In this article we will discuss about the Regulation and Renal Threshold for Blood Glucose.
Regulation of the Blood Glucose:
The stable blood glucose level is maintained by the role of liver, skeletal muscle, kidney, muscular exercise and hormones.
Role of Liver:
1. Liver is the pivot of carbohydrate metabolism of the whole body. The presence of glucose-6-phosphatase in the liver converts glucose-6-phosphate to glucose which diffuses into the blood stream to form the constant and the only source of glucose of blood unless and until glucose is available from the intestine from carbohydrate diet.
2. Muscle glycogen cannot be converted to glucose due to the lack of the enzyme glucose-6-phosphatase. Therefore, glycogen is converted to lactic acid which by “Cori Cycle” or “Lactic Acid Cycle” is converted to glucose in the liver and the glucose is diffused to the blood stream.
3. The liver cells, like other cells, require the oxidation of organic substances to maintain their own vital functioning. In the absence of fuel glucose, glycogen is diminished and the oxidation of fat occurs forming keto acids. Some of the keto acids are utilized for cellular energy. But if the concentration of keto acids is increased, the keto acids diffuse into the blood stream and accumulate producing ketosis.
4. When the glycogen reservoir diminishes, the amino acids of the body proteins are utilized by the liver for gluconeogenesis.
Role of Skeletal Muscle:
1. Extra-hepatic tissues are relatively impermeable to glucose and, therefore, insulin is required for the uptake of glucose to these cells.
2. Increased blood glucose promotes glycogenesis and oxidation of glucose in muscles. Muscle glycogen does not serve directly as a source of glucose during hypoglycemia. But glucose is supplied to the blood form muscle glycogen by “Cori Cycle” or “Lactic Acid Cycle”.
Role of Kidney:
1. There is considerable evidence that the kidney is able to form glucose from a number of carbohydrate intermediates and also from amino acids.
2. It possesses some capacity for gluconeogenesis, although the capacity is minor as compared to that of the liver.
3. When the blood glucose level exceeds the renal threshold level (160-180 mg/100 ml), renal tubules are incapable of reabsorbing all the filtered sugar in glomeruli and the excess glucose is excreted in urine. This results in the decrease of blood glucose concentration.
Role of Muscular Exercise:
Muscular exercise promotes the entry of glucose into muscle cells and the glucose is utilized by the muscle. Thus, it lowers blood glucose level.
Role of Hormones:
1. It is produced by the p-cells of the islets of Langerhans in the pancreas and is liberated into the blood by the direct response to hyperglycemia. Insulin is released by amino acids, free fatty acids, ketone foodies, glucagon, secretin and tolbutamide. Epinephrine and norepinephrine block the release of insulin.
2. It increases the rate of uptake of glucose to tissues.
3. It promotes glycogenesis by stimulating hexokinase and glycogen synthetase and oxidation of glucose by stimulating phosphofructokinase.
4. It decreases hepatic glycogenolysis and gluconeogenesis.
5. It stimulates lipogenesis and protein synthesis.
6. It inhibits ketogenesis.
B. Anterior pituitary hormones:
1. The anterior pituitary gland secretes hormones that elevate the blood sugar level by antagonizing the action of insulin. They are growth hormone, ACTH and other ‘diabetogenic’ principles.
2. Growth hormone secretion is stimulated by hypoglycemia. It decreases glucose uptake in certain tissues, e.g., muscle.
3. Growth hormone mobilizes free fatty acids from adipose tissue which themselves inhibit glucose utilization.
4. It produces hyperglycemia which stimulates secretion of insulin causing exhaustion of β-cells.
5. ACTH enhances the release of free fatty acids from adipose tissue and inhibits glucose utilization. It also increases blood glucose level by stimulating the secretion of adrenal cortex hormones.
C. Adrenal cortex hormones:
1. Glucocorticoids (11-oxy-steroids) are important in carbohydrate metabolism. They lead to gluconeogenesis by the increased protein catabolism in the tissues, increased hepatic uptake of amino acids and increased activity of transaminases in the liver.
2. Glucocorticoids inhibit the utilization of glucose in extra-hepatic tissues. They are antagonists to insulin.
3. Glucocorticoids also increase the formation of glucose in the liver by stimulating glucose-6-phosphatase and fructose-1, 6-bisphosphatase.
1. Epinephrine, secreted by the adrenal medulla, stimulates glycogen breakdown in muscle by increasing phosphorylase activity.
2. In muscle, glycogen is converted to lactic acid instead of glucose due to the lack of glucose-6-phosphatase. This lactic acid is converted to glucose in the liver by “Cori Cycle” and diffuses into the blood.
3. It diminishes the release of insulin from pancreas.
1. Glucagon is produced by the alpha cells of the islets of Langerhans of the pancreas, being stimulated by hypoglycemia. It causes glycogenolysis by activating phosphorylase in the liver.
2. It stimulates gIucose-6-phosphatase in the liver to form glucose from glucose-6-phosphate.
3. It enhances gluconeogenesis from amino acids and lactate.
F. Thyroid hormone:
1. Thyroxine has diabetogenic action.
2. It increases the rate of absorption of hexoses and accelerates gluconeogenesis.
3. It stimulates hepatic glycogenolysis with consequent rise in blood sugar. This is due to increased sensitivity to epinephrine.
G. Sex hormones:
Estrogens cause increased liberation of insulin and thus decrease blood sugar level. Testosterones also decrease blood sugar level.
Renal Threshold for Glucose:
Glucose is continually filtered by the glomeruli when the blood sugar rises to a high level. But it is completely returned to the blood by the renal tubular reabsorption. The reabsorption is influenced by phosphorylation by enzymes.
The capacity of the renal tubules to reabsorb glucose is limited to the rate of about 350 rag/minute. If the filtrate contains more glucose than can be reabsorbed, the excess passes into the urine to produce glycosuria.
Glycosuria occurs in the individuals when the venous blood sugar exceeds 160-180 mg/100 ml. This level of the venous blood sugar is said to be the renal threshold for glucose. The maximal rate of reabsorption of glucose by the tubule (TmG- the tubular maximum for glucose) is constant. Hence, it is a more accurate measurement than the renal threshold which varies with the change of glomerular filtration rate.