Centrally acting alpha-adrenoceptor agonists in hypertension: mechanisms and their role in therapy.

Experimental work in animals has shown that both methyldopa and clonidine lower the blood pressure predominantly by an action on the central nervous system. This action is due to alpha-adrenoceptor stimulation by clonidine and by alpha-methyl-noradrenaline. The evidence for a central site of action for these drugs in man is indirect but persuasive. Both drugs lower blood pressure and heart rate without causing much postural or exercise hypetension. Sympathetic responses like the overshoot of pressure following Valsalva's manoeuvre are reduced but not abolished. Failure of ejaculation in the male is much less common than with drugs that are known to blockade sympathetic adrenergic neurones in the periphery.