After reading this article you will learn about:- 1. Chemistry of Insulin 2. Biosynthesis of Insulin and Regulation of its Secretion 3. Physio-Pharmacologic Actions of Insulin 4. Deficiency of Insulin 5. Preparations, their Peak Hours, Duration of Action and Doses 6. Clinical Applications.
Chemistry of Insulin:
Insulin is a polyptide with a molecular wt. of about 6000. It consists of two amino acids chain, α and β linked together by two disulphide bridges (Fig. 22.14). The α-chain contains 21 amino acids while the β-chain contains 30 amino acids.
The sequence of the amino acids is fully known by now. The disulfide bridges are essential for biological activity. It is soluble in water but coalesces at pH 3.2 & pH 10. It is relatively insoluble at pH 4 to 7. The biological activity of the hormone is prolonged if combined with protein (protamine) and/or zinc.
The amino acids of insulin obtained from different species varies at 8th and 10th of α-chain and 30th of β-chain as given below:
Biosynthesis of Insulin and Regulation of its Secretion:
Biosynthesis of insulin occurs in response to activation of the endoplasmic ribosomes of the β-cells in presence of glucose. Glucose probably serves as stimulus. This leads to synthesis of m- RNA and consequent biosynthesis of preproinsulin.
The preproinsulin immediately undergoes cleavage to form pro-insulin (mol. wt. 9000). Pro-insulin further undergoes cleavage in the Golgi apparatus to form insulin. Insulin so formed is stored in membrane limited storage granules to be released on demand.
Higher level of glucose in circulation serves as stimulus for release of insulin. Glucose molecules with the help of its carriers enters inside the Fig. 22.14 Structure of Insulin cells where it is metabolised and raises the concentration of ATP. This leads to closure of the K-channel in the cell membrane and thereby its de-polarisation.
De-polarisation above threshold value opens Ca-channels and thereby influx of calcium into the cell. Under influence of increased Ca2+ions the synthesised insulin granules fuse together to form bigger grannules and also migrate towards the periphery to be released by a process called emiocytosis (exocytosis).
The sensitivity of insulin releasing mechanism to glucose is dependent upon carbohydrate intake. In general fasting depresses the mechanism of release while short lived hyper-glycaemia favours the same. But sustained hyper-glycaemia (glucotoxicity) causes selective insensitivity of the β-cells. Many other factors also stimulate/inhibit release of insulin. Alloxan destroys the β-cells.
In ruminants the level of butyrates or a metabolite of it stimulate release of insulin. Acetates and propionates do not stimulate release of insulin.
Physio-Pharmacologic Actions of Insulin:
Insulin affects metabolism of almost all important metabolites in the system, however, the function for which it is known even to common people is hypoglycaemia that it produces.
It favours entry of glucose and amino acids, in almost all cells of the body except in liver, neurons, erythrocytes, cells of G.I. tract and kidney tubules. Under influence of the hormone uptake of metabolites by the tissue cells increases. This increase though is enzymatic, is popularly known as through widening of the pores for entry of metabolites.
The mechanism by which it works as hypoglycaemic and anabolic agent can be briefly stated as below:
(i) Action on Liver cells:
Insulin does not favour entry of glucose inside the liver cells however it favours glycogenesis. Glucose finds easy entry inside the liver cells even without the help of insulin. It favours production of hepatic glycogen-synthatase which in turn enhances the rate of glycogenesis.
It further disfavours formation of enzymes responsible for gluconeogenesis and thereby decrease in the rate of formation of glucose from non-glucose sources. It further suppresses the enzymatic process of glycogenolysis. Since it favours anabolism of protein in the different cells of the body, the process of deamination runs slow in the liver. This keeps production of urea at low rate.
(ii) Action on Muscles:
Under influence of the hormone glucose and amino acids find easy entry inside the skeletal and cardiac muscles. This favours deposition of muscle glycogen and protein anabolism. For the purpose of protein anabolism the hormone seems to influence ribosomes of the cells. It disfavours catabolism of muscle protein.
(iii) Action of Adipose Tissues:
Since insulin favours easy entry of glucose in the adipose tissues and on metabolism it gives rise to fatty acid and glycerol, the two products combine to produce triglycerides. The esterification of fatty acid with glycerol is also influenced by insulin.
(iv) Insulin has also been claimed to reduce catabolism.
Deficiency of Insulin:
It occurs due to failure of β-cells to produce insulin. Insulin deficiency whether induced experimentally or otherwise leads to hyper-glycaemia, glucosuria, diuresis, polydipsia, increased catabolism of carbohydrate, protein and fat, ketogenesis, -ve N balance, increased gluconeogenesis, weight loss, loss of resistance to infection, coma and death.
The condition is popularly called Diabetes Mellitus. The disease is often observed in dogs. Herbivores rarely suffer from such conditions. Birds other than carnivores do not appear to suffer from this disease. Diabetes mellitus is the most common disease of endocrine disorder.
It is metabolic diserder characterized by:
(i) Hyperglycemia
(ii) glycosuria
(iii) Hyperlipemia
(iv) Negative nitrogen balance
(v) Ketonemia.
Thickening of capillary basement membrane and other vascular changes are associated with:
(i) Atherosclerosis
(ii) Coronary artery disease,
(iii) Inter-capillary glomerulosclerosis
(iv) Retinopathy
(v) Neuropathy and gangrene of the extremities.
Types of Diabetes Mellitus:
(i) Insulin Dependent Diabetes mellitus (IDDM) Type-I
(ii) Non-Insulin Dependent Diabetes mellitus (NIDDM)-Type-IT
Type I:
Abrupt on set of diabetes in persons below 20 years who are under weight. The important symptom is ketoacidosis. Family history of diabetes is positive in only 10% of the cases.
In type I insulin secretion is deficient or absent and therefore, treatment with insulin is necessary.
Type II:
On set of diabetes is after 40 years of age in men and women. 80% of diabetic cases are associated with obesity and family history is positive in all the cases. β-cell mass in the pancreatic islets and serum insulin remain normal or increased. There is reduction in number and sensitivity of receptors in the peripheral tissues (down regulation).
Diagnosis:
Fasting Blood glucose → > 120 mg/dl
Post prandial Blood glucose → > 180 mg/dl
Treatment:
Type I:
Insulin
Type II:
(i) Diet control and weight reduction by exercise, accompanied by oral hypoglycemic.
(ii) When Diet and oral hypoglycemic fail to maintain glycemic control, addition of insulin in the regimen is essential.
Mode of Action of Insulin:
Trans membrane insulin receptors are glycoprotein with two α and two β subunits linked by disulfide bonds. These receptors are present in three target issues – liver, muscles and fat. However, gonads, R.B.C. and other tissues also possess insulin receptors in small numbers.
Insulin molecules bind with alpha-subunits and stimulates tyrosine kinase activity of β-subunit resulting in auto- phosphoryration of insulin receptors and auto-phosphoryration of insulin receptors and phosphorylation of other cellular proteins.
There after translocation of glucose trans-protein on the cell surface are mediated by the cascade of phosphorylation and dephosphorylation reactions.
Other mechanisms of insulin action includes lowering of intracellular cAMP, elevation of cGMP concentration and activation of phospholipase C to produce inositol triphosphate and 1-2 di-acetyl-glycerol which ultimately regulate intracellular enzyme. The following figure has been given to explain the mode of action of insulin.
Preparations, their Peak Hours, Duration of Action and Doses:
A number of insulin preparations are available in the market under different trade names and it is not possible to mention them here. The list below shows some of the preparations and other associated important factors with it.
Clinical Applications:
Since animals other than canine and felines rarely suffer from insulin deficiency the drug has not gained veterinary importance. However in cases where diagnosis indicate its use, the drugs should be used after careful judgement.