INSULIN

MECHANISM OF INSULIN SECRETION

Glucose is the principal stimulus to insulin secretion in human beings. The molecular mechanism of insulin secretion is follows: --------------------

Glucose enters the β cell by facilitated transport through GLUT 2, a specific subtype of glucose transporter.

The sugar is then phosphorylated by glucokinase, or hexokina敳桴湥

se, then undergoes metabolism and oxidation to CO2 and H2O. An offshoot of this process, probably a rising ATP/ADP ratio.

As a result, the ATP-sensitive K+ channel is blocked which cause membrane depolarization.

This opens voltage-dependent Ca2+ channels, leading to Ca2+ influx. Ca2+ activates phospholipase A2 and phospholipase C, that results in the formation of arachiodonic acid, IP3 (Inositol tris-phosphate) and DAG (Diacylglycerol).

The new altered ionic balance within the

 cells facilitates a contraction of the subcellular microtubule microfilament system, which is involved in the migration of insulin containing secretory granules into contact with the cell membrane. Fusion of the granule and cell membranes permits the rel慥敳漠⁦

ease of insulin.

MOLECULAR MECHANISM OF INSULIN ACTION

Insulin exerts its action by the following mechanism: ----------------------

Insulin binds to a specific receptor on the surface of its target cells. This receptor is a large transmembrane glycoprotein complex consisting of two ( and two

 subunits linked by disulfide bridges. The (-subunits are entirely extra cellular and each caries an insulin binding sites, whereas the β-subunits are transmembrane proteins with tyrosine kinase activity.

Binding of insulin to the (-subunits of the heterotetrameric insulin receptor leads to the rapid intra-molecular auto-phosphorylation of several tyrosine residues in the β-subunits and results in substantial enhancement of the receptors tyrosine kinase activity.

The activated receptor kinase then lead⁳潴琠敨

s to the phosphorylation of insulin receptor substrate proteins (IRS 1 to 4). This phosphorylated IRS-2 serves as a docking protein for other proteins tat contain so called Src-homology-2 (SH-2) domains. One of these SH-2 domain proteins is phosphoinositide-3-kinase (PI). PI-3-kinase catalyze the addition of phosphate to phosphoinositides on the 3-position of the D-myoinositol ring and these compounds apparently are involved in signal transduction.

Again insulin receptor complex also activate a most potent mitogenes called RAS oncoprotein. Activated Ras binds to Raf-1, a serine/threonine protein kinase, which activates the mitogen-activated protein (MAP) kinase cascade, which in turn activates several nuclear transcription factors leading to the expression of genes that are involved both with cell growth and with intermediary metabolism.          

FUNCTIONS OF INSULIN

On Carbohydrate Metabolism: Insulin increases uptake, uses, storage of glucose by the liver, muscle and all the cells of the body and decreases blood glucose level as follows: -----

Insulin increases the activity of enzyme glucokinase which enhances the uptake of glucose from blood and initiate phosphorilation of glucose in liver.

Insulin inhibits the enzyme phosphorilase thus prevents break down of liver glycogen.

Insulin stimulate the enzyme glycogen synthetase thus promotes glycogenesis.

Insulin promotes conversion of excess liver glucose into fatty acid.

Insulin inhibits gluconeogenesis.

 

On Fat Metabolism:

Insulin promotes fatty acid synthesis.

Insulin increases fat storage in the adipose tissue.

Insulin act as fat sparer by increasing utilization of carbohydrate I presence of fat. 

On Protein Metabolism:

It increases the transport of amino acid to cells.

It increases translation of mRNA on ribosome thus forming new protein.

Insulin promotes the rate of transcription of DNA to form RNA, which increases protein synthesis.

Insulin inhibits gluconeogenesis.

Insulin depressed protein catabolism. 

DIABETES

Diabetes is a collection of disorders characterized by defective regulation of carbohydrate, lipid and protein metabolism or a general term referring to characterize excessive urine excretion.

TYPES OF DIABETES

Diabetes is classified as -------------

Primary diabetes.

Secondary diabetes.

Maturity-onset Diabetes of Youth (MODY)

MODY 1 – hepatic nuclear factor 4( gene mutations.

MODY 2 – glucokinase gene mutations.

MODY 3 – hepatic nuclear factor 1( gene mutations.

MODY 4 – pancreatic determining factor X gene mutations.

MODY X – unidentified gene mutations.

Diabetes secondary to pancreatic disease.

Chronic pancreatities.

Surgery

Tropical diabetes. 

Diabetes secondary to other endocrinopathies.

Glucocorticoid administration.

Cushing’s diseases. 

Diabetes secondary to immune suppression.

Diabetes associated with genetic syndromes.

Diabetes associated with drug therapy.

Again diabetes is classified as -----------

Diabetes insipidus: It is a clinical condition characterized by the passage of large volume of watery urine. It is due to failure of ADH secretion. Most frequently as a result of a tumor of the hypothalamus that destroy the portion of hypothalamus that control the ADH secretion.

Diabetes Mellitus: Diabetes mellitus refers to a group of disorders characterized by absent or deficient insulin secretion of peripheral insulin resistance, resulting in hyperglycemia and impaired metabolism.  

There are two main forms of diabetes mellitus: ----------

Type 1 diabetes (also known as insulin dependent diabetes mellitus – IDDM – or Juvenile onset diabetes.)

Type 2 diabetes (also known as non insulin dependent diabetes mellitus – NIDDM – maturity onset diabetes.)

INSULIN THERAPY

Insulin replacement therapy is indicated for all type I and for type II diabetes whose hyperglycemia dose not respond to dietary or sulfonylurea therapy. Generally insulin is destroyed in the gastrointestinal tract must be given in parenterally – usually subcutaneously but intravenously or occasionally intramuscularly in emergencies.

Types:

Preparation of insulin can be classified according to their: ---------------

Species of origin

Duration of action.

SPECIES OF ORIGIN:

Human insulin: Human insulin is widely produced by recombinate DNA techniques. It is now carried out by inserting the human pro-insulin gene cDNA in Escherichia coli or yeast and treating the extracted Pro-insulin to form the human insulin.

Since threonine contains a hydroxyl group that makes it more polar and thus more hydrophilic than porcine insulin. Thus human insulin tends to be absorbed more rapidly and have a slightly shorter duration of action.

Example:

Human insulin injection (Novolin R)

Monotard Human insulin (Novolin L)

Porcine insulin: Porcine insulin differs from human insulin by one amino acid (alanine instead of threonine at the C-terminal of the B chain i.e. in position B-30)

Bovine insulin: Bovine insulin differs from human insulin by three amino acid (alanine instead of threonine in position B-30 and threonine and isoleucine in position A-8 and A-10 are replaced by alanine and valine respectively.

Mixture of Bovine and Porcine: 70% Bovine and 30% Porcine.

DURATION OF ACTION:

The three major types of insulin differ in onset and duration of action: -----------------------------

Short and rapid acting insulin.

Intermediate acting insulin.

Long acting insulin.

Short and rapid acting:

Onset of action: .05-.07hr.

Duration of action: 5-8hr.

Regular:

It dissolved usually in neural buffer PH

It usually should be injected 30-45 min before meals through intravenously or intramuscularly.

If metabolic conditions are stable, it usually is given subcutaneously in combination with an intermediate or long acting preparation.

Example: Crystalline Zinc insulin.

Semilente:

It is an amorphous precipitate of insulin with zinc ions in acetate buffers.

Example: Prompt insulin zinc suspension.

Intermediate acting insulin:

Onset of action: 1-2hr

Duration of action: 18-24hr.

Isophane insulin suspension: NPH insulin is a suspension of insulin in a complex with zinc and protamine in a phosphate buffer.

Lente insulin: Lente insulin is a mixture of crystallized (ultralente) and amorphous (semilente) insulin in an acetate buffer.

Long acting insulin:

Onset of action: 4-6hr.

Duration of action: 20-36hr

Ultralente: A long crystalline form with a high zinc content in an acetate buffers. It is also known as extended insulin zinc suspension.

Protamine zinc insulin suspension: Adding the basic protein protamine to crystalline zinc insulin causes the formation of large crystals. This produces a compound that is less soluble. When injected, this formulation serves as a tissue depot, producing slow absorption into the blood stream.

Insulin mixture:

Some diabetes need a mixture of insulin types (e.g. rapid acting insulin to control morning hyperglycemia and an intermediate acting insulin to later hyperglycemia) 

Example:  20% regular/80% NPH and 25% lispro/75% NPH

In addition, the above types of insulin preparation, there is a number of short acting insulin analogue. The major advantages of these analog are that they can easily retain a monomeric or diametric configuration from their hexamer after administration. Thus these analogs are absorbed three times more rapidly from subcutaneous site s than in human insulin, that leading to earlier hypoglycemic response.

Human insulin lispro: This analog is identical to human insulin in which a lysine and a praline residue are switched at B-29 and B-28 position respectively. It has two therapeutic advantages as compared to human insulin.

The prevalence of hypoglycemia is reduced by 20%-30% with lispro than Human insulin.

Glucose control as assessed by hemoglobin Aic is moderately but significantly improved by 0.3%-0.5% with lispro than Human insulin.

Insulin aspert: Insulin aspert is formed by the replacement of prolein at B-28 with aspartic acid.

ADVERSE REACTION

Hypoglycemia: The most common adverse reaction to insulin is hypoglycemia. This may result from: ------

An inappropriate large dose.

Food intake.

Superimposion of additional factors that increase sensitivity to insulin and insulin-dependent glucose uptake.

The worst sequela of hypoglycemia is insulin shock, characterized by abnormalities of the CNS, including hypoglycemic convulsions.

Early symptoms of hypoglycemia, such as sweating, tachycardia and hunger are thought to be brought about by the compensatory secretion of Epinephrine. 

Lipoatrophy and Lipohypertrophy: Irritation at the site of insulin injection can lead to Lipoatrophy and Lipohypertrophy. Thus site of injection should be rotated. Subcutaneous infusion can result in infection and local allergic reactions to components of the infusion system.

Insulin edema: Some degree of edema, abdominal bloating and blurred vision develops in many diabetes patients with severe hyperglycemia or ketoacidosis that is brought under control with insulin. Edema is attributed primarily to retention of Na+, although increased capillary permeability associated with inadequate metabolic control also may contribute.

Antigenic response: With the development of new, more highly purified animal insulin and the advent of human insulin, the production of insulin antibodies and the hypersentivity reaction are less of a problem. Insulin antibodies may attenuate responses to regular insulin injected subcutaneously and delay recovery from hypoglycemia.

Weight gain: is an undesirable effect of intensive insulin therapy.

Growth promoting properties of insulin may be a factor in the microvascular complications of diabetes.

DIABETIC’S COMPLICATIONS

Ketoacidosis: Under normal condition, insulin: -------

Inhibit lipolysis

Stimulate fatty acid synthesis (thereby increasing the concentration of malonyl CoA)

Decrease the hepatic concentration of carnitine.

------------ All these factors decrease the production of ketonbodies.

Conversely, Glucagon stimulates ketonbody production by increasing fatty acid oxidation and decreasing concentration of malonyl CoA. So, in the diabetic patient, the consequences of insulin deficiency and glucagon excess provide a hormonal milieu that favors ketogenesis that may lead to ketonemia and acidosis. 

Dehydration and Polydipsia: In the diabetic condition the glucose concentration in urine is increased. When the renal threshold for glucose reabsorption is exceeded, glucose spills over into the urine (glycosuria) and causes an osmotic diuresis (polyuria) which in turn results in dehydration, thirst and increased drinking (Polydipsia).

Microvascular complications: Thickening of the capillary basement membrane and other vascular changes occur during diabetics conditions. These toxic effects may be results of accumulation of non-enzymatically glycosylated products. Glycosylated products may eventually form cross linked proteins termed advanced glycosylation end products. These products form vascular matrix and can cause vascular complication including: --------

Premature atherosclerosis

Intercapillary glomeruloscleorsis.

Retinopathy

Ulceration

Gangrene of the extremities.

Diabetics Neuropathy: Diabetics neuropathy is associated with accumulation of poorly metabolized osmotically active metabolites of glucose such as sorbitol, produced by the action of aldose reductase. Thus increased osmotic pressure of the tissue and causes pain in the nerve ending.

In addition, in neural tissue and perhaps in other tissues, glucose competes with myoinositol for transport into cells. Reductions of cellular concentrations of myoinositol may contribute to altered nerve function and neuropathy. 

ORAL HYPOGLYCEMIC DRUG

SULFONYLUREAS:

First Generation

Second Generation

Tolbutamide

Glipizide

Acetohexamide

Gliclazide

Chlorpropamide

Glimepiride

Tolazamide

Glyburide

Mechanism of action:

Sulfonylurea stimulates the release of insulin from the functioning pancreatic (-cells by facilitating Ca2+ transport across the (-cell membranes and decrease hepatic glucose output. This is mediated by an increase in the intracellular ratio of ATP/ADP which block ATP sensitive K+ channel. This cause a depolarization of the cell membrane and open the voltage dependent Ca2+ channel and leading to Ca2+ influx. Thus insulin is released from the pancreatic (-cell.

Sulfonylurea has been shown to enhance glycogen synthesis and inhibit glycogenolysis & gluconeogenesis in the liver.

Sulfonylurea also stimulate the synthesis the glucose transporters. As a result it may improve the peripheral glucose uptake by muscle. 

Adverse effects:

Hypoglycemia including coma

Cuteneous reaction including rashes and photosensitivity.

GIT reaction including nausea and vomitting.

Hematologic reaction: Leukopenia, Agranulocytosis, Thrombocytopenia, Aplastic anemia, Hemolytic anemia has occurred.

Transient chloestatic jaundices occurs rarely.

Weight gain (Obesity).

BIGUANIDES:

Metformin

Phenformin 

Buformin

Mechanism of action:

Decrease hepatic glucose production. I.e. it prevents the glycogenolysis and keeps the stored glucose intake in liver.

Increases the insulin tissue sensitivity to insulin thus increases insulin action in muscles and adipose tissue.

Reduces the absorption of glucose from the intestine.

Reduces plasma concentration of LDL and VLDL.

Decreases gluconeogenesis process by which more glucose formed.

Induce the gene of glucose transporter.

Advantages:  

Does not cause hypoglycemia

Usually decrease in plasma triglyceride, cholesterol and fibrinogen concentration.

Platelate stickiness is reduced

Does not cause weight gain

Reduceses microvascular microvascular complication.  

Adverse effects:

Anorexia

Nausea

Vomitting

Diarrhoea

Metallic taste

Abdominal discomfort 

Impaired renal function

Lactic acidosis

THIAZOLIDINEDIONS:

Pioglitazone

Troglitazone

Rosiglitazone

Mechanism of action:

Thiazolidinedions are selective agonist for nuclear peroxisome proliferators activated receptor gamma (PPAR(). Bindings the drug with PPAR( activates insulin responsive genes that regulate carbohydrate and lipid metabolism.

Decreases insulin resistance in peripheral tissue.

Reduces glucose production by the liver.

Increases glucose transport into muscle and adipose tissue by enhancing the synthesis and translocation of glucose transporter protein.

Activates the genes that regulate free fatty acid metabolism in peripheral tissue.

Adverse effects:

Anemia

Weight gain

Edema

Plasma volume expansion

Hepatotoxicity

Mild LDL

D- GLUCOSIDES INHIBITOR:

Acarbose

Miglitol

Mechanism of action:

Reduceses the absorption of starch, dextrin and disaccharides by inhibiting the action of

(-glucosidase. Inhibition of this enzyme slows the absorption of carbohydrate.

Competitively inhibit glucoamylase and sucrase but have weak effects on pancreatic

(-amylase. Thus reduces the postprandial plasma glucose levels in types I Diabetes Mellitus and types II Diabetes Mellitus.    

Adverse effects:

Flatulence

Diarrhoea

Abdominal pain and bloating

Cause high osmotic pressure when the amount of disaccharides increases in GIT.