title: Determination of Henry's constant, the dissociation constant, and the buffer capacity of the bicarbonate system in ruminal fluid.
abstract: Despite the clinical importance of ruminal acidosis, ruminal buffering continues to be poorly understood. In particular, the constants for the dissociation of H2CO3 and the solubility of CO2 (Henry's constant) have never been stringently determined for ruminal fluid. The pH was measured in parallel directly in the rumen and the reticulum in vivo, and in samples obtained via aspiration from 10 fistulated cows on hay- or concentrate-based diets. The equilibrium constants of the bicarbonate system were measured at 38°C both using the Astrup technique and a newly developed method with titration at 2 levels of partial pressure of CO2 (pCO2; 4.75 and 94.98 kPa), yielding mean values of 0.234 ± 0.005 mmol ∙ L(-1) ∙ kPa(-1) and 6.11 ± 0.02 for Henry's constant and the dissociation constant, respectively (n/n = 31/10). Both reticular pH and the pH of samples measured after removal were more alkalic than those measured in vivo in the rumen (by ΔpH = 0.87 ± 0.04 and 0.26 ± 0.04). The amount of acid or base required to shift the pH of ruminal samples to 6.4 or 5.8 (base excess) differed between the 2 feeding groups. Experimental results are compared with the mathematical predictions of an open 2-buffer Henderson-Hasselbalch equilibrium model. Because pCO2 has pronounced effects on ruminal pH and can decrease rapidly in samples removed from the rumen, introduction of a generally accepted protocol for determining the acid-base status of ruminal fluid with standard levels of pCO2 and measurement of base excess in addition to pH should be considered.
authors.affiliation: Institute of Veterinary Physiology, Faculty of Veterinary Medicine, Freie Universität Berlin, 14163 Berlin, Germany.
authors.forename: Katharina T
authors.initials: KT
authors.lastname: Hille
authors.affiliation: Department of Animal Physiology, Systems Neurobiology and Neural Computation, Humboldt-Universität zu Berlin (Germany), 10115 Berlin, Germany.
authors.forename: Stefan K
authors.initials: SK
authors.lastname: Hetz
authors.affiliation: Institute of Veterinary Physiology, Faculty of Veterinary Medicine, Freie Universität Berlin, 14163 Berlin, Germany.
authors.forename: Julia
authors.initials: J
authors.lastname: Rosendahl
authors.affiliation: Institute of Veterinary Physiology, Faculty of Veterinary Medicine, Freie Universität Berlin, 14163 Berlin, Germany.
authors.forename: Hannah-Sophie
authors.initials: HS
authors.lastname: Braun
authors.affiliation: Institute of Animal Nutrition, Faculty of Veterinary Medicine, Freie Universität Berlin, 14195 Berlin, Germany.
authors.forename: Robert
authors.initials: R
authors.lastname: Pieper
authors.affiliation: Institute of Veterinary Physiology, Faculty of Veterinary Medicine, Freie Universität Berlin, 14163 Berlin, Germany. Electronic address: stumpff@zedat.fu-berlin.de.
authors.forename: Friederike
authors.initials: F
authors.lastname: Stumpff
mesh_terms: D000138:Acidosis / Q000662:veterinary; D000818:Animals; D001639:Bicarbonates / Q000737:chemistry*; D001826:Body Fluids / Q000737:chemistry; D002021:Buffers; D002417:Cattle / Q000502:physiology*; D004032:Diet / Q000662:veterinary; D005260:Female; D006863:Hydrogen-Ion Concentration; D012159:Reticulum / Q000378:metabolism; D012417:Rumen / Q000378:metabolism*
chemical_list: D001639:Bicarbonates; D002021:Buffers
keywords: Henry’s constant; bicarbonate; dissociation constant; rumen