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The vasorelaxant effect of glaucine, the major alkaloid of Platycapnos spicata (L.) Bernh., was studied. At concentrations of 10 microM-0.3 mM in normal Krebs solution it was almost equieffective in relaxing K(+)-induced and noradrenaline-induced tension in rat aortic rings without endothelium, with IC50 values of 160 +/- 16 microM and 90 +/- 14 microM respectively. In experiments in a calcium-free medium, 10 microM glaucine strongly inhibited noradrenaline-induced contractions. Glaucine (0.3 mM) did not affect basal uptake of 45Ca, but induced uptake was reduced to 100% (K+) and 97.7% (noradrenaline) of the basal value. These results suggest that glaucine has an intracellular effect and also acts on the cell membrane by blocking voltage-dependent and receptor-operated calcium channels.