Effect of the phosphodiesterase 5 inhibitors sildenafil, tadalafil and vardenafil on rat anococcygeus muscle: Functional and biochemical aspects

The anococcygeus muscle is part of the erectile machinery in male rodents. Phosphodiesterase (PDE) 5 inhibitors enhance and prolong the effects of cGMP, which has a key role in penile erection. The aim of the present study was to provide a functional and biochemical comparison of the three PDE5 inhibitors, namely sildenafil, tadalafil and vardenafil, in the rat anococcygeus muscle. Muscle strips were mounted in 4 mL organ baths and isometric force recorded. Levels of cGMP were measured using an enzyme immunoassay kit. Western blots were used to determine PDE5 protein expression. The PDE5 inhibitors concentration-dependently relaxed carbachol-precontracted anococcygeus muscle; however, vardenafil was more potent (pEC₅₀ = 8.11 ± 0.05) than sildenafil (7.72 ± 0.06) or tadalafil (7.69 ± 0.05). Addition of N^G-nitro-l-arginine methyl ester (100 μmol/L) or 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (10 μmol/L) to the organ baths caused significant rightward shifts in concentration-response curves for all PDE5 inhibitors. Sildenafil, tadalafil and vardenafil (all at 0.1 μmol/L) caused leftward shifts in the glyceryl trinitrate (GTN) concentration-response curves (by 4.0-, 3.7- and 5.5-fold, respectively). In addition, all three PDE5 inhibitors significantly potentiated relaxation responses to both GTN (0.01-10 μmol/L) and electrical field stimulation (EFS; 1-32 Hz), with vardenafil having more pronounced effects. All three PDE5 inhibitors reduced EFS-evoked contractions in a concentration-dependent manner over the concentration range 0.001-1 μmol/L. There were no significant differences between the effects of the three PDE5 inhibitors. Vardenafil (0.01-0.1 μmol/L) was more potent in preventing cGMP degradation in vitro than sildenafil (0.01-0.1 μmol/L) and tadalafil (0.01-0.1 μmol/L). Under control conditions, the expression of PDE5 was higher in the anococcygeus muscle than in the corpus cavernosum. In conclusion, PDE5 inhibitors enhance exogenous and endogenous nitric oxide-mediated relaxation in the rat anococcygeus muscle. The potency of vardenafil was greater than that of either sildenafil or tadalafil.