Glucose is derived from digestion of dietary carbohydrates, breakdown of glycogen in the liver (glycogenolysis) and production of glucose from amino acid precursors in the liver (gluconeogenesis). In ruminants, the main source of glucose is gluconeogenesis from volatile fatty acids (prioponate) absorbed from rumen by bacterial fermentation. Glucose is the principal source of energy for mammalian cells. Uptake is mediated by a group of membrane transport proteins, called glucose transporters (GLU), some of which are insulin-dependent, e.g. GLU-4.

The blood glucose concentration is influenced by hormones which facilitate its entry into or removal from the circulation. The hormones affect glucose concentrations by modifying glucose uptake by cells (for energy production), promoting or inhibiting gluconeogenesis, or affecting glycogenesis (glycogen production) and glycogenolysis. The most important hormone involved in glucose metabolism is insulin.

• Insulin: Insulin enables energy use and storage and decreases blood glucose concentration. Insulin is produced by beta cells in the pancreatic islets. Insulin release is stimulated by glucose, amino acids and hormones (e.g. glucagon, growth hormone, adrenaline). Release is inhibited by hypoglycemia, somatostatin, and noradrenaline.
  • Insulin decreases blood glucose by promoting glucose uptake through GLU-4 and its use in metabolism (e.g. energy production, protein production) by liver, muscle and other tissue cells. Insulin also inhibits glucose production by inhibiting gluconeogenesis and glycogenolysis.
  • Insulin increases fatty acid and triglyceride synthesis (through stimulation of endothelial lipoprotein lipase), thus increasing fat stores (adipogenesis), and enhances glyogen synthesis and storage in the liver.
  • Insulin induces the cellular uptake of K+, phosphate and Mg+.

Several hormones oppose the action of insulin and, therefore, will increase blood glucose. The main hormones that mediate this effect are glucagon, growth hormone, catecholamines, and corticosteroids. The increase in blood glucose can occur through inhibition of insulin release, stimulation of glucose-yielding pathways (glycogolysis, gluconeogenesis), or decrease of glucose uptake or use by tissues. Collectively, increases in these hormones can induce a state of insulin resistance. Insulin resistance can also be mediated by inflammatory cytokines (TNF-alpha), obesity and pregnancy. Inflammatory cytokines are thought to be responsible for insulin resistance observed in sepsis. Hyperglycemia in critical care patients has been associated with a poor outcome and has prompted the use of glucose monitoring in such patients in human and veterinary medicine. In pregnancy, hormones such as progesterone can cause insulin resistance (this is thought to be mediated through growth hormone release) and results in gestational diabetes in humans. Pregnancy-associated hormones may also contribute to insulin resistance and hyperlipidemic syndromes in pregnant horses, ponies and camelids.

• Glucagon: Glucagon causes an increase in blood glucose, by stimulating gluconeogenesis and glycogenolysis and facilitating glucose release from hepatocytes. Low blood glucose is the main stimulus for glucagon release from alpha cells in pancreatic islets.
• Catecholamines (epinephrine/norepinephrine): Epinephrine from the adrenal medulla acts via beta-adrenergic receptors, whereas norepinpherine is released from nerve endings and acts on alpha2-adrenergic receptors. Norepinephrine and epinephrine have somewhat opposing effects on insulin release (norepinephrine inhibits, epinephrine stimulates), but the net effect of both is increased blood glucose. This occurs via stimulation of glycogenolysis and release of glucose from hepatocytes (epinephrine), and indirectly through inhibition of insulin release (norepinephrine), and release of growth hormone (epinephrine) and ACTH (which increases cortisol). The increase in glucose in response to catecholamines is usually transient (primarily due to intermittent release of catecholamines) and can be quite pronounced in cats, cattle and camelids.
• Growth hormone (GH): This increases blood glucose by inhibiting glucose uptake by cells. It also promotes glycogenolysis in muscle tissue. Progesterone may cause insulin resistance by stimulating secretion of GH. Growth hormone is released from the pituitary by growth hormone-releasing hormone, which is secreted by the hypothalamus usually in response to low blood glucose and epinephrine.
• Corticosteroids: These increase blood glucose by inducing glucose release from hepatocytes and inhibiting glucose uptake by cells (through decreasing GLU-4). Corticosteroids also stimulate gluconeogenesis and glucagon secretion (which also increases blood glucose).

The table below summarizes the effects of these different major hormones on physiologic processes that affect blood glucose concentrations.

Increase
(indirect by
insulin inhibition)