[Studies on the mechanism of action of clonidine (author's transl)]

The effects of 3.7-120 microgram 2-(2,6-dichloro-anilino)-2-imidazoline (clonidine, Catapresan)/kg i.v. on the arterial blood pressure and the central nervous activities of sympathetic and phrenic nerves were studied in 42 anaesthetized cats which had been relaxed and artificially respirated. At the dosage 3.7-30 microgram/kg clonidine produced a moderate fall of blood pressure and a more distinct sympathetic inhibitory effect. At a dosage of 60-120 microgram/kg an increase of the hypotensive action was not observed whereas the central nervous inhibition increased with the dosage. The activity of the respiratory centre remained constant. The pressor effects as well as the sympathetic stimulation effects of nicotine, nicetamide or asphyxia were reduced by the treatment with clonidine. At a dosage of 3.7-30 microgram clonidine/kg, i.e. including the 6fold therapeutic dosage, the inhibition of the evoked sympathetic activity was, however, considerably weaker than the inhibition of the sympathetic activity at rest. Therefore clonidine at a therapeutic dosage probably causes an activation of the central part of the pressor-receptor reflexes and as a result a decrease of the central sympathetic activity and the moderate fall in blood pressure. At a higher dosage the absence of increasing hypotensive effect in spite of increasing sympathetic inhibition is probably caused by a stimulation of peripheral postsynaptic alpha-adrenoreceptors.