A 52-year-old man with hypertension is prescribed an α1-adrenergic antagonist. Which signaling cascade is blocked by this drug, and how does it differ from the cascade activated by β1-adrenergic agonists in the same vascular smooth muscle cell?

Question

Accepted Answer

A. α1-antagonist blocks Gq-phospholipase C-IP3/DAG; β1-agonist activates Gs-adenylyl cyclase-cAMP — opposing vascular effects. ## α1 vs β1 Adrenergic Signaling in Vascular Smooth Muscle

### Opposing Second Messenger Pathways

**Key Point:** α1 and β1 adrenergic receptors activate antagonistic signaling cascades in vascular smooth muscle, leading to opposite functional outcomes.

| Feature | α1-Adrenergic | β1-Adrenergic |
|---------|---------------|---------------|
| G-protein | Gq | Gs |
| Effector enzyme | Phospholipase C (PLC) | Adenylyl cyclase (AC) |
| Second messenger | IP3 and DAG | cAMP |
| IP3 target | IP3 receptor on SR → Ca²⁺ release | — |
| DAG target | PKC activation | — |
| cAMP target | — | PKA activation |
| Net effect on [Ca²⁺]~i~ | ↑↑ (contraction) | ↓ (relaxation) |
| Vascular tone | **Vasoconstriction** | **Vasodilation** |

### Mechanism of α1-Mediated Vasoconstriction

1. Norepinephrine binds α1-GPCR
2. Gq protein activated → PLC stimulation
3. PLC cleaves PIP~2~ → **IP3 + DAG**
4. IP3 diffuses to sarcoplasmic reticulum (SR)
5. IP3 binds IP3 receptor (ligand-gated Ca²⁺ channel)
6. **↑ Intracellular Ca²⁺** → MLCK activation → myosin light chain phosphorylation → **contraction**
7. DAG also activates PKC → further phosphorylation events

### Mechanism of β1-Mediated Vasodilation

1. Epinephrine binds β1-GPCR
2. Gs protein activated → AC stimulation
3. AC converts ATP → **cAMP**
4. cAMP activates **PKA**
5. PKA phosphorylates:
   - **MLCK** (inactivates it) → ↓ myosin light chain phosphorylation
   - **IP3 receptor** (desensitizes it) → ↓ Ca²⁺ release
   - **Sarcoplasmic reticulum Ca²⁺-ATPase (SERCA)** → ↑ Ca²⁺ reuptake
6. **↓ Intracellular Ca²⁺** → **relaxation**

### Clinical Correlation

**High-Yield:** In vascular smooth muscle:
  - **α1 blockade** → ↓ IP3/DAG → ↓ [Ca²⁺]~i~ → vasodilation → ↓ blood pressure
  - **β1 activation** → ↑ cAMP → ↑ PKA → vasodilation → ↓ blood pressure

**Clinical Pearl:** α1-antagonists (prazosin, doxazosin, terazosin) are used in hypertension because they block the IP3/DAG-mediated vasoconstriction. β1-agonists are NOT used for vasodilation in hypertension because they increase heart rate and contractility (unwanted side effects); instead, β1-antagonists are used.

**Mnemonic:** **Gq-IP3-Contract** vs **Gs-cAMP-Relax**
  - Gq couples to PLC → IP3/DAG → contraction
  - Gs couples to AC → cAMP → relaxation

Answer

B. α1-antagonist blocks cAMP; β1-agonist activates IP3/DAG — both cause vasodilation

Answer

C. Both pathways activate cAMP but α1 causes vasoconstriction while β1 causes vasodilation

Answer

D. α1-antagonist blocks IP3/DAG pathway; β1-agonist activates cAMP pathway — opposite effects on vascular tone

A 52-year-old man with hypertension is prescribed an α1-adrenergic antagonist. Which signaling cascade is blocked by this drug, and how does it differ from the cascade activated by β1-adrenergic agonists in the same vascular smooth muscle cell?

Explanation

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