Sildenafil, Sildenafil drug. I think it is a phosphodiesterase inhibitor. I am thinking that. What is the mechanism of action? I mean, cyclic AMP or cyclic GMP. Yes.
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sildenafil mechanism of action PDE5 cyclic GMP cGMP pathway pharmacology
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That diagram is perfect. Here is a full, authoritative answer:
Sildenafil - Mechanism of Action
You are correct that it is a phosphodiesterase inhibitor, and the second messenger is cyclic GMP (cGMP) - not cAMP.
The Full Pathway (Step by Step)
1. Sexual stimulation triggers NO release
- Non-adrenergic, non-cholinergic (NANC) nerves in the penis release nitric oxide (NO)
- Acetylcholine from cholinergic nerves also stimulates endothelial cells to activate eNOS (endothelial nitric oxide synthase), converting arginine + O₂ → NO
2. NO activates guanylyl cyclase
- NO diffuses into the vascular smooth muscle cell and binds soluble guanylyl cyclase (sGC)
- sGC converts GTP → cGMP
3. cGMP causes smooth muscle relaxation
- cGMP activates cGMP-specific protein kinase (PKG)
- PKG opens K⁺ channels and promotes Ca²⁺ reuptake into the endoplasmic reticulum, reducing intracellular Ca²⁺
- With less Ca²⁺, myosin heads detach from actin → smooth muscle relaxation → vasodilation → penile erection
4. PDE5 normally terminates the signal
- Phosphodiesterase type 5 (PDE5) breaks down cGMP → 5'-GMP, ending the relaxation signal
5. Sildenafil blocks PDE5
- Sildenafil competitively occupies the active site of PDE5
- cGMP is not hydrolyzed as quickly → it accumulates to higher levels and persists longer
- The net result: amplification of the NO-cGMP pathway → prolonged, enhanced smooth muscle relaxation
Key Points to Remember
| Feature | Detail |
|---|---|
| Drug class | Selective PDE5 inhibitor |
| Second messenger | cGMP (NOT cAMP) |
| Enzyme blocked | Phosphodiesterase type 5 (PDE5) |
| Where PDE5 is enriched | Corpus cavernosum, vascular smooth muscle, lung |
| Net effect | Sustained cGMP elevation → vasodilation |
| Related drugs | Tadalafil (Cialis), Vardenafil (Levitra) - same class |
Clinical Uses
- Erectile dysfunction (ED) - trade name Viagra
- Pulmonary arterial hypertension (PAH) - trade name Revatio - because PDE5 is also enriched in the pulmonary vasculature
- Achalasia - off-label; reduces lower esophageal sphincter (LES) pressure (as noted in [Sleisenger & Fordtran's GI textbook])
Critical Drug Interaction
Because sildenafil amplifies the NO-cGMP pathway, combining it with nitrates (nitroglycerin, isosorbide dinitrate, sodium nitroprusside) - which are exogenous NO donors - can cause severe, life-threatening hypotension. This combination is contraindicated.
Sources: Katzung's Basic & Clinical Pharmacology, 16e | Barash's Clinical Anesthesia, 9e | Murray & Nadel's Respiratory Medicine | Fuster & Hurst's The Heart, 15e
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