Xun-Bin Huang, Cheng-Liang Xiong, Cheng-Gao Yu, Jie-Ling Zhou, Ji-Yun Shen

There are plenty of reports on clinical studies on sildenafil in erectile dysfunction (ED) in men [1, 2], however, animal experiments are scarce [3, 4]. The present paper was aimed at the effects of sildenafil on the penile size, the corpus cavernosal electromyogram (EMG) and the intra-corpus cavernosal pressure (ICP) in rhesus macaques.

Twenty Chinese rhesus macaques (Macaca mulatta, provided by the Provincial Experimental Center of Non-human Primates, Fuzhou, China), weighing 5 - 11 kg, were randomly divided into 2 groups with 10 animals each.

Sildenafil citrate (Viagra) 100 mg tablets were purchased from the Pfizer Pharmaceuticals, China. It was given orally at a dose of 4 mg dissolved in 0.2 mL distilled water (2 %).

Monkeys were anesthetized with ketamine, 10 mg/kg, i.m. and laid supine with limbs bending down at the edge of the operating table. Sildenafil or an equivalent volume of distilled water was dripped into the oral cavity. The length and diameter of the penis, the intra-corpus cavernosum pressure (ICP) and the electromyogram (EMG) of the corpus cavernosum were examined be fore and at 15, 30, 45 and 60 min after administration of Sildenafil. EMG was performed by attaching the reference electrode on the inner part of the thigh and the needle recording electrode into the corpus cavernosum at a 45^o angle and a depth of 0.8 - 1.0 cm. Signals were amplified through a DC Preamplifier (Type F2G-1, Nanjing Liuhe Wireless Gadget Factory, China) and recorded through a self-poise recorder (Type XWT200, Shanghai No.2 Automechanism Factory, China). For ICP recording a needle electrode (gauge 8) was penetrated into the corpus cavernosum 1 - 1.5 cm away from the EMG electrode and connected to a pressure energy transformer (Type BPM-1, Baoji Wireless Factory, China) connected to a self-poise recorder (XWT200, Shanghai No. 2 Automatic Factory, Shanghai, China).

The penile diameter increased significantly at 15 - 45 min after administration of sildenafil (P<0.05), Table 1.

The EMG dropped significantly at 15 - 60 min after administration of sildenafil compared with the pre-treatment value (P<0.05 - 0.01, Table 2). The ICP in the sildenafil group was increased significantly (P<0.05, Table 3).

The present result shows that sildenafil significantly increases the penile diameters and the ICP with a reduction in the EMG of the cavernous body, which indicates relaxation of the corpus cavernosum smooth muscle. The most obvious evidence of ED is the absence of a fully expanded penis. Though the penile length do not show significant change before and after sildenafil, the latter significantly increases the penile diameter.

Recently, it was indicated that sildenafil enhanced penile erection upon electro-stimulation in rats and with sildenafil, penile erection could be induced by weaker stimuli [3]. Taher et al [4] showed that sildenafil relaxed diabetic penile smooth muscle of cynomolgus monkey in vitro. Sildenafil also relaxed the corpus cavernosum of rabbits in vitro [5].

The present study is the first report on the observation of the relaxation effect of sildenafil on the corpus cavernosum in vivo in the monkey, thus creating a model in non-human primate for the objective study of sildenafil effect.