In vivo and in vitro response of corpus cavernosum to phosphodiesterase-5 inhibition in the hypercholesterolaemic rabbit

To investigate the effects of hypercholesterolaemia (HC) on rabbit corpus cavernosa in vivo and in vitro, and evaluate the efficacy of vardenafil and sildenafil in normal and HC rabbits, as the phosphodiesterase-5 (PDE-5) inhibitors vardenafil and sildenafil are widely used for treating erectile dysfunction (ED) and most organic causes of ED are associated with vascular risk factors like HC. Male New Zealand White rabbits were randomly divided into two groups; 11 HC rabbits were fed a 2% cholesterol diet, and 12 age-matched control rabbits received a regular diet. After 12-14 weeks, erectile responses to intravenous sodium nitroprusside (SNP) and PDE-5 inhibitors were evaluated for 2 h in conscious rabbits. Penile length was measured and the area under the curve calculated. Relaxant responses of corpus cavernosal strips to electrical-field stimulation (EFS) were measured before and after exposure to PDE-5 inhibitors and the nitric oxide synthase inhibitor N'-nitro- l-arginine methyl ester. HC rabbits had a lower erectile response to SNP than controls; in both control and HC rabbits there was a greater erectile response after simultaneous exposure to SNP and vardenafil, or SNP and sildenafil. However, the responses of the HC rabbits were still significantly less than those of the controls. Corpora from control rabbits responded to EFS with greater relaxations at all frequencies, except 1 Hz. Corpora from both HC and control rabbits had greater responses to EFS after exposure to vardenafil and sildenafil; N'-nitro-l-arginine methyl ester diminished the response to EFS. There was a significantly lower in vivo and in vitro erectile response in HC rabbits than in controls; erectile function measured in conscious rabbits can be used to assess quantitatively the efficacy of different agents, e.g. sildenafil and vardenafil, in pathological animals. In addition, both agents improve in vitro responses of erectile tissue from HC rabbits to EFS.