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

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-alpha]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 x 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 x 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 release of nitric oxide, prostaglandins or by the activation of potassium channels.