Role of neural NO synthase (nNOS) uncoupling in the dysfunctional nitrergic vasorelaxation of penile arteries from insulin-resistant obese Zucker rats.

Ana Sánchez,Sara Benedito,Medardo Hernández,Cristina Contreras,Dolores Prieto,Belén Climent,María Pilar Martínez,Albino García-Sacristán,Carlo Gaetano

doi:10.1371/journal.pone.0036027

Abstract

ObjectiveErectile dysfunction (ED) is considered as an early sign of vascular disease due to its high prevalence in patients with cardiovascular risk factors. Endothelial and neural dysfunction involving nitric oxide (NO) are usually implicated in the pathophysiology of the diabetic ED, but the underlying mechanisms are unclear. The present study assessed the role of oxidative stress in the dysfunctional neural vasodilator responses of penile arteries in the obese Zucker rat (OZR), an experimental model of metabolic syndrome/prediabetes.Methods and ResultsElectrical field stimulation (EFS) under non-adrenergic non-cholinergic (NANC) conditions evoked relaxations that were significantly reduced in penile arteries of OZR compared with those of lean Zucker rats (LZR). Blockade of NO synthase (NOS) inhibited neural relaxations in both LZR and OZR, while saturating concentrations of the NOS substrate L-arginine reversed the inhibition and restored relaxations in OZR to levels in arteries from LZR. nNOS expression was unchanged in arteries from OZR compared to LZR and nNOS selective inhibition decreased the EFS relaxations in LZR but not in OZR, while endothelium removal did not alter these responses in either strain. Superoxide anion production and nitro-tyrosine immunostaining were elevated in the erectile tissue from OZR. Treatment with the NADPH oxidase inhibitor apocynin or acute incubation with the NOS cofactor tetrahydrobiopterin (BH4) restored neural relaxations in OZR to levels in control arteries, while inhibition of the enzyme of BH4 synthesis GTP-cyclohydrolase (GCH) reduced neural relaxations in arteries from LZR but not OZR. The NO donor SNAP induced decreases in intracellular calcium that were impaired in arteries from OZR compared to controls.ConclusionsThe present study demonstrates nitrergic dysfunction and impaired neural NO signalling due to oxidative stress and nNOS uncoupling in penile arteries under conditions of insulin resistance. This dysfunction likely contributes to the metabolic syndrome-associated ED, along with the endothelial dysfunction also involving altered NO signalling.

Highlights

Erectile dysfunction (ED) is currently considered as an early clinical sign of vascular disease due to its high prevalence in patients with cardiovascular risk factors including diabetes, hypertension and hyperlipidemia [1,2]
Under non-adrenergic non-cholinergic (NANC) conditions, Electrical field stimulation (EFS) (0.5 to 16 Hz) induced frequency-dependent relaxations in arteries pre-contracted with Phe which were significantly reduced in obese Zucker rats (OZR) compared with lean Zucker rats (LZR) (Figures 1A and B)
The substrate of nitric oxide (NO) synthesis L-arginine (361023 M) significantly reversed the inhibition induced by L-NOARG in both groups (Figures 2A and B), this effect of L-arginine being larger in arteries from obese animals where NO-dependent relaxations were restored to levels similar to those in control arteries (Figure 2D). These results indicate that the vasodilatation induced by EFS in penile arteries during erection is largely mediated by NO production and impaired in OZR

Summary

Introduction

Erectile dysfunction (ED) is currently considered as an early clinical sign of vascular disease due to its high prevalence in patients with cardiovascular risk factors including diabetes, hypertension and hyperlipidemia [1,2]. Diabetic patients have an increased risk of vascular and nerve dysfunction and both autonomic neuropathy and endothelial dysfunction are considered the main etiological factors in the diabetic ED [9,10]. Impairment of the NO-mediated neural and endothelial relaxations was first demonstrated as a cause of ED in human erectile tissue from diabetic patients [12]. Likewise, decreased erectile responses to pelvic nerve stimulation and impaired NANC nervemediated relaxations in cavernosal tissue have been found in prediabetic obese Zucker rats (OZR) [17] and in type 2 diabetic db/db mice, respectively [18,19]. Enhanced corporal NANC-mediated relaxations despite impaired erectile function have been reported in Goto-Kakizaki non-obese type 2 diabetic rats and suggested as a compensatory mechanism to overcome restricted pre-penile arterial blood supply [20]

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Discussion

Conclusion