Ca 2+ channel blocking effect of iso- S-petasin in rat aortic smooth muscle cells

Abstract

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 μM) 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 Ca 2+-induced vasoconstriction in a concentration-dependent manner in Ca 2+-depleted/high K +-depolarized ring segments, indicating that iso- S-petasin inhibited Ca 2+ 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 μM) appeared to directly inhibit the L-type voltage-dependent Ca 2+ channel (VDCC) activity. Intracellular Ca 2+ concentration ([Ca 2+] 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 pentaacetoxymethyl ester) showed suppression of the KCl-stimulated increase in [Ca 2+] i by iso- S-petasin (10, 100 μM). In conclusion, these results suggest that Ca 2+ antagonism of the L-type VDCC in vascular smooth muscle cells might largely account for the hypotensive action of iso- S-petasin.