Lower urinary tract symptoms (LUTS) in men with benign prostatic hyperplasia (BPH) are associated with erectile dysfunction. Alpha-1-adrenoceptor antagonists are effective drugs for treating symptomatic BPH. Clinical data show improvements in LUTS by phosphodiesterase 5 inhibitors. This study aimed to evaluate effects of silodosin, a highly selective α1A-adrenoceptor antagonist, alone or in combination with the phosphodiesterase 5 inhibitor tadalafil on contractions of isolated human and rat prostates. In organbath studies, effects of increasing concentrations of silodosin (1 nM-1 µM) and tadalafil (100 nM-100 µM) on contractions by electrical field stimulation or phenylephrine of human and rat prostate strip preparations were investigated. The combination silodosin and tadalafil reduced electrically-induced contractions of human prostate preparations better than single drugs alone. At any frequencies (1-32 Hz), inhibitory effects of combined therapy (P-values vs single drug) in human tissue were 26-42% (1 nM silodosin+100 nM tadalafil; P<0.05), 40-58% (10 nM silodosin+1 µM tadalafil; P<0.001-0.05), 56-67% (100 nM silodosin+10 µM tadalafil; P<0.01-0.05), and 33-55% (1 µM silodosin+100 µM tadalafil P<0.01-0.05). Similar findings were obtained in rat prostate preparations. In human and rat prostate tissue, the drug combination exerted similar inhibitory effect on phenylephrine contractions as silodosin alone. Silodosin plus tadalafil had greater potency than each drug alone to inhibit prostate contractions to electrical field stimulation but not to phenylephrine. This study supports the clinical application of a combination of an α1A-adrenoceptor antagonist and a phosphodiesterase 5 inhibitor for symptomatic BPH and suggests that the drug combination requires endogenous nerve-activity for optimal effect.
Keywords: Alfa-1A-adrenoceptor; Contraction; Inhibition; Phosphodiesterase 5; Prostate; Smooth muscle.
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