GLUCONEOGENESIS IN RUMINANTS
In ruminants, microbial fermentation of carbohydrates results little glucose being absorbed directly from the GIT. Therefore, in ruminants glucose required at tissue level is provided via gluconeogenic pathways from propionate, amino acid, glycerol and lactate. Five tissues which require glucose in ruminants are nervous tissue, muscle, adipose tissue, mammary gland and foetus.
The major metabolic difference between ruminant and non-ruminant is the extent to which ruminant utilizes acetate instead of glucose as the major precursor of energy storage and oxidation and their relying on gluconeogenesis for glucose during both fate and fasting conditions. The rate of gluconeogenesis in ruminant is highest shortly after meal when gluconeogenic precursors are plentiful. Propionate is the only VFA which makes a net contribution of glucose synthesis (about 27 – 54 %). Amino acids can be used for gluconeogenesis except for lysine, leucine and tourine (synthesized from cysteine except in cat). Glycerol is the 3rd compound used for gluconeogenesis. Most of the glycerol is bound with fatty acids in the triglycerides and is released only during glycolysis. One third of the glycerol is used for gluconeogenesis, 10% is oxidized directly to CO2 and the remaining for synthesis of triglycerides. 5% of the glucose needs can also be originated from lactate via Cori cycle.
Dietary and Hormonal control of gluconeogenesis
The quantity of glucogenic precursors available to liver is the major factor determining the amount of glucose formed. Amount of propionate and lactate available for gluconeogenesis are directly related to amount of non-structural CHO present in the diet.and amount of diet consumed. When there is glucose deficiency, glucose receptors in the hypothalamus will be stimulated and send impulses to the adrenal medulla which increases secretion of epinephrine. Reduced insulin and increased glucagon and epinephrine increase glucogenolysis and cause glycolysis in adipose tissue resulting in mobilization of glycerol and free fatty acids. The increase glucagon-insulin ratio stimulates amino acid released from muscle specially alanine and glutamine. Amino acid uptake by liver is increased by glucagon, cortisol and growth hormone. Glucagon increases uptake of alanine, glutamine and other glucogenic amino acids. The uptake of lactate by liver is increased by glucagon.
The overall metabolic pathways leading to glucose synthesis are controlled by five reactions. The key enzymes which control these reactions in ruminants are :-
1. Pyruvate carboxylase – for formation of oxaloacetate from pyruvate.
2. NAD malate dehydrogenase – conversion of malate to oxaloacetate.
3. Phosphoenol pyruvate carboxykinase – for conversion of oxaloacetate to phosphoenol pyruvate.
4. Fructose 1,6-diphosphate – for conversion of fructose 1,6-diphosphate to 6-phosphate.
5. Glucose 6-Phosphatase – for release of free glucose into the blood.
All these reactions are influenced by factors which alter enzyme activation or synthesis and are altered by factors such as diet, fasting, lactation, pregnancy and hormonal status. As a rule, glucagon and gluco-corticoids increase reaction rates while insulin depresses reaction rates of all five enzymes.