Effects of eupatilin on the contractility of corpus cavernosal smooth muscle through nitric oxide-independent pathways

Abstract

Eupatilin (5,7-dihydroxy-3,4,6-trimethoxyflavone) is one of the main compounds present in Artemisia species. Eupatilin has both antioxidative and anti-inflammatory properties and a relaxation effect on vascular contraction regardless of endothelial function. We evaluated the relaxant effects of eupatilin on the corpus cavernosum (CC) of rabbits and the underlying mechanisms of its activity in human corpus cavernosum smooth muscle (CCSM) cells. Isolated rabbit CC strips were mounted in an organ bath system. A conventional whole-cell patch clamp technique was used to measure activation of calcium-sensitive K+-channel currents in human CCSM cells. The relaxation effect of eupatilin was evaluated by cumulative addition (10(-5) M similar to 3 x 10(-4) M) to CC strips precontracted with 10(-5) M phenylephrine. Western blotting analysis was performed to measure myosin phosphatase targeting subunit 1 (MYPT1) and protein kinase C-potentiated inhibitory protein for heterotrimeric myosin light chain phosphatase of 17-kDa (CPI-17) expression and to evaluate the effect of eupatilin on the RhoA/Rho-kinase pathway. Eupatilin effectively relaxed the phenylephrine-induced tone in the rabbit CC strips in a concentration-dependent manner with an estimated EC50 value of 1.2 +/- 1.6 x 10(-4) M (n = 8, p ˂ 0.05). Iberiotoxin and tetraethylammonium significantly reduced the relaxation effect (n = 8, p ˂ 0.001 and p = 0.003, respectively). Removal of the endothelium or the presence of L-NAME or indomethacin did not affect the relaxation effect of eupatilin. In CCSM cells, the extracellular application of eupatilin 10(-4) M significantly increased the outward currents, and the eupatilin-stimulated currents were significantly attenuated by treatment with 10(-7) M iberiotoxin (n = 13, p ˂ 0.05). Eupatilin reduced the phosphorylation level of MYPT1 at Thr853 of MLCP and CPI-17 at Thr38. Eupatilin-induced relaxation of the CCSM cells via NO-independent pathways. The relaxation effects of eupatilin on CCSM cells were partially due to activation of BKCa channels and inhibition of RhoA/Rho-kinase.