The cGMP-specific phosphodiesterase PDE5A is highly expressed in vascular and bronchial smooth muscle, in renal tubules and in platelets. It is prominent in the retina and the corpus cavernosa, the pair of erectile tissues in the penis. It catalyses the degradation of cGMP in these tissues, facilitating muscle contraction. Clinically used PDE5A inhibitors target PDE5A in vascular muscle to treat erectile dysfunction (Ventimiglia et al. 2016, Gong et al. 2017) and bronchial smooth muscle to treat pulmonary arterial hypertension (Palacios et al. 2020).
Erectile dysfunction drugs such as Viagra (sildenafil), Cialis (tadalafil) and Levitra (vardenafil) inhibit PDE5A (Boolell et al. 1996, Ballard et al. 1998, Moreland et al. 1998), increasing cGMP levels and thereby allowing smooth muscle relaxation and vasodilation. This allows blood flow into the corpus cavernosa leading to erection (Raheem & Kell 2009).