(2A) Pharmacodynamics: Receptors II – G-protein coupled and tyrosine kinase

What are the 2 types of membrane receptors we talk about?

• GPCR

• RTK

What topics we cover in membrane receptors? (GPCR)

I. Membrane receptors

→ II. GPCR structure + activation + signaling

→ III. Gs, Gq, Gi pathways

→ IV. How GPCR signaling gets shut down/regulates itself

Membrane receptors are:

Transmembrane proteins that bind extracellular ligands and convert those extracellular signals into intracellular responses.

Ligands can include:

• Drugs

• Hormones

• Neurotransmitters

GPCRs

G-protein coupled receptors (GPCRs) are membrane-anchored receptors that trigger signaling cascades when ligands bind.

GPCR Structure

Important structural features:

• 7 transmembrane α-helices

• N-terminus: extracellular ligand-binding region

• C-terminus: intracellular G-protein interaction region

The associated G protein contains:

• α subunit

• β subunit

• γ subunit

GPCR

Resting state

• No activating ligand bound.

• G protein inactive.

• Gα associated with GDP.

GPCR

Activation

Ligand binds GPCR

↓

Receptor changes conformation

↓

GPCR activates G protein

↓

GDP exchanged for GTP on Gα

↓

G protein becomes active

↓

Intracellular signaling begins

Signal Transduction

Definition

transmission of a signal through a cell as a series of molecular events.

Signal Transduction

Core sequence:

Drug binds receptor

↓

Conformational change

↓

Intracellular signaling cascade

↓

Cellular biochemistry/function changes (MOLECULAR EVENTS)

↓

Physiological response

First Messenger vs. Second Messenger

The professor emphasized that the drug does not necessarily need to…

 enter the cell.

A drug can remain outside:

Drug outside cell

→ receptor activation

→ intracellular molecules activated

→ response

first Messenger vs. Second Messenger

• First messenger - Drug/ligand

• Second messenger - intracellular substance that carries the signal after receptor activation.

• The professor explicitly said:

  • Second messenger ≠ the drug.

SKIP

Examples of signal transduction

1. Calcium

2. Digoxin

SKIP

Signal transduction

Calcium

In the professor's muscle-cell example:

Drug binds outside cell

↓

Signaling cascade

↓

Ca²⁺ released from sarcoplasmic reticulum

↓

↑ cytoplasmic Ca²⁺

↓

Muscle contraction

SKIP

Signal transduction

Digoxin

The professor used digoxin to demonstrate the same concept.

She stated:

• Digoxin has a narrow therapeutic index.

• It can produce toxic effects relatively easily.

• Drug binding outside the cardiac cell can initiate events that increase intracellular calcium.

• Increased intracellular calcium produces stronger contraction.

She connected this example with systolic heart failure, where contraction is impaired.

The purpose of the example was to reinforce:

Extracellular drug → intracellular signaling → final physiological effect

 Three G Proteins

G protein	Main action	Result
Gs	Stimulates adenylyl cyclase	↑ cAMP (second messenger)
Gq	Activates Phospholipase C (PLC)	IP₃ + DAG IP₃ → Ca²⁺ release DAG → PKC Ca²⁺ → modulates cellular processes
Gi	Inhibits adenylyl cyclase	↓ cAMP (second messenger)

Gs signaling pathway

Epinephrine/stress
→ binds an adrenergic receptor
→ that receptor activates Gs
→ Gs increases cAMP
→ cAMP carries the signal inside the cell → protein kinase A (PKA)
→ eventually the heart rate increases

Homeostasis

The professor described homeostasis as the body's tendency toward balance.

If one signaling pathway becomes strongly activated, the body needs mechanisms that can limit or oppose that stimulation

SKIP

Homeostasis

example

• If epinephrine kept stimulating the heart for hours without a brake, heart rate could remain excessively elevated.

• This introduced the β-arrestin brake principle.

 β-Arrestin Brake Principle

This mechanism limits continued GPCR signaling.

β-Arrestin Brake Principle

Complete pathway

Agonist activates GPCR

↓

GRK phosphorylates activated GPCR

↓

β-arrestin recruited

↓

β-arrestin blocks further G-protein coupling

↓

Desensitization

Desensitization

• decreased receptor responsiveness after continued stimulation.

• Increasing drug concentration does not necessarily keep increasing the effect indefinitely.

⭐ PROFESSOR EMPHASIS

This mechanism helps explain why drug effects may reach a maximum/limit.

What’s the next step after β-arrestin shuts the receptor down?

β-Arrestin-Mediated Internalization

β-arrestin also links the GPCR to clathrin machinery.

This leads to:

Endocytosis/internalization

The receptor is removed from the membrane and brought into the cell.

What Happens to the Internalized Receptor?

→ Recycling = resensitization (Receptor becomes functional/available again at the membrane)

OR

→ Degradation = downregulation (Fewer receptors total)