RUMINANT NUTRITION SYMPOSIUM: Role of fermentation acid absorption in the regulation of ruminal pH12

Abstract

Highly fermentable diets are rapidly converted to organic acids [i.e., short-chain fatty acids (SCFA) and lactic acid] within the rumen. The resulting release of protons can constitute a challenge to the ruminal ecosystem and animal health. Health disturbances, resulting from acidogenic diets, are classified as subacute and acute acidosis based on the degree of ruminal pH depression. Although increased acid production is a nutritionally desired effect of increased concentrate feeding, the accumulation of protons in the rumen is not. Consequently, mechanisms of proton removal and their quantitative importance are of major interest. Saliva buffers (i.e., bicarbonate, phosphate) have long been identified as important mechanisms for ruminal proton removal. An even larger proportion of protons appears to be removed from the rumen by SCFA absorption across the ruminal epithelium, making efficiency of SCFA absorption a key determinant for the individual susceptibility to subacute ruminal acidosis. Proceeding initially from a model of exclusively diffusional absorption of fermentation acids, several protein-dependent mechanisms have been discovered over the last 2 decades. Although the molecular identity of these proteins is mostly uncertain, apical acetate absorption is mediated, to a major degree, via acetate-bicarbonate exchange in addition to another nitrate-sensitive, bicarbonate-independent transport mechanism and lipophilic diffusion. Propionate and butyrate also show partially bicarbonate-dependent transport modes. Basolateral efflux of SCFA and their metabolites has to be mediated primarily by proteins and probably involves the monocarboxylate transporter (MCT1) and anion channels. Although the ruminal epithelium removes a large fraction of protons from the rumen, it also recycles protons to the rumen via apical sodium-proton exchanger, NHE. The latter is stimulated by ruminal SCFA absorption and salivary Na(+) secretion and protects epithelial integrity. Finally, SCFA absorption also accelerates urea transport into the rumen, which via ammonium recycling, may remove protons from rumen to the blood. Ammonium absorption into the blood is also stimulated by luminal SCFA. It is suggested that the interacting transport processes for SCFA, urea, and ammonia represent evolutionary adaptations of ruminants to actively coordinate energy fermentation, protein assimilation, and pH regulation in the rumen.

Relative contribution of ruminal buffering systems to pH regulation in feedlot cattle fed either low- or high-forage diets

“…A major objective of our study was to determine whether changes in the ruminal SCFA profile when the supply of rapidly fermentable CHO in beef cattle is altered has an effect on ruminal epithelial permeability to SCFA. Passive diffusion of protonated (HSCFA) and protein-mediated transport of anionic SCFA (SCFA − ) are thought to be the major pathways for absorption of ruminal acid (Aschenbach et al, 2011). Compared with passive diffusion, limited research has been carried out to characterize the different protein-mediated transport systems involved, and their functional relevance to pH regulation.…”

Methane production and diurnal variation measured in dairy cows and predicted from fermentation pattern and nutrient or carbon flow

“…Endogenous C flow was estimated from endogenous CP and fat flow using the C content of bacterial fat and protein according to Reichl and Baldwin (1975). Possible bicarbonate flow at the duodenum was not considered as it was assumed that all bicarbonate is converted into CO 2 in the abomasum (Aschenbach et al, 2011;Dijkstra et al, 2012). According to glucose fermentation stoichiometry, 3 mol C are used for 1 mol Ac (2 mol C for Ac, 1 mol C for CO 2 ) and for 1 mol Pr, and 6 mol C for 1 mol Bu (4 mol C for Bu, 2 mol C for CO 2 ) and for 1 mol Va.…”

Metabolic adaptation and reticuloruminal pH in periparturient dairy cows experiencing different lipolysis early postpartum

“…The reason for the lower pH of the Low group can be due to an increased DMI found in this group of cows. Because ruminal pH is a product of both fermentation and neutralization processes, with chewing activity and inherent absorption and metabolization capacity of the rumen epithelium playing a role (Aschenbach et al, 2011), these factors might also have contributed to the differences in ruminal pH among groups.…”

“…Rumen pH is routinely used as an indicator of rumen health, as pH reflects both the ruminal milieu needed for proper ruminal functioning and also indicates the fermentation output available as energy source for gluconeogenesis and milk production (Aschenbach et al, 2010 and, thus likely modulating the metabolic adaptation of the transition cow. Nevertheless, as ruminal pH is also affected by neutralisation processes (Aschenbach et al, 2011), these factors might also contribute to rumen health and metabolic adaptation of periparturient dairy cows. We have recently demonstrated that transition dairy cows show different responses of ruminal pH despite similar feeding management (Humer et al, 2015a), posing them at different risks of rumen fermentation disorders postpartum.…”