The hypotensive activity and side effects of methyldopa, clonidine, and guanfacine.

Clonidine (Catapres, Catapresan), guanfacine (Estulic), and methyldopa (Aldomet) are the prototypes of centrally acting antihypertensive drugs. Clonidine and guanfacine are lipophilic drugs that readily penetrate into the brain, where they stimulate alpha-adrenergic receptors in the pontomedullary region. The stimulation of these central alpha-adrenergic receptors has been shown to activate an inhibiting neuron, which causes a reduction of peripheral sympathetic tone and a subsequent fall in arterial blood pressure and heart rate. Both a centrally initiated reduction of vagus reflex activity and the activation of presynaptic alpha 2-adrenergic blocking agents in the heart may contribute to the bradycardia. Studies indicate that methyldopa also penetrates into the brain, where it is converted into alpha-methylnorepinephrine. This amine may stimulate the same central alpha-adrenergic receptors as those activated by clonidine, which will result in a hypotensive effect. Possibly, alpha-methyldopamine might also play a role. Accordingly, the modes of action of clonidine and alpha-methyldopa probably are very similar at a basic level. The central adrenergic receptors probably are located postsynaptically. Their receptor demand corresponds more closely to that of the alpha 2-subtype. Central alpha 1-adrenergic receptors might possibly play a part in the modulation of vagally induced baroreflex bradycardia. A discussion on the pharmacological basis of the side effects of the centrally acting antihypertensives has been limited to those adverse reactions that are somehow related to alpha-adrenergic receptors. Sedation, a common side effect, appears to be mediated by central alpha 2-adrenergic receptors, at least in animal models.(ABSTRACT TRUNCATED AT 250 WORDS)