Figure 715

Central (^-adrenergic agonists. Central (^-adrenergic agonists cross the blood-brain barrier and stimulate (^-adrenergic receptors in the vasomotor center of the brain stem [6,9]. Stimulation of these receptors decreases sympathetic tone, brain turnover of norepinephrine, and central sympathetic outflow and activity of the preganglionic sympathetic nerves. The net effect is a reduction in norepinephrine release. The central ( 2-adrenergic agonist clonidine also binds to imidazole receptors in the brain; activation of these receptors inhibits central sympathetic outflow. Central ( 2-adrenergic agonists may also stimulate the peripheral ( 2-adrenergic receptors that mediate vasoconstriction; this effect predominates at high plasma drug concentrations and may precipitate an increase in blood pressure. The usual physiologic effect is a decrease in peripheral resistance and slowing of the heart rate; however, output is either unchanged or mildly decreased. Preservation of cardiovascular reflexes prevents postural hypotension.


Generic (trade) name

First dose, mg

Usual daily dose

Maximum daily dose

Duration of action

a-Methyldopa (G) (Aldomet)

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