Screening for Rho-kinase 2 inhibitory potential of Indian medicinal plants used in management of erectile dysfunction

Abstract

The aim of the study

Activation of Rho-kinase 2 (ROCK-II) results in contraction of corpus cavernosum smooth muscle and ROCK-II inhibitors relax corpus cavernosum in vitro and in vivo hence, plant extracts capable of inhibiting ROCK-II enzyme may be useful in management of erectile dysfunction (ED). The aim of the study was to screen selected Indian medicinal plants, having similar ethnopharmacological use for ROCK-II inhibition.

Materials and methods

Some Indian medicinal plants reported as aphrodisiacs in Ayurveda and modern scientific literature were collected, authenticated and extracted. Direct methanol and successive aqueous extracts of these plants were screened for ROCK-II inhibitory activity using HTRF^® KinEASE™ STK S2 Kit (Cisbio Bioassays). Relaxant effect of potent extract was recorded on isolated rat corpus cavernosum.

Results

Methanolic and successive aqueous extracts of 30 plants were screened for ROCK-II inhibition and 15 herbal extracts showed inhibition ranging between 50 and 88% at 50 μg/mL. While IC₅₀ of Y-27632, a standard ROCK-II inhibitor, was found to be 163.8±1.2 nM. The Methanolic extract of Terminalia chebula (METC) with IC₅₀ value of 6.09±0.17 μg/mL was found to be most potent and relaxed isolated rat corpus cavernosum significantly (p<0.01). Chebulagic and chebulinic acid of METC were found to inhibit ROCK-II and might be responsible for the inhibitory potential of the extract.

The traditional use of plants like Butea frondosa, Syzygium aromaticum, Butea superba, Chlorophytum borivilianum and Mucuna pruriens, as aphrodisiacs and for male sexual disorder (MSD) might be in part due to the ROCK II inhibitory potential of these plants.

Conclusion

Some of the Indian medicinal plants have ROCK-II inhibitory potential and those deserve further investigation.

Introduction

Erectile dysfunction (ED) is a male sexual disorder where male patients fail to accomplish and retain adequate erection to allow pleasing sexual intercourse (NIH consensus development panel on impotence, 1993). Erectile dysfunction is known to be a major health problem and can cause considerable distress, unhappiness and relationship problems (Morgentaler, 1999). ED is also considered as a marker of cardiovascular disorder and it is known to decrease quality of life (Burchardt et al., 2001, Billups, 2005). It affects elder males and those with diabetes and high blood pressure (Carson, 2004, Vickers and Wright, 2004). A male aging study showed that 52% of respondents have some degree of erectile dysfunction, with 35% of men aged 40–70 reporting moderate or complete impotence (Morgentaler, 1999).

Relaxation of corpus cavernosum smooth muscle (CCSM) and penile arteries leads to trapping of blood in penis resulting in increased intracavernous pressure (ICP) that is manifested as penile erection (Andersson and Stief, 1997). Impaired endothelial dysfunction results in an impaired relaxation and is the primary reason for ED (Maas et al., 2002).

Rho-kinase 2 (ROCK-II) inhibitor, Y-27632 was found to relax rabbit and human corpus cavernosum in vitro (Rees et al., 2001). Further, intracavernous administration of Y-27632 increased ICP in aged rat, which is a manifestation of increased erectile function (Gao et al., 2007) and Y-27632 when applied to glans penis of rat caused penile erection (Dai et al., 2004).

Plants based medicines have been in use since eons for general health as well as to cure a specific illness. Eurycoma longifolia (Tongkat Ali), a South East Asian plant has been used as an aphrodisiac and is known to increase male sexual prowess and be effective in treating ED (Ang et al., 2001). Based on experimental studies in rats, Eurycoma longifolia was found to increase the number and frequency of sexual intercourse episodes, duration of sexual performance and ejaculations (Ang et al., 2001, Ang et al., 2000). Unfortunately, studies on are lacking to support its aphrodisiac activity. Lepidium meyenii Walp. also known as Maca, is a root vegetable cultivated in the central Peruvian Andes and is known for its aphrodisiac activities. Research on male rats and mice indicates that Maca positively affects sexual behavior parameters such as the latency to and frequency of mounting and latency to ejaculation after 15 day of treatment (Cicero et at., 2001).

Another plant, Pausinystalla yohimbe (Yohimbine), an indigenous of West African countries has been used as a pharmacological agent in the treatment of ED and the drug is an alpha-2-adrenergic receptor antagonist (MacKay, 2004). Yohimbine improves sexual function by displacing epinephrine from alpha-2 adrenergic receptors in the pelvic areas and it increases the proneness to arousal through supplying epinephrine from the alpha-2 receptors to the central nervous system (brain) where it is active as a neurotransmitter (Riley, 1994).

In Ayurveda, the herbs that are used to increase male sexual potency/ dynamics are called ‘Vajikaran’ herbs or aphrodisiacs (Sharma, 1996, Thakur and Dixit, 2007). Several Indian medicinal plants have been reported as aphrodisiac or Vajikaran herbs (Sharma et al., 2000, Sharma et al., 2001a, Sharma et al., 2001b, Sharma et al., 2002a, Sharma et al., 2002b, Billore et al., 2004, Levekar et al., 2007, Sharma, 1996) and are used in the management of sexual disorder including ED.

We found that inclusion of these traditional plants appears to be based mainly on personal testimonials, cultural beliefs, or one or two isolated scientific reports rather than on a body of consistent scientific evidence. Either the research has not been done, or the evidence is too preliminary and nonspecific to support their general use in the treatment of erectile dysfunction.

The mechanism of action of Indian medicinal plants (Table 1) used in the study has not been elucidated so far, which may be at least in part due to ROCK-II inhibitory activity, hence the present study.