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- Original Article -

Relaxation mechanisms of neferine on the rabbit corpus cavernosum tissue in vitro

Jun Chen^1,2,†, Jun Qi^1,†, Fang Chen¹, Ji-Hong Liu², Tao Wang², Jun Yang², Chun-Ping Yin³

¹Department of Urology, Xinhua Hospital, School of Medicine, Shanghai Jiaotong University, Shanghai 200092, China
²Department of Urology, ³Department of Pharmacy, Tongji Hospital, Tongji Medical College, Huazhong University of Science and Technology, Wuhan 430030, China

Abstract

Aim: To investigate the relaxation mechanisms of neferine (Nef) on the rabbit corpus cavernosum tissue in vitro. Methods: Strips of rabbit corpus cavernosum were mounted in organ chambers. The effects of Nef were examined on isolated muscle strips precontracted with phenylephrine (PE) alone, in the presence of N^W-nitro-L-arginine (LNNA, a nitric oxide synthase inhibitor), 1-H-[1,2,4]oxadiazolo[4,3-α]quinoxalin-1-one (ODQ, a guanylyl cyclase inhibitor), indomethacin (cyclooxygenase inhibitor), tetraethylammonium (Ca²⁺-activated K⁺ channel blocker), 4-aminopiridine (4-AP ,voltage dependent K⁺ channel blocker) and glibenclamide (ATP sensitive K⁺channel blocker). The effects of Nef on KCl-induced contraction of isolated muscle strips were also investigated. The procedure of calcium absence-calcium addition was designed to observe the effect of Nef on two components of the contractile responses to PE based on the source of Ca²⁺ (extracellular vs. intracellular).  Results: Corpus cavernosum strips relaxed in response to Nef (10^_9_10^_4 mol/L) in a concentration-dependent manner with an IC₅₀ of 4.60 × 10^_6 mol/L. However, they were not affected by LNNA, ODQ, indomethacin or K⁺-channel blockers. Nef (10^_6 mol/L, 10^_5 mol/L) concentration dependently reduced the maximal contraction response of isolated strips induced by KCl to 79.3% ± 5.5% and 61.5% ± 3.2%, respectively (P < 0.01). In the calcium absence_calcium addition procedure, Nef 10^-5 mol/L inhibited both intracellular calcium-dependent and extracellular calcium-dependent contraction induced by PE (2 × 10^-5 mol/L) (P < 0.05). The inhibition ratios were 26.2% ± 5.4% and 48.3% ± 7.6%, respectively.  Conclusion: The results of the present study suggest that Nef possesses a relaxant effect on rabbit corpus cavernosum tissues, which is attributable to the inhibition of extracellular Ca²⁺ influx and the inhibition of release of intracellular stored Ca²⁺, but not mediated by the release of nitric oxide, prostaglandins or by the activation of potassium channels.(Asian J Androl 2007 Nov; 9: 795_800)

Keywords: neferine; corpus cavernosum; relaxation

Correspondence to: Prof. Fang Chen, Department of Urology, Xinhua Hospital, School of Medicine, Shanghai Jiaotong University, Shanghai 200092, China.
Tel: +86-21-6515-1555 Fax: +86-21-6579-5173
E-mail: chenfang007@hotmail.com
Prof. Jun Qi, Department of Urology, Xinhua Hospital, School of Medicine, Shanghai Jiaotong University, Shanghai 200092, China.
Tel: +86-21-6579-0000 ext.7805 Fax: +86-21-6579-5173
E-mail: jasonqi@sh163.net
^†These two authors contributed equally to this work.
Received 2006-12-27 Accepted 2007-06-04

DOI: 10.1111/j.1745-7262.2007.00321.x

1 Introduction

Erectile dysfunction (ED) is a common problem affecting approximately 50% of men aged 40_70 years [1]. Current pharmacological treatment for ED includes the oral, intracavern

osal and intraurethral administration of erectogenic drugs. Oral pharmacotherapy is the most effective therapy for ED with the highest patient preference. Oral phosphodiesterase type 5 (PDE5)-inhibitors (sildenafil, tadalafil and vardenafil) are superior in effectiveness to centrally acting drugs (apomorphine and yohimbine). Local pharmacotherapy (intracaver-nosal and intraurethral treatments) is a second line therapy in cases of failure or contraindica-tions for oral pharmacotherapy [2]. Although many drugs are now available for treating ED, finding a new drug for treating ED and understanding its mechanism of action is still important.

Many traditional Chinese medicines are effective as treatments for ED. Because of the complex chemical ingredients, it remains unclear which ingredients exactly, and by which mechanisms, have the chemical effect in the treatment of ED. Some extracts from traditional Chinese medicines, of the alkaloids, coumarin and saponin series, relax the smooth muscle of corpus cavernosum [3_8], which provides an open window for developing new drugs for the treatment of ED.

Neferine (Nef) is a bis-benzylisoquinoline alkaloid extracted (isolated) from the green seed embryo of Nelumbo nucifera Gaertn, which is effective in preventing the onset of reentrant ventricular tachyarrhy-thmias [9_10]. Nef can inhibit very low density lipoprotein oxidation [11] and platelet aggregation [12], protect vascular endothelial cells from damage induced by oxygen free radicals [13] and increase sensitivity to anticancer drugs [14]. In the course of our studies on the development of naturally occurring agents for the treatment of ED, we found that Nef induced relaxation on the phenylephrine (PE)-precontracted corpus cavernosum [6]. In the present study, the relaxation mechanisms of Nef on the isolated rabbit corpus cavernosum were investigated.

2 Materials and methods

All animal experiments were carried out with the approval of the Institute for Animal Care and Use Committee of Xinhua Hospital (Shanghai, China).

2.1 Materials

Nef was kindly provided by Prof. Jia-Ling Wang (Department of Pharmacology, Tongji Medical College, Huazhong University of Science and Technology, Wuhan, China). The purity of Nef was greater than 99.8%. It was dissolved in HCl 0.1 mol/L, then diluted with Krebs' solution to the desired concentration. The following drugs were all obtained from Sigma Chemical Company (St. Louis, MO, USA): N^W-nitro-L-arginine (LNNA, a nitric oxide synthase inhibitor), 1-H-[1,2,4]oxadiazolo[4,3-α]quinoxalin-1-one (ODQ, a guanylyl cyclase inhibitor), indomethacin (cyclooxygenase inhibitor), tetraethylammonium (TEA, Ca²⁺-activated K⁺ channel blocker), 4-aminopiridine (4-AP, voltage dependent K⁺ channel blocker) and glibenclamide (ATP sensitive K⁺channel blocker). Phenylephrine (PE) was from Shanghai Harvest Pharmaceutical (Shanghai, China). The Krebs' solution (in mmol/L) consists of: NaCl 118, KCl 4.7, CaCl₂ 2.5, MgSO₄ 1.2, KH₂PO₄ 1.2, NaHCO₃ 25 and glucose 11 (pH 7.4). In calcium-free solution, CaCl₂ was omitted from the Krebs' solution with the addition of 0.05 mmol/L egtaic acid (EGTA).

2.2 Tissue procurement

Adult male (4_6 months) New Zealand white rabbits (2.5_3.0 kg) were killed with pentobarbital sodium (50 mg/kg). The penis was surgically removed en bloc with care being taken to keep the tunica albuginea intact in all animals. The corpus spongiosum and urethra were excised. The corpus cavernosum tissue was then carefully dissected free from the surrounding tunica albuginea. Each rabbit provided two strips of corpus cavernosum tissue. The strips of corpus cavernosum with dimensions of approximately 2 × 2 × 6 mm were mounted in 10 mL Organ-bath chambers containing Krebs buffer solution (pH 7.4) at 37ºC, and aerated with 95% oxygen and 5% CO₂. The strips were suspended with silk ties to a force-displacement transducer (ML T0201/D, AD Instruments, Sydney, Australia). Changes in isometric tension were measured by Powerlab/4sp (AD Instruments, Sydney, Australia). The strips of corpus cavernosum were stretched to a resting force of 2.0 g and were equilibrated for at least 60 min. During this period, the tissue was washed with fresh solution at 37ºC, for a consecutive 15 min, and tension was adjusted if necessary.

2.3 Organ bath experiments

2.3.1 Effects of Nef on PE-induced rabbit corpus cavernosum smooth muscle responses alone, in the presence of LNNA, ODQ, indomethacin, TEA, 4-AP and glibenclamide

Strips were precontracted with 10^_5 mol/L PE. After muscle strips were stabilized, relaxation responses to cumulative concentrations of Nef (10^_9_10^_4 mol/L) were assessed. When the influence of antagonists on the Nef-induced relaxation was evaluated, inhibitors (LNNA 10^_4 mol/L, ODQ 10^_4 mol/L, indomethacin 10^_4 mol/L, TEA 10^_4 mol/L, 4-AP 10^_4 mol/L or glibencla-mide 10^_5 mol/L) were added to the organ bath 20 min before addition of PE in different experiments.

2.3.2 Effects of Nef on KCl-induced rabbit corpus cavernosum smooth muscle responses

The addition of cumulative concentrations of KCl (2 × 10^_2, 4 × 10^_2, 6 × 10^_2, 8 × 10^_2, 10 × 10^_2 mol/L) was performed and concentration response was recorded. After washout by Krebs solution and reequilibration, each cavernosal strip was incubated with different concentrations (10^_6 mol/L, 10^_5 mol/L) of Nef and equilibrated for 20 min. Cumulative concentration responses of KCl were repeatedly recorded.

2.3.3 Effects of Nef on the two contractile elements of isolated corpus cavernosum smooth muscle strips induced by PE

In Ca²⁺-free medium, PE induced rapid transient contraction, presumably because of release of intracellular Ca²⁺; and the sustained contraction when extracellular Ca²⁺ was present presumably the result of Ca²⁺ influx [15]. After being washed with Ca²⁺-free Krebs solution three times and being equilibrated for 20 min, the strips were contracted by 2 × 10^_5 mol/L PE (due to release of intracellular Ca²⁺). After a stable higher tension baseline was established, 2.5 × 10^_3 mol/L CaCl₂ was added, strips contracted again and a peak was reached (as a result of entry of Ca²⁺). This was followed by thorough washout by Krebs solution and refilling of intracellular Ca²⁺ stores by repeated stimulation of 6 × 10^_2 mol/L KCl in Ca²⁺ containing Krebs solution three times. After washing with Ca²⁺-free Krebs solution, 10^_5 mol/L Nef was added and incubated for 20 min, and then the above-mentioned experiment was repeated. The effects of Nef on the two contractile elements of isolated strips induced by PE were observed.

2.3.4 Data analysis

All data were expressed as mean ± SEM. Responses were expressed as a percentage of active muscle tone induced by PE or KCl. Statistical analysis was performed with one-way analysis of variance using SPSS 12.0 software (SPSS Ltd., Chicago, IL, USA). P < 0.05 was considered a significant difference.

3 Results

3.1 Effects of Nef on PE-induced rabbit corpus cavernosum smooth muscle responses alone, in the presence of LNNA, ODQ, indomethacin, TEA, 4-AP and glibenclamide

Corpus cavernosum strips relaxed in response to Nef (10^_9_10^_4 mol/L) in a concentration-dependent manner with an IC₅₀ of 4.60 × 10^_6 mol/L (n = 8, P < 0.05) (Figure 1). However, the response of isolated muscle strips was not affected by nitric oxide synthase inhibitor (LNNA), guanylyl cyclase inhibitor (ODQ), cyclooxy-genase inhibitor (indomethacin), Ca²⁺-activated K⁺ channel blockers (TEA), voltage dependent K⁺ channel blocker (4-AP) and ATP sensitive K⁺channel blocker (glibenclamide) (n = 8, P > 0.05) (Figure 2).

3.2 Effects of Nef on KCl-induced contraction of isolated corpus cavernosum smooth muscle strips

Nef (10^_6 mol/L, 10^_5 mol/L) concentration dependently reduced the maximal contraction response of isolated strips induced by KCl to 79.3% ± 5.5% and 61.5% ± 3.2%, respectively (n = 8, P < 0.01) (Figure 3).

3.3 Effects of Nef on the two contractile elements of isolated corpus cavernosum smooth muscle strips induced by phenylephrine

Nef 10^_5 mol/L inhibited both intracellular calcium-dependent and extracellular calcium-dependent contraction induced by PE (2 × 10^_5 mol/L) (n = 6, P <  0.05). The inhibition ratios were 26.2% ± 5.4% and 48.3% ± 7.6%, respectively (Figure 4).

4 Discussion

A number of mechanisms have been identified for the local regulation of penile smooth muscle contractility and, therefore, penile erection. Molecules participating in these pathways can be considered targets for the development of new drugs to treat ED.

Many studies have demonstrated that nitric oxide (NO) is the most important factor for immediate relaxation of penile vessels and corpus cavernosum. During sexual stimulation, NO releases from the non-adrenergic non-cholinergic nerve terminals and endothelium within the erectile tissue of the penis. It activates guanylyl cyclase, resulting in an increased conversion of guanosine triphosphate to cyclic guanosine monophosphate (cGMP). cGMP in turn activates protein kinase G, which phosphorylates certain proteins and ion channels. This process facilitates the reduction of intracellular calcium, enabling the smooth muscle to relax, resulting in erection [16, 17]. To investigate whether relaxation induced by Nef was attributable to an interaction with NO-cGMP pathway, corpus cavernosum tissues were pretreated with LNNA (a nitric oxide synthase inhibitor) and ODQ (a guanylyl cyclase inhibitor), respectively. The treatment of corpus cavernosum tissues with these inhibitors did not significantly affect the relaxant activity of Nef. These findings indicate that the relaxant action of Nef on the rabbit corpus cavernosum is not mediated by NO-cGMP pathway.

Prostaglandin (especially PGI₂ and PGE₁) is derived from arachidonic acid by the action of cyclooxygenases. Prostaglandin inhibits the release of noradrenaline from penile adrenergic nerves and increases intracellular cAMP levels in corpus cavernosum smooth muscle [18, 19]. This process promotes muscle relaxation through intracellular reduction of calcium concentration, resulting in erection. Therefore, the possibility that relaxation induced by Nef was a result of an interaction with prostaglandins was further investigated. However, the treatment of corpus cavernosum tissues with cyclooxygenase inhibitor (indomethacin) did not affect the relaxant activity of Nef. Therefore, the corpus cavernosum tissues relaxation effect of Nef was not related to prostaglandins.

Potassium channels (particularly Ca²⁺-activated K⁺ channel, voltage dependent K⁺ channel, and ATP sensitive K⁺channel) play an important role in the modulation of corpus cavernosum smooth muscle cell tone. Potassium channel opening is be expected to cause hyperpo-larzation, inhibit Ca²⁺ channel activity, lower intracellular Ca²⁺ concentration and result in relaxation [20_23]. However, in the present experiment, we found that the treatment of corpus cavernosum tissues with TEA (Ca²⁺-activated K⁺ channel blocker), 4-AP (voltage dependent K⁺ channel blocker) and glibenclamide (ATP sensitive K⁺channel blocker) did not change the relaxant activity of Nef. These findings suggest that the relaxant effect of Nef is unrelated to Ca²⁺-activated, voltage dependent and ATP sensitive K⁺ channels.

It is understood that the contractile activity of corpus cavernosum smooth muscles is regulated by the concentration of free Ca²⁺ in the cytosol, which depends on voltage-dependent Ca²⁺ influx, receptor-operated Ca²⁺ influx, and release of intracellularly stored Ca²⁺ [24]. High extracellular K⁺ induced the membrane depolarization rapidly, which opened the voltage-dependent Ca²⁺ channel and brought about the influx of the extracellular Ca²⁺. PE elicited [Ca²⁺]_i elevation by: (i) activating α₁-adrenoceptor-operated Ca²⁺ channel to induce extracellular Ca²⁺ entry; and (ii) activating G protein and then facilitating the formation of IP3, which acts on IP3 receptor and induces intracellular Ca²⁺ release. In the present study, the contraction of corpus cavernosum tissues induced by PE or KCl was reduced by Nef. Furthermore, Nef also concentration-dependently relaxed PE-induced contraction of isolated muscle strips in a calcium-free solution. The results suggest that Nef relax corpus cavernosum tissues by inhibiting both extracellular Ca²⁺ influx and release of intracellular stored Ca²⁺.

In conclusion, the results of the present study suggest that Nef possesses a relaxant effect on rabbit corpus cavernosum tissues, which is attributable to the inhibition of extracellular Ca²⁺ influx and the inhibition of release of intracellular stored Ca²⁺, but is not mediated by the release of nitric oxide, prostaglandins or by the activation of potassium channels. Further experiments are required to establish whether cAMP signal transduction pathway, Rho-A/Rho-kinase pathway and cell-cell communication contributes to Nef induced relaxation or not. Because in the experiment on muscle strips in vitro, the influence of arterial inflow, venous outflow, autonomic innervation and a multitude of local and total body factors were negligible, further pharmacological research and toxicology tests are needed in vivo to clarify whether Nef can be developed into a drug for ED.

Acknowledgment

The authors thank Prof. Jia-Ling Wang for kindly supplying the Nef. The technical assistance from Mr Jun Pan (Department of Urology, Xinhua Hospital, School of Medicine, Shanghai Jiaotong University, Shanghai, China) is also greatly appreciated. This study was sponsored in part by the National Natural Science Foundation of China (No. 30471736).

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