Mechnism of action of sildenafil -

Ans. is 4b' i.e., Inhibition of phosphodiesterase-5 Phosphodiesterase-5 (PDE-5) inhibitorso Inhibition of PDE-5 prevents degradation of cGMP which results in accumulation of cGMP and marked potentiation of NO action - Relaxation of smooth muscle in corpus cavemosum and blood vessels supplying it - Erection of penis.o These are the drugs of choice for pharmacological treatment of erectile dysfunction.o The drugs in this group are siledenafil, todafil and vardenafil.o These drugs only influence the erectile ability and overall sexual satisfaction, and does not increase libido or lead to ejaculatory or argasmic changes.o Siledenafil is more selective for pulmonary PDE-5, and thus is the drug of choice for pulmonary hypertension. It also inhibits (weakly) PDE-6 which is involved in phototransduction of retina, resulting in impaired colour vision especially blue green discrimnation.o Todafil is longest acting and more potent. It has less affinity for PDE; causes visual disturbances less frequentlyo Vardenafil is similar to sildenafil, but can cause prolongation of QT interval, and therefore should be avoided in patients with hyperkalemia and/or patients receiving class IA or class III antiarrhythmics.