• Diabetes2Animals

Type 2 Diabetes in Animals: Experimental and Clinical Aspects

1. Introduction

2. Background

   1. Insulin production

   2. Insulin release

   3. Binding of insulin

3. Types of diabetes mellitus

   1. Type 1

   2. Type 2

   3. Gestational diabetes

   4. Feline diabetes

   5. Canine diabetes

4. Symptoms and consequences

   1. Early symptoms

   2. Micro- and Macrovascular complications

   3. Atherosclerosis

   4. Retinopathy

   5. Cataracts

   6. Diabetic nephropathy, disease of the kidney

   7. Diabetic ketoacidosis

5. Feline diabetes diagnosis and medication

   1. Diagnosis

      • The glucose tolerance test

   2. Medication

      • Hypoglycemics

Diabetes is a chronic disease and a metabolic illness, in which body is failing to metabolize glucose (a C6 sugar), into a usable energy source. Hyperglycemia (high sugar levels) will occur and glucose will leave the body with the urine giving it a sweeter smell (which gave this illness its latin name, honey-sweet). It may results in excess thirst, excess urination, lethargy, depression and can cause symptoms as severe as liver failure, kidney failure, blindness, stroke or death if gone untreated. In this paper, we will discuss mostly Type 2, but for easier understanding, also touch on Type 1 and some of the subtypes known.

Background:

• The body functions only with energy. The main source providing this energy is glucose, a sugar resulting from the digestion of carbohydrates (sugars and starches). The glucose then circles in the bloodstream, ready to use, but it can only enter the cells with help of a hormone called insulin.

Insulin production:

Insulin is produced in the pancreas, by the beta cells. Beta cells are situated in clusters, called islets of Langerhans. The mRNA first translates an inactive protein, called preproinsulin, which contains an amino-terminal signal sequence. This is important for it to pass through the membrane of the ER, for its post-translational processing. Here, the signal sequence is clipped, specific peptidases cleave the protein, three disulfid-bonds occur creating proinsulin. Lastly peptidases clip out another inactive sequence, forming the final product, Insulin. This is then stored in secretory granules until the signal for its release is given.

Insulin release:

Insulin release is mediated by a rise in glucose concentration. GLUT2 transporters (trans-membrane protein) let glucose into the beta cells, glucose is phosphorylated by glucokinase, further creating ATP (energy source). As the ATP level inside the cells rises, ATP-gated potassium channels are closed, raising the intracellular positive charge. The cell is depolarized, causing the calcium channels to open and transport calcium into the cell, leading to the export of the insulin-storing granules: exocytosis.

Binding of insulin

Insulin circulates in the bloodstream until they find their receptors, to which they then promptly bind. The GLUT4 receptors are insulin sensitive, by binding insulin, they undergo a conformation change. Glucose is let into the cell through facilitated diffusion(against a concentration gradient). Different tissues have different usages for glucose. The skeletal muscle burns it for energy and adipose tissue converts it to triglycerides for storage.

Regulation

It is highly important to regulate both the glucose level in the blood (hyper- and hypoglycemia are dangerous) as well as regulating the insulin levels. The enzyme insulinase breaks down circulating insulin, giving it a half-life period of approximately six minutes. This ensures that that the level of circulating insulin is kept in check, as not to result in hypoglycemia (low sugar levels). [1]

Types of Diabetes mellitus:

Type 1 Diabetes

• This is an autoimmune disease, sometimes called 'Juvenile diabetes' or 'Insulin dependent diabetes'. Here, little or no insulin is being produced, due to the self-destruction (autoimmune T-cell mediated attack) of the beta-cells in the pancreas. This results in hyperglycemia. Its onset is sudden and mostly diagnosed at an early age, also it does not seem to be effected by obesity or malnutrition. This type is medicated by daily insulin injections and in constant need of monitoring the glucose levels in the blood.

Type 2 Diabetes

• Also called 'Age on-set' or 'Non-insulin dependent' diabetes. It is often seen as a more milder version of diabetes, due to its slow onset (develops over a long period of time), but the after-effects are just as dangerous as for type 1. It is a more complicated and variable illness, having to do with lower insulin levels in the blood as well as insulin resistance. This is a result of beta-cell dysfunction, levels such as 50% productivity have been recorded, (due to apoptose of the beta-cells but has not been fully researched at this time) which results in less insulin production and secretion.[6] The GLUT4 receptors lowered ability to bind insulin, results in less glucose being transported into the cell. Hereby inhibiting glycolysis and resulting in hyperglycemia. Also, insulin resistance in adipose cells reduces the uptake of circulating lipids, leading to increased hydrolysis of stored triglycerides. This then elevates the level of free fatty acids in the bloodstream. Insulin resistance in the liver, causes reduced glycogen production, as well as storage, resulting in failure to suppress glucose production. Hepatic glucose production is also heightened, when there is a large amount of free fatty acids in the blood plasma. This, as well as reduced muscle glucose uptake and increased liver glucose production all contribute to hyperglycemia. Beta-cells can not make up for the high glucose values in the blood by producing enough insulin, they attempt to keep hyperinsulinemic values which come with a price on the pancreas. This can result in hypertension as well, leading to an increased risk of coronary artery disease. [5][4][3][2][8]

Gestational diabetes

• An important subtype seems to be the gestational diabetes, which results in 2-5% of all pregnancies. It is a non-permanent type of diabetes, often, if treated resulting in the birth of a healthy baby and a healthy mother. It is a result of beta-cell dysfunction, causing insufficient insulin production of the pancreas, resulting in hyperglycemia. Although if untreated, the baby is born overweight especially upper bodyweight, due to increased glucose uptake during the pregnancy. This can lead to complications at the birth, for example macrosomia (the head and shoulder region of the foetus are too large to fit into the birth canal), caesarean section will need to be made. Most babies are then born with hypoglycemia, which can lead to brain damage if it goes unnoticed, also the lungs in these children mature later and they are at a risk for jaundice (hyperbilirubinia: high levels of bilirubin). There is a high risk (20-50%) for females congesting gestational diabetes, resulting in a later on type 2 diabetes (in a 12 year period). [7]

Feline diabetes mellitus

• It is a common endocrine disorder in cats, with a reported 2.45 cases per 1000 cats. Although it seems to be under-diagnosed, due to the owners not identifying the symptoms since they are not too well developed. Some symptoms are increased hunger, thirst and urination and obesity. About 80-95% of felines affected have type 2 diabetes. [8]

Canine diabetes mellitus

• The typical canine diabetic is female, middle age and overweight. The number of diagnosed patients has increased three fold in the last thirty years. Canines are most likely to have type 1 diabetes, which is the insulin dependent type. Diabetes typically occurs in dogs between 5 and 12 years of age, and is uncommon under 3 years of age. Breeds predisposed to diabetes include the Samoyed, Tibetan Terrier and Cairn Terrier, while others such as the Boxer and German Shepherd Dog seem less susceptible. [9]

Symptoms and consequences

• Diabetes can lead to many secondary diseases. Therefore it is important to be aware of early symptoms.

Diagnostic symptoms of diabetes:

• weight loss
• polydipsia (excess thirst)
• polyurina (excess urination)
• polyphagia (increased appetite)
• dehydration and
• acetone smell from the animals breath
  • Dogs often develop cataracts which can lead to blindness. Cats more often develop nephropathy or nerve damage that cause weak hind legs.[19](Many owners believe that this is caused by age). Other secondary effects can be damage and inflammation of the pancreas, urinary tract infection, damage of liver and stroke.

Micro- and Macrovascular complications

• Diabetes 2 can lead to many other serious illnesses due to micro- and macrovascular complications. The main pathological mechanism in macrovascular disease is the atherosclerosis.

Atherosclerosis

• Atherosclerosis is the thickening of an arterial wall caused by damage or chronic inflammation. Because of injury, lipids accumulate at the walls, and macrophages accumulate in order to form foam cells of the oxidized lipids. These again proliferate macrophage stimulation and attracts T-lymphocytes. The t-lymphocytes increase smooth muscle proliferation and collagen accumulation. The final result of these processes is a lipid atherosclerotic lesion, with a fibrous cap. [20]. Among microvascular complications, the diabetic retinopathy may be the most frequently complication related to the diabetes disease.[20]

Retinopathy

It is considered a main reason for blindness in humans as well as for dogs with diabetes.[21] Diabetic retinopathy can be divided in proliferative and background retinopathy. In background retinopathy, small hemorrhages appear in the retina. Hard exudates coming from lipid deposition often occur near the hemorrhages. They appear as small dots and can decrease the vision. Proliferate retinopathy, is the formation of new capillaries on the retina, that normally would not be there. They can lead to vitreous hemorrhage, which can cause blindness.[20]

• There are various factors influencing the development of retinopathy. One example is the aldose reductase enzyme. This enzyme is helping glucose to be converted to glucose alcohol; sorbitol. In case of higher glucose concentration, more glucose will be converted to sorbitol. This leads to a sorbitol accumulation in the cells, resulting in osmotic stress. Osmotic stress occurs when concentration, hence water movement around a cell, changes rapidly. Osmotic stress has been considered as a mechanism important in diabetic microvascular complications. However, studies where inhibitors of the aldose reductase enzyme have been used, have not been very successful.[20]

Cataracts

• As mentioned before dogs with diabetes very often develop cataracts, and as many as 80% in the first 16 months after diagnosis.[21] Cataracts are a clouding of the eye lens that obstructs the light going to the retina.[22] They can occur because of old age, eye injury, or due to inherited genes, but are often associated with diabetes.[23]

Diabetic nephropathy, disease of the kidney

• Diabetic nephropathy is a microvascular complication of the kidney. The diabetic nephropathy is connected with long-term disease. The percent of incidents in canines is unknown, but it is more frequently observed in feline diabetes.[21] The kidney contains glomeruli, small capillaries that are responsible for filtration of blood. Hyperglycemia will increase intraglomerular hypertension (abnormally increased blood pressure in the glomeruli) and renal hyperfusion (abnormally increased perfusion) and thus the glomeruli will be damaged.[24] The increased pressure will result in glomerulosclerosis and renal failure.[24] The kidneys can not work properly and protein will leak to the urine. The following parameters can be measured in order to define diabetic renal disease; increased excretion of the protein albumin in the urine; microalbuminuria. This is classified if the secretion is between 50-299 mg/l. This may be followed by macroalbuminuria; albumin concentration in urine higher than 299mg/l. Plasma creatinine can also be measured, where increased values indicates damage of nephrons in the kidney. At the point of macroalbuminuria the kidneys are still working, but several glomeruli are destroyed, and the performance decreases. Diabetic nephropathy is a main cause of ESRD (End-stage renal disease). The incident rate of ESRD in humans with diabetes type 2 is about 4-20%.[24]

When ESRD is reached, only 10% of the kidney functions normally, and they don’t work sufficiently enough for everyday life dialysis or transplantation is needed.[25] Diabetes can increase the chance of developing ESRD approximately twelve-fold. [26]

Diabetic ketoacidosis

• One of the symptoms previously mentioned is the acetone smell of the animals breath. This is because of ketones in the body. If the level of insulin is too low, the body cannot utilize glucose as a source of energy, and it will start to use fat as an energy source instead. When fat is degraded, glycerol and fatty acids are obtained. Acids called ketones are increased in the blood and in the urine. Too high concentrations of ketones are toxic for the organism. This condition is called ketoacidosis. [27] Although this is rare in diabetes type 2.

Feline diabetes diagnosis and medication

• As with humans, felines are prone to diabetes type 2, due to obesity and inactivity. [12] Above all factors mostly older cats are diagnosed with diabetes type 2 [8]. Also certain kinds of medication may lead to the onset of diabetes, like glucocorticoids and some hormones used on cats in heat [4].

Diagnosis

The diagnosis is based on the previously mentioned symptoms as well as hyperglycemia. Although stressed cats are prone to hyperglycemia, glucose in the urine is also measured [8].

• For this diagnosis, test strips are used. The test strips are placed into a fresh urine sample and then removed. The later indicated colour on the the test strip shows the levels of glucose that are present.

The picture below illustrates a typical test used to measure the presence of glucose in the urine, where the values below 250 mg/dl are a negative test.

• The normal value of blood glucose level in the cat is 80-120 mg/dl. A diabetic cat may have values higher than 250 mg/dl and some even up to 600 mg/dl [16].

The glucose tolerance test

• The glucose tolerance test is another way to diagnose diabetes and is often used on patients with few or no symptoms. It is a time consuming test compared to other options, so it is not regarded as a patient friendly diagnostic tool [28]. It can be done either orally (OGTT) or intravenous (IGTT) but the latter is rarely used. Normally the OGTT is used when there are indications of gestational diabetes, insulin resistance and hypoglycemia, and also pre-diabetes [30].

This test will give information about how fast the glucose is removed from the blood and how it is tolerated and broken down [29]. Furthermore it will show how the intake of glucose changes the fasting glucose level in the body. Blood samples will be taken prior to the intake of glucose but also in intervals after to see how the levels are changing [30]. The longer it takes for the animal to metabolize the given glucose, the more it indicates problems with maintaining glucose homeostasis. [31]

Medication

Cats with diabetes can have a long and normal life, if the diet and activity of the pet are regulated properly. Normally, the cat will be put on a high fiber diet [16]. There is also the need for one or two injections of insulin, which are made daily by the owner [8]. It is important that insulin is given as an injection, as it is a protein and would be digested by the stomach if given orally [13]. It is important that the veterinarian helps the owner and shows how and where to inject the insulin, as well as how to shake and heat the insulin, so that it is not destroyed.

• There are three types of insulin used to treat diabetes. The short acting insulin, normally used on animals that are very sick with diabetes (vomiting). This is given until the animal is eating again. Intermediate acting types are NPH and Lente, a long lasting type is PZI (protamine zink insulin) [8].

Hypoglycemics

• Some medications, like hypoglycemics, normally used on humans, work as an oral treatment on cats as well, but are not used on dogs [13]. A hypoglycemic is a tablet form of medication, only used on patients with diabetes type two. There are four classes of hypoglycemics used to treat diabetes type two; Sulfonylureas, Metformin, Thiazolidinediones and Alpha-glucosidase inhibitors [14].

  Sulfonylureases are the most commonly used drugs when it comes to the treatment of diabetes type two. Sulfonylureas stimulates the beta cells and increases the level of insulin released to the blood, hereby lowering the glucose levels in the blood. There are generally few side effects with the use of sulfonylureas, but hypoglycemia may occur, due to the heightened levels of insulin secreted [14]. Metformin's function is to increase the insulin action and only work when insulin is present. Exactly how Metformin works is not clear, but it appears to have an effect on the liver, by suppressing the output of glucose [14]. The drug lowers the fasting glucose level by approximately 20%, compared to the use of sulfonylureas. The fasting glucose level is normally less than 120 mg/dl [14], but above 200 mg/dl in diabetic cats [16]. It is also observed to have a weight reducing effect. However there are side effects of this medication on the gastrointestinal tract and the risk of lactic acidosis [14]. Thiazolidinediones have an effect on the utilization of glucose. The drug acts on different tissues, like muscles and fat, and reverses the insulin resistance. Its effect is enhanced in combination with Metformin, but this is still under drug trial and not used by doctors yet [14]. Actos and Avandia are both thiazolidinediones used today, with some side effects on the liver. Rezulin is also a thiazolidinediones, but had a negative effect on the liver, causing death, so it was taken off the market.