Effects of dissolved carbon dioxide on the integrity of the rumen epithelium: An agent in the development of ruminal acidosis
The pH drop leads to a fermentation imbalance and affects the function and integrity of the epithelial wall, leading to cell oedema, rumenitis and cicatrizing of the epithelium (Dirksen, 1985, 1986; Gäbel, Bell, & Martens, 1989; Nagaraja & Titgemeyer, 2007). The pH threshold of intraruminal fermentation coincides with the vulnerability threshold of the ruminal epithelium because early inflammatory responses may occur when the ruminal pH is <5.6 for >1 hr (Gozho, Plaizier, Krause, Kennedy, & Wittenberg, 2005; Khafipour, Krause, & Plaizier, 2009).
However, there have been several hints that the drop in pH, that is the increase in the H+ concentration, is not the only cause of changes in the rumen wall. Additional factors/substrates must be present. Studies by Gäbel (1988) suggested that the epithelium is much more resistant to a drop in the ruminal pH when short‐chain fatty acids (SCFA) are absent. This resistance is probably due to the fact that SCFA act as direct or indirect proton carriers by shifting H+ across the cell membrane (if the pH is low enough to provide larger amounts of undissociated SCFA [HSCFA]), which leads to acidification of the cell interior and burdens the pH regulatory systems, especially the sodium/proton exchange proteins (NHE) (Müller, Aschenbach, & Gäbel, 2000).
However, HSCFA may act as carriers of not only protons but also CO2. Due to decarboxylation processes in the rumen and due to intraruminal conversion of HCO3− (provided by saliva and the rumen wall (Aschenbach et al., 2011)) into H2CO3, a large amount of CO2 is released. According to Emmanuel, Lawlor, and McAleese (1969), the concentration of total CO2 (CO2 + H2CO3 + HCO3−) may reach up to 65 mmol/L when roughage‐concentrate ratios are factored in. Laporte‐Uribe (2016) calculated an intraruminal concentration of total CO2 that was as high as 100 mmol/L.
Regarding the concentration of dissolved CO2 (dCO2) in the ruminal fluid, Henry's constant of CO2 was determined to be 0.244 ± 0.01 mmol L−1 kPa−1 (Hille et al., 2016) which means that the value is approximately 24‐fold higher than Henry's constant of O2 as measured in human blood (Jelkmann, 2010). According to Laporte‐Uribe (2016), Henry's constant of CO2 dissolved in ruminal fluid was even underestimated and might be higher due to the influences of viscosity and other physicochemical factors. CO2 is regarded as highly membrane permeable (Missner, Kugler, Antonenko, & Pohl, 2008; Missner & Pohl, 2009). Consequently, when it is released in the rumen fluid, CO2 is dissolved and then transferred from the ruminal contents into the cytosol, where conversion to H2CO3 occurs due to the high intracellular activity of carbonic anhydrase (Aafjes, 1967; Bondzio et al., 2011; Carter, 1971; Kaseda, Ichihara, Nishita, Amasaki, & Asari, 2006). Due to the pKa value of the acid/base system of ~6.1 (Aschenbach et al., 2011), which is below the physiological pH of ~7.