Mechanism of hypotensive effects of verticillatine

Verticillatine (Vtc) is a new alkaloid first isolated in our institute from the water-soluble fraction of the root of Rauwolfia verricillata (Lour. ) Baill. In anesthetized cats. iv Vtc 0.5-2.0 mg/kg produced dose-dependent ganglion blocking effects and the ED50 of blocking action on the sympathetic and parasympa-thetic ganglion were l.03 and 0.8 mg/kg. respectively. The affinity of Vtc to a-adrenoceptor was studied using radioligand binding assay in the rat brain cortex. The competitive curves showed that the apparent dissociation constant (Ki) of Vtc for ai and az-adrenoceptor were 0.22 and 0.11 pmol/L, respectively. Vtc antagonized NE-induced contraction of the rat anococcygeus and rabbit aorta (postsynaptic ai-adrenoceptor) and pAz value were 4.3 and 4.6, respectively. Vtc antagonized the inhibiting effects of clonidine on field stimulation twitch of the rat vas deferens (presynaptic az-adrenoceptor) and pAz value was 4.3. Vtc and hexamethonium (Ca) manifested neither the central antihypertensive effect nor direct peripheral vasodilating action. C6 was found to be only a ganglionic blocking agent in this case. Vtc. in addition to its gangjion blocking action, showed a-adrenoceptor blocking properties.