Download Insulin and its preparations and more Summaries Pharmacology in PDF only on Docsity!
Introduction:
Insulin is a polypeptide hormone produced by β -cells of islets of langerhans ofcells of islets of langerhans of pancreas. It has profound Influence on metabolism of carbohydrates, fats and proteins. It was considered as the anabolic hormone. Insulin was the first hormone to be isolated, purified and synthesized. Insulin was also the first hormone to be sequenced. Insulin was also the first hormone to be prepared by recombinant DNA technology.
Structure of insulin:
Insulin is a two chain polypeptide having 51 amino acids and molecular weight about
The A chain usually is composed of 21 amino acid residues, and the B chain has 30. There are two interchain disufide bridge at A7-cells of islets of langerhans ofB7 & A20-cells of islets of langerhans ofB19. There is also a intrachain disulfide link in chain A at 6 and 11 amino acids. Although the amino acid sequence of insulin has been highly conserved in evolution, there are significant variations that account for differences in both biological potency and immunogenicity (De Meyts, 1994). There is a single insulin gene and a single protein product in most species. Fig-cells of islets of langerhans of1. Structure of insulin
History of insulin:
Insulin was discovered in1921 by Dr. Friedrich Banting and Dr. Charles Best. It was first obtained in pure crystalline form in 1926 by Abel. The chemical structure was fully worked out in 1956 by Sanger. In 1923 Banting and Best was awarded with noble prize for insulin. Hodgkin and coworkers elucidated insulin’s three-cells of islets of langerhans ofdimensional structure in 1972.
Synthesis:
Gene for protein synthesis is located on Chr 11. Insulin is synthesized in the β cells of pancreatic islets as a single chain peptide Preproinsulin (110 AA). After translocation through the membrane of the rough endoplasmic reticulum, the 24-cells of islets of langerhans ofamino-cells of islets of langerhans ofacid N-cells of islets of langerhans ofterminal signal peptide of preproinsulin is cleaved rapidly to form proinsulin. Thereafter, proinsulin folds, and the disulfide bonds form. The connecting or ‘C’ peptide (35 AA) is split off by proteolysis in Golgi apparatus; both insulin and C peptide are stored in granules within the cell. The C peptide is secreted in the blood along with insulin.
Regulation of Insulin secretion:
Insulin secretion is a tightly regulated process designed to provide stable concentrations of glucose in blood during both fasting and feeding. Under basal condition ~1U insulin is secreted per hour by human pancreas. Secretion of insulin from β cells is regulated by chemical, hormonal and neural mechanisms.
- Chemical: The β cells have a glucose sensing mechanism dependent on entry of glucose into β cells (through the aegis of GLUT2) and its phosphorylation by glucokinase. Other nutrients that can evoke insulin release are—amino acids, fatty acids and ketone bodies, but glucose is the principal regulator and it stimulates synthesis of insulin as well.
- Hormonal: A number of hormones, e.g. growth hormone, corticosteroids, thyroxine modify insulin release in response to glucose.
- Somatostatin inhibits release of both insulin and glucagon.
regulation of pancreatic exocrine and endocrine secretions. Pancreatic polypeptide released following a meal may reduce further food consumption; however, it is also released in response to fasting.
Mechanism of insulin action:
Insulin acts on specific receptors located on the cell membrane of practically every cell, but their density depends on the cell type: liver and fat cells are very rich. The insulin receptor is a heterotetrameric glycoprotein consisting of 2 extracellular α and 2 transmembrane β subunits linked together by disulphide bonds. It is oriented across the cell membrane as a heterodimer (Fig. 19.3). The α subunits carry insulin binding sites, while the β subunits have tyrosine protein kinase activity. Binding of insulin to α subunits induces aggregation and internalization of the receptor along with the bound insulin molecules. This activates tyrosine kinase activity of the β subunits → pairs of β subunits phosphorylate tyrosine residues on each other → expose the catalytic site to phosphorylate tyrosine residues of Insulin ReceptorSubstrate proteins (IRS1, IRS2, etc). In turn, a cascade of phosphorylation and dephosphorylation reactions is set into motion resulting in stimulation or inhibition of enzymes involved in the rapid metabolic actions of insulin. Certain second messengers like phosphatidyl inositol trisphosphate (PIP3) which are generated through activation of a specific PI3-cells of islets of langerhans ofkinase also mediate the action of insulin on metabolic enzymes. Insulin stimulates glucose transport across cell membrane by ATP dependent translocation of glucose transporter GLUT4 and GLUT1 to the plasma membrane as well as by increasing its activity. Over a period of time it also promotes expression of the genes directing synthesis of GLUT4. Genes for a large number of enzymes and carriers have been shown to be regulated by insulin primarily through MAP kinases. Activation of transcription factors also promotes proliferation and differentiation of specific cells. The internalized receptor-cells of islets of langerhans ofinsulin complex is either degraded intracellularly or returned back to the surface from where the insulin is released extracellularly. The relative preponderance of these two processes differs among different tissues: maximum degradation occurs in liver, least in vascular endothelium.
Insulin Rapid acting Short acting Intermediate acting Long acting Fig-cells of islets of langerhans of3. mechanism of insulin action
Actions of insulin on various tissues:
On Liver: Insulin inhibits glucose uptake and glycogen synthesis. It also inhibits glycogenolysis and glucose output’ Inhibits gluconeogenesis from protein, pyruvate, and glycerol. It also decreases glucose trnsport, increases glycolysis, triglyceride synthesis, protein synthesis. On Muscle: Insulin increases glucose uptake and utilization, Insulin also inhibits proteolysis and release of amino acids, pyruvate, lactate into blood which form substrate for gluconeogenesis in liver. Insulin increases glycogen deposition, protein synthesis. On Adipose : Increases glucose uptake and storage as fat and glycogen. Inhibits lipolysis and release of FFA + glycerol which form substrate for gluconeogenesis in liver. Increases lipogenolysis and lipoprotein lipase activity.
Classification and Insulin preparations:
Page | 5
It is a buffered solution of unmodified insulin stabilized by a smallamount of zinc. At the concentration of the injectable solution, the insulin molecules self aggregate to form hexamers around zinc ions. After s.c. injection, insulin monomers are released gradually by dilution, so that absorption occurs slowly. Peak action is produced only after 2–4 hours and action continues upto 6–8 hours. The absorption pattern is also affected by dose; higher doses act longer. When injected s.c. just before a meal, this pattern often creates a mismatch between need and availability of insulin to result in early postprandial hyperglycaemia and late postprandial hypoglycaemia. Regular insulin injected s.c. is also not suitable for providing a low constant basal level of action in the interdigestive period.
Intermediate acting Insulin:
1) Insulin zinc suspensions: Two types of insulin-cells of islets of langerhans ofzinc suspensions have been produced. The one with large particles is crystalline and practically insoluble in water (ultralente or ‘extended insulin zinc suspension’). It is longacting. The other has smaller particles and is amorphous (semilente or ‘prompt insulin zinc suspension’), is short-cells of islets of langerhans ofacting. Their 7:3 ratio mixture is called ‘Lente insulin’ and is intermediate-cells of islets of langerhans ofacting. 2) Isophane (Neutral Protamine Hagedorn or NPH) insulin: Protamine is added in a quantity just sufficient to complex all insulin molecules; neither of the two is present in free form and pH is neutral. On s.c. injection, the complex dissociates slowly to yield an intermediate duration of action.
Long acting Insulin:
1) Protamine zinc suspension: It contains excess of protamine, so that the complexed insulin is released more slowly at the site of s.c. injection and a long-cells of islets of langerhans ofacting preparation results. It is rarely used now. 2) Insulin glargine: This long-cells of islets of langerhans ofacting biosynthetic insulin has 2 additional arginine residues at the carboxy terminus of B chain and glycine replaces asparagine at A 21. It remains soluble at pH4 of the formulation, but precipitates at neutral pH encountered on s.c. injection.
A depot is created from which monomeric insulin dissociates slowly to enter the circulation. Onset of action is delayed, but relatively low blood levels of insulin are maintained for upto 24 hours. A smooth ‘peakless’ effect is obtained. Thus, it is suitable for once daily injection to provide background insulin action. Fasting and interdigestive blood glucose levels are effectively lowered irrespective of time of the day when injected or the site of s.c. injection.
Immunopathology of insulin therapy:
1) Insulin allergy:
It is an immediate hypersensitivity reaction and also is a rare condition in which local or systemic urticaria reults from histamine release from mast cells senstized by anti-cells of islets of langerhans ofinsulin IgE antibodies. In severe cases anaphylaxis results also. Because sensitivitynis often to noninsulin protein contaminants, the human and analog insulins have markedly reduced the incidence of insulin allergy, especially local reactions.
2) Immune-insulin resistance:
A low titer of circulating IgG anti-cells of islets of langerhans ofinsulin antibodies that neutralize the action of insulin to a negligible extent develops in most insulin treated patients. Rarely, the titer of insulin antibodies leads to insulin resistance.
Insulin delivery devices:
- Insulin syringes
- External insulin pumps
- Implantable insulin pumps
- Insulin pens
- Insulin jet injectors
- Insulin inhalers
References:
- Diabetes control and complications trial (DCCT) Group: Effect of intensive therapy on the microvascular complications of type 1 diabetes mellitus; JAMA 287 : 2563-cells of islets of langerhans of 2569, 2002.
- Essentials of Medical Pharmacology, 7th^ Edition by KD Tripathi.
- Goodman And Gillman’s, The Pharmacological Basis of Therapeutics, 13th^ Edition.
