Communication in Animals LECTURE 5

Adrenlaine hormonal response

• Stimulates glycogen hydrolysis in the muscle cells

Concentration of Adrenaline in the blood

• Very small 10^-9M

  → similar to the binding constant for the hormones to the receptor

What is the binding constant for the hormones to the receptor

• concentration of the hormones

• at which 50% of the receptor binding sites are occupied

What does this concentration ensure?

• cellular response to the presence of the hormones will be directly related to the hormone concentration

Sequence of events for stimlation of glycogen hydrolysis with adrenaline

1. Adrenaline binds to receptor

2. Actiavtes Gs

3. Activates adenylate cyclase→ synthesise cyclic AMP

4. cAMP→ activates Protein Kinase

5. Activates phosphorylase kinase→ into active form

   • NB Phosphorylase kinase is also activated by Ca2+

   • SECOND SIGNALLING PATHWAY

   • from the ACh stimulation for muscle contraction

   • Nicotinic ACh receptor, Na+ Ion channel

   • E.g we have Interconnecting pathways! CONVERGENCE

→ Level of metabolism is linked to the energy needs of the contractile machinery

6. Active phosphoylase kinase→ Activates glycogen phosphoylase byt phosphoylation

7. Resulting glucose-1-phosphate is fed into glycolytic pathway

8. Produces ATP

Heterotrimeric G proteins

• E.g Gs and Gq

What is the strucutre of Gs

3 protein subunits

• G alpha

• G beta

• G gamma

→ Heterotrimetic G protein

Mechanism of Heterotrimeric G- protein action

1. Unstimulated→ Exists as a trimer with GDP bound to Ga

   • unable to interact with or activate adenylaate cyclase

2. Hormones binds→

   • receptor conformation change

   • receptor associates with G protein complex

   • GDP replaced with GTP on Ga

3. Ga dissociate from Gb and Gy

4. Ga binds and activates adenylate cylcase

5. Adenylate cyclase synthesis cAMP

How is the Gs protein reset?

1. GTP cleaves to form GDP + Pi

   • by GTP hydrolysing activity of Ga subunit

2. Ga subunit detaches from adenylate cyclase

3. Re-associates with Gb and Gy

   → inactive complex again

Wide ranging role of G-protein

• Many signal transpductions

  • visual excitation

  • olfaction

• → Associated with mantt inhertited disorders

Example of G-protein ingerited disorder

• single amino acid substitution

• forms G-protein isoform

• interacts with luteinising hormone receptor

  → temperature sensitive response

Classes of Ga proteins

• have different 2nd messenger

• have different receptors

• associated with different effector proteins

  • Adenylate cyclase

  • phopholpase C

  • cGMP phosphodiesterase

Characteristic of Adrenaline signalling pathway

1. Amplification

2. Lateral mobility of membrane

3. Down regulation

1. Amplification

Amplified 10^6 forld from 10^-9

Achieved by

1. Single receptors can activate multiple G-protein complexes

   • e.g like in cAMP for dictocelum slime mould etc

2. Adenylate cylcase acting catalytically to produce cAMP

3. Protein kinase and phosphorylase kinase acting cataltyically

2. Lateral mobility of membrane proeins

• Essential for G protein function

3. System down regulation

Needed for when hormones concentration in blood stream falls. How?

1. Hydrolysis of GTP bound to Ga→ GDP
   Hydrolysis of cAMP by phosphodiesterase

2. Dephosphorylation of phosphorylase kinase and glycogen phosphorylase

   • by action of phosphatases

IMPORTANCE: glycogen breakdown only persist in the continued presence of

• hormone

or

• Ca2+ signals

How can down regulation be comprimised?

1. Caffeine

   • (cAMP elevation)

2. Bacterium Vibrio cholerae produces cholera toxin

   • (GTP bound elevation)

1. Caffeine→ what does it do

1. Acts as inhibitor of phosphodiesterase

2. Elevated cAMP levels

→ Alertness + muscle tremor

2. Cholera toxin→ in intestine

1. Protein toxin ADPribosylates the Ga subunit

2. Inhibits GTPase activise

3. GTP permanently bound

4. Adenylate cyclase permanently active

5. 100fold increase in cyclic AMP concentration in

   • epithelial epithelial cell

6. Unpleast flow of Cl- and water into intestinal lumen

   → Dehydration→ death

Role of NO

• regulation of blood pressure

  → maintained by state contraction of smooth muscle cells

  • surrounding blood vessels

How is NO formed

1. Arginine

2. NO synthetase actiavted by Ca2+

3. Argineine→ NO

NO synthesis pathway

1. Blood pressure too high→ stretch activated Ca2+ channels in endothelium

2. promote Ca2+ entry

3. Stimulates NO formation

also

4. ACh elevates internal Ca2+ levels

   • Ach at musculerinic receptors

   • Go G protein→ PLC→ IP3→ Ca2+→ NO synthase

NO action pathway

1. NO diffuse to the smooth muscle cells

2. Induces muscle relaxation via cGMP-dependent mechanism

Down regulation:

1. cGMP rapidly degraded by cGMP phosphodiesterase (PDE)

   • (target of the active ingredient in Viagra)

NB type of receptor used

ACh bind to muscarinic receptors

not

nicotinic ACh→ used for ACh- Gated Na+ channels

Self -assessment questions

1. What is convergence of signaling?

2. How does the Gs class of G proteins contribute to adrenaline signalling?

3. How does NO regulate blood pressure?