Ketosis
Two forces rule cellular homeostasis. The proton motive force, PMF (Abad, 2011) and pH (Occhipinti & Boron, 2019). The ruminal epithelium is driven by HCO3- transport and high intracellular HCO3- content (Bilk et al., 2005; Lee & Hong, 2020), i.e., the high intracellular HCO3- gradient with the exterior contrast with the high ruminal dCO2 concentration with the and low intracellular CO2.
Aquaporins assist in transporting both water and CO2 into cells (Endeward et al., 2017; Zhong et al., 2020), and transform both molecules into HCO3- and protons to keep this gradient. This process is helped by the intracellular carbon anhydrase (CA), see previous post (Rabbani et al., 2021; Vilas et al., 2015). Therefore, the CO2/HCO3- gradient better explains cellular homeostasis, the pH scale cannot measure concentrations, it is a quotient.
High ruminal CO2 favours HCO3- dehydration (rumen) and high intracellular HCO3- favours CO2 hydration (cells). However, cells evolving in rich CO2 and HCO3- environments, required PMF to promote cellular homeostasis. i.e., RBC accumulates Cl- to increase HCO3- carrying capacity (Klocke, 1987), futile cycles help bacteria to survive under extreme conditions (Buurman et al., 1991) hence the apparently coupling of pH and PMF.
Why is this relevant? catabolism enhances HCO3- generation to replenish the intracellular pool, and high HCO3- increases cellular anabolism. Thus, intracellular HCO3- homeostasis has a profound effect on energy metabolism and cell function (Alka & Casey, 2014; Blombach & Takors, 2015). Since, the ruminal epithelium depends on the ruminal CO2 absorption for HCO3- formation (Rackwitz & Gabel, 2018), a decline in ruminal CO2 and H2O absorption via aquaporins (dehydration) might trigger cellular catabolism due to cellular dehydration.
Ketosis
Ruminant ketosis poses a significant economic burden in the cattle industry. While, we considered primary ketosis (post-calving) and secondary ketosis (starvation-induced) separate entities (Baird et al., 1972; Oetzel, 2007), they might share a similar mechanism: cellular dehydration
Secondary ketosis: Krebs in 1960 described the effect of starvation on ketogenesis of liver samples. Further research suggest that acidification of the intracellular cells might trigger this response.
Primary ketosis: many theories have been proposed but what we know is that AMPK activation (cellular energy gauge) trigger ketogenesis, even when normal levels of energy are present.
The link between both might be “cellular dehydration” due to CO2 holdup and/or starvation. The concomitant intracellular “HCO3- depletion” triggers catabolism to replenish this intracellular pool and might lead to Ketosis.
Cellular dehydration the unifying theory
Normally the ruminal epithelium requires for normal function the joint absorption of ruminal water and CO2 via aquaporins. Intracellularly both molecules are combined to form HCO3- and H+ via carbon anhydrase. The cells use preferably these molecules to drive Na+ and SCFAs absorption (Rackwitz and Gäbel, 2018). In this way HCO3- and H+ are excreted back into the rumen to be reconstituted into CO2 and H2O. Up to 80% of all CO2 transactions in the rumen epithelium are done using this “aquaporin cycling” (Veenhuizen et al., 1988). In fact, dCO2 promotes greater SCFAs absorption (Ash and Dobson, 1963).
During starvation, both SCFAs and dCO2 formation are limited, leading to a reduction in intracellular HCO3- and H3O+ formation and cellular dehydration.
After calving, the exposure to production diets can trigger CO2 holdup, which is a condition where the physicochemical properties of the ruminal fluids changes and CO2 cannot longer effervescent freely from the fluid leading to its accumulation as liquid dissolved CO2. CO2 holdup might cause ruminal acidosis (Laporte Uribe, 2023), but high dCO2 leads to hyperosmolarity (Ash & Dobson, 1963) and reduces H2O absorption (Dobson, 1984; Dobson et al., 1971; Lodemann & Martens, 2006). Ketosis, or the formation of ketone bodies due to intracellular catabolism (White, 2015) might be triggered by CO2 holdup and its effect on ruminal hyperosmolarity and cellular dehydration.
Consequences
Reduced HCO3- Formation: Within rumen epithelial cells, carbonic anhydrase converts CO2 and water into bicarbonate (HCO3-) and protons (H3O+). However, with limited CO2 and water uptake, HCO3- formation declines leading to intracellular acidification.
Impaired SCFA and Na+ Absorption: HCO3- and H+ plays a crucial role in the absorption of SCFAs and sodium (Na+) from the rumen. A decrease in HCO3- availability consequently hampers SCFA and Na+ uptake leading to SCFA accumulation.
AMPK activation: reduced HCO3- formation and decreased SCFA absorption leads to AMPK activation. AMPK, acting as a cellular fuel gauge, triggers fat mobilization and ketone production (ketogenesis) as an alternative energy source.
Ketogenesis is likely a compensatory mechanism to replenish, intracellular H2O and CO2 pools for HCO3- and protons formation lost due to the impaired rumen function.
While the initial triggers for primary and secondary ketosis differ (dietary changes vs. starvation), the proposed mechanism suggests a common pathway leading to ketosis and offers a unifying perspective, emphasizing the critical role of ruminal dCO2 formation and transport.
Continuous dCO2 Monitoring as a Preventive Measure:
The theory highlights the importance of continuously monitoring ruminal dCO2, as a potential preventive measure for ketosis. By identifying early signs of CO2 hold-up and low fermentative activity (starvation) dietary interventions can be implemented to minimize the risk of cellular dehydration and subsequent ketogenesis.
