Insulin

The hormone Insulin is the key regulator of blood glucose levels and is released from specialized beta cells within the Islets of Langerhans of the endocrine pancreas. Whilst the Islets may comprise only a small percentage of the pancreas (1-2 %), they serve the major physiological function in homeostatic control of blood glucose concentrations and cellular metabolism. The beta cells of the pancreas release stored insulin in response to a range of neural, nutrient, hormonal and chemical stimuli but primarily in response to glucose stimulation. Insulin secretion is increased by high blood glucose levels and inhibited by falling glucose levels. The overall effect of Insulin is to stimulate anabolic (energy storage) processes to lower blood levels of glucose, fatty acids, and amino acids and to promote their conversion to the respective storage forms of glycogen, triglycerides and protein.

(CC) Image: David E. Volk
X-ray crystal structure of human insulin.

Mechanism of secretion

Primary mechanism of Insulin secretion by the pancreas

• Glucose enters beta cells of the Isletes of Langerhans by facilitated diffusion involving GLUT-2 glucose transporters.
  
• This cytosolic glucose is metabolized to ATP increasing the intracellular ratio of ATP:ADP.
  
• Elevation in the intracellular ATP concentration induces closure of cell-surface ATP-sensitive K⁺ (K_ATP) channels. This reduces repolarizing current density leading to cell membrane depolarization.
  
• Voltage-dependent Ca²⁺ channels open in response to membrane depolarization, facilitating movement of extracellular Ca²⁺ into the beta cell.
  
• Increasing cytosolic calcium concentrations activate phospholipase C stimulating the production of inositol 1,4,5-triphosphate (IP₃) and diacylglycerol (DAG) from membrane phospholipids.
  
• IP₃ binds to intracellular receptors on the membrane of the endoplasmic reticulum (ER) initiating the release of large amounts of stored calcium into the cytoplasm
  
• The rising cytosolic Ca²⁺ triggers exocytosis of insulin containing vesicles within the beta cell into nearby blood vessels. The extensive vascularization of the pancreatic islets region ensures rapid diffusion of insulin into blood circulation and subsequent delivery to target cells and tissues.

In diabetes

Main Article: Diabetes mellitus

Patients suffering from diabetes mellitus have abnormal insulin function. In Type-I (or Juvenile), the pancreas produces insufficient amounts of insulin, resulting in high blood sugar levels. In Type-II diabetes, insulin receptors on cells become less responsive to insulin in the blood and there is a general alteration in the way cells respond to this hormone. Thus, while the pancreas may be producing enough insulin, the hormonal signal does not get through, resulting in high blood sugar levels.

Insulin Therapy

Types of insulin

The commonly used types of insulin are:

Rapid-acting

Rapid-acting insulins include three analogues:

• insulin lispro – begins to work within 5 to 15 minutes and is active for 3 to 4 hours.
• insulin aspart
• insulin glulisine

Short-acting

Short-acting insulins include regular insulin – starts working within 30 minutes and is active about 5 to 8 hours.

Intermediate-acting

Intermediate-acting act within 1 to 3 hours and are active 16 to 24 hours.

• neutral protamine Hagedorn (NPH) insulin
• lente insulin
• neutral protamine lispro (NPL) insulin

Long-acting

Long-acting insulins include:

• ultralente insulin – starts working in 4 to 6 hours, and is active 24 to 28 hours.
• Insulin glargine (LANTUS™) – starts working within 1 to 2 hours and continue to be active, without major peaks or dips, for about 24 hours, although this varies in many individuals.
• Insulin detemir (Levemir™) – starts working within 1 to 2 hours and continue to be active for about 24 hours, although typically not as long acting as glargine.

Premixed insulins

Mixtures can be of two types of human insulins or two insulin analogues. Human mixtures and analogue mixtures have similar efficacy. Mixtures of human insulin:

• A mixture of NPH and regular insulin – starts working in 30 minutes and is active 16 to 24 hours. There are several variations with different proportions of the mixed insulins.

Mixtures of analogue insulin:

• NPL/lispro 75/25

Modes of administration

Unlike many medicines, insulin cannot be taken orally. Like nearly all other proteins introduced into the gastrointestinal tract, it is reduced to fragments (even single amino acid components), whereupon all 'insulin activity' is lost.

Subcutaneous

Insulin is usually taken as subcutaneous injections by single-use syringes with needles, an insulin pump, or by repeated-use insulin pens with needles.

Insulin pump

Insulin pumps are a reasonable solution for some. Advantages to the patient are better control over background or 'basal' insulin dose, bolus doses calculated to fractions of a unit, and calculators in the pump that help with dosing 'bolus' infusions. The limitations are cost, the potential for hypoglycemic and hyperglycemic episodes, catheter problems, and no "closed loop" means of controlling insulin delivery based on current blood glucose levels.

Insulin pumps may be like 'electrical injectors' attached to a semi-permanently implanted catheter or cannula. Some who cannot achieve adequate glucose control by conventional (or jet) injection are able to do so with the appropriate pump.

As with injections, if too much insulin is delivered or the patient eats less than he or she dosed for, there will be hypoglycemia. On the other hand, if too little insulin is delivered, there will be hyperglycemia. Both can be life-threatening. In addition, indwelling catheters pose the risk of infection and ulceration. These risks can be minimized by keeping infusion sites clean. Insulin pumps require care and effort to use correctly. However, some diabetics are capable to keep their glucose in reasonable control only on a pump.

Inhalation

In 2006 the U.S. Food and Drug Administration approved the use of Exubera, the first inhalable insulin. Inhaled insulin has similar efficacy to injected insulin, both in terms of controlling glucose levels and blood half-life. Currently, inhaled insulin is short acting and is typically taken before meals; an injection of long-acting insulin at night is often still required. When patients were switched from injected to inhaled insulin, no significant difference was found in Hb_A1c levels over three months. Accurate dosing is still a problem, but patients showed no significant weight gain or pulmonary function decline over the length of the trial, when compared to the baseline. Following its commercial launch in 2005 in the UK, it has not (as of July 2006) been recommended by National Institute for Health and Clinical Excellence for routine use, except in cases where there is "proven injection phobia diagnosed by a psychiatrist or psychologist".

Transdermal

There are several methods for transdermal delivery of insulin. Pulsatile insulin uses microjets to pulse insulin into the patient, mimicking the physiological secretions of insulin by the pancreas. Jet injection (also sometimes used for vaccinations) had different insulin delivery peaks and durations as compared to needle injection. Some diabetics find control possible with jet injectors, but not with hypodermic injection.

Both electricity using iontophoresis and ultrasound have been found to make the skin temporarily porous. The insulin administration aspect remains experimental, but the blood glucose test aspect of 'wrist appliances' is commercially available.

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.

Intranasal insulin

Intranasal insulin is being investigated.

Oral insulin

The basic appeal of oral hypoglycemic agents is that most people would prefer a pill to an injection. However, insulin is a protein. Protein hormones, like meat proteins, are digested in the stomach and gut.

The potential market for an oral form of insulin is enormous and many laboratories have attempted to devise ways of moving enough intact insulin from the gut to the portal vein to have a measurable effect on blood sugar. One can find several research reports over the years describing promising approaches or limited success in animals, and limited human testing, but as of 2004, no products appear to be successful enough to bring to market.

Pancreatic transplantation

Another improvement would be a transplantation of the pancreas or beta cell to avoid periodic insulin administration. This would result in a self-regulating insulin source. Transplantation of an entire pancreas (as an individual organ) is difficult and uncommon. Generally, it is performed in conjunction with liver or kidney transplant. However, it is possible to do a transplantation of only the pancreatic beta cells. It has been highly experimental (for which read 'prone to failure') for many years, but some researchers in Alberta, Canada, have developed techniques with a high initial success rate (about 90% in one group). Nearly half of those who got an islet cell transplant are insulin-free one year after the operation; by the end of the second year that number drops to about one in seven. Beta cell transplant may become practical in the near future. Additionally, some researchers have explored the possibility of transplanting genetically engineered non-beta cells to secrete insulin. Clinically testable results are far from realization. Several other non-transplant methods of automatic insulin delivery are being developed in research labs, but none is close to clinical approval.