Review/Oorsig Volume 22, Issue 03 - Page 26

Oorsig/Review removed via the production of ketone bodies. directly to the liver but rather are first converted in the muscle to the transport amino acids alanine, glutamine and aspartate. They form an important part of the substances contributing to gluconeogenesis. The substances used to form glucose through the gluconeogenic pathway (i.e. propionate, lactate, glycerol alanine, aspartate and glutamine) enter the energy pathways in several positions. Butyrate will also enter the ß oxidation pathway and be transformed into triglycerides or ketone bodies. Most of the butyrate will in fact already be converted to β hydroxy butyrate by the rumen epithelium as it is absorbed from the rumen. Acetate and butyrate are thus ketogenic in nature. As ruminants cannot rely on a dietary glucose source of any magnitude, they need to produce large amounts of newly formed glucose (gluconeogenesis). This happens mainly in the liver through the gluconeogenic pathway. Skeletal muscle acts as an amino acid depot. Most amino acids from muscle are not transported The glucose formed through the gluconeogenic pathway is mainly used to manufacture lactose in lactating animals. Milk contains about 4.5 % lactose i.e. 50 kg of milk will contain 2.25 kg of lactose, nearly all of which must come from glucose provided by gluconeogenesis. RUMEN ADIPOSE TISSUE MUSCLES Proteins Triglycerides VFA's: Hormone Alanine Glutamine Aspartate P ropionate B utyrate 25% Acetate 10% 65% sensitive Lipase Glucose Glycerol Amino Acids C3 Acetyl CoA (C2) Oxaloacetate (C4) FFA C6 Excreted as lipoproteins CO2 G lucose (C 6) C5 LIVER FFA * Stored as TAG C4 CO2 Acetoacetyl CoA Aceto- Acetate B eta-O H - Acetone B utyrate KETONE BODIES Insulin * This reaction does not take place in the liver, but can take place in heart, muscle, kidney, a.o. tissues in the presence of sufficient C4 Diagram illustrating the pathways of energy metabolism in lactating dairy cows. 26 BST Prolactin Placental lactogen