Mechanisms of inhibitory activity of root extract of Carpolobia lutea G. Don on in vitro contractile responses of rabbit corpus carvernosum

Introduction

Carpolobia lutea root extract (CLRE) has been reported to enhance penile erection. However, the mechanism involved is poorly understood. We investigated in vitro mechanisms of CLRE action on contractile activity of rabbit corpus cavernosum (CC).

Methods

Corpus cavernosum strips from four healthy male New Zealand rabbits (2.5–3.0 kg) were mounted on an organ chamber and contracted with phenylephrine (PE) (10⁻⁹ to 10⁻⁵ M) and Potassium Chloride (KCl) (10–50 mM) before treatment with various concentrations of CLRE (0.1–1.2 mg/ml). Interactions between CLRE and a Nitric Oxide Synthase (NOS) inhibitor (N-nitro-l-arginine methyl ester – l-NAME 10⁻⁴ M); guanylyl cyclase inhibitors (Oxalodiazolo 4,3-a quinoxalin-1-one – ODQ 10 μM, 20 μM, 30 μM), and (methylene blue 10–30 μM); a cyclooxygenase inhibitor (10⁻⁴ M indomethacin); potassium-channel inhibitors (100 μM tetraethyl ammonium TEA), (100 ηM apamin) and (glibenclamide 10 μM and 20 μM); and a calcium-channel inhibitor (−10⁻⁴ M nifedipine) were investigated.

Results

Maximal contractions of KCl and PE contracted CC strips were significantly reduced in a concentration-dependent manner (40.8 ± 3.6% and 38.6 ± 4.0% from 64.6 ± 2.9% and 98.1 ± 4.2% respectively). Relaxant effect of CLRE was significantly reduced by ODQ (38.6 ± 4.0% to 6.4 ± 1.3% and 38.6 ± 4.0% to 7.2 ± 1.2%), nifedipine (38.6 ± 4.0% to 21.1 ± 2.7%) and glibenclamide (40.8 ± 3.6% to 31.5 ± 3.3%). However l-NAME, indomethacin, methylene blue, TEA and apamin did not inhibit relaxation by CLRE.

Conclusion

Concentration-dependent relaxant effect of CLRE in rabbit CC involves the soluble guanylate cyclase/cyclase Guanosine Monophosphate system, and activation of ATP-dependent K⁺ channels.