Title:
Effects of ruminal ammonia and butyrate concentrations on reticuloruminal epithelial blood flow and volatile fatty acid absorption kinetics under washed reticulorumen conditions in lactating dairy cows.

Abstract:
The effect of reticuloruminal epithelial blood flow on the absorption of propionate as a volatile fatty acid (VFA) marker in 8 lactating Holstein cows was studied under washed rumen conditions. The cows were surgically prepared with ruminal cannulas and permanent catheters in an artery and mesenteric, right ruminal, and hepatic portal veins. The experiment was designed with 2 groups of cows: 4 cows adapted to high crude protein (CP) and 4 to low CP. All cows were subjected to 3 buffers: butyric, ammonia, and control in a randomized replicated 3 × 3 incomplete Latin square design. The buffers (30 kg) were maintained in a temporarily emptied and washed rumen for 40 min. The initial concentration of VFA was 84.2 mmol/L. Butyrate was increased from 4 to 36 mmol/L in butyric buffer by replacement of acetate, and ammonia (NH(3)) was increased from 2.5 to 22.5 mmol/L in ammonia buffer by replacement of NaCl. Increasing amounts of deuterium oxide (D(2)O) were added to the buffers as the order of buffer sequence increased (6, 12, and 18 g of D(2)O). Ruminal clearance of D(2)O was used to estimate epithelial blood flow. To increase accuracy of the epithelial blood flow estimates, data of ruminal liquid marker (Cr-EDTA), and initial and final buffer volumes were fitted to a dynamic simulation model. The model was used to estimate ruminal liquid passages, residual liquid, and water influx (saliva and epithelia water) for each combination of cow and buffer (n=24). Epithelial blood flow increased 49±11% for butyric buffer compared with control. The ruminal disappearance of propionate (marker VFA) was affected by buffer and followed the same pattern as for epithelial blood flow. The correlation between ruminal disappearance of propionate and epithelial blood flow (r=0.56) indicates that the removal of propionate can be limited by epithelial blood flow. The ruminal disappearance of propionate increased 30±12% for the butyric compared with ammonia buffer and 12.5±8% when compared with control. The net portal flux of propionate increased 32±6% in butyric compared with control. In conclusion, rumen epithelial blood flow is positively correlated with ruminal disappearance of propionate and affects the kinetics of ruminal VFA absorption.