Effect of (-)-stepholidine on blood pressure and alpha-adrenoceptor agonists-, KCl- and CaCl2-evoked contractions of aortic strips

Iv (-)-SPD lowered the blood pressure in anesthetized rat, the ED50 value was 5.1 +/- 2.5 mg.kg-1. In the experiments of rat and rabbit aortic strips, (-)-SPD 0.3-100.0 mumol.L-1 inhibited the contraction initiated by clonidine (alpha 2) and phenylephrine (alpha 1) and shifted the dose-response curve to the right parallely without change in maximum response. The inhibitory ratio of (-)-SPD acting on alpha 2/alpha 1 adrenergic receptors was about 7.2, and (-)-SPD thus was predominant inhibition on alpha 2 adrenergic receptors. In the experiment of aortic strips from reserpinized rabbits, the inhibition of (-)-SPD on contraction evoked by clonidine was diminished markedly. The results suggest that (-)-SPD stimulated mainly the alpha 2-adrenergic receptors of presynaptic nerve endings. Moreover (-)-SPD 1 mumol.L-1 inhibited the release of intracellular Ca2+ initiated by NE. (-)-SPD 3-30 mumol.L-1 blocked the voltage-dependent Ca2+ channel.