9-Pancreatic Function and Carbohydrate Metabolism

Pancreatic Islets (Islets of Langerhans)

Endocrine cells of the pancreas

Types of Pancreatic Islets (what do they each secrete?)

• Beta cells secrete insulin

• Alpha cells secrete glucagon

• Delta cells secrete somatostatin

• F cells secrete pancreatic polypeptide

Digestive enzymes of the pancreas (exocrine functions)

• Lipase

• Peptidases

• Amylases

Exocrine pancreas things:

• Digestive enzymes

• Bicarb

70-130 mg/dL

Typical blood glucose value!

Insulin

• increases after each meal and decreases as time passes

• THIS and glucose are positively related to each other

Glucagon

• decreases after each meal and increases as time passes

• THIS and glucose are negatively related to each other

Peptide Hormones

Insulin and Glucagon are both THIS.

They both go through the typical processing for THIS:

• preprohormone → prohormone → hormone

Blood glucose is above 80 mg/dL

When insulin is secreted from pancreatic beta cells:

Glucose

Insulin causes liver and skeletal muscle to absorb and metabolize THIS

Blood glucose is below 70 mg/dL

Glucagon is secreted from pancreatic alpha cells when:

Release Glucose

Glucagon causes the liver to do THIS:

Hypoglycemia (fasting)

In prolonged states of THIS, ketone synthesis from acetyl groups (2 C) becomes increasingly more significant

GLUT 1 and GLUT 3

Location:

• Erythrocytes

• Neurons

• Cornea

• Placenta

Transport:

• Passive & Bidirectional

NOT Regulated by Insulin

Function:

• Allow locations to access plasma glucose at all times

GLUT 2

Location:

• Pancreatic Beta Cells

• Liver

• Kidney

• SI

Transport:

• Passive & Bidirectional

NOT regulated by Insulin

Function:

• Allows beta cells to detect/regulate blood glucose

• Allows hepatocytes to absorb/metabolize glucose

GLUT 4

Location:

• Skeletal Muscle

• Cardiac Muscle

• Adipose

Transport:

• Passive & Bidirectional

REGULATED BY INSULIN!

Function:

• Via insulin, allows absorption/storage of glucose in locations

GLUT 5

Location:

• Spermatocytes

• GI Tract

Transport:

• Fructose!

NOT regulated by Insulin

Function:

• Allows absorption of fructose by locations

Action of GLUT 2 on Pancreatic Beta Cells

1. Glucose enters beta cell via GLUT 2

2. Glucose metabolized → Increases ATP

3. Increased ATP → K leak channels close → Cell depolarizes

4. Depol. → Voltage-Gated Ca channels open → More depol.

5. Depol. → Exocytosis of Insulin!

Recently Fed State GLUT 2

• GLUT 2 present

• Plasma glucose high

• Insulin metabolizes glucose

• Hepatocytes absorb glucose

Fasted State GLUT 2

• GLUT 2 present

• Plasma glucose low

• Glucagon causes glucose synthesis (no insulin present)

• Hepatocytes are glucose source

Recently Fed State GLUT 4

• Insulin present

• Plasma glucose high

• Insulin causes exocytosis of GLUT 4

• Glucose enters cell, converts to glycogen/fat

Fasting State GLUT 4

• Insulin absent

• Plasma glucose low

• No insulin → endocytosis of GLUT 4

• Glucose cannot exit or enter cell

• Cell breaks down stored glycogen/fat

Insulin Deficiency

• Recently Fed State

  • Blood glucose high

  • Blood insulin low

  • GLUT 4 stays inside the cell

  • Muscles can’t absorb glucose from ECF

  • Body stuck in catabolic state (muscle wasting!)

• T1D

  • Starvation in the midst of plenty

Hypoglycemia

• Too little glucose!

• Severe → Death

• Mild/Moderate → Common

• Caused by:

  • Excessive insulin

  • Poor diet

  • Genetics

C Peptide

• Byproduct of insulin secretion

• Indicates how much insulin is in blood

Hypoglycemia

• Too little glucose!

• Reactive THIS is triggered by glucagon secretion by the pancreas

• Attempts to increase blood glucose (tremors, tachycardia, etc.)