NO LACTIC-ACID ABSORBED FROM THE CECUM AND RUMEN OF SHEEP

A comparative study of the absorption of lactic acid, volatile fatty a cids (VFA), glucose, and ammonia from caecal and rumen pouches was und ertaken in anaesthetised sheep. Test solutions varying in pH, osmolari ty, lactic acid concentration, and with fixed concentrations of VFA, a mmonia, and glucose (100, 7, and 4 mmol/L, respectively) were introduc ed into the clean, surgically sealed pouches. Studies were undertaken in 9 sheep, each with 2 pouches in the caecum and 1 in the rumen. Samp les were taken at 10-min intervals for 50 min to determine rate of abs orption. 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 du e to conversion of propionic acid and tissue metabolism during the exp eriment. 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 rat es 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 absorbe d from rumen pouches (0.18 mu mol/cm(2).min), but not from caecal pouc hes (-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 tha t from rumen pouches (0.24 mu g N/cm(2).min) (P < 0.0001). The mean ab sorption 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 de crease in pH, osmotic pressure, or the concentration of lactic acid re sulted in corresponding increases in the absorption of VFA.