Insulin and Diabetes

Question 1

Insulin Secretion

Answer 1

1. Produce ATP through glycolysis and citric acid cycle.
2. ATP inhibits the K+/ATP pump, so K+ builds up inside the cell, causing depolarization.
3. Depolarization opens the voltage-gated Ca2+ channels.
4. The influx of Ca2+ stimulates insulin vesicle exocytosis.

Question 2

K+/ATP pump

Answer 2

4 subunits of Kir6.2 that forms a pore through the membrane.
Kir6.2 binds ATP.
There are 4 submits of SUR1.
Mg2+-ADP binding to SUR1 activates the channel, which opens it and allows K+ conductance; inhibiting insulin secretion.

Question 3

Insulin at Target Tissues

Answer 3

Insulin binds to receptors on the surface of target cells.
All tissues express insulin receptors; energy-storing tissues (LIVER, MUSCLE, ADIPOSE) have much higher receptor levels and are main target tissues.

Question 4

Insulin Receptor

Answer 4

Glycoprotein consisting of 4 disulfide-linked subunits.
2 extracellular alpha subunits.
2 transmembrane beta subunits.
Intracellular tyrosine kinase domain.

Question 5

Activating the insulin receptor

Answer 5

Binding of insulin (or drugs) to the extracellular portion of the insulin receptor (alpha subunits) activates the intracellular tyrosine kinase.
Tyrosine kinase autphosphorylates tyrosine on the beta subunit AND phosphorylates the insulin receptor substrate proteins (IRS-proteins) and Shc.
This recruits and activates second messenger proteins like Grb-2, SHP-2, SOS, and PI3-Kinase.

Question 6

Energy Homeostasis major pathways

Answer 6

1. Glycogen metabolism: shorter term stores; finite capacity.
   Major carbohydrate reserve; muscle glycogen (source of glucose for muscle) and liver glycogen (maintains blood glucose).
2. Lipid metabolims: longer-term energy stores, unlimited capacity.

Question 7

Glycogenesis and glycogenolysis

Answer 7

The liver and kidney both have glucose 6-phosphatase (the enzyme needed to allow glucose export from cells).
MUSCLE DOES NOT HAVE THIS ENZYME, SO GLUCOSE REMAINS INTRACELLULAR.

Question 8

Glucose uptake

Answer 8

Stimulated by insulin.
1. Insulin causes GLUT4 glucose transporters to move to the cell membrane facilitating glucose uptake.
2. Hexokinase and glucokinase (liver) MUST phosphorylate glucose to glucose 6-phosphate for cell utilization.
In order to get an initial rise in the ATP/ADP ratio to secrete more insulin, basal insulin concentrations have to stimulate glycolysis to produce cellular energy (ATP).
Hexokinase locks glucose in the cell by phosphorylating it, it cannot go back through the transporters.
Key regulatory enzymes that are stimulated by high glucose and insulin include phosphofructokinase and pyruvate kinase.

Question 9

Gluconeogenesis

Answer 9

Major substrates: glucogenic AA, lactate, glycerol, noncarbohydrate sources.
Liver and kidney are the major gluconeogenic tissues.
Glucose export into the blood following gluconeogenesis:
Need glucose 6-phosphatase, which is present in the liver and kidneys, but absent from muscle and adipose tissue (cannot export glucose into the bloodstream).

Question 10

Lipid storage in adipose tissues

Answer 10

Insulin favors net deposition of triglycerides.
Glucose is metabolized to FA palmitate via pyruvate and acetyl-CoA intermediates.
Insulin increases the transport of glucose into adipose cells via translocation of GLUT4 to cell membrane.
Lipogenic enzymes are activated by insulin such as pyruvate kinase, pyruvate DH, acetyl-CoA carboxylase, and glycerol phosphate acyltransferase.
FA synthesis is also stimulated in the liver and other tissues and can be transported to adipose tissues via the circulation.

Question 11

Glucagon Action

Answer 11

Activates a G protein-coupled receptor on the plasma membrane to increase cAMP and activate protein kinase A.
Acts in the liver to promote glycogenolysis and gluconeogenesis.
Acts in adipose tissue to promote lipolysis.
Degraded by the liver and kidneys.

Question 12

Somatostatin

Answer 12

Secreted by the pancreatic delta cells, the GI tract, and the hypothalamus.
14 AA peptide
DECREASES SECRETION OF BOTH INSULIN AND GLUCAGON.
Inhibits GI tract motility.
Inhibits secretion of some non-pancreatic hormones.
Stimuli for somatostatin secretion are like those for insulin: high plasma glucose, AA, FA.
Circulating half life is 2 minutes.

Question 13

Glucagon-like peptide 1 (GLP-1)

Answer 13

Produced primarily in enteroendocrine cells of the ileum.
Produced by alternative cleavage of proglucagon.
Released from L cells during nutrient absorption in the GI tract.
Increases insulin secretion.
Suppresses glucagon secretion.
Delays gastric empyting.
Decreases appetite.
Circulating half life is 1-2 minutes and is degraded by dipeptidyl peptidase-4.

Question 14

Leptin

Answer 14

Regulates the long-term energy balance and the neuroendocrine response to energy storage.
Secreted from fat cells.
Signals to the CNS the amount of energy (fat) stored.
No leptin in mice=obese mice.
Decreases appetite.
Leptin allows metabolism and the endocrine system to spend energy on growth, reproduction, and maintenance of a high metabolic rate.
Low leptin causes increased appetite and impair energy-expensive functions.

Question 15

Classification of insulin preparations-T1D

Answer 15

Onset and duration of action vary with preparation and chemical structure.
Recombinant versions based on human insulin dominant the US marketplace.
Animal-derived insulin is still available outside the US and for some vet applications.

Question 16

Administration of insulin

Answer 16

Subcutaneous to create a depot of insulin at the site of injection.
Same with an insulin pump.
IV injection only in medical setting.
Inhaled forms where absorption occurs in the lung.

Question 17

Factors affecting rate of insulin absorption

Answer 17

Solubility of insulin preparation.
Local circulation site-to-site variability.
Person-to-person variability.
Faster absorption gives faster onset of action and shorter duration of action.
When you create the insulin depot, it absorbs over time.

Question 18

Regular Insulin

Answer 18

Short-acting preparation.
Structurally identical to the hormone insulin.
Regular insulin aggregates into hexamers.
Dissociation of the hexameters to monomers is the RATE-LIMITING STEP for absorption.

Question 19

Insulin lispro, insulin aspart, and insulin glulisine

Answer 19

Ultrarapid-acting insulins
Designed to keep the molecule in a monomeric form to speed absorption.
Structurally similar to regular insulin but with some sequence changes.
These can be injected minutes before a meal.
Administered subcutaneously.
They have the same mechanism of action as regular insulin.
The difference between these and regular insulin is in absorption.

Question 20

NPH (neutral protamine Hagedorn) insulin

Answer 20

Intermediate-acting preparation.
Insulin is combined with protamine (protein from fish sperm).
This prolongs the time required for absorption of insulin.

Question 21

Insulin glargine, insulin detemir, insulin degludec

Answer 21

Long-acting preparations
Steady absorption without a peak.
Mimics the basal insulin secretion.
Modifications increase the presence of the hexamer form.
Modifications from regular insulin are based on sequence changes.

Question 22

Basis of the glycohemoglobin (HbA1c) test

Answer 22

Glucose in the blood nonenzymatically glycosyltates blood proteins.
This non enzymatic glycosylation in RBC generates HbA1c.
This occurs at a rate proportional to the level of glucose in the blood.
Based on the lifespan of a RBC being approx 120 days..
These levels yield an estimate of the average blood glucose level over several weeks.

Question 23

Type 2 Diabetes

Answer 23

Begins with a state of insulin resistance.
The tissues become relatively refractory to insulin action and require increased insulin levels to respond appropriately.
Eventually, beta cells fail to keep pace with the demand for insulin.
The degree to which insulin resistance vs. reduced beat cell function contributes to the disease varies among patients.
Patients have increased circulating insulin levels.
Common variants are thought to confer a small increased risk and these may act in a cumulative fashion.
Maturity onset diabetes of the young MODY; autosomal mutations that can cause disease on their own; mutations impair beta cell function (apoptosis, ROS, glucotoxicity, ER stress, inflammation).

Question 24

Diabetic hyperosmolar syndrome

Answer 24

Blood glucose >600 mg/dL
Kidney can no longer recover all the glucose.
Leads to excess urine and severe dehdyration.
Can occur in both types, but associated with infections in Type 2 patients.

Question 25

Long-term vascular pathology in T1D and T2D and the aldose reductase and the polyol pathway

Answer 25

Premature atherosclerosis
Retinopathy
Nephropathy
Neuropathy
Sorbitol builds up causing osmotic problems in some tissues that damage the microvasculature (retina, kidney, nerves).
Affected tissues have glucose uptake in the absence of insulin action.

Question 26

Pharmacological agent mechanisms of action

Answer 26

Increase insulin secretion by pancreatic beta cells.
Sensitize target cells to the actions of insulin.
Inhibit glucose recovery from the urine.
Insulin therapy.
Slow the absorption of sugars from the GI tract.

Question 27

Biguanides

Answer 27

Act by increasing insulin sensitivity in target tissues.
Metformin
Common choice for initial treatment of T2D.
Half-life of 1.5-3 hours.
Not bound to plasma proteins or metabolized in humans.
Excreted unchanged by the kidneys.
Oral route delivery.
Mechanism of action:
1. Activate AMP-dependent protein kinase (AMPPK).
2. Increase adenosine monophosphate production.
3. Blocks the breakdown of FA.
4. Inhibits hepatic gluconeogensis and glycogenolysis.
5. Increased activity of the insulin receptor.
6. Increased glucose uptake and metabolic responsiveness multiple tissues.
Therapeutic effects:
Leads to lowering of blood glucose and insulin levels.
Beneficial intreating T2D and insulin resistance/hyperinsulinemia associated with PCOS.
DOES NOT induce hypoglycemia.
Lower serum lipids and decreases weight.
Adverse effects:
Mild GI distress is common; transient and can be minimized by adjusting dose.
Lactic acidosis (cause of nausea and vomiting).
Decreases the flux of metabolic acids through gluconeogenic pathways, so lactic acid can accumulate.
Risk factors: hepatic disease, heart failure, respiratory disease, alcohol abuse.
Renal disease can reduce clearance because the drug is not metabolized; patients over 65-70 years may have retail impairment that reduces drug clearance.

Question 28

Sulfonylurea drugs

Answer 28

Treat T2D by stimulating insulin release from pancreatic beta cells.
Raises circulating insulin levels to overcome insulin resistance.
Inhibit the K+/ATP channel.
These drugs bind to the SUR1 subunit of the K+/ATP channel by possibly displacing Mg2+/ADP that activate the channel.
Adverse effects:
Hypoglycemia resulting from TOO MUCH INSULIN SECRETION.
Weight gain due to increased actions at adipose tissue.
Tolbutamide, Tolazamide, Acetohexamide, Chlorpropamide are other sulfonylurea drugs.
These first generation agents have the same mechanism of action as the second generation agents.
Vary for unique pharmacokinetic properties (duration of action coincides with incidence of hypoglycemia, tolbutamide has a short duration of action so hypoglycemia is less common).

Question 29

Glyburide, Glipizide, and Glimepiride

Answer 29

Second generation sulfonylureas.
Oral delivery.
Lower blood glucose and cause metabolic improvements by increasing insulin action.
Metabolized by the liver.
These vary in their duration of action, although glyburide has the longest duration.

Question 30

Meglitinides

Answer 30

Oral delivery.Similar to sulfonylureas in absoprtion, metabolism, and adverse effects.
These stimulate insulin releases by binding to SUR1 in the beta cell K+/ATP channel.
These bind at a different site on SUR1 than the sulfonylureas.

Question 31

Repaglinide and Nateglinide

Answer 31

Rapidly absorbed from the intestine.
COMPLETE METABOLISM in the liver to inactivate metabolites.
Half life of less than 2 hours.
The short half life reduces the risk of hypoglycemia compared to sulfoylurea drugs, but it is also implies more frequent dosing.
Adverse effects:
hypoglycemia and weight gain.

Question 32

Thiazolidinediones (TZDs)

Answer 32

Rosiglitazone and Pioglitazone.
Oral delivery.
Lead to lowering of blood glucose and insulin levels.
Treats PCOS and T2D.
These act by sensitizing peripheral tissues to insulin.
Act as agonists for a nuclear hormone receptor called the PEROXISOME PROLIFERATOR ACTIVATED RECEPTOR-gamma (PPARgamma).
PPARgamma forms a heterodimer (2 different proteins form a dimer) with the retinoid X receptor (RXR); once activated, it activates transcription of genes involved in glucose and lipid metabolism.
PPARgamma is mainly expressed in ADIPOSE TISSUE.
TZDs increase insulin sensitivity in adipose tissue.
Adverse effects:
Weight gain, edema, heart failure, hepatotoxicity.
These drugs DO NOT increase insulin secretion or cause hypoglycemia.

Question 33

Thiazolidinedione site of action

Answer 33

PPARgamma is expressed at low levels in the liver and muscle; TZDs have little effect on insulin sensitivity in liver and muscle cells in vitro, which are the primary sites of insulin resistance in T2D.
Indirect therapeutic effects such as decreased liver glucose output.

Question 34

GLP-1 Agonists and Mimetics

Answer 34

Incretin (metabolic hormone that causes a decrease in blood glucose levels).
Released from the small intestine following nutrient absorption.
GLP-1 causes increased insulin release, decreased glucagon release, slowing of gastric emptying, and decrease in appetite.
Very short half life and is broken down by dipeptidyl-peptidase 4 (DPP-4).
DPP-4 inhibitors have been developed to increase the hormone half life.

Question 35

Exenatide

Answer 35

Peptide drug based on a Gila monster hormone (lizard).
Long-acting analogue (comparable) of GLP-1.
Small increase in secretion of insulin by beta cells (when glucose levels are elevated).
Suppresses the secretion of glucagon by alpha cells.
Decreases appetite (like GLP-1).
Slows the rate of nutrient entry into the circulation due to the slowing of gastric emptying.
MUST BE INJECTED (usually twice a day).
Typically used in combination with other drugs.
Adverse effects: Nausea and risk of hypoglycemia.

Question 36

Albiglutide (Tanzeum), liraglutide (Victoza), and lixisenatide (adlyxin) are recently approved analogs of GLP-1.

Answer 36

Albiglutide half life is 4-7 days.
Liraglutide half life is 13 hours.
Dosing of these drugs can be less frequent than exenatide.

Question 37

Sitagliptin

Answer 37

Oral use.
Selective inhibitor of DPP-4.
This elevates circulating GLP-1 levels and insulin concentration.
This decreases glucagon concentration.
Increases the responsiveness of insulin release to glucose.
Clearance by the kidney.
Adverse reactions:
Increase in respiratory tract infections, nasopharyngitis, and headaches.
Not a significant risk for hypoglycemia.
Other selective DPP-4 inhibits have the same mechanism of action: Saxagliptin (Onglyza), Linagliptin (Tradjenta), Alogliptin (Nesina).

Question 38

Amylin Analogs

Answer 38

Amylin is secreted along with insulin by the beta cells.
It works along with insulin to suppress glucagon secretion.
Slows CHO absorption by slowing gastric emptying.

Question 39

Pramlintide

Answer 39

Synthetic analog of amylin (islet amyloid polypeptide).
Give subQ typically before meals and with insulin.
This drug binds to re ephors in the CNS, including parts of the brain that receive neural input from the GI tract.
Reduces food intake and depresses GI motility.
Suppresses postprandial (after dinner or lunch) release of glucagon acting indirectly via the CNS.
Adverse effects:
Hypoglycemia-requires lower insulin dose than of insulin was used alone.
Nausea.

Question 40

New drug with a new mechanism of action approved in 2013!!!

Answer 40

Canagliflozin (Invokana)
This drug is a sodium glucose co-transporter 2 (SGLT2) inhibitor.
Reduces glucose readsorption of glucose by the kidney, which increases glucose excretion in the urine to lower the blood glucose.
Dapagliflozin (Farxiga) and Empagliflozin (Jardiance): these are additional drugs with the same mechanism of action.
They cause dehydration and thirst as side effects.
Also an increased incidence of some lower UTI.

Question 41

Alpha-Glucosidase inhibitors

Answer 41

Oral delivery.
Acarbose has a tetrasaccharide structure and is NOT ABSORBED.
Moglitol is structurally similar to glucose and IS ABSORBABLE.
Glucosidases include maltase, isomaltase, sucrase, and glucoamylase.
Normally, these enzymes cleave complex CHO to glucose and other simple sugars.
Intestinal brush border enzymes.
Mechanism of action:
The absence of cleavage of complex CHO inhibits CHO uptake.
These drugs increase the time required for absorption of CHO (starch, dextrin, disaccharides).
CHO are absorbed in smaller quantities and over a prolonged time throughout the length of the intestine.
This REDUCES THE POSTPRANDIAL PEAK IN BLOOD SUGAR.
Adverse effects:
Flatulence, bloating, abdominal discomfort and diarrhea.
These result from gas released by bacterial action on CHO that remain in the GI tract.
No hypoglycemia or weight gain.

Question 42

Excess insulin and hypoglycemia

Answer 42

Most commonly induced by medication use.
Also can be caused by insulin-secreting tumors (surgical excision).
Diazoxide: beta cell K+/ATP channel binding drug that stabilizes the open state to reduce insulin secretion by these tumors.
Octreotide: a synthetic form of somatostatin that reduces hormone release from insullinomas, glucagonomas, and thyrotropin-secreting pituitary adenomas.

Question 43

An additional mechanism of insulin secretion

Answer 43

1. cAMP is generated at the plasma membrane from ATP
2. cAMP-dependent pathways activated.
3. This leads to the release of intracellular stores of Ca that promotes exocytosis of insulin levels.
   (The other mechanism is that involving the closing of the K/ATP pump and influx of extracellular Ca).