Ca2+ channel blocking effects of hirsutine, an indole alkaloid from Uncaria genus, in the isolated rat aorta.

Ca2+ channel blocking activity of hirsutine and its pharmacological features were studied. Hirsutine (10(-6) to 3 x 10(-5) M) produced a dose-dependent relaxation of the isolated rat aorta contracted by norepinephrine and high K+ concentration. This effect was exhibited in the aorta strips with or without the endothelium, suggesting an involvement of vasodilative mechanisms not dependent on the endothelium. Hirsutine also inhibited the contractions induced by serotonin and Ca2+ channel activator YC-170, but not by Ca2+ ionophore A23187. The pA2 value of hirsutine was 6.6 +/- 0.1 (mean +/- S.E.; n = 4) in antagonizing cumulative dose-response curve for Ca2+ in the depolarized aorta strips. It is concluded that hirsutine apparently exhibits Ca2+ channel blocking activity mainly through inhibition of the voltage-dependent Ca2+ influx.