Mesaconitine-induced relaxation in rat aorta: role of Na +/Ca 2+ exchangers in endothelial cells

Abstract

Previously, we reported that mesaconitine, an aconite alkaloid, increased intracellular Ca 2+ concentration ([Ca 2+] i) level in endothelium and caused relaxation in rat aorta via nitric oxide production. In the present study, we investigated the mechanisms of increase in the [Ca 2+] i level induced by mesaconitine in rat aorta and in human umbilical vein endothelial cells (HUVECs). Treatment with the low Na + buffer delayed the 30 μM mesaconitine-, but not 10 μM acetylcholine-, induced relaxation in rat aorta. Treatments with an inhibitor of Na +/Ca 2+ exchangers (20 μM 3′,4′-dichlorobenzamil) and a reversed mode (Ca 2+ influx) inhibitor of the exchangers (30 μM 2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl]isothiourea methanesulfonate, KBR7943) showed similar effects. In HUVECs, 30 μM mesaconitine increased the [Ca 2+] i level in the presence of extracellular CaCl 2 and NaCl, and the response was inhibited by KBR7943. Mesaconitine increased intracellular Na + concentration level in HUVECs. The [Ca 2+] i response by mesaconitine was inhibited by 100 μM d-tubocurarine (an inhibitor of nicotinic acetylcholine receptors), but was not inhibited in the glucose-free buffer and by inhibitors of Na +/H + exchangers. These findings suggest that mesaconitine stimulated Ca 2+ influx via the Na +/Ca 2+ exchangers in endothelial cells and caused relaxation in the aorta. The possibility of d-tubocurarine-sensitive Na + channels as target(s) of mesaconitine is discussed.