Module 3: Messengers and Receptors & Mitosis

Long-Range Signalling: Hormones

• hormones are chemical messengers used for long-distance communication between different parts of the body

• this type of signalling is called endocrine signalling

How does endocrine signalling work?

• hormones are produced by endocrine tissues

  • travel from sending to receiving cells via the circulatory system

• secreted directly into the bloodstream

• as they circulate, they bind to specific receptors on target tissues

• their life span ranges from a few seconds to many hours

4 chemical classifications of endocrine hormones

1. Amino acid derivative

2. Peptides

3. Proteins

4. Steroids

2 examples of hormones

• both are types of adrenergic hormones

1. Epinephrine (adrenaline)

2. Norepinephrine

Adrenergic Hormones

• type of endocrine hormone

• produced by the adrenal glands

• stimulates breakdown of glycogen → supply glucose to muscles

• activate flight-or-fight response

  • puts body functions on hold and redirects sources in stressful situations

2 types of adrenergic hormone receptors

1. α-adrenergic receptors

   • binds both epinephrine and norepinephrine

   • located on smooth muscles of visceral organs

   • activates Gq proteins

   • stimulates effector cells

   • cause constriction of blood vessels

2. β-adrenergic receptors

   • binds epinephrine better than norepinephrine

   • located on smooth muscles in hearts, lungs, skeletal muscles

   • activates Gs proteins

     • stimulates cAMP signal transduction pathway

   • relaxes effector cells

   • cause dilation of blood vessels

How do adrenergic hormones function?

• epinephrine binds to beta-adrenergic receptors on liver or muscle cells

  • this activates Gs protein, stimulating adenylyl cyclase

• adenylyl cyclase converts ATP → cAMP (second messenger)

• cAMP activates Protein Kinase A (PKA)

• PKA phosphorylates and activates phosphorylase kinase

  • PKA can also phosphorylate glycogen synthase and inactivate it

• phosphorylase kinase activates glycogen phosphorylase a

  • less active form → more active form

  • leads to an increased rate of glycogen breakdown

• glycogen phosphorylase a breaks down glycogen → glucose-1-phosphate

why do α-adrenergic receptors use IP₃ pathway?

• α-adrenergic receptors stimulate the formation of IP3 and DAG

  • this increases calcium concentration

  • leads to smooth muscle contraction,

    • constricts blood vessels and reduce blood flow

2 other hormone examples

• both are secreted by islets of Langerhans in the pancreas

  1. Glucagon

     • increases blood sugar through glycogen breakdown

  2. Insulin

     • lowers blood sugar by promoting glycogen synthesis into muscle and adipose cells

2 examples of diabetes

• Type I Diabetes

  • body cannot make insulin

    • loss of insulin-producing cells in the islets of Langerhans

    • can be successfully treated with insulin

• Type II Diabetes

  • the body resists insulin

    • it produces it but cells do not respond well

    • cannot be treated with insulin

How does insulin function?

• insulin binds to receptor tyrosine kinases on the cell surface

• this causes auto-phosphorylation of the receptor

• the receptor then phosphorylates IRS-1 (insulin receptor substrate 1)

2 pathways for insulin signalling