A model of ruminal volatile fatty acid absorption kinetics and rumen epithelial blood flow in lactating Holstein cows

Ruminal absorption of volatile fatty acids (VFA) is quantitatively the most important nutrient flux in cattle. Historically, VFA absorption models have been derived primarily from ruminal variables such as chemical composition of the fluid, volume, and pH. Recently, a mechanistic model incorporated the control of VFA absorption from epithelial surface area of the reticulorumen. In the present study, we hypothesized that ruminal absorption of VFA was controlled through epithelial permeability to VFA and rumen epithelial capillary blood flow. The objective of the study was to construct a model of VFA exchange across the rumen wall that incorporates epithelial blood flow as a driving force for ruminal VFA removal. The bidirectional fluxes between the ruminal and epithelial pool of VFA were assumed mass action driven, given that passive diffusion of nonionized VFA is the dominant transmembrane VFA flux. Parameter estimates were derived by fitting the model to observed data. The model provided reliable unbiased estimates of ruminal VFA absorption and rumen epithelial blood flow. Blood flow was modeled using an equation that considered the effect of butyrate and dietary crude protein intake per kilogram of body weight. The rate constants related to the flux from ruminal fluid to epithelium were in the order isobutyrate < acetate < propionate < butyrate (0.32±0.02, 0.72±0.2, 0.91±0.06, and 0.97±0.02 /h, respectively). The rate constants for fluxes of isobutyrate, acetate, propionate, and butyrate from the rumen epithelium to the ruminal fluid, relative to the pool size of the epithelium, were 4.78, 10.6, 13.4, and 14.3 /h, respectively. Ruminal concentrations of acetate, propionate, butyrate, and isobutyrate were predicted with root mean square prediction errors as percentage of the observed means (RMSPE) of 5.86, 5.75, 11.3, and 4.12, respectively. The epithelial blood flow was predicted with 26.3% RMSPE. Sensitivity analyses indicated that when ruminal butyrate concentration increased from 4.0 to 37.4mmol/L, blood flow of the epithelium increased 47% and the ruminal disappearance rate of propionate increased 11%. The concentration gradient of propionate between ruminal fluid and epithelium was no more than 3:1 and increased with increasing blood flow. In conclusion, a dynamic model based on rumen epithelial blood flow and bidirectional fluxes of VFA between ruminal fluid and epithelium gave unbiased predictions with low residual error of ruminal VFA absorption