Factors affecting Carbohydrate Metabolism

 Factors affecting Carbohydrate Metabolism

Role of Endocrines on Carbohydrate Metabolism

Glucose level of blood at any given time is always determined by the balance between the amount of glucose entering the blood stream and the amount leaving it. Liver takes an important part by allowing net uptake of glucose when the blood glucose is high and by allowing net release of glucose when the glucose level is low. Liver thus plays as a glucostat in maintaining a constant circulating glucose level by the relative activities of the above processes. These processes are not automatic but are controlled by different hormones. A brief description is given below-

1. Insulin- Insulin increases the utilization of glucose by most tissues. The most important effect of insulin is to increase the rate of glycogen formation. Insulin is secreted from the β- cells of the islets of Langerhans of pancreas. Degree of insulin activity and the actual production of insulin by the β-cells of the pancreatic islets are affected by the level of blood sugar. Hyperglycemia stimulates the pancreas to produce the increased quantity of insulin and if the hyperglycemia is maintained for a longer period then the permanent damage to the β-cells may ensue and thus permanent diabetes prevails. But it is difficult to ascertain that the hypoglycemia leads to decrease in insulin secretion in same level. Because during such state adrenaline is secreted and this hormone thus masks the effect of insulin on liver glycogen.

There are other factors which either suppress the production of insulin or may render its action less effective. Growth hormone, Glucocorticoids (cortisone and hydrocortisone) and also thyroxin act in such process. Growth hormone and Glucocorticoids inhibit Phosphorylation of glucose by affecting hexokinase activity. These two hormones have got no action on the entry of glucose into the cells. Glucagon, the α-cell hormone of pancreatic islets and also of gastro intestinal tract seems to counteract the insulin by exhaustion atrophy of β-cells.

The insulin is mostly concerned with the utilization of glucose by the tissues and this involves the Phosphorylation in which the chain of conversions of glucose and its combination is controlled by a series of enzymes of which hexokinase is an important one. Insulin stimulates the catalytic action of hexokinase.

Insulin increases the glycogen synthetase activity in muscle. Blood sugar is converted to fatty acids and eventually deposited in the fat depots by insulin in greater quantity than that which is turned into tissue glycogen. Insulin increases the conversion of sugar to fatty acids.

Furthermore, formation of liver glycogen is quantitatively higher than the formation of tissue glycogen.

2. Glucagon-Glucagon is also known as hyperglycaemic-glycogenolytic factor (HGF). Main effect of glucagon on carbohydrate metabolism is to increase the breakdown of liver glycogen to glucose and hence hyperglycemia. It does not cause the breakdown of muscle glycogen. Glucagon is secreted from the α-cells of the islets of Langerhans of pancreas as also walls of the duodenum and stomach. Glucagon raises the blood glucose level by stimulating the adenyl cyclase in the liver leading to the formation of cyclic AMP that activates the phosphorylase.

Glucagon has got no effect on muscle phosphorylase. Due to action of glucagon on adenyl cyclase, cyclic AMP is formed from ATP. The cyclic AMP thus activates the Phosphorylation process of liver glycogen and thus glucose is formed.

Besides this, glucagon also stimulates the process of Gluconeogenesis from available amino acids in the liver. Thus increased activity of glucagon increases the blood glucose level which may indirectly stimulate the β-cell activity for the production of excess insulin. β-cells may become damaged due to prolonged glucagon activity

3. Growth hormone - Growth hormone opposes the hexokinase mechanism, so that the Phosphorylation of glucose is depressed causing hyperglycemia. This hyperglycemia causes secretion of insulin from the β-cells. Prolonged effect of growth hormone may eventually exhaust the β-cells but α-cells remain unaffected.

4. Epinephrine-Epinephrine or adrenalin increases the blood sugar level in three ways: (1) by mobilizing the carbohydrate stores of the liver; (2) by indirect formation of glucose from muscle glycogen: (3) by excessive formation of Glucocorticoids indirectly through liberation of ACTH.

5. Posterior pituitary hormones-Large doses of vasopressin and oxytocin raise the blood sugar level temporarily.

6. Thyroid hormones- Thyroid hormones increase the glucose absorption from the intestine. The hormone also depletes some liver glycogen.

7. Anterior pituitary hormones-The anterior pituitary hormones ACTH and TSH may have some indirect role on the glucose metabolism through acting on the respective target organs.

8. Prolactin- Prolactin has got some anti-insulin effect. It reduces the sensitivity of cell to insulin.

9. Sex hormones-Female sex hormones- oestrone and oestradiol act by stimulating the secretion of insulin. Male sex hormones, testosterone acts opposite.

Role of Vitamins in Carbohydrate Metabolism

1. Thiamine (vitamin B₁) - Thiamine acts as a coenzyme of the carboxylase which helps in oxidative decarboxylation of pyruvic acid. It has a potential role in the oxidation of sugar in tissues including brain. In the tissues thiamine exists as thiamine pyrophosphate ester and helps in decarboxylation of α-ketonic acid as a coenzyme.

2. Riboflavin (vitamin B2)- Since riboflavin is related with tissue oxidation, so it takes part in carbohydrate metabolism. In the tissues this vitamin exists as FMN and FAD. These two coenzymes in combination with apoenzyme play a great role in a number of enzyme systems.

3. Nicotinic acid (Niacin) - It remains as a prosthetic group of two enzymes. NAD and NADP, and takes part in tissue oxidation. Niacin helps in the formation of fats from carbohydrates.

4. Pantothenic acid (vitamin B3) - Since pantothenic acid is a component of coenzyme A, so it takes part in carbohydrate metabolism.

5. Cyanocobalamin (vitamin B12) - This vitamin acts as a cofactor (cobamide) for the enzyme methyl malonyl CoA isomerase which is concerned for the conversion of methyl malonyl CoA to succinyl CoA or succinyl CoA to methyl malonyl CoA. Thus this vitamin is essential in the biochemical conversion of carbohydrate to fat or fat to carbohydrate.

6. Ascorbic acid (vitamin C)- Ascorbic acid takes part in the tissue oxidation probably by acting as hydrogen-carrier.