Blunted non-nitric oxide vasodilatory neurotransmission in penile arteries from renal hypertensive rats

https://doi.org/10.1016/j.vph.2006.01.014Get rights and content

Abstract

The present study was designed to explore whether there are any effects on neurogenic responses in penile small arteries during the development of hypertension in a one-kidney, one-clip (1K1C) model, a non-renin-dependent model of renovascular hypertension. Five weeks after surgery, male Sprague–Dawley rats were given vehicle, bendroflumethiazide (7.5 mg/kg/day), or l-arginine (2 g/kg/day) in their drinking water for five weeks. Experiments were performed on penile small artery rings (150–200 μm) mounted on microvascular myographs for electrical field stimulation (EFS), and erectile tissue was processed for immunohistochemistry. Maximal neurogenic contractions were unmodified in penile preparations. Relaxations induced by EFS were reduced in the presence of ADMA. In 1K1C rats, neurogenic vasorelaxation mediated by nitric oxide (NO) was unaltered, while relaxation resistant to NO synthase inhibition was blunted. l-arginine and bendroflumethiazide lowered blood pressure in 1K1C rats, but vasodilation was still blunted in the penile arteries. Immunoreactivity for factor VIII and neuronal NO synthase was unaltered in penile arteries from 1K1C animals. Endothelium-dependent vasorelaxation evoked by acetylcholine was also blunted in preparations from 1K1C rats, while exogenous NO relaxation was unaffected. Plasma concentrations and urinary excretion of ADMA did not differ among the experimental animals. Our findings indicate that the reduced release of a non-NO vasodilatory neurotransmitter accounts for the impaired neurogenic vasodilation of the penile arteries. Although ADMA inhibits penile vasorelaxation, it is unlikely to affect erectile function in 1K1C rats.

Introduction

Penile erection is initiated by activation of parasympathetic nerves leading to arterial dilation followed by filling of the sinusoids and entrapment of pressurized blood in the corpus cavernosum of the penis (Andersson, 2003, Simonsen et al., 2002). Hypertension is an independent risk factor for the development of erectile dysfunction (Johannes et al., 2000). Moreover, it has been suggested that increased resistance due to structural and functional changes in the corpus cavernosum contribute to decreased erectile function in spontaneously hypertensive rats (SHR) (Okabe et al., 1999, Toblli et al., 2000, Dorrance et al., 2002, Ushiyama et al., 2004, Behr-Roussel et al., 2003). However, whether vasodilation in the penile arteries supplying the corpus cavernosum with blood is altered has not yet been addressed.

Nitric oxide (NO) is the main neurotransmitter mediating the relaxation of erectile smooth muscle (Andersson, 2003, Ignarro et al., 1990, Simonsen et al., 2002, Toda et al., 2005), while both NO and an unknown neurotransmitter contribute to neurogenic vasodilation in penile arteries (Simonsen et al., 2002, Prieto et al., 1998). Recent studies have shown that both neurogenic and endothelium-dependent NO-mediated relaxation is impaired in the erectile smooth muscle of SHR rats (Behr-Roussel et al., 2003, Ushiyama et al., 2004). However, in addition to NO, an endothelium-derived hyperpolarizing factor (EDHF)-type mechanism of relaxation contributes to acetylcholine vasorelaxation in rat penile arteries (Kun et al., 2003). Further, this mechanism is impaired in penile arteries obtained from diabetic men (Angulo et al., 2003). It is not known, however, whether vasodilatory pathways are also impaired in the penile vasculature of hypertensive subjects. Herein, we explore whether neurogenic vasodilation is affected in penile arteries from one-kidney, one-clip (1K1C) hypertensive rats. We chose this non-renin-dependent model of hypertension to investigate mechanisms independent of angiotensin II. In addition, the rats were administered bendroflumethiazide to investigate whether this would enhance the hypertension-associated impairment of penile arterial vasodilation, and whether supplementation with the NO synthase substrate l-arginine might improve neurogenic relaxation in penile arteries. Finally, we evaluated whether endogenous inhibitors of NO synthase, such as asymmetric dimethyl-l-arginine (ADMA), could contribute to the impaired vasodilation observed in renal hypertensive rats.

Section snippets

Animal model

Male Sprague–Dawley rats (7–8 weeks, 180–200 g) were obtained from the Møllegaard Breeding Center, Skensved, Denmark. The rats were allowed to settle for one week before starting the experimental procedures. Anesthesia was induced in the animals with methohexital (7 mg/kg/i.p.) and further methohexital was given if needed during the operation. The rats were operated on by a ventral procedure described previously (Stankevicius et al., 2002). In brief, the left renal artery was freed from

Animal data

The body weight was similar in the hypertensive and normotensive rats and was not affected by treatment with BDF or l-arginine (Table 1). In 1K1C rats, systolic blood pressure steadily increased to reach a plateau of 175–180 mm Hg, 8–10 weeks after the renal artery had been clipped. From 3 weeks after surgery, systolic blood pressure was significantly higher in 1K1C rats compared to the sham-operated animals. Treatment with BDF and l-arginine lowered the systolic blood pressure in hypertensive

Discussion

The present study provides the first direct evidence of impaired vasodilatory neurotransmission in penile small arteries from renal hypertensive rats. NO-mediated neurogenic relaxations were unaltered and the response to exogenously added NO was similar in the smooth muscle of specimens from control and hypertensive rats. However, relaxations evoked by EFS differed, even in the presence of ADMA, in penile arteries from renal hypertensive versus normotensive rats, suggesting that the blunted

Acknowledgements

We thank Karen K. Busch and Helle Zibrandtsen for technical assistance. We thank Dr. Sara Benedito for helpful discussion of the manuscript. This study was supported by grants from the Danish Heart Foundation (99-2-2-35-22742), Danish Research Council, and the Aarhus University Research Foundation.

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