Q.28 The drug sildenafil (Viagra) works by:
1. Relaxation of smooth muscles and vasodilation by activating geranylyl cyclase
2. Contraction of smooth muscles and vasodilation by activating adenylyl cyclase
3. Relaxation of smooth muscles and vasodilation by inhibiting cGMP phosphodiesterase
4. Contraction of smooth muscles and vasoconstriction by activating cGN’1P phosphodiesterase

Correct Answer: Option 3

Sildenafil (Viagra) works by inhibiting cGMP phosphodiesterase type 5 (PDE5), which increases cGMP levels to promote smooth muscle relaxation and vasodilation in the corpus cavernosum during sexual stimulation.

Option Analysis

Option 1: Incorrect

Sildenafil does not activate guanylyl cyclase (also called guanylate cyclase); nitric oxide (NO) activates it to produce cGMP. Sildenafil instead prevents cGMP breakdown.

Option 2: Incorrect

Sildenafil targets PDE5 (cGMP-specific), not adenylyl cyclase (which produces cAMP). It causes relaxation and vasodilation, not contraction.

Option 3: Correct

Sexual stimulation releases NO, activating guanylate cyclase to raise cGMP, which relaxes smooth muscle for penile blood inflow. Sildenafil selectively inhibits PDE5, blocking cGMP degradation and enhancing vasodilation.

Option 4: Incorrect

Sildenafil inhibits (does not activate) PDE5 and causes relaxation/vasodilation, not contraction or vasoconstriction. “cGN’1P phosphodiesterase” appears to be a misspelling or error.​

Introduction to Sildenafil Viagra Mechanism

Sildenafil (Viagra) treats erectile dysfunction via its sildenafil viagra mechanism, selectively inhibiting cGMP phosphodiesterase type 5 (PDE5) to boost cGMP for smooth muscle relaxation and vasodilation. This enhances penile blood flow only with sexual stimulation.

How Sildenafil Inhibits PDE5

NO from penile nerves activates guanylate cyclase, producing cGMP that relaxes corpus cavernosum smooth muscle. PDE5 normally degrades cGMP; sildenafil binds competitively, elevating cGMP levels and amplifying NO effects without direct relaxation.

Clinical Effects and Requirements

Vasodilation increases arterial inflow and traps blood in erectile tissue, requiring arousal for efficacy. It’s also used for pulmonary hypertension via similar pulmonary vessel relaxation.