HORMONAL REGULATION OF BLOOD GLUCOSE

During fast, decline in blood glucose is prevented by glycogenolysis and gluconeogenesis (liver and kidney). These organs contain glucose-6-phosphatase, necessary to convert glucose-6-phosphage to glucose. With more prolonged fasting (>24 hr), gluconeogenesis accounts for all of the glucose production. In contrast after meal the absorbed glucose is converted to glycogen or fat for storage. 

Due to these regulatory mechanisms blood glucose is maintained within narrow range. These regulatory mechanisms includes insulin, which decreases blood glucose, and the counter regulatory hormones (glucagon, epinephrine, cortisol and growth hormone), which increase blood glucose concentration.

(Source: Tietz Clinical Chemistry, 4th Edition)

(Source: Tietz Clinical Chemistry, 4th Edition) 

INSULIN

BIOSYNTHESIS

This peptide hormone of 51 amino acids (5.8 kDa) with 2 peptide chain (A and B) was discovered by Banting and Best in 1921. Two peptide chain (A with 21 amino acids and B with 30 amino acids) linked by two disulphide bonds. It is secreted by β cells of islets of Langerhans in pancreas (there are about 1 million β cells). The 23-26 amino acids in β chain are conserved as they are required for biological activity. Insulin is first protein hormone to be sequenced, to be measured by RIA, first compound produced by recombinant DNA technology. 

It is synthesized as preproinsulin (100 amino acids) which undergoes post translational modification. Peptide cleavage produces proinsulin with 86 amino acids. Proinsulin is stored in secretory granules prior to release from beta cells by exocytosis. 90% proinsulin is converted to mature insulin by removal of C-peptide (31 amino acid) prior to secretion, by prohormone convertase proteins. C-peptide is co-secreted in equimolar amounts with mature insulin. Proinsulin also has 8-15% activity of insulin and has 3-5 times longer half life than insulin and is the major storage form of insulin. In insulinoma and T-2 diabetes, C-peptide and proinsulin level increases. Substance stimulating the synthesis and storage of insulin include glucose, mannose, Leucine, arginine. Glucose facilitates synthesis and secretion but amino acids facilitates only release of stored insulin.

SECRETION AND PHARMACOKINETICS

In beta cell glycolysis leads to the production of ATP which causes closure of ATP dependent sulphonylurea sensitive K⁺ channels. This causes depolarization inside the cell leading to opening of voltage sensitive Ca²⁺ and calcium influx which cause exocytosis of granules containing insulin. Hyperglycemia, mannose, lactate, arginine, Leucine, Incretions (GIP, cholecystokinin, vasoactive intestinal peptide), sulphonylureas, etc. also stimulates insulin release. These are secretogogues of insulin. Release is inhibited by hypoglycemia, somatostatin and various drugs like diazoxide, phenytoin, nicotinic acid, etc.).

Due to cephalic and gastric influences, oral glucose is potent stimulus to insulin secretion than equivalent amount of IV glucose. This difference is known as incretion effect and is mediated by gut derived hormones like glucagon-like peptide-1 (GLP-1), ghrelin, etc.

Drugs that augment the release of insulin via ATP sensitive K⁺ channels of beta cells are sulphonylurea, meglitinides and GLP-1 receptor analogs.

Insulin is secreted in pulsatile manner with pulse periodicity of 11-15 min. The first release of stored insulin occurs 1-2 minutes after IV dose of glucose and ends within 10 minutes. Second phase of release of synthesized insulin, occurs after 11-15 minutes lasting until normoglycemia is restored usually within 60 to 120 minutes. Both first-phase response and normal pulsatile secretion are lost in type 2 diabetes mellitus and in obese subjects, and more in type I DM. 

Approximately 30-40 U of insulin secreted per 24 h in healthy subjects with basal secretion of 0.25-1.0 U/h until glucose concentration exceed a threshold of about 90 mg/dl, and secretion becomes maximal at glucose concentration of 270-360 mg/dl. Insulin has T1/2 of 3-5 minutes but C-peptide 35 minutes. Released insulin go to liver and approximately 50% is extracted and remaining 50% go to general circulation and binds to receptor. Insulin is also degraded in PCT of kidney. But C-peptide is only metabolized in kidney.

ACTION OF INSULIN:

The liver

§  Inhibit hepatic glucose output: By inhibition of glycogenolysis and gluconeogenesis.

§  Stimulate glycogen storage – stimulation of glycogen synthase

§  Stimulate glycolysis – stimulation of PFK

§  Stimulation of lipogenesis and glucose oxidation – stimulation of PDH that form acetyl CoA à fatty acid synthesis/TCA cycle. Glycerol formed from the intermediate of glycolysis which in turn can form triacylglycerol.

Skeletal muscle

§  Stimulate glucose transport – activation of GLUT4

§  Stimulation of glycogen synthesis – activation of glycogen synthase

§  Stimulation of glycolysis – stimulation of PFK.

Adipose tissue

§  Inhibition of lipolysis – inhibition of hormone sensitive lipase.

§  Promote re-esterification – increased supply of glycerol 3-phosphate

§  Stimulation of lipolysis (circulation) – stimulation of lipoprotein lipase.

§  Increased glucose uptake GLUT4

Central nervous system

§  Stimulates satiety, postprandial thermogenesis, other.

Other

§  Promotes DNA, RNA synthesis

§  Stimulation of amino acid uptake

§  Na⁺-K⁺-ATPase stimulation – increasing intracellular energy availability

§  Sodium retention, Na⁺/H⁺ antiport activation.

At present only 2 receptors are identified that respond to insulin: the insulin receptor and IGF receptor. The median effective dose (ED₅₀) for insulin antilipolytic action of adipose tissue is <20 mU/L, for inhibition of hepatic glucose output is 30-50 mU/L and for stimulation of glucose uptake into skeletal muscle is 50-70 mU/L. Due to these effect most individuals with type 2 diabetes do not develop ketoacidosis for many years. Since the basal secretion level of insulin is 3-15 mU/L this only inhibit lipolysis but do not affect hepatic glucose output and extrahepatic uptake.

INSULIN RECEPTOR

These are glycoproteins with molecular weight of 350 kDa and consist of 2α and 2β chains linked by disulfide bridges present in plasma membrane. Approximately there are 20, 000 insulin receptors present in one adipose tissue. The α-subunit (MW 135 kDa) is located on the outer surface of membrane and contains the insulin binding site. The β-subunit (MW 95 kDa) extends extracellularly through plasma membrane and contains an intrinsic tyrosine kinase which is activated upon insulin binding to α-subunit. Binding of insulin causes the conformational change activating intrinsic tyrosine kinase activity in β-subunit which will phosphorylate tyrosine residue on several proteins and on itself.

In addition to phosphorylating itself, the insulin receptor catalyzes the tyrosine phosphorylation of a number of specific intracellular proteins. These include the four members of the family of insulin-receptor substrate (IRS) proteins (termed IRS-1, 2, 3 and 4), Shc, and Gab-1. The phosphorylated tyrosine on these target proteins acts as docking site for intracellular signal transducer proteins.

 
After activation insulin-receptor complexes are internalized by endocytosis; receptors are later recycled to cell surface. Internalization helps the signal to reach to nucleus and is the route for clearance of insulin from circulation. 

INSULIN-LIKE GROWTH FACTORS AND THEIR RECEPTORS:

At least 2 protein hormones, IGF-1 (somatomedin C) and IGF-2 have some actions of insulin. They are produced by liver in response to GH. These are weak agonists for insulin glucoregulatory and antilipolytic action. They have growth-promoting effect. They acts as somatomedins and induced by growth hormone and mediate its growth-promoting effects.

IGF-I acts through IGF receptors or the insulin receptor but IGF-II lacks tyrosine kinase activity.