Confusion Resolved by Juice Case

Question 1

What are normal and pathological ranges of glood glucose concentrations before and after eating?

Answer 1

Normal fasting [glucose] = 70-100 mg/dL

Hypoglycemia: < 54 mg/dL

Hyperglycemia: > 110 mg/dL

Question 2

What circulating hormones normally regulate blood glucose concentration?

Which one tries to lower blood glucose?

Which tried to raise blood glucose?

Answer 2

Insulin tried to lower blood glucose.

Glucagon tried to increase blood glucose

Question 3

What are the effects of insulin?

Answer 3

1. Stimulate cells to take up glucose from the blood, reducing blood glucose concentration
2. increaes glycogen synthesis
3. increases protein synthesis
4. increases glycolysis and fatty acid synthesis
5. blocks glucagon release

PROMOTES STORAGE/ANABOLISM

Question 4

What does glucagon do?

Answer 4

1. blocks cellular glucose uptake
2. increases glygoenolysis and lipolysis
3. increases gluconeogenesis

LIBERATES GLUCOSE FROM STORAGE/UPREGULATES CATABOLISM

Question 5

What controls insulin secretion?

In other words, what are the steps of insulin secretion?

Answer 5

Insulin secretion is DIRECTLY related to high blood glucose levels.

1. In the fasted state when glucose concentration is below 5 mM, you don’t want insulin secretion, so the insulin is held in secretory granules within the pancreatic beta cell.
2. Also in the fasted state there is a polarization of the cell membrane that is driven by APT dependent potassium pumps that pump K out of the cell, creating a negative charge on the inside and a positive charge on the outside.
3. After a high carb meal (in the fed state), the glucose concentration in the blood increases. Glucose is therefore brought into the pancreatic beta cell (just like any other cell) through GLUT1 and GLUT2 transporters.
4. WIthin the pancreatic beta cell, the glucose will enter glycolysis, then the TCA cycle, then the ETC. This will increase the concentration of ATP within the beta cell.
5. The increase in ATP causes allosteric inhibition of the ATP dependent K+ pump. This causes a depolarixation of the membrane because the charge differential across the membrane disperses.
6. The depolarization opens up voltage gated Ca++ channels, allowing Ca++ to rush into the beta cell from the cytosol.
7. The Ca++ influx causes Ca++ to bind to proteins embedded in the secretory vesicle membrane.
8. These secretory vescicle proteins will then interact with proteins on the beta cell’s plasma membrane, resulting in a fusion between the vesicle and the plasma membrane
9. This causes the insulin to be dumped into circulation.

Question 6

What organ / cell type produces insulin?

Answer 6

The endocrine pancrease

the beta cells

Question 7

How does insulin cause a decrease in blood glucose concentration?

Answer 7

1. Insulin binds insulin receptors on cell surfaces, which summon GLUT4 transporters to the membrane, making it easier to bring glucose into the cells and out of the blood.
2. Phosphatidylinositol on the membrane is activated, which starts a signalling cascase involving phosphatases that will phosphorylate in order to activate fuel storage enzymes and inactivate catabolism enzymes.
3. Glycogen synthase kinase is also activated that will then shut off glycogenolysis.

Question 8

What are the steps for insulin binding to the receptor? What happens int he cascade?

Answer 8

The insulin receptor has an alpha and a beta section and lives at the membrane as a dimer. There are also two proteins associated on the plasma side: IRS-1.

1. When insulin binds, the insulin receptor acts as a ligand-activated tyrosine kinase, meaning it phoshorylates tyrosine residues on the beta chain and on the IRS-1, creating docking sites for aggregations of other proteins, including Grb2, PLCy, and PI3K
2. Grb2 is an adapter that activatesa series of MAP kinases which go off in a sequential phosphorylation cascases where one kinase phosphorylates the next kinase, etc. THe ultimate downstream goal of this is that the MAP Kinases will phosphorylate and activate transcription factors that will enter the nucleus and transcribe mRNA necessary for proteins involved in anabolic processes.

Question 9

Describe the steps of insulin effect through the phospatidyl inositol pathway.

Answer 9

1. In resposne to insulin binding the receptor, phosphoinositol 3-kinase phosphorylates the phosphatidylinositol to make phosphoinositol 3,4,5-triphosphate.
2. This activates PDK1 kinase.
3. PDK1 phosphorylates and activatesa PKB
4. PKB then phosphorylates and INACTIVATES glycogen synthase kinase 3. And is phosphorylates and ACTIVATES protein phosphatase 1.

So just remember that insulin activates phosphatases.

Question 10

Describe the process of insulin maturation.

Answer 10

Insulin is a protein hormone.

It starts out as a preproinsulin that has a signal peptide directing it to go to the lumen of the ER

In the ER the signal peptide is removed, leaving Proinsulin.

THe proinsulin binds into a confirmation with mtiple alpha helices connected through disulfide bonds.

Within the golgi, the C-peptide portion of the proinsulin is cleaved off to form insulin.

Insulin then complexes with zinc ion to form the insulin hexamer which is the hormone that is stored in secretory vesicles to await Ca++ signalling.

Question 11

What is the significance of the insulin:c-peptide ratio?

Answer 11

Because every proinsulin in the body must have C-peptide cleaved off to form mature insulin, any increase in insulin within the body will have a related increase in c-peptide.

So any ratio higher than 1:1 will suggest that the hypoglycemia was caused by insulin overdose through injection, not through endogenous insulin

Question 12

Why are liver cells alone able to excrete glucose?

Answer 12

They express glucose 6-phosphatase.

This allows them to do the reverse reaction of hexokinase/glucokinase in removing the phosphate from the 6C, yielding glucose that can be exported from the cell.

Question 13

What stimulates glucagon release into the circulation?

Answer 13

Glucagon release into circulation is stimulated by low levels of insulin and low levels of glucose in the blood

It will also be released through stimulation by cortisol, epinephrine, and norepinephrine

Question 14

How is mature glucagon made?

Answer 14

Somewhat similarly to mature insulin…

It’s a protein initially in the form of proglucagon. After proteolytic processing, introns are spliced out, yielding glucagon which is packaged into secretary vesicles in the pancreatic alpha cells. It is stored there until low blood glucose stimulates its release.

Question 15

What two molecules prevent glucaon secretory vesicles from fusin with the plasma membrane and releasing glucagon?

Answer 15

Insulin and Glucose

Question 16

Insulin activates phosphatases.

What does glucagon activate? What are the steps?

Answer 16

Kinases!

Glucagon binds to its heterotrimeric G protein-coupled receptor (GPCR)

The alpha subunit of the GPCR detaches and activates adenylate cycle, increasing the concentration of cAMP.

cAMP activates PKC

PKC will activate pyruvate kinase (PK-L) to increase the conversion of PEP to pyruvate.

PKC will aso phosphorylate enzymes like glycogen phosphorylase, phosphoglucomutate, and glucose 6 phosphatase in order to increase glycogenolysis and glyconeogenesis to make new glucose.

Question 17

What was the cause of the patient’s symptoms?

Answer 17

An insulinoma was releasing insulin constitutively so that any glucose she took in was immediately entering the cells.

Because insulin was constantly being secreted, glucagon was never secreted and the liver therefore never received a signal that blood glucose was low, therefore there was no gluconeogenesis or glycogenolysis.

Question 18

What would an insulinoma look like under an immunostain designed to detect the presence of peptide hormones?

Answer 18

It would stain positive for insulin and negative for glucagon.