Glucose

Glucose

Definition

• A simple sugar, C6H12O6, found in most foodstuffs

• It is the only monosaccharide present in significant amounts in blood and body fluids.

• Glucose is the major source of energy through the process of oxidative metabolism.

Synonyms

d-Glucose, C₆H₁₂O₆, glycogen (storage form)

Physiology

• Glucose in the blood is derived from three main sources:

Intestinal absorption

Glucose is the end-product of carbohydrate digestion, absorbed by enterocytes.

Increased blood glucose concentrations occur 2 to 4 hours after a meal in simple-stomached animals.

Hepatic production

Gluconeogenesis and glycogenolysis within hepatic cells produce glucose when metabolically necessary.

Gluconeogenesis converts noncarbohydrate sources, primarily amino acids (from protein) and glycerol (from fat), in simple-stomached animals.

Glycogenolysis converts glycogen (poly-glucose) stored in hepatocytes to glucose through hydrolysis.

Kidney production

Gluconeogenesis and glycogenolysis within renal epithelial cells can result in the formation of glucose when metabolically necessary.

• The plasma concentration of glucose is controlled by a number of hormones, in particular, insulin and glucagon. The physiology of glucose homeostasis is controlled primarily by insulin release in response to elevated glucose levels (postprandial), although in birds, glucagon appears to serve as the primary regulator. Significant species variations in glucose levels have been noted. In general, levels are lowest in reptiles (60 to 100 mg/dL) and highest in birds (200 to 500 mg/dL), with mammals in between (100 to 200 mg/dL).

Glucose that is not needed for energy is stored in the form of glycogen as a source of potential energy, readily available when needed. Most glycogen is stored in the liver and in muscle cells. When these and other body cells are saturated with glycogen, excess glucose is converted to fat and is stored as adipose tissue.

Typical Normal Range

The typical normal range for this laboratory test varies greatly among species. The reader is referred to the following Elsevier publications for additional information:

Carpenter J: Exotic animal formulary, ed 4, St Louis, 2013, Saunders.

Mader D: Reptile medicine and surgery, ed 2, St Louis, 2006, Saunders.

Clinical Applications

Causes of Abnormally High Levels

• Hyperglycemia

Postprandial hyperglycemia

Diabetes mellitus

Increased glucocorticoid concentrations

Hyperadrenocorticism

Therapeutic corticosteroids

• Catecholamine release

Pheochromocytoma

• Increased growth hormone (growth hormone–producing tumor)

• Increased glucagon (glucagon-producing tumor)

• Increased progesterone production (diestrus in female)

• Pancreatitis

• Drugs (see later)

• Hyperthyroidism

• Moribund animals

Hyperglycemia is generally defined by blood glucose concentrations exceeding 500 mg/dL.

Hyperglycemia most often results from catecholamine release from stress, glucocorticosteroid excess from administration of corticosteroids, and diabetes mellitus.

Exertion, excitement, and extreme temperatures stimulate the release of catecholamines, resulting in a mild to moderate increase in the blood glucose concentration. Stress hyperglycemia can occasionally produce a strong positive on urine glucose dipstick, similar to diabetes mellitus.

Excess glucocorticosteroids normally cause a mild to moderate increase in the blood glucose concentration (≤600 mg/dL) in birds.

Concentrations greater than 700 mg/dL are suggestive of diabetes mellitus in birds.

The pathophysiology of diabetes mellitus in birds is variable, however, and appears to be associated with excess glucagon in the presence of hyperglycemia. In psittacine birds, pancreatitis and pancreatic islet cell tumors are known causes. In some species (e.g., tucans [Ramphastidae]), diabetes occurs commonly and may be related to diets rich in fruits. Budgerigars and cockatiels are predisposed to diabetes associated with hepatic lipidosis. Birds suffering from diabetes mellitus demonstrate polyuria and urinary glucose concentrations exceeding 1 mg/dL.

Reproductively active cockatiel hens can present with “pseudodiabetes.” Plasma glucose levels are elevated but remain under 1000 mg/dL. The cause appears to be reduced pancreatic function from inflammation associated with yolk peritonitis.

The normal blood glucose concentration of most reptiles ranges between 60 and 100 mg/dL, but this is subject to marked physiologic variation.

The most common cause of hyperglycemia is iatrogenic delivery of excessive glucose.

Although a persistent, marked hyperglycemia and glucosuria are suggestive of diabetes mellitus, the disorder is rarely observed in reptiles.

Hyperglycemia also occurs with glucocorticosteroid excess.

Hyperglycemia most often is transient and due to postprandial increases.

Glucocorticoid excess (e.g., chronic stress, exogenous corticoids, hyperadrenocorticism) produces mild elevations in glucose concentration (150 to 200 mg/dL).

Stress-induced catecholamine release can result in higher elevations of glucose (200 to 300 mg/dL) and simulates diabetes mellitus.

Diabetes mellitus is relatively rare in ferrets and most often is iatrogenic and associated with surgical removal of pancreatic insulin-secreting neoplasms, or may be associated with use of drugs such as megestrol acetate. Glucose concentrations are greater than 400 mg/dL and frequently are greater than 1000 mg/dL.

Hyperglycemia is a relatively common finding in rabbits and can be accompanied by glycosuria.

Stress-induced catecholamine release is believed to be the most common cause. Handling pet rabbits or warm temperatures result in increased blood glucose.

Glucocorticoid excess (e.g., stress induced, exogenous corticoids, hyperadrenocorticism) is also possible.

Hyperglycemia is associated with severe gastrointestinal distress ranging from acute obstruction to chronic stasis.

Diabetes mellitus is rarely a cause of hyperglycemia in rabbits and is rarely diagnosed in pet rabbits. Rare hereditary diabetes occurs in some populations. Herbivorous animals withstand the absence of insulin more readily than carnivorous ones. Management of hyperglycemia in herbivorous animals is done via diet modification.

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Tags: Clinical Veterinary Advisor Birds and Exotic Pets

Jul 29, 2016 | Posted by admin in EXOTIC, WILD, ZOO | Comments Off

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