[Modification of the effect of cardio-accelerator nerve stimulation in dogs by clonidine and several...

In dogs anaesthetized with pentobarbital (30 mg. kg -1 i.v.), clonidine (0,01 mg.kg-1 i.v.) reduced the tachycardia induced at low frequencies by stimulation of the cardiac nerve. The effects of some alpha-adrenoceptor blocking agents on this effect have been studied. Small doses of yohimbine (0.3 mg. kg-1 i.v.) or piperoxan (0.3 mg. kg-1 i.v.) increased the effects of the stimulation and in addition antagonized the inhibitory effects of clonidine and reversed the pressor response to adrenaline. Thymoxamine (1 mg.kg-1 i.v.) and prazosin (1 mg.kg-1 i.v.) did not increase the effect of the stimulation of the cardiac nerve, but reduced the effect of clonidine. ARC239 (0.05 mg.kg-1) reversed the pressor response to adrenaline but even at high doses did not increase the effects of the stimulation of the cardiac nerve or the effects of clonidine. These observations afford further evidence for a dissimilarity between pre and post-synaptic alpha-adrenoceptors.