Calcium channel blockade in vascular smooth muscle cells: Major hypotensive mechanism of S-petasin, a hypotensive sesquiterpene from Petasites formosanus

In vivo and in vitro studies were carried out to examine the putative hypot ensive actions of S-petasin, a sesquiterpene extracted from the medicinal p lant Petasites formosanus. Intravenous S-petasin (0.1-1.5 mg/kg) in anesthe tized rats produced a dose-dependent hypotensive effect. In isolated aortic ring, isometric contraction elicited by KCl or the L-type Ca2+ channel ago nist Bay K 8644 was reduced by S-petasin (0.1-100 mM), an action not affect ed by the cyclooxygenase inhibitor indomethacin, nitric-oxide synthase inhi bitor N-omega-nitro-L-arginine, guanylyl cyclase inhibitor methylene blue, or removal of vascular endothelium. Pretreatment with S-petasin for 10 min shifted the concentration-response curve for KCl (15-90 mM)-induced contrac tion to the right and reduced the maximal response. In Ca2+-depleted and hi gh K+-depolarized aortic rings preincubation with S-petasin attenuated the Ca2+-induced contraction in a concentration-dependent manner, suggesting th at S-petasin reduced Ca2+ influx into vascular smooth muscle cells (VSMCs). Moreover, in cultured VSMCs, whole-cell patch-clamp recording indicated th at S-petasin (1-50 muM) inhibited the L-type voltage-dependent Ca2+ channel (VDCC) activities. Intracellular Ca2+ concentration ([Ca2+](i)) estimation using the fluorescent probe 1-[2-(5-carboxyoxazol-2-yl)-6-aminobenzofuran- 5-oxy]-2-(2 ' -amino-5 ' -methylphenoxy)-ethane-N,N,N,N-tetraacetic acid pe ntaacetoxymethyl ester indicated that S-petasin (10, 100 muM) suppressed th e KCl-stimulated increase in [Ca2+](i). Taken together, the results suggest ed that a direct Ca2+ antagonism of L-type VDCC in vascular smooth muscle m ay account, at least in part, for the hypotensive action of S-petasin.