Halichlorine is a novel L-type Ca2+ channel inhibitor isolated from the marine sponge Halichondria okadai Kadota

Halichlorine, isolated from a marine sponge Halichondria okadai Kadota, has a unique structure and its physiological activity is virtually unknown. In the present study, we investigated the direct effect of halichlorine on vascular contractility. In endothelium-denuded rat aorta, while the treatment of halichlorine (0.01–10 μM) did not induce vascular contraction, halichlorine (0.01–10 μM) dose-dependently inhibited both the steady-state precontractions induced by high K+ (65.4 mM) and phenylephrine (1 μM). The vasodilator effect of halichlorine (10 μM) on high K+ (65.4 mM)-induced contraction was more potent than that on phenylephrine (1 μM)-induced contraction (65.4 mM high K+: 72.7 ± 3.4%; 1 μM phenylephrine: 34.7 ± 2.3%). To investigate the mechanism underlying the suppressive effect of halichlorine on vascular contractility, we examined the effect of halichlorine on intracellular Ca2+ concentration in vascular smooth muscle with a fluorescent Ca2+ indicator, fura-2. Treatment of halichlorine (10 μM) significantly inhibited the sustained [Ca2+]i elevation induced by high K+ (65.4 mM) (45.3 ± 5.5%). Furthermore, current measurements by whole-cell mode patch-clamp recording in rat aortic smooth muscle cells (A7r5 cells) demonstrated that halichlorine (10 μM) decreased the current density of the L-type Ca2+ channel (peak Ca2+-channel current densities: − 2.09 ± 0.27 pA/pF for control; − 0.58 ± 0.07 pA/pF for halichlorine). These results suggest that halichlorine inhibits L-type Ca2+ channels in vascular smooth muscle cells, which inhibit intracellular Ca2+ influx, and then reduce vascular contractions.