Insulin (from Latin insula, "island", as it is produced in the Islets of Langerhans in the pancreas) is a polypeptide hormone that regulates carbohydrate metabolism. Apart from being the primary effector in carbohydrate homeostasis, it also has a substantial effect on small vessel muscle tone, controls storage and release of fat (triglycerides) and cellular uptake of both amino acids and some electrolytes. In this last sense, it has anabolic properties. Its concentration (more or less, presence or absence) has extremely widespread effects throughout the body.
Insulin is used medically in some forms of diabetes mellitus. Patients with type 1 diabetes mellitus depend on exogenous insulin (injected subcutaneously) for their survival because of an absolute deficiency of the hormone; patients with type 2 diabetes mellitus have either relatively low insulin production or insulin resistance or both, and a non-trivial fraction of type 2 diabetics eventually require insulin administration when other medications become inadequate in controlling blood glucose levels.
Insulin has the empirical formula C257H383N65O77S6.
Insulin structure varies slightly between species of animal. Its carbohydrate metabolism regulatory function strength in humans also varies. Pig insulin is particularly close to the human one.
Structure and production
Insulin is synthesized in humans and other mammals within the beta cells (β-cells) of the islets of Langerhans in the pancreas. One to three million islets of Langerhans (pancreatic islets) form the endocrine part of the pancreas, which is primarily an exocrine gland. The endocrine part accounts for only 2% of the total mass of the pancreas. Within the islets of Langerhans, beta cells constitute 60–80% of all the cells.
Insulin is synthesized from the proinsulin precursor molecule by the action of proteolytic enzymes known as prohormone convertases (PC1 and PC2). Active insulin has 51 amino acids and is one of the smallest proteins known. Beef insulin differs from human insulin in three amino acid residues, and pork insulin in one residue. Fish insulin is also close enough to human insulin to be effective in humans. In humans, insulin has a molecular weight of 5808. Insulin is structured as 2 polypeptide chains linked by 2 sulfur bridges (see figure shown above), with one additional sulfur bond in the A chain (not shown). Chain A consists of 21, and chain B of 30 amino acids. Insulin is produced as a prohormone molecule – proinsulin – that is later transformed by proteolytic action into the active hormone.
The remaining part of the proinsulin molecule is called C-peptide. This polypeptide is released into the blood in equal amounts to the insulin protein. Since exogenous insulins contain no C-peptide component, serum levels of C-peptide are good indicators of endogenous insulin production. C-peptide has recently been discovered to have itself biological activity; the activity is apparently confined to an effect on the muscular layer of the arteries.
Actions on cellular and metabolic level
The actions of insulin on the global human metabolism level include:
Control of cellular intake of certain substances, most prominently glucose in muscle and adipose tissue (about 2/3 of body cells).
Increase of DNA replication and protein synthesis via control of amino acid uptake.
Modification of the activity of numerous enzymes (allosteric effect).
The actions of insulin on cells include:
Increased glycogen synthesis – insulin forces storage of glucose in liver (and muscle) cells in the form of glycogen; lowered levels of insulin cause liver cells to convert glycogen to glucose and excrete it into the blood. This is the clinical action of insulin which is useful in reducing high blood glucose levels as in diabetes.
Increased fatty acid synthesis – insulin forces fat cells to take in glucose which is converted to triglycerides; lack of insulin causes the reverse.
Increased esterification of fatty acids – forces adipose tissue to make fats (ie, triglycerides) from fatty acid esters; lack of insulin causes the reverse.
Decreased proteinolysis – forces reduction of protein degradation; lack of insulin increases protein degradation.
Decreased lipolysis – forces reduction in conversion of fat cell lipid stores into blood fatty acids; lack of insulin causes the reverse.
Decreased gluconeogenesis – decreases production of glucose from various substrates in liver; lack of insulin causes glucose production from assorted substrates in the liver and elsewhere.
Increased amino acid uptake – forces cells to absorb circulating amino acids; lack of insulin inhibits absorption.
Increased potassium uptake – forces cells to absorb serum potassium; lack of insulin inhibits absorption.
Arterial muscle tone – forces arterial wall muscle to relax, increasing blood flow, especially in micro arteries; lack of insulin reduces flow by allowing these muscles to contract.
Regulatory action on blood glucose
Despite long intervals between meals or the occasional consumption of meals with a substantial carbohydrate load (e.g., half a birthday cake or a bag of potato chips), human blood glucose levels normally remain within a narrow range. In most humans this varies from about 70 mg/dl to perhaps 110 mg/dl (3.9 to 6.1 mmol/litre) except shortly after eating when the blood glucose level rises temporarily. In a healthy adult male of 75 kg with a blood volume of 5 litre, a blood glucose level of 100 mg/dl or 5.5 mmol/l corresponds to about 5 g (1/5 ounce) of glucose in the blood and approximately 45 g (1 1/2 ounces) in the total body water (which obviously includes more than merely blood and will be usually about 60% of the total body weight in men). This homeostatic effect is the result of many factors, of which hormone regulation is the most important.
There are two groups of mutually antagonistic metabolic hormones affecting blood glucose levels:
catabolic hormones (such as glucagon, growth hormone, and catecholamines), which increase blood glucose
and one anabolic hormone (insulin), which decreases blood glucose
Mechanisms which restore satisfactory blood glucose levels after hypoglycemia must be quick and effective because of the immediate serious consequences of insufficient glucose. This is because, at least in the short term, it is far more dangerous to have too little glucose in the blood than too much. In healthy individuals these mechanisms are indeed generally efficient, and symptomatic hypoglycemia is generally only found in diabetics using insulin or other pharmacologic treatment. Such hypoglycemic episodes vary greatly between persons and from time to time, both in severity and swiftness of onset. In severe cases prompt medical assistance is essential, as damage (to brain and other tissues) and even death will result from sufficiently low blood glucose levels.
Beta cells in the islets of Langerhans are sensitive to variations in blood glucose levels through the following mechanism (see figure to the right):
Glucose enters the beta cells through the glucose transporter GLUT2
Glucose goes into the glycolysis and the respiratory cycle where the high-energy ATP molecule is produced by oxidation
Dependent on blood glucose levels and hence ATP levels, the ATP controlled potassium channels (K+) close and the cell membranes depolarise
On depolarisation, voltage controlled calcium channels (Ca2+) open and calcium flows into the cells
An increased calcium level causes activation of phospholipase C, which cleaves the membrane phospholipid phosphatidyl inositol 4,5-bisphosphate into inositol 1,4,5-triphosphate and diacylglycerol.
Inositol 1,4,5-triphosphate (IP3) binds to receptor proteins in the membrane of endoplasmic reticulum (ER). This allows the release of Ca2+ from the ER via IP3 gated channels. This further raises the cell concentration of calcium.
Significantly increased amount of calcium in the cells causes release of previously synthesised insulin, which has been stored in secretory vesicles
The calcium level also regulates expression of the insulin gene via the calcium responsive element binding protein (CREB).
This is the main mechanism for release of insulin and regulation of insulin synthesis. In addition some insulin synthesis and release takes place generally at food intake, not just glucose or carbohydrate intake, and the beta cells are also somewhat influenced by the autonomic nervous system.
Substances that stimulate insulin release are also acetylcholine, released from vagus nerve endings (parasympathetic nervous system), cholecystokinin, released by enteroendocrine cells of intestinal mucosa and gastrointestinal inhibitory peptide (GIP). The first of these act similarly as glucose through phospholipase C, while the last one acts through the mechanism of adenylate cyclase.
Sympathetic nervous system (α2 adrenergic agonists) inhibits the release of insulin.
When the glucose level comes down to the usual physiologic value, insulin release from the beta cells slows or stops. If blood glucose levels drop lower than this, especially to dangerously low levels, release of hyperglycemic hormones (most prominently glucagon from Islet alpha cells) forces release of glucose into the blood from cellular stores. The release of insulin is strongly inhibited by the stress hormone adrenalin (epinephrine).
The brain and hypoglycemia
Though other cells can use other fuels for a while (most prominently fatty acids), neurons are dependent on glucose as a source of energy in the non-starving human. They do not require insulin to absorb glucose, unlike muscle and adipose tissue and they have very small internal stores of glycogen. Glycogen stores in liver cells (unlike glycogen stores in muscle cells) can be converted to glucose and released into the blood, and the glycerol backbone in triglyeride can also be used to produce blood glucose. Exhaustion of these sources can, either temporarily or on a sustained basis, if reducing blood glucose to a sufficiently low level, first and most dramatically manifest itself in impaired functioning of the central nervous system – dizziness, speech problems, even loss of consciousness, are not unknown. This phenomenon is known as hypoglycemia or, in cases producing unconsciousness, "hypoglycemic coma" (formerly termed "insulin shock" from the most common causative agent). Because endogenous causes of insulin excess (such as an insulinoma) are extremely rare naturally, the overwhelming majority of hypoglycemia cases are caused by human action (e.g. iatrogenic, caused by human action, eg medicine), and are usually accidental. There have been a few cases reported of murder, attempted murder or suicide using insulin overdoses, but most insulin shock appears to be due to mismanagement of insulin (didn't eat as much as anticipated, or exercised more than expected), or a mistake (e.g. 200 units of insulin instead of 20).
Possible causes of hypoglycemia include:
Oral hypoglycemic agents (eg, any of the sulfonylureas, or similar drugs, which increase insulin release from beta cells in response to a particular blood glucose level).
External insulin (usually injected subcutaneously).
Ingestion of low-carbohydrate sugar substitutes (animal studies show these can trigger insulin release according to a report in Discover magazine August 2005, p18).
Diseases and syndromes caused by an insulin disturbance
There are several conditions in which insulin disturbance is pathologic:
Diabetes mellitus – general term referring to all states characterized by hyperglycemia.
Type 1 – autoimmune-mediated destruction of insulin producing beta cells in the pancreas resulting in absolute insulin deficiency.
Type 2 – multifactoral syndrome with combined influence of genetic susceptibility and influence of environmental factors, the best known being obesity, age, and physical inactivity, resulting in insulin resistance in cells requiring insulin for glucose absorption. This form of diabetes is strongly inherited.
Other types of impaired glucose tolerance (see the diabetes article).
Insulinoma or reactive hypoglycemia.
Metabolic syndrome – precondition first called Metabolic Syndrome X by Gerald Reaven, sometimes called prediabetes. The precondition is characterized by elevated blood pressure, dyslipidemia (disturbances in blood cholesterol forms and other blood lipids), and increased waist circumference in Western populations. The basic underlying cause is insulin resistance, a dimished capacity of insulin response tissues (muscle, fat, liver) to respond to insulin. Untreated, Metabolic Syndrome can lead to morbidities such as essential hypertension, obesity, Type 2 diabetes, and cardiovascular disease (CVD).
Insulin is absolutely required for all animal (including human) life. The mechanism is almost identical in nematode worms (e.g. C. elegans), fish, and in mammals. In humans, insulin deprivation due to the removal or destruction of the pancreas leads to death in days or at most weeks. Insulin must be administered to patients in whom there is a lack of the hormone for this, or any other, reason. Clinically, this is called diabetes mellitus type 1.
The initial source of insulin for clinical use in humans was from cows, pigs or fish pancreases. Insulin from these sources are effective in humans as they are nearly identical to human insulin (two amino acid difference for bovine insulin, one amino acid difference for porcine). Insulin is obviously a protein which has been very strongly conserved across evolutionary time. Differences in suitability of beef, pork, or fish insulin preparations for particular patients have been primarily the result of preparation purity and of allergic reactions to assorted non-insulin substances remaining in those preparations. Purity has improved more or less steadily since the 1920s, but allergic reactions have continued though slowly reducing in severity. Though insulin production from animal pancreases was widespread for decades, there are very few patients today relying on insulin from these sources.
Human insulin is now manufactured for widespread clinical use using genetic engineering techniques, which significantly reduces impurity reaction problems. Eli Lilly marketed the first such insulin, Humulin, in 1982. Humulin was the first medication produced using modern genetic engineering techniques, in which actual human DNA is inserted into a host cell (E. coli in this case). The host cells are then allowed to grow and reproduce normally, and due to the inserted human DNA, they produce actual human insulin.
Genentech developed the technique Lilly used to produce Humulin. Novo Nordisk has also developed a genetically engineered insulin independently. Most insulins used clinically are produced this way, for they avoid most of the allergic reaction problem.
Modes of administration
Unlike many medicines, insulin cannot be taken orally. It is treated in the gastrointestinal tract precisely as any other protein; that is, reduced to its amino acid components, whereupon all 'insulin activity' is lost. There are research efforts underway to develop methods of protecting insulin from the digestive tract so that it can be taken orally, but none has yet reached clinical use. Instead insulin is usually taken as subcutaneous injections by single-use syringes with needles, or by repeated-use insulin pens with needles.
There are several difficulties with the use of insulin as a clinical treatment for diabetes:
Mode of administration.
Selecting the 'right' dose and timing.
Selecting an appropriate insulin preparation (typically on 'speed of onset and duration of action' grounds).
Adjusting dosage and timing to fit food amounts and types.
Adjusting dosage and timing to fit exercise undertaken.
Adjusting dosage, type, and timing to fit other conditions as for instance the increased stress of illness.
The dosage is non-physiologic in that a subcutaneous bolus dosage of only insulin is given instead of the pancreas releasing insulin and C-peptide gradually and directly into the portal vein.
It is simply a nuisance for patients to inject themselves once or several times a day.
It may be dangerous in the case of mistake (most especially 'too much' insulin).
There have been several attempts to improve upon this mode of administering insulin as many people find injection awkward and painful. One alternative is jet injection (also sometimes used for some vaccinations) which has different insulin delivery peaks and durations as compared to needle injection of the same amount and type of insulin. Some diabetics find control possible with jet injectors, but not with hypodermic injection. There are also 'insulin pumps' of various types which are 'electrical injectors' attached to a semi-permanently implanted needle (i.e. a catheter). Some who cannot achieve adequate glucose control by conventional injection (or sometimes jet injection) are able to with the appropriate pump.
An insulin pump is a reasonable solution for some. However there are several major limitations - cost, the potential for hypoglycemic episodes, catheter problems, and, thus far, no approvable means of controlling insulin delivery in the field based on blood glucose levels. If too much insulin is delivered or the patient eats less than normal, there will be hypoglycemia. On the other hand, if too little insulin is delivered by the pump, there will be hyperglycemia. Both of these can lead to potentially life-threatening conditions. In addition, indwelling catheters pose the risk of infection and ulceration. However, that risk can be minimized by keeping catheter sites clean. Thus far, insulin pumps require considerable care and effort to use correctly. However, some diabetics are able to keep their glucose in reasonable control only on a pump.
Researchers have produced a watch-like device that tests for blood glucose levels through the skin and administers corrective doses of insulin through pores in the skin of the patient. Both electricity and ultrasound have been found to make the skin temporarily porous. The insulin administration aspect remains experimental at this writing. The blood glucose test aspect of such 'wrist appliances' is, at this writing, commercially available essentially as described.
Another 'improvement' would be to avoid periodic insulin administration entirely by installing a self-regulating insulin source. For instance, pancreatic, or beta cell, transplantation. Transplantation of an entire pancreas (as an individual organ) is technically difficult, and is not common. Generally, it is performed in conjunction with liver or kidney transplant surgery. However, transplantation of only pancreatic beta cells is a possibility. It has been highly experimental (for which read 'prone to failure') for many years, but some researchers in Alberta, Canada, have developed techniques which have produced a much higher success rate (about 90% in one group). Beta cell transplant may become practical, and common, in the near future. Additionally, some researchers have explored the possibility of transplanting genetically engineered non-beta cells to secrete insulin, as an alternative to beta cell transplantation. Clinically testable results are far from realization. Several other non-transplant methods of automatic insulin delivery are being developed in the research labs as this is written. None is currently close to clinical approval.
Inhaled insulin is under active investigation as are several other insulin administration techniques. Currently the only inhalable insulin approved by the FDA for use is Exubera.
Dosage and timing
The central problem for those requiring external insulin is picking the right dose of insulin and the right timing.
Physiological regulation of blood glucose, as in the non-diabetic, would be best. Increased blood glucose levels after a meal is a stimulus for prompt release of insulin from the pancreas. The increased insulin level causes glucose absorption and storage, reducing glycogen to glucose conversion, reducing blood glucose levels, and so reducing insulin release. The result is that the blood glucose level rises somewhat after eating, and within an hour or so returns to the normal 'fasting' level. Even the best diabetic treatment with human insulin, however administered, falls short of normal glucose control in the non-diabetic.
Complicating matters is that the composition of the food eaten (see glycemic index) affects intestinal absorption rates. Glucose from some foods is absorbed more (or less) rapidly than the same amount of glucose in other foods. And, fats and proteins both cause delays in absorption of glucose from carbohydrate eaten at the same time. As well, exercise reduces the need for insulin even when all other factors remain the same, since working muscle has some ability to take up glucose without the help of insulin.
It is in principle impossible to know for certain how much insulin (and which type) is needed to 'cover' a particular meal in order to achieve a reasonable blood glucose level within an hour or two after eating. Non-diabetics' beta cells routinely and automatically manage this by continual glucose level monitoring and adjustment of insulin release. All such decisions by a diabetic must be based on general experience and training (ie, at the direction of a physician or PA, or in some places a specialist diabetic educator) and, further, specifically based on the individual experience of the patient. It is not straightforward and should never be done by habit or routine, but with care can be done quite successfully in practice.
For example, some diabetics require more insulin after drinking skim milk than they do after taking an equivalent amount of fat, protein, carbohydrate, and fluid in some other form. Their particular reaction to skimmed milk is different than other diabetics', but the same amount of whole milk is likely to cause a still different reaction even in that same person. Whole milk contains considerable fat while skimmed milk has much less. It is a continual balancing act for all diabetics, especially for those taking insulin.
Insulin dependant diabetics require a base level of insulin (Basal Insulin), as well as extra short acting insulin to cope with meals (Bolus Insulin). Maintaining the basal rate and the bolus rate is a continuous balancing act that all insulin diabetics have to manage each day. This is normally achieved through regular blood tests, although there is work being undertaken on continuous blood sugar testing equipment.
It is important to notice that diabetics generally need more insulin than the usual -not less- during physical stress like infections or surgeries.
Medical preparations of insulin (from the major suppliers – Eli Lilly and Novo Nordisk -- or from any other) are never just 'insulin in water'. Clinical insulins are specially prepared mixtures of insulin plus other substances. These delay absorption of the insulin, adjust the pH of the solution to reduce reactions at the injection site, and so on. Some recent insulins are not even precisely insulin, but so called insulin analogs. The insulin molecule in an insulin analog is slightly modified so that they are:
Absorbed rapidly enough to mimic real beta cell insulin (Lilly's is 'lispro', Novo Nordisk's is 'aspart').
Steadily absorbed after injection instead of having a 'peak' followed by a more or less rapid decline in insulin action (Novo Nordisk version is 'Insulin detemir' and Aventis' version is 'Insulin glargine').
All while retaining insulin action in the human body.
The management of choosing insulin type and dosage / timing should be done by an experienced medical professional working with the diabetic.
Allowing blood glucose levels to rise, though not to levels which cause acute hyperglycemic symptoms, is not a sensible choice. Several large, well designed, long term studies have conclusively shown that diabetic complications decrease markedly, linearly, and consistently as blood glucose levels approach 'normal' patterns over long periods. In short, if a diabetic closely controls blood glucose levels (ie, on average, both over days and weeks, and avoiding too high peaks after meals) the rate of diabetic complications goes down. If glucose levels are very closely controlled, that rate can even approach 'normal'. The chronic diabetic complications include cerebrovascular accidents (CVA or stroke), heart attack, blindness (from proliferative diabetic retinopathy), toehr vascular damage, nerve damage from diabetic neuropathy, or kidney failure from diabetic nephropathy. These studies have demonstrated beyond doubt that, if it is possible for a patient, so-called intensive insulinotherapy is superior to conventional insulinotherapy. However, close control of blood glucose levels (as in intensive insulinotherapy) does require care and considerable effort, for hypoglycemia is dangerous and can be fatal.
A good measure of long term diabetic control (over approximately 90 days in most people) is the serum level of glycosylated hemoglobin (HbA1c). A shorter term integrated measure (over two weeks or so) is the so-called fructosamine level, which is a measure of similarly glyclosylated proteins (chiefly albumin) with a shorter half life in the blood. There is a commercial meter available which measures this level in the field.
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