No lactic acid absorbed from the caecum and rumen of sheep

A comparative study of the absorption of lactic acid, volatile fatty acids (VFA), glucose, and ammonia from caecal and rumen pouches was undertaken in anaesthetised sheep. Test solutions varying in pH, osmolarity, lactic acid concentration, and with fixed concentrations of VFA, ammonia, and glucose (100, 7, and 4 mmol/L, respectively) were introduced into the clean, surgically sealed pouches. Studies were undertaken in 9 sheep, each with 2 pouches in the caecum and 1 in the rumen. Samples were taken at 10-min intervals for 50 min to determine rate of absorption. Neither L-lactic acid nor D-lactic acid was absorbed from the caecal or rumen pouch and there was a slight increase in both isomers of lactic acid (0.39%/min of L-lactic acid and 0.24%/min of D-lactic acid averaged for the caecum and rumen values) which was presumably due to conversion of propionic acid and tissue metabolism during the experiment. The rate of increase per unit area in the caecum (0.06 mu mol/cm(2).min) was much greater than that in the rumen (0.015 mu mol/cm(2).min) based on the average of L-and D-lactic acid. The absorption rates of acetic, propionic, and butyric acids from the caecum (0.49, 0.17, and 0.08 mu mol/cm(2).min, respectively) and the rumen (0.48, 0.16, and 0.08 mu mol/cm(2).min, respectively), based on absorptive surface area of the pouches, were very similar. Glucose was apparently absorbed from rumen pouches (0.18 mu mol/cm(2).min), but not from caecal pouches (-0.01 mu mol/cm(2).min). The mean absorption rate of ammonia from caecal pouches (0.60 mu g N/cm(2).min) was 2.5 times greater than that from rumen pouches (0.24 mu g N/cm(2).min) (P < 0.0001). The mean absorption rate of ammonia for the caecal and rumen pouches was about 2.6 times higher at pH 6.5 (0.99%/min) than that at pH 4.5 (0.38%/min) (P < 0.0001) at the same osmolarity and lactic acid concentration. A decrease in pH, osmotic pressure, or the concentration of lactic acid resulted in corresponding increases in the absorption of VFA.