Ca2+ channel blocking effect of iso-S-petasin in rat aortic smooth muscle cells

The purpose of the present study was to examine the mechanisms underlying the putative hypotensive actions of iso-S-petasin, a sesquiterpene extract of Petasites formosanus through both in vivo and in vitro experiments. Intravenous administration of iso-S-petasin elicited dose-dependent (0.1 - 1.5 mg/kg) hypotensive and bradycardiac responses in anesthetized rats. Isometric tension recording in isolated thoracic aorta revealed that iso-S-petasin (0.01-100 muM) inhibited KCl- or Bay K 8644 (1,4-dihydro-2,6-dimethyl-5-nitro-4-[2'(trifluoromethyl)phenyl]-3-pyridinecarboxylic acid methyl ester)-induced vasoconstriction independent of endothelium. Iso-S-Petasin also attenuated Ca2+-induced vasoconstriction in a concentration-dependent manner in Ca2+-depleted/high K+-depolarized ring segments, indicating that iso-S-petasin inhibited Ca2+ influx into vascular smooth muscle cells. This was confirmed by whole-cell patch-clamp recording in cultured vascular smooth muscle cells where iso-S-petasin (10-100 muM) appeared to directly inhibit the L-type voltage-dependent Ca2+ channel (VDCC) activity. Intracellular Ca2+ concentration ([Ca2+](i)) measurements using the fluorescent probe fura-2/AM (1-[2-(5-carboxyoxazol-2-yl)-6-aminobenzofuran-5-oxy]-2-(2'-amino-5'-methylphenoxy)-ethane-N,N,N',N'-tetraacetic acid pentaacetoxy-methyl ester) showed suppression of the KCl-stimulated increase in [Ca2+](i) by iso-S-petasin (10, 100 muM). In conclusion, these results suggest that Ca2+ antagonism of the L-type VDCC in vascular smooth muscle cells might largely account for the hypotensive action of iso-S-petasin. (C) 2002 Elsevier Science B.V. All rights reserved.