[Dynorphin and atriopeptin may be involved in the depressor mechanism of clonidine]

Intravenous injection (iv) of clonidine significantly induced depressor and bradycardiac responses in rats. These responses were found to be blocked by iv alpha-adrenoreceptor blocker phentolamine, intraperitoneous injection (ip) of naloxone (10mg/kg) or iv antidynorphin-IgG(80 microliters). Furthermore, clonidine (iv) induced a significant increase of urine volume, which was also partly blocked by naloxone (10mg/kg). A dramatic increase of rat plasma immunoreactive atriopeptin induced by clonidine (iv) was partly antagonised by pretreatment with naloxone (4mg/kg) or phentolamine (10mg/kg). These results show that dynorphin and atriopeptin may be involved in the depressor mechanism of clonidine.