Release and circulation of insulin

The most important physiological stimulus is raised blood glucose level: within 30–60 seconds of absorption of glucose after a meal, there is increased release of insulin. This occurs via an excitation–secretion coupling process with depolarisation of cell membranes and calcium influx, then exocytosis of insulin-containing secretory vesicles, with a rapid initial rise due to release of stored insulin, then a slower delayed phase over 60–90 minutes when both stored and newly synthesised insulin are released to deal with the raised sugar level in the blood. A fall in blood glucose directly inhibits insulin secretion, which shifts metabolism to the post-absorptive pattern levels such as occur during fasting.

Other stimuli to insulin secretion include:

Insulin is released via capillaries into the portal circulation to the liver. There is a low basal level of release in pulses every 15–30 minutes, with increased release in response to stimuli.

Insulin release is inhibited by somatostatin and by adrenaline (via α₂ receptors). Deficiencies of release occur in pancreatic disorders (diabetes mellitus, pancreatitis, tumours) and other endocrine disorders (Cushing’s disease, acromegaly), and can be caused by drugs, including alloxan (of mainly experimental interest) and the thiazide diuretics.

Insulin is circulated bound to β-globulin. As a protein, it is rapidly digested in the gut if given orally, with a half-life of only a few minutes (this explains why the hormone must be administered parenterally to treat diabetes). It has much longer biological duration of action (2–4 hours), however, as it is taken up and bound to receptors in the tissues where it acts.

Actions of insulin

Overall, insulin facilitates removal of glucose from the blood into muscle and fat cells and promotes storage of nutrients. After binding of insulin to specific receptors on membranes of target cells, glucose transporters are recruited from intracellular storage pools to the cell membrane, where they facilitate the active uptake of glucose into the cell where it is ‘trapped’ as glucose-6-phosphate. A great variety of biochemical reactions and processes are mediated by insulin: in summary, it affects uptake, utilisation and storage of carbohydrates, fats and amino acids in liver, adipose and muscle cells, so that nutrients are stored as glycogen, triglycerides and fatty acids, and proteins. It thus controls intermediary metabolism, promotes the anabolic state (building up) and has long-term effects on cell proliferation and growth regulation. Glycogenolysis, lipolysis and proteolysis are inhibited.

The actions of insulin are physiologically antagonised by the catabolic hormones, i.e. adrenocorticotrophic hormone (ACTH), glucocorticoids, adrenaline, growth hormone (GH) and thyroxine. Hence, low insulin levels (and diabetes) can occur secondary to other endocrine disorders, including acromegaly and Cushing’s disease.

Mechanism of action

The mechanism of action of insulin is via binding to membrane receptors on target cells and activation of a tyrosine kinase enzyme. This initiates cascades of phosphorylation reactions leading to many kinase and phosphatase activities, as well as DNA transcription and cell replication. Intracellular vesicles containing a glucose transporter (GLUT-4) fuse with the plasma membrane and the transporter is inserted, leading to a rapid 10-to 30-fold increase in glucose uptake by the cell. Cells in the brain, exercising muscle and liver are not dependent on insulinmediated glucose uptake.

Epidemiology

The self-reported prevalence of diabetes mellitus in the adult Australian population is around 3.5%; the prevalence increases with age, so in those over 65 years of age the prevalence is more like 14%. However, there may be as many people again in whom the disorder has not been diagnosed. Of people with diabetes, approximately 83% report having type 2. Diabetes is the ninth leading cause of death in Australia (Australian Bureau of Statistics 2007); it is the main underlying cause in 2.8% of registered deaths and is a contributing factor in almost 10% of all deaths. The cost to the Australian community of treating diabetes in 2000/01 was estimated to be about $784 million. It is recommended that all people over 55 years be screened for glucose intolerance, plus younger people with added risk factors.

Worldwide, the incidence of type 2 diabetes is increasing; risk factors include disadvantaged socioeconomic status, overweight and obesity, lack of exercise and hypertension. Increasing incidence is occurring in the Australian Aboriginal and Torres Strait Islander population (see Clinical Interest Box 36-2) and in the Polynesian population in New Zealand (Clinical Interest Box 36-3). In developing countries, the mortality rate is unacceptably high, often due to inadequate, expensive and irregular supplies of insulin. In many Pacific Island countries, prevalence is increasing rapidly, due partly to a shift away from traditional diet (vegetables and fruit) to Western-type diet, fatty meats and ‘junk foods’, with the consequent rise in obesity.

Pathology

Uncontrolled diabetes is a devastating disease that is a major cause of blindness, end-stage renal disease, neuropathies, accelerated atherosclerosis and lower limb amputations. The two general classifications for diabetes mellitus are type 1 diabetes (formerly known as insulindependent diabetes mellitus, IDDM, or juvenile-onset) and type 2 diabetes (non-insulin-dependent diabetes mellitus, NIDDM, maturity-onset). The features of the two types are summarised in Table 36-1.

Table 36-1 Features of type 1 and type 2 diabetes

The complex disorder of carbohydrate, fat and protein metabolism is primarily a result of a complete lack of insulin (type 1) or a relative lack of insulin or defects of the insulin receptors (type 2). Glucose cannot be readily taken up into cells and glycogen fails to store in the liver, although the conversion of glycogen back to glucose and the formation of glucose from other substances (gluconeogenesis) are not necessarily impaired. As a result, the blood glucose level rises rapidly. When it exceeds a critical level (the renal threshold), the excess is secreted by the kidney (glycosuria). Symptoms include increased appetite (polyphagia), thirst (polydipsia), weight loss, increased urine output (polyuria), weakness (fatigue) and itching (e.g. pruritus vulvae).

Diagnosis is by the signs and symptoms described above and by measurement of high blood glucose levels (casual 11.1 mmol/L, fasting 7.0 mmol/L). Glucose tolerance testing is a more stringent criterion, in which a standard dose (75 g) of glucose is administered after overnight fasting, and the level of glucose is measured in a venous blood sample 2 hours later (11.1 mmol/L or more indicates diabetes).

Course and complications

The course of untreated diabetes mellitus is progressive. The symptoms of diabetic coma and acidosis are directly or indirectly the result of the accumulation of ketone bodies. Respiration becomes rapid and deep, the breath has an odour of acetone, the blood glucose level is elevated, the patient becomes dehydrated, and stupor and coma develop unless treatment is prompt.

The long-term complications of diabetes mellitus can lead to an increase in morbidity and mortality, despite treatment with insulin (type 1) or diet modification and oral hypoglycaemic agents (type 2). Most of the complications have been shown to be due to thickening of the basement membrane of small blood vessels (microvascular disease), leading to ischaemia, neuropathies, nephropathy and diabetic retinopathy, which can include vitreal haemorrhage, retinal detachment and blindness. Macrovascular disease (atherosclerosis and thrombosis of larger vessels) may result in coronary artery disease, strokes, gangrene leading to amputations and cardiomyopathy leading to heart failure. There is increased risk of infections (due to poor circulation and high blood glucose levels) and impaired wound healing; foot infections may lead to osteomyelitis, gangrene and amputation (see review by Bowen [2007]).

Comorbidities with diabetes are very common, partly because the risk factors for diabetes are the same risk factors for many cardiovascular disorders. Elderly patients with diabetes frequently also suffer from hypertension, cerebrovascular disease, arthritis, asthma or mental health problems. Thus there is a high likelihood of polypharmacy for the many pathologies and of escalating drug interactions.

Treatment

It is recommended that large-scale community intervention programs focus on increased physical activity and healthier food options (fruit, vegetables, whole grains, dairy products and unsaturated fats), especially for children. Weight-reduction surgery is very effective in treatment, suggesting the central role of obesity in the syndrome. Some dietary supplements and alternative therapies that have been shown to have some benefit include eicosapentaenoic acid and docosahexaenoic acid (in fish oils), soy proteins, dietary fibre, polyphenolic compounds, modest wine intake and green tea. However, overall ‘there is no substitute for therapeutic lifestyle changes, including healthful eating and increased physical activity’ (Hollander & Mechanick 2008).

Treatment with hyperglycaemic agents

In mild to moderate hypoglycaemia, a readily available sugar source should be taken, such as jelly beans, honey or a sweet drink. This is followed with a complex carbohydrate that is more slowly absorbed, such as bread or dried fruit. In severe hypoglycaemia, in which the patient is unconscious or cannot take oral glucose, glucagon 1 mg (adult) or 0.5 mg (children <5 years) is administered SC or IM (see Drug Monograph 36-1). It is recommended that all patients at risk of developing hypoglycaemia carry a glucagon injection kit with them, and their families or carers should know how to administer it. In the hospital or clinic situation, glucose 50% or 10% solution is administered IV. Unconscious patients with hypoglycaemia should wake within 4–6 minutes of these therapies.

Treatment with glucose

Glucose is a monosaccharide that is absorbed from the intestine and then either used or stored by the body. Medically, it is indicated to treat or manage hypoglycaemia and is administered orally or parenterally. It is also present in many oral and parenteral electrolyte solutions, peritoneal dialysis solutions, formulations for rehydration, food supplements and blood glucose monitoring kits. In adults, about 10–20 g is administered orally and repeated in 10 minutes if necessary. The only adverse effects have been some reports of nausea. No significant drug interactions are reported.

Glycosylated haemoglobin

A better indicator of long-term management of diabetes is that of glycosylated (glycated) haemoglobin (HbA_1C), a form of haemoglobin that has a glucose molecule attached. This form of haemoglobin (normally about 7%) is increased in the red blood cells of people with poorly controlled diabetes; the glucose remains attached to haemoglobin for the life of the red blood cell, so the level of glycosylated haemoglobin reflects the average blood glucose concen tration over the past 3 months. It is commonly recommended that glycosylated haemoglobin be measured every 3 to 6 months in diabetes; levels of HbA_1C above 9% show poor control.

Sugar intake

Chronic hypoglycaemia is particularly dangerous for young people with diabetes, as the developing brain is dependent on glucose as an energy source; management of children with diabetes thus errs on the side of hyperglycaemia. Parents and carers need to be aware of the sugar content of medicines as well as that of foods and drinks; many medicines formulated for children have a high sugar content to encourage compliance (as Mary Poppins sang so engag ingly, ‘A spoonful of sugar helps the medicine go down’). Syrup, elixir and suspension formulations frequently have 50%–70% w/v (g/100 mL) sucrose and may contain other calorigenic sweetening agents such as sorbitol or honey. The carbohydrate content may have to be considered in the diet, particularly if the medicines are for another chronic condition, such as epilepsy or asthma. The sugars can also have deleterious effects on teeth if taken chronically. Other sweeteners may be more appropriate for people with diabetes or those on a strict diet. Lists are sometimes published of the sugar content of medicines, or of sugar-free formulations; see Clinical Interest Box 36-6 for some ‘sugar-free’ liquid formulations.

Sources

Insulin preparations available in Australia now are either human insulin (synthesised in the laboratory by chemical alteration of pork insulin or by recombinant DNA technology) or bovine insulin (derived from beef pancreas)—rarely used now. Beef insulin differs from human insulin by three amino acids, whereas the pork insulin differs from human insulin by a single amino acid. The human (or recombinant) insulin is identical to the insulin produced by the human pancreas; see Drug Monograph 36-2. Insulins produced by recombinant DNA technology are identified by the suffixes rys, recombinant yeast Saccharomyces cerevisiae, or rbe, recombinant bacteria Escherichia coli.

Many diabetic patients continue to be effectively treated with beef insulin if they have not developed insulin resistance, insulin allergies or lipodystrophy (a breakdown of subcutaneous fat occurring after repeated injections) at the insulin injection sites. There is a higher degree of immunogenicity reported with beef insulin than with human insulin. Subcutaneously administered human insulin may be absorbed faster and have a shorter duration of action than the animal insulins. It is standard practice now to prescribe human insulin whenever possible. Patients switched from animal to human insulin should be closely monitored initially because a dosage adjustment may be necessary. When human insulin was introduced, there was some suggestion that patients found it harder to predict when a ‘hypo’ (i.e. a hypoglycaemic state) was coming on; however, this has not been proven to be a problem clinically.

Time-course of formulations

Insulins, whether human or bovine, have been formulated in many different ways to alter the pharmacokinetic properties of the mixture; the formulations are described as ultra-short-acting, short-acting, intermediate-acting or long-acting (all contain 100 IU/mL; see Clinical Interest Box 36-7). The wide range of formulations available allows careful titration of dosage to maximise blood glucose control and minimise fluctuations.

The chemical nature of the protein is not changed (except in the ‘bolus’ and ‘basal’ analogues in which minor changes are made in the amino acid sequences), only the rate at which it acts and is inactivated. Table 36-3 lists some types of insulin formulations, their approved names and pharmacokinetic characteristics; the formulations are also compared graphically in Figure 36-2. (Note that the time-course of action may vary among individuals, or at different times in the same individual, and is dependent on the site of injection, blood supply, body temperature, physical activity etc.) Premixed formulations are also available, i.e. 20/80, 30/70 and 50/50 mixes of short- and intermediate-acting types, giving a wide range of possible insulins.

Table 36-3 Characteristics of insulin preparations after subcutaneous administration

Figure 36-2 Insulin formulations’ durations of action (see also Table 36-3). Curves indicate approximate times of onset, peak and duration of hypoglycaemic activity of the main types of insulin formulations. The ultra-short formulation, insulin glulisine, has peak action at about 1 hour after injection and duration 2–3 hours.

Insulin stocks not in use are normally stored in the refrigerator, but the vial or cartridge currently in use is stable at room temperature. Freezing the solution denatures and inactivates the protein. Note that, whereas the oldertype insulin formulations were all chemically compatible and could be mixed together, some newer forms cannot, especially insulin glargine.

New, ultra-short-acting synthetic insulin analogues, insulin lispro, insulin aspart and insulin glulisine, have been developed. In these insulins two or three amino acids in human insulin have been changed. The primary advantage of the new insulins is that they have a more rapid onset of action than regular insulin; therefore they are administered as a ‘bolus’ immediately before a meal. They also have an earlier peak effect and shorter duration of action, so people with type 1 diabetes will usually require concurrent use of an intermediate- or long-acting insulin product as well to provide basal insulin activity. Insulin lispro and insulin aspart are also formulated crystallised with protamine, which prolongs the release and biological actions.

Recently, two new insulin analogues have been approved for use as ‘basal’ insulins, as their long, flat absorption profiles give them a long-acting effect more reproducible than those of the older long-acting formulations (insulin zinc suspension [IZS] and NPH¹). Insulin glargine has a slightly different amino acid sequence, which makes it soluble in the acidic solution in the vial but, after injection, microcrystals form in the tissues, from which insulin is slowly released. This formulation may cause pain on injection and must not be mixed with other insulins. Insulin detemir is a form in which a fatty acid compound is attached to the insulin molecule; after administration, the complex binds to albumin in the plasma and tissues, and insulin is slowly released. The detemir formulation may cause local reactions, but is less likely to cause weight gain than other insulins.

Routes of administration

Insulin is usually administered ‘ac, SC’, i.e. before meals (15–30 minutes before), subcutaneously. Injection sites are alternated, rotating around the abdomen and thighs, to minimise adverse local effects from the injections. In an emergency (diabetic ketoacidosis), short-acting insulins can be administered IV by infusion or IM.

Insulins are supplied in vials, all in the strength 100 IU/ mL. Many formulations are also available prepacked in cartridge ‘pens’, which facilitate injection and improve convenience and compliance with therapy. Vials or cartridges of insoluble preparations (i.e. all except regular insulin) should be rotated between the hands and inverted gently before a dose is withdrawn, to resuspend the protein; the container should not be shaken vigorously because this could denature the protein.

Portable insulin pumps have improved the metabolic state of some type 1 patients who did not have adequate diabetic control after intensive dietary restrictions and multiple daily injections of insulin. The insulin pump is battery-operated and computer-programmed to release small amounts of ultra-short-acting insulin per hour. It does not analyse the BGL, but is programmed based on the individual’s estimated daily insulin needs, diet and physical exercise. The patient can also push a button that releases a bolus dose to cover each meal consumed. Pumps are expensive, subject to mechanical failure and require careful maintenance. However, continuous insulin infusion has been shown to reduce slightly the glycosylated haemoglobin level in adults with type 1 diabetes; effects in type 2 are less clear.

Clinical trials of oral, nasal and inhaled insulin formulations are continuing; an effective non-injected form would be a wonderful improvement for people with diabetes.

Insulin dosage

There is no standard dose of insulin for the diabetic person; requirements depend on many factors, including lifestyle, weight, diet, exercise levels, stress, illness and pregnancy. (During pregnancy, insulin is the drug of choice to control diabetes; insulin requirements may drop for 24–72 hours after delivery and slowly return to pre-pregnancy levels in about 6 weeks.) Each patient’s needs must be determined individually to avoid hypoglycaemia and hyperglycaemia. A typical daily dose might be in the order of 0.7 IU/kg per day (reflecting the normal pancreatic production of about 50 IU per day) split into 2–4 injections and possibly two or three different types of insulin.

Insulin dosages should not be considered to be a fixed regimen; the dosage may need adjustment as a result of physical growth (child growing into adolescent then adult), illness, the development of anti-insulin antibodies, concomitant administration of certain medi cations, changes in lifestyle, missing a meal or doing unexpected exercise. Specific instructions may be required regarding insulin administration for a preoperative patient because of the alteration in the patient’s dietary patterns and metabolic requirements as the result of the surgical procedure. Treatment programs need to be reviewed regularly and adjusted as necessary, with the prescriber, other health-care professionals and patient working closely together to manage hypoglycaemia and hyperglycaemia and, if possible, avoid long-term complications.

Dosage regimens

As described earlier, the DCCT and UKPDS trials showed the importance of tight control of BGL, and hence techniques (‘insulin algorithms’) were developed to help determine the exact amount of insulin required at any time. Patients are taught to carry out self-monitoring of blood glucose (SMBG), and to adjust their insulin doses if the SMBG values are too high or low.

Biguanides: metformin

A biguanide OHA for the treatment of type 2 diabetes is metformin (Drug Monograph 36-3). Its mechanism of action is not completely understood; it is known to:

It does not affect the pancreatic beta cells; therefore it does not increase insulin release and is unlikely to cause hypoglycaemia. It is preferred to sulfonylureas in overweight patients as it is less likely to cause weight gain and improves the plasma lipid profile (see review by Bosi [2009]). Metformin is now available in combination tablet formulations with glibenclamide or with rosiglitazone.

The first drug released in this chemical category was phenformin, but it was withdrawn from the market due to its association with potentially life-threatening lactic acidosis. Metformin subsequently fell from favour temporarily, but it has been associated only rarely with this complication and is now the drug of first choice in type 2 diabetes. However, metformin is contraindicated in patients at risk of lactic acidosis, i.e. those with liver or kidney disease, the elderly and those taking alcohol or drugs that raise metformin levels.

Sulfonylureas

The sulfonylureas were developed as a spin-off from the sulfonamide antibacterial agents: it was noticed that some patients taking sulfonamides had lowered blood sugar levels. The sulfonylureas enhance the release of insulin from the beta cells in the pancreas and increase the cellular sensitivity to insulin in body tissues. Their mechanism of action is to bind to receptors and block ATP-sensitive potassium channels (K-ATP) that normally open when [K] is low, causing increased potassium conductance and hence inhibition of insulin secretion. Modulation by sulfonylureas closes the channels, thus blocking potassium efflux and causing cell depolarisation, calcium entry and insulin secretion. Hence they decrease glycogenolysis and gluconeogenesis and reduce blood glucose concentration in people with a functioning pancreas.

The first-generation sulfonylureas included chlorpropamide and tolbutamide, both of which have now been withdrawn. The second-generation agents include glibenclamide (see Drug Monograph 36-4), glipizide, gliclazide and glimepiride; they are more potent than the earlier drugs. All are taken with meals, to minimise risk of hypoglycaemia. Choice is made on the basis of pharmacokinetic parameters, as those with long half-lives are more risky in elderly patients, those that are excreted as active drug or metabolites are more risky in patients with renal disease, and those that are eliminated mainly by hepatic inactivation are more risky in people with liver disease.

Common adverse effects include hypoglycaemia, weight gain, GIT and taste disturbances and rashes. Serious blood disorders and allergic reactions occur rarely. Interactions are common with drugs that are metabolised by similar pathways, including sulfonamides, non-steroidal antiinflammatory drugs, coumarins, alcohol and monoamine oxidase inhibitors, and with drugs that compete for protein-binding sites.

Thiazolidinediones (‘glitazones’)

Two new agents for use in treatment of type 2 diabetes are pioglitazone and rosiglitazone. These drugs enhance the sensitivity of peripheral tissues and the liver to insulin and thus reduce insulin resistance, which is common in type 2 diabetes. Their mechanism of action is via activation of the peroxisome proliferator-activated (PPAR)-gamma receptor, a nuclear receptor that regulates gene transcription especially in adipocytes, where it regulates glucose and lipid metabolism via proteins including GLUT-4 (the princi pal glucose transporter protein), lipoprotein lipase and transporter and binding proteins for fatty acids. Circulating free fatty acid levels are thus reduced.

Reductions in BGLs occur soon after starting treatment, but the full effect on insulin sensitivity is not seen for several weeks. The drugs can be used in combination with metformin and sulfonylureas and are recommended in type 2 diabetes uncontrolled by diet alone.

Adverse effects include anaemia, peripheral oedema, weight gain and increased risk of heart failure and peripheral limb fractures. Pioglitazone appears to be safer with respect to patients with increased cardiovascular risk factors; rosigliatazone is more likely to increase total triglyceride levels and LDL particle size, so its prescribing is restricted. An earlier drug in this class (troglitazone) was withdrawn due to hepatic toxicity, so patients taking the newer glitazones are monitored carefully for signs of hepatic or heart failure. If no significant improvement in glycaemic control occurs after 8 weeks of therapy, the glitazone should be stopped and insulin therapy started.

Incretin enhancers

As described earlier, incretins are peptides that are released from the digestive tract in response to food; those currently known are glucose-dependent insulinotropic (poly)peptide (GIP) and glucagon-like peptide-1 (GLP-1). They stimulate glucose-dependent insulin release, enhance β-cell proliferation, reduce post-prandial glucose levels especially, delay gastric emptying and reduce appetite; people with type 2 diabetes have a markedly lowered response to incretins. Thus new drugs that enhance incretin actions are useful in type 2 diabetes, and provide a new group of drugs with different modes of action. The long-term effects of these drugs are as yet unknown; they are currently used as add-on therapy, usually with metformin (see reviews by Prins [2008], Reutens [2008] and McGill [2009]).

Exenatide is called an incretin-mimetic, as it is a peptide with potent agonist activity at GLP-1 receptors; as a peptide, it must be given by injection. Liraglutide (not yet available in Australia or New Zealand), another peptide GLP-1 receptor agonist, has a longer half-life and is proving safe and effective in recent clinical trials in patients unable to maintain glycaemic control with diet, exercise and standard OHAs. The GLP-1 agonists induce weight loss but frequently cause transient nausea, dyspepsia and diarrhoea; hypoglycaemia can occur, and immune reactions against the peptides. Pancreatitis is a serious potential adverse event that is being monitored.

Sitagliptin and vildagliptin inhibit dipeptidyl peptidase 4, the enzyme that inactivates GLP-1 and GIP; they have long half-lives and can be administered orally once daily. They increase incretin levels and β-cell glucose sensitivity in patients with type 2 diabetes to normal levels, and also reduce glucagon secretion and preserve β-cell mass. They appear to have few adverse effects (mainly gastrointestinal and musculoskeletal complaints) and low incidence of hypoglycaemia unless used with a sulfonylurea. Serious hypersensitivity reactions have been reported.

Other oral hypoglycaemic agents

Miscellaneous OHAs include an α-glucosidase inhibitor (acarbose), repaglinide and orlistat. These drugs will be discussed briefly; their pharmacokinetic characteristics and mechanisms of action are summarised in Table 36-4 and Figure 36-3, respectively. Complementary and alternative medicine methods relevant to the treatment of diabetes are discussed in Clinical Interest Box 36-8.