[The effect of clonidine and guanfacine on blood pressure]

Imidazoline derivatives, clonidine and guanfacine, were reviewed with respect to their effect on blood pressure. Excitation of the central alpha-adrenoceptors induced by clonidine or guanfacine produced a simultaneous decrease in peripheral sympathetic tone, hypotension and bradycardia. Both ponto-medullary regions and the hypothalamus are important action sites for these two drugs. With respect to the effects of clonidine or guanfacine on central alpha-adrenoceptors, two possibilities were investigated. First, we presumed that the excitation of central alpha-adrenoceptors induced by these two drugs appears to be located at postsynaptic sites. Thus, the alpha 2-subtype was considered. Secondarily, stimulation of the central presynaptic alpha 2 receptor produced by clonidine or guanfacine could not be excluded completely. It has been reported that after abrupt cessation of guanfacine, a withdrawal syndrome infrequently occurs which is moderate as compared with that caused by the abrupt cessation of clonidine. In comparison with clonidine, the characteristics of guanfacine pharmacokinetics, including its large volume of distribution, long biological half life and long duration of action, appear to be associated with the infrequent occurrence of withdrawal syndrome after guanfacine cessation.