Insulin

• Beta cell: No 1 – contains insulin, proinsulin and C peptide, pancreatic amylin and GABA

• Alpha cell: contains glucagon

• Delta : somatostatin

• Insulin lowers sugar whereas glucagon and somatostatin both increase sugar by different mechanisms

Alpha cells --- produce glucagons

PAX 4 --- give rise to beta cell

PAX 6 --- give rise to alpha cell

GENES OF INTEREST

• PAX 4 AND 6 – required for development of beta and alpha cells

• TRANSCRIPTION FACTOR:IPF insulin-promotor-factor-1*

• Hepatic nuclear factor -4 alpha*

• Hepatic nuclear factor – 1 beta*

• Glucokinase gene*

*Associated with MODY (maturity offset diabetes of young)

Birth of a Hormone

• Pre pro hormone (RER) degraded to

• Pro hormone – pro-insulin (Golgi apparatus). Have Zn proinsulin hexamers

• Degraded to Zn-Insulin hexamer comprised of A and B chain joined by disulfide bonds and C-peptide. (Secretary granules )

• Stored in Crystal form (Mature Granules)

• Equamolar amounts of CP and insulin secreted together – C peptide can be measured to check insulin level

PreProInsulin --- ProInsulin --- GammaBeta chain and C chain

Beta chain structure pro-lys causes hexamer formation new administered insulin switched to lys-pro cannot form hexamer and is absorbed faster. Asp-insulin replaces pro with Asp also doesn’t aggregate.

“minutia”

• proinsulin 7% of biological activity, tumor can cause large amounts of proinsulin.

• C peptide is a marker of endogenous secretion – young child with diabetes check C chain production in Beta cells.

• Insulin – 50% picked by liver

Insulin acts on three major organs: liver, muscle, fat

Liver produces glucose all the time – monitored by insulin. Sugar increases and insulin increases then sugar is deposited in muscle and fat. There are glucose transporters need carrier proteins that transport glucose across cell membrane (impermeable).

Glucose Transporters

Glut 1- for brain

Glut 2- only in Beta cells glucose sensor, only after meal in liver, prevents hepatic uptake or inappropriate insulin secretion

Glut 4- found in muscle and fat cells, major insulin target organs, needs insulin action for mobilization to cell membrane

Type II diabetes defect in Glut 4 is not mobilized, leads to glucose levels remaining high in blood.

2 Phase of Insulin Release

Sharp increase followed by sharp decrease, then a gradual prolonged increase in insulin levels.

1^st phase is connected with a rapid release of insulin, due to prestored insulin

2^nd phase is indicative of prolonged secretion phase in concert with increased synthesis of insulin.

Loss of 1^st phase is earliest sign of compromise in Beta cells, in older persons it is also a sign of heart disease. Risk factors associated with this are obesity, smoking, lack of exercise.

Regulation of insulin release

Stimulation

Nutrients= glucose and leucine (amino acid)

Gastric Hormones =

Neural Beta adrengic= acetylcholine

Drugs= sulfonia – insulin secretalog causes insulin secretion special receptors

Inhibition

Neural= alpha adrengic stimuli, tumor of adrenal gland

Goliath disease is related to the inhibition of insulin

Humeral= somatostatin- associated with hyperglycemia

Drugs- Diazoxide. Thiazide, diuretics,

Hypoglycemia protective mechanism

Mechanism of Insulin Release

Glut 2- glucose into cell

Glucokinase is activated and ATP increased by metabolism of glucose; defective gene leads to familial diabetes.

Potassium channels close = depolarization

Calcium channels open- myofibrils

Insulin is extruded and sugar does down

Berne and Levy chart

G-protein mediated effects acetylcholine activates phospholipase C which then activated both IP3, which increases calcium, and DAG. DAG then activates Protein Kinase C, activating adenylcyclase, which in turn increases calcium.

This is example of multiple mechanisms that activate the release of insulin.

Diabetes can be caused by:

Abnormal insulin

Lack of proinsulin converting to insulin (Familial Hyperproinsulinemia)

insulin receptor abnormality

Antibody developed against insulin receptor, autoimmune disease

Feedback relationship between Insulin and nutrients. Nutrients (Glucose, Amino Acids, Fatty Acids/ketoacids, and Potassium) stimulate insulin secretion; insulin in turn stimulates the uptake, storage, and metabolism of these nutrients.

Signal Transmission

The insulin receptor has both intercellular and extracellular components. It consists of two alpha chains and two beta chains joined by disulfide bonds. Refer to p. 833 for schematic representation. The beta chain crosses the plasma membrane of the cell and contains the inactive tyrosine kinase site. The alpha chain resides on the extracellular portion of the plasma membrane and is responsible for the inactivation of the tyrosine kinase; it is also where the insulin hormone binds. When insulin binds to the alpha portion of the receptor, the inhibition over tyrosine kinase is released on the beta chain. Autophosphorylation then takes place on the beta subunit at three tyrosines with ATP as the substrate.

Intercellular Leviators

IRS (insulin receptor substrates) - IRS-1 and IRS-2, are phosphorylated tyrosines that are phosphorylated by the activated tyrosine kinases. These serve as docking sites or activating sites for several different cascades, protein phosphatases and faciliatory proteins via serine and therionine.

PI-3K (phosphotidylinositol-3-kinase)-, one example of the cascade activation, an enzyme responsible for regulation and catalytic activation of phosphatidylinositol 3-4 and 3,4,5 phosphates.(These phosphates activate S6 kinases as Protein Kinase B /AKT to GSK to glycogen synthesis, Glut 4. In obese diabetics this is defective.

GRB-2- ( growth receptor binding protein-2)-is as IRS-1 initiated step which activates the binding of GTP to RAS, this activates cell growth and differentiation; glycogen, lipid, and protein synthesis; and MAP kinase activation.

Sites of insulin action

Liver -activates enzymes glucokinase and glycolytic enzymes, and inhibits enzymes for gluconeogensis (glucagons mediated). Decreases ketogenesis, increases protein and lipid synthesis.

Muscle- stimulates Glut-4 for the uptake of glucose, increases glycogen synthesis, increases amino acid uptake, increases protein synthesis in ribosomes, and decreases protein catabolism.

Adipose- stimulates Glut-4 for the uptake of glucose, leads to a build-up of fat by; increasing glycerol phosphatases, increases triglyceride deposition, and inhibition of lipolysis.

Lack of insulin leads to an excess of glucagons, which leads to gluconeogensis during fasting leading to hyperglycemia.

Between meals liver mediates glucose levels via glucagons

After eating sugar increases leading to and increase in insulin.

Glycogen Regulation (which regulated blood glucose levels)

Glycogen synthase is activated by insulin, which dephosphorylates the enzyme, leading to the synthesis of glycogen.

Glycogen phosphorylase is activated by glucagon, which phosphorylates the enzyme, leading to the breakdown of glycogen. ( which raises blood glucose level)

Glycolysis is controlled by insulin, because the rate-limiting enzymes for glycolysis are activated by insulin. (Glucokinase, Phosophofrucktokinase-1 (PFK-1), pyruvate kinase (PK)).

Gluconeogensis is activated by glucagons, by activating the rate-limiting enzymes for gluconeogensis. (Pyruvate carboxylase, F-D-Pase, glucose phosphatase, and phosphoenolpyruvate carboxykinase (PEPCK).

( the enzymes were important for the American Boards)

Fat Synthesis

Is insulin dependent. Pyruvate is produced during glycolysis, Pyruvate dehydrogenase then converts pyruvate to acetyl-CoA, Acetyl CoA then becomes Malonyl CoA. Malonyl CoA is then converted to Fatty Acetyl CoA, that then produces triglycerides. Malonyl CoA also inhibits breakdown of fatty acids because it inhibits carnitine acyltransferase (CAT), which is responsible for transferring fatty acids into the mitochondrial matrix for metabolism. Thus insulin is responsible for the synthesis and prevention of breakdown of fatty acids.

Lipoprotein lipase

Is another insulin dependent pathway for the production of triglycerides. Free Fatty acids are released by lipoprotein lipase, which then cross the membrane of the fat cell and are resynthesised into triglycerides for storage.

During fasting glucagon increases the amount of free fatty acids in the blood. The carnitine acyltransferase then transports the free fatty acids into the mitochondrial matrix where they are metabolized. Fatty acyl CoA is combined with pyruvate to form acetyl CoA(for citric acid cycle) and ketone bodies (beta-hydroxy butyrate).

Fasting State

Alanine(a major gluconeogenic precursor) is sent to the liver to form glucose.

Muscles release glutamine to kidney and gut.

Fed State

Branched Amino Acids (leucine, isoleucine, and valine) go to muscle for protein synthesis.

The rest of the Amino Acids go to the liver.

Insulin Action

• Inhibit tissue lipase, stimulate lipoprotein lipase

• Stimulate cellular uptake of potassium, phosphorous, and magnesium

• One of the normal regulators of potassium balance

• Increase absorption of potassium, phosphorous, and sodium by kidneys

• Insulin receptors in hypothalamus suppress appetite.

Glucagon Secretion

Stimulated by:

Cortisol and Amino Acids

Inhibited by:

Glucose, insulin, and somatostatin.

Somatostatin

Shuts off both insulin and glucagon

Glucagon turns on somatostatin, insulin turns off somatostatin.

In the anterior pituitary it suppresses GH and TSH

Pancreas it decreases secretion of insulin.

In stomach it decreases gastric motility.

In small intestine it drops secretin

In kidney it decreses renin secretion

In Thyroid it decreases calcintonin secretion.

What happen if you take in glucose and don’t have insulin?

Blood glucose levels remain high and Amino acid levels in the blood remain high.

After a fat meal hypertriglyceremia will result.

After awhile glucose toxicity will result where high glucose levels decrease the sensitivity of the receptor.

Fasting with insulin deficiency

Decreased glucose uptake leads to hyperglycemia. This causes glycouria which leads to osmotic diuresis. This causes severe dehydration.

Because of the lack of the uptake of glucose cells will increase protein catabolism, lipolysis, and plasma fatty acids which cause polyuria and polydipsis (drink and pee). This leads to ketogenesis leading to ketoacidosis. Resulting in coma and eventual death.

Diabetes

Diabetes comes from many sources:

Autoimmune disease type I is associated with antibody produced resulting in complete destruction of the beta cell. This results in a total lack of insulin (insulin defiency), making the patient insulin dependent (must receive insulin from an exogenous source or they will die). Many times this disease is in conjunction with other autoimmune diseases; such pernis anemia and hyperparathyroidism. There are genes involved with this form, if both parents have gene 50% chance offspring will have diabetes. The disease may also be due to a viral attack on pancreas. Geographical location has also shown revelence to occurrence of the disease, possible toxic element (Finland and Corsica). If a treatment (drugs) is used against immune destructive process, a reduction in the appearance of diabetes. This disease normally hits children, but can occur at any age. LADA-late autoimmune diabetes of adult (is a much slower process).

Type II-Insulin resistance

This is a progressive disease. It goes from impaired fasting, impaired glucose tolerance, and diabetes metellius. This disease can be prevented by exercising, not smoking, and watching their weight. Impaired fasting glucose is indicated by a fasting glucose of 110-125 mg/dl. Impaired glucose tolerance is indicated by a glucose tolerance test. This test consists of administrating a 75 g of glucose and 2 hours later measuring glucose level. A plasma glucose level greater 140 mg/dl is positive for impaired glucose tolerance. Diabetes metellius is indicated is a fasting glucose level of greater than 125 mg/dl. Early treatment of hyperglycemia helps to slow beta-cell destruction. Also early insulination prevents the loss of beta-cell.

Etiological classification DM

Type I – B-cell destruction leading to absolute insulin deficiency)

• immune mediated

• idiopathic

Type 2 (combination of insulin resistance and relative insulin deficiency in any combination) weight loss acts on resistance

LADA- latent autoimmune diabetes in result- Type 2, not obese, can take years to evolve. Treat patients with insulin to preserve B-cell reserve.

Other types

• Genetic defects of beta-cell function- MODY- maturity onset diabetes of the young

• Genetic defects in insulin action

• Diseases of exocrine pancreas-severe, recurrent pancreatitis, alcohol destroys pancreas

• Endocrinopathies growth hormone antagonizes insulin. Here excess growth hormone gives you diabetes. Excess glucagon increases sugar- glucagonoma-excess glucagon leads to hyperglycemia

• Somatostatinoma- leads to hyperglycemia

• Pheochromocytoma-disease of adrenal medulla excreting excess norepinephrine, and alpha antagonist. An alpha antagonist shuts off insulin leading to hyperglycemia.

• Low Potassium leads to impaired insulin release. Conn’s syndrome is associated with excess aldostrone, a hormone that leads to sodium retention with hypertension and loss of potassium causing to hypokalemia. Hypokalemia leads to impaired insulin release and hyperglycemia

In each of these diseases there is impaired insulin release or action

• Kushing’s syndrome- hyperglycemia from excess ACTH

• Drugs- Diazoxide doesn’t allow potassium channels to close so no insulin release. Treats insulinoma.

• Infections

• Gestational- during pregnancy, have impaired insulin action, sugar is high, resulting in a larger baby.

Type-1 genetic factors

• D3 or D4 antigen or both

• Asp 57 in DQ beta chain is a protective factor

Family Studies

• Monozygotic twins – type 1 50% or less concordance. Only 50% because of environmental factors also contribute to development of disease. Type-2 near 100% concordance

• Single family member with type 1 diabetes metullis, increased risk for other family members

Highest Diabetes Metullis occurrence is in Scandinavia and Sardinia

Viral Factors

• Congenital rubella can induce diabetes met. In 20% of affected people, possibly molecular mimicry.

• Caxack virus

• Entroviral infections

Type 1 Diabetes Met. – Islet cell auto antigens

• GAD- glutamic acid decarboxylase

• Cytoplasmic islet cell antibody

• Insulin

• P69/ABBO’s

• HSP 65- heat shock protein 65

• Insulin receptor (rare)

Genetic implication in diabetes-progression through stages. First antibody are negative, then they get positive. Initially, give IV glucose and everything is normal. Then lose first phase of insulin release. Then, fasting blood sugar goes up. Then when you take glucose load, blood sugar goes up. Then blood sugar is high all the time. C-peptide is present. At the end, no c-peptide, no insulin, and the patient must get insulin or die.

Type 1 is progressive but type 2 also is. Give insulin earlier and maintain better control by keeping glucose levels normal, avoid glucosetoxicity.

Development of Type 1

• One may have abnormal antibody, but may not develop disease

• Loss of first stage of glucose induced insulin release is the earliest observable evidence of beta cell function impairment.

• Subjects with abnormal antibody and loss of first stage have highest incidence for progression to full blown disease

Prevention

• stop autoimmune reaction- we don’t know how

Type 2 Diabetes Met.

• Insulin resistance and deficiency

• Progressive disease and increasing numbers of patients need insulin to control hyperglycemia

• Longer you wait to treat, greater beta cell destruction you get.

Resistance caused by:

• Adipocyte produces tumor necrosis factor that impairs insulin effect.

• Hormone resistin is secreted by fat cell

• Adiponectin hormone is involved

• Fat cell producing peptide that impairs insulin receptor function, can’t get glucose into cell, weight loss helps.

Normalglycemia= sugar level less than 100

Impaired fasting glucose= sugar level greater than 100, start looking at other elements that are risk factors for CV disease. Don’t give insulin yet.

Diabetes= sugar level greater than 125 on 2 occasions, or random sugars greater than 200.

When you eat, pancreas secretes insulin. Challenge with Glucose Tolerance test, can pancreas meet need. Give 75 mg glucose and wait 2 hours, normal sugar less than 140, diabetes greater than 200. Impaired glucose tolerance 140-200. A high percentage of people with impaired glucose tolerance develop diabetes. This can be prevented by lifestyle change.

High sugar leads sugar hooking on to proteins, including hemoglobin. We can measure the amount of glycosylated hemoglobin. This equals hemoglobin A_1c . Amount of hemoglobin A_1c tells us the amount of sugar in the past 3 months. Normal A_1c less than 5.8%. Get below 6.3% to avoid heart disease.

Symptoms of diabetes for both Type 1 and Type 2

• Polyuria and polydypsia and weight loss (catabolic state from no insulin)

• Alteration in vision (blurred vision)

Classical presentation of polyuria, polydypsia, and weight loss.

Type 2- asymptomatic picture, high sugar picked up in routine testing.

Complication- Heart disease

Microvascular complications

• Retinopathy-end stage=blindness

• Nephropathy-end stage= dialysis and transplantation

• Neuropathy- disabling – sensory, motor, cranial, mono and poly, autonomic, cranial nerve palsy- check for diabetes.

Macrovascular complication

• Coronary heart disease: MI and CHF

• Cerebrovascular disease: stroke and disability

• Peripheral vascular disease: intermittent clavdication (hurts when you walk)

• Impotence because of decreased vascular supply of neuropathy

• Diabetic foot- caused by neuropathy, infection, gangrene, amputation

Pathogenic risk factors

• Hypertension- normal BP is less than 130/80 and in presence of renal damage less than 125/75

• Hyperlipidemia- classical is hypertriglyceridemia and low HDL

Category: Physiology Notes