ORIGINAL RESEARCH
The Functional and Structural Consequences of Cavernous Nerve Injury are Ameliorated by Sildenafil Citrate

https://doi.org/10.1111/j.1743-6109.2008.00794.xGet rights and content

ABSTRACT

Introduction

Radical prostatectomy (RP) is associated with erectile dysfunction (ED). A single, placebo-controlled, human study has assessed the effects of regular sildenafil use after RP and demonstrated an increased chance of preservation of preoperative erectile function.

Aim

This study was undertaken to define the effects of such a regimen in an animal model.

Methods

Using the cavernous nerve (CN) crush injury model, animals were divided into a number of groups: no CN injury (sham), bilateral CN injury exposed to either no sildenafil (control) or sildenafil at two doses (10 and 20 mg/kg) subcutaneously daily for three different durations (3, 10, 28 days).

Main Outcome Measures

At these time points, CN electrical stimulation was used to assess erectile function by mean intracavernosal pressure (ICP)/mean arterial pressure (MAP) ratio. For the structural analyses, whole rat penes were harvested. Staining for Masson's trichrome was utilized to calculate the smooth muscle-collagen ratio. Immunohistochemical antibody staining was performed for endothelial (CD31 and eNOS) and neural (GAP43, NGF, and nNOS) factors and immunoblotting was performed to analyze the AKT/eNOS pathway. Terminal deoxynucleotidyl transferase biotin-dUTP nick end labeling (TUNEL) assay was used for the assessment of apoptotic indices and the CN architecture was evaluated by transmission electron microscopy (TEM).

Results

Erectile function was improved with sildenafil in a time- and dose-dependent fashion with maximization of erectile function recovery occurring with daily 20 mg/kg at the 28-day time point. Sildenafil use resulted in smooth muscle-collagen ratio protection and CD31 and eNOS expression preservation. Sildenafil reduced apoptotic indices significantly compared with control. Animals exposed to sildenafil had increased phosphorylation of akt and eNOS. Tem demonstrated distinct differences in architecture between control and sildenafil groups toward an increased amount of myelinized nerve fibers.

Conclusions

Sildenafil use in the CN crush injury model preserves erectile function that appears to be mediated predominantly through preservation of smooth muscle content and endothelial function as well as through reduction in apoptosis. Mulhall JP, Müller A, Donohue JF, Mullerad M, Kobylarz K, Paduch DA, Tal R, Li PS, Cohen-Gould L, and Scardino PT. The functional and structural consequences of cavernous nerve injury are ameliorated by sildenafil citrate.

Introduction

Approximately 50,000 radical prostatectomies (RP) are performed each year in the United States for the treatment of prostate cancer. This operation is associated with at least transient erectile dysfunction (ED), with ED rates ranging from 20% to 90% depending upon the study reviewed 1, 2, 3, 4. It is postulated that the development of post-RP ED is due predominantly to a combination of temporary erectile (cavernous) nerve injury and damage to the erectile tissue secondary to neuropraxia and potentially absence of cavernosal oxygenation [5].

A single human, randomized, placebo-controlled study has been conducted examining the role of nightly sildenafil for a 36-week time period following RP [6]. This study demonstrated the ability of this regimen to increase the rate of preservation of preoperative erectile function at approximately 1 year postoperatively in the sildenafil arm compared with the placebo arm [6]. In another noncontrolled study, Schwartz et al. demonstrated that regular use of sildenafil post-RP preserved cavernosal smooth muscle content [7].

The presumed mechanism of this apparent protective effect of sildenafil was originally believed to be cavernosal oxygenation related to penile erection; however, more recently, some have postulated that sildenafil may have an endothelial or neuroprotective effect [8]. The rat cavernous nerve (CN) injury model is believed to simulate the neural injury that occurs during RP and is designed as a model to study the mechanisms of ED post-RP as well as to explore ED-minimizing strategies [9]. This study was undertaken to generate animal data in support of human studies and to explore the mechanisms by which sildenafil may preserve erectile function in this animal model.

Section snippets

Animal Groupings and Sildenafil Administration

Sprague-Dawley rats, initially weighing 250–300 g, were randomly divided into four groups: (i) sham (no CN crush, no sildenafil); (ii) control (C; bilateral CN crush, no sildenafil); and two treatment groups (bilateral CN crush, sildenafil sc): (iii) sildenafil 10 mg/kg (S10); and (iv) sildenafil 20 mg/kg (S20) subcutaneously daily commencing day of CN crush until 24 hours prior to sacrifice. Within each of the four groups, three time subgroups were analyzed, 3 days, 10 days, and 28 days after CN

Erectile Hemodynamics (Figure 1)

The mean ICP/MAP ratios in the sham group was 70 ± 6% and remained significantly higher compared with all other groups (P < 0.01). However, daily subcutaneous sildenafil commencing the day of CN crush injury, administered at 10 mg/kg (S10) and 20 mg/kg (S20) showed an improvement in the mean ICP/MAP ratio compared with control at all three time points. The control group (C) demonstrated improvement in erectile function over time with the ICP/MAP ratio rising from 18 ± 10% at 3 days to 31 ± 13% at 10

Discussion

The rat model of CN injury was first described by Quinlan in 1989 [10] and has been used extensively to investigate techniques and interventions to prevent ED following RP. We documented in prior experiments the use of this animal model in the assessment of functional and structural changes of erectile tissue after CN injury conducted using various techniques [9]. Our findings in this current series of experiments showed a highly significant reduction in ICP/MAP ratios in animals after CN

Conclusion

We have demonstrated, in the CN crush injury model, that sildenafil citrate preserves erectile function and acts by preserving smooth muscle content, endothelial integrity and reducing apoptosis. At this time, based on TEM data, while there are clear differences in CN architecture between treated and untreated animals, it is unclear if these changes represent a genuine neuroregenerative effect. The pharmacological effect on erectile function recovery is dose- and time-dependent. Furthermore, it

Category 1

  • (a)

    Conception and Design

  • John P. Mulhall

  • (b)

    Acquisition of Data

  • John P. Mulhall; Alexander Müller; John F. Donohue; Michael Mullerad; Raanan Tal; Keith Kobylarz; Darius A. Paduch; Leona Cohen-Gould

  • (c)

    Analysis and Interpretation of Data

  • John P. Mulhall; Alexander Müller; Michael Mullerad; Leona Cohen-Gould

Category 2

  • (a)

    Drafting the Article

  • Alexander Müller; Michael Mullerad; John F. Donohue; John P. Mulhall

  • (b)

    Revising It for Intellectual Content

  • John P. Mulhall; Michael Mullerad; Alexander Müller

Category 3

  • (a)

    Final Approval of the Completed

Acknowledgements

This study was funded in part by the Dow Wallace Fund and a research grant from Pfizer Inc.

References (24)

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