permeability of RE and further reported that the presence of VFA were responsible for the increase the permeability when coupled with low ruminal pH. Thus, there is a growing body of research suggesting that changes occurring concurrently with decreased ruminal pH during SARA may damage RE barrier function. However, the specific role of individual components is not clear.
In the present study, the Isc of the RE was increased by HIS compared with CON, which is similar to the changes of physiological parameters of RE during SARA where increased Isc and Gt were observed (Klevenhusen et al., 2013). However, these results differ from Aschenbach and Gäbel (2000) where no effect of HIS was detected on Isc. Aschenbach et al. (2000) illustrated a very efficient intraepithelial catabolism at a mucosal pH of 7.4. The catabolism of histamine seems to comprise a complex enzymatic pathway initiated by the diamine oxidase enzyme (DAO) (Sjaastad, 1967; Dickinson and Huber, 1972). Sun et al. (2017) demonstrated that histamine can activate of the NF-κB inflammatory pathway and upregulate the expressions of the inflammatory cytokines (TNF-α, IL-6, and IL-1β), a then induce the inflammatory response in bovine rumen epithelial cells. Thus, both efficient intraepithelial catabolism of histamine and induced inflammatory response in rumen epithelium seemingly indicated an induced metabolism in rumen epithelium which may be partly account for the increased Isc of RE under histamine.
The apparent permeability of HIS and LPS in RE were compared and the results showed that the Papp of HIS was less than that for LPS. Aschenbach et al. (2000) suggested that, at a mucosal pH of 7.4, permeability of the ruminal epithelium to histamine was very low. In addition, their study also demonstrated a very efficient intraepithelial catabolism of histamine (mucosal to serosal direction, 98.7%) at mucosal pH 7.4 and a significant secretory mechanisms from serosal to mucosal side (Aschenbach et al., 2000). Thus, their results in vitro approach established that the intact ruminal epithelium is a very effective barrier to luminal histamine (Aschenbach and Gäbel, 2000). LPS is thought to enter circulation by transport across the intestinal epithelium via not only paracellular pathways through the openings of intestinal tight junctions between two epithelial cells, but also by a transcellular pathway through lipid raft membrane domains involving receptor-mediated endocytosis (Berg, 1995; Drewe et al., 2001; Mani et al., 2012). Transcellular passive transportation is the predominate pathway of LPS absorption by intestinal mucosa (Drewe et al., 2001) and specific transport system for LPS was observed in colonic epithelial cells (Tomita et al., 2004). Furthermore, significantly increased translocation of LPS from the mucosal to the serosal side of rumen tissues under the presence of mucosal side LPS was observed by Emmanuel et al. (2007). Thus, in the present study, the higher Papp of LPS than HIS indicated that LPS seemingly can more easily pass through the gastrointestinal tract than HIS.
Supporting past research, we did not observe an effect of HIS on Gt under incubation conditions with a pH of 7.4 (Aschenbach and Gäbel, 2000). While Gt was not affected, HIS increased Papp and flux rate to FITC suggesting a direct role of HIS on altering RE permeability. Aschenbach et al. (2000) also reported that HIS receptors are broadly distributed and evidence of their localization within smooth muscle of the rumen (Ohga and Taneike, 1978) causing cessation of rumen contractions, increased vascular blood flow, and increased vascular permeability. In addition, HIS has been reported to increase permeability of the intestinal tract in rabbits (Kingham and Loehry, 1976; Miller et al., 1992) and permeability of HIS to cross the RE barrier increases with exposure to acidic pH (Aschenbach and Gäbel, 2000; Aschenbach et al., 2000); however, when measured in vivo under anesthesia, permeability of the rumen to HIS was low (Kay and Sjaastad, 1974). Overall, the results of the present study support previous research that HIS exposure may have a causative role in reducing the barrier function of the RE. In addition, the mRNA abundance of OCLN, one of the tight junctions, was downregulated in HIS compared with CON (Fig. 3). Epithelial barrier function is primarily dependent on tight junction (TJ) proteins that limit paracellular permeation (Marchiando et al., 2010). A variation in epithelial permeability can be related to a change in the general abundance of TJ proteins, including the localization and the interaction of the proteins (Markov et al., 2015, 2017). The study of Liu et al. (2013) suggested that downregulation of TJ protein (CLDN-4 and OCLN) as well as redistribution of CLDN-1, CLDN-4, and OCLN out of the TJ caused the increased RE permeability. Thus, the downregulated mRNA abundance of OCLN of the present study further implied a disrupted permeation barrier of RE by HIS.
Little effects of LPS on permeability and electrophysiological character of rumen tissues was observed at pH 7.4 in the present study, which is similar with the results of Emmanuel et al. (2007) where the presence of LPS did not affect permeability of rumen tissue to 3H-mannitol at pH values of 5.5 and 6.5. However, at pH 4.5, the permeability of rumen tissues to 3H- mannitol was increased more than 6-fold by the presence of LPS (Emmanuel et al., 2007). Similar phenomenon also occurred in colon tissues, where no difference permeability of 3H- mannitol due to LPS at pH of 6.5 and 7.4, but permeability was increased at pH 5.5 (Emmanuel et al., 2007). Thus, consistent with previous results, this study suggested that separately increased mucosal side LPS without acidic pH has no significant effect on the permeability of rumen tissues.