A protein hormone secreted by beta cells of the Islets of Langerhans in the pancreas. It is composed of 2 chains of 21 (A chain) and 30 (B chain) amino acids linked by 2 sulphide bonds; with a molecular weight of 5808 it is one of the smallest known proteins.
It is regarded as the hormone of the 'fed' state. Its primary action is the regulation of cellular uptake of glucose, but has important actions on fat and protein metabolism as well.
C-peptide and Insulin production in the body
Insulin is produced in the cell as a single protein, the A and B sections are linked and plausibly aligned for assembly by a section denoted C which is excised, activating the molecule, around the time of secretion. Some C-peptide appears in the blood for a while, and thus the question of whether insulin is being produced in a specific body can be answered by measuring C-peptide. No C-peptide is in exogenous Insulin, therefore treatment of the partial or total Insulin deficiency state does not interfere with the test. It is fiddly and requires rapid cooling of specimens or delivery to the lab and thus may not be available at all locations.
The main use of Insulin as a drug is to replace failed or relatively insufficient production in Diabetes.
(See treatment in Diabetes and Homeostasis).
In clinical terms, 1 unit of insulin is defined as the activity contained in 1/22 milligram of the international standard of zinc-insulin crystals.
Routes of Administration
Insulin is currently given by subcutaneous injection or infusion in the ambulant patient, and in hospitals sometimes also by iv infusion or injection. An inhaled insulin preparation has obvious attractions but as of 2006 is not in common use.
The ideal insulin replacement would depend upon glucose measurements from moment to moment, require no conscious intervention by the patient and work from a substantial reservoir of insulin. We currently lack durable implanted glucose sensors to replace the islet cells' surface, and the reliability of pumps and infusion systems also fall well short of ideal.
Recent developments in insulin formulations have provided basal insulins which can be injected once daily and are mobilised from that depot at a reasonably constant rate over a period in excess of 24 hours.
In the insulin dependent diabetic, to deal with meals bolus doses of insulin must be added to these, with an estimation of the amount required based on what is to be eaten and to an extent what is to be done afterwards being made before each dose. Rapid onset short acting insulins - prototypically "soluble insulin" - are used for this.
Alternative regimes include twice-daily injections of a mixture of rapid and slower acting insulins. These are clearly less logical and less flexible and generally less effective as well as not making it clear to the patient that there is an interplay between their eating, testing and choice of insulin dose. Nevertheless they have been presented as being easier to start diabetics on, rather than as persisting becuase of inertia and tradition.
Devices and systems
There are far too many different Insulin dosing devices, and the manufacturers guard their own designs, avoid compatibility of cartridges with injectors, and actively seek to differentiate their products. This is not helpful. See incompatible devices.
Requirements may rise due to gluconeogenic effect of stress response.
Aim for return to standard subcutaneous dosing when able to tolerate diet.
There is overwhelming evidence for its effectiveness and it is life saving in type 1 diabetes.
- Compared to metformin and possibly sulphonylureas in type 2 diabetes associated with heart failure it has a higher mortality
- Causes weight gain.