[The blocking and partial agonistic actions of jatrorrhizine on alpha-adrenoceptors]

Jatrorrhizine (Jat) competitively antagonised the effects of phenylephrine on rabbit aortic rings and clonidine on electrically stimulated rat vas deferens with pA2 values of 5.73 and 5.86, respectively. It (5 mg/kg iv) also diminished the pressor responses induced by either thoracic sympathetic stimulation (T7-T9) or noradrenaline (2 micrograms/kg iv) in pithed rats. Furthermore the dose-response curves for methoxamine and B-HT 920 were shifted parallely to the right by iv Jat 10 mg/kg. Below 1 mumol/L, Jat alone showed the depression of the contraction of rat vas deferens stimulated electrically. This effect, however, was less evident at higher concentrations of Jat and was reversed by yohimbine (0.1 mumol/L). Similarly, at very low concentration (1 nmol/L), Jat shifted the dose-response curves for clonidine parallely to the left. In pithed rats, the increase of heart rate evoked with electrical stimulation of spinal segment (C7-T1) was attenuated by Jat (1 mg/kg) and clonidine (30 micrograms/kg), but argumented by large doses of Jat (10 mg/kg) and yohimbine (50 micrograms/kg). The above results showed that: firstly, Jat has blocking actions on both alpha 1- and alpha 2-adrenoceptors, which may be relevant to its hypotensive and anti-arrhythmic actions; secondly, the demonstration of dualistic actions of Jat on adrenoceptors, which has never been documented before, may be also present in other homologues of the protoberberines.