Modelling fluxes of volatile fatty acids from rumen to portal blood.

Abstract

A model was developed that describes the relationships between the production, the ruminal concentrations and the fluxes of volatile fatty acids (VFAs) across portal drained viscera (PDV). This study presents some preliminary results. Firstly, general steady state relationships between production and portal appearance of VFA were determined by fitting equations to published data coupling both intraruminal infusions and measurement of VFA portal fluxes. With a simple model consisting of one digestive and one blood compartment, saturation in uptake of arterial acetate (Ac) by PDV when arterial supply was very good, linearity between production and portal net absorption for each VFA, and saturation in ketogenesis by PDV were evidenced. Secondly, a dynamic model was used to assess the effect of pH on the VFA fluxes in the rumen between two meals (12 h interval). The fractional absorption rate of each VFA by the rumen wall (%/h) was fitted to pH according to their dissociation constant, from data obtained with the emptied washed rumen method. With a diet inducing large postprandial variations of pH, the effects of fractional absorption rate ‐ either constant (related to mean pH) or variable (related to postprandial variations of pH) ‐ on ruminal VFA concentrations and kinetics of VFA absorbed by the rumen wall were quantified.