title: Postjunctional vascular smooth muscle alpha-2 adrenoreceptors in human autonomic failure.
abstract: Animal studies have suggested that vasoconstrictor alpha-2 adrenoreceptors exist on vascular smooth muscle cells. We tested this hypothesis in a patient with severe autonomic failure who demonstrated a pressor response to oral clonidine (a selective alpha-2 adrenoreceptor partial agonist). After clonidine 0.8 mg orally, mean arterial pressure rose by 54 mm Hg. After pretreatment with prazosin (a selective alpha-1 adrenoreceptor antagonist) and confirmation of alpha-1 blockade, clonidine 0.8 mg still raised mean arterial pressure by 43 mm Hg. After pretreatment with yohimbine (a selective alpha-2 adrenoreceptor antagonist), clonidine 0.8 mg elevated mean arterial pressure by 13 mm Hg. Since alpha-1 antagonism does not block, and alpha-2 antagonism does not block the pressor effect of clonidine, we conclude that clonidine raised blood pressure in this severely affected autonomic failure patient by vascular alpha-2 adrenoreceptor stimulation. Thus, this provides pharmacological evidence that postjunctional vascular smooth muscle alpha-2 adrenoreceptors exist in man and can modulate blood pressure.
mesh_terms: D000368:Aged; D001342:Autonomic Nervous System Diseases / Q000378:metabolism*; D001792:Blood Platelets / Q000032:analysis; D001794:Blood Pressure; D003000:Clonidine; D004305:Dose-Response Relationship, Drug; D006339:Heart Rate; D006801:Humans; D008297:Male; D009130:Muscle, Smooth / Q000378:metabolism*; D009638:Norepinephrine / Q000097:blood; D011187:Posture; D011224:Prazosin; D011942:Receptors, Adrenergic, alpha / Q000378:metabolism*; D015016:Yohimbine
chemical_list: D011942:Receptors, Adrenergic, alpha; D015016:Yohimbine; D003000:Clonidine; D009638:Norepinephrine; D011224:Prazosin