A 68-year-old man presents with a complaint of light-headedness on standing that is worse after meals and in hot environments. Symptoms started about 4 years ago and have slowly progressed to the point that he is disabled. He has fainted several times, but always recovers consciousness almost as soon as he falls. Review of symptoms reveals slight worsening of constipation, urinary retention out of proportion to prostate size, and decreased sweating. He is otherwise healthy with no history of hypertension, diabetes, or Parkinson's disease. Because of his urinary retention, he was placed on the α1 antagonist tamsulosin but he could not tolerate it because of worsening of orthostatic hypotension. Physical examination revealed a blood pressure of 167/84 mm Hg supine and 106/55 mm Hg standing. There was an inadequate compensatory increase in heart rate (from 84 to 88 bpm), considering the degree of orthostatic hypotension. Physical examination is otherwise unremarkable with no evidence of peripheral neuropathy or parkinsonian features. Laboratory examinations are negative except for plasma norepinephrine, which is low at 98 pg/mL (normal is 250–400 pg/mL for his age). A diagnosis of pure autonomic failure is made, based on the clinical picture and the absence of drugs that could induce orthostatic hypotension and diseases commonly associated with autonomic neuropathy (eg, diabetes, Parkinson's disease). What precautions should this patient observe in using sympathomimetic drugs? Can such drugs be used in his treatment?
The sympathetic nervous system is an important regulator of virtually all organ systems. This is particularly evident in the regulation of blood pressure. As illustrated in the case study, the autonomic nervous system is crucial for the maintenance of blood pressure even under relatively minor situations of stress (eg, the gravitational stress of standing).
The ultimate effects of sympathetic stimulation are mediated by release of norepinephrine from nerve terminals, which then activates adrenoceptors on postsynaptic sites (see Chapter 6). Also, in response to a variety of stimuli such as stress, the adrenal medulla releases epinephrine, which is transported in the blood to target tissues. In other words, epinephrine acts as a hormone, whereas norepinephrine acts as a neurotransmitter.
Drugs that mimic the actions of epinephrine or norepinephrine have traditionally been termed sympathomimetic drugs. The sympathomimetics can be grouped by mode of action and by the spectrum of receptors that they activate. Some of these drugs (eg, norepinephrine and epinephrine) are direct agonists; that is, they directly interact with and activate adrenoceptors. Others are indirect agonists because their actions are dependent on their ability to enhance the actions of endogenous catecholamines. These indirect agents may have either of two different mechanisms: (1) they may displace stored catecholamines from the adrenergic nerve ending (eg, the mechanism of action of tyramine), or they may decrease the clearance of released norepinephrine either by (2a) inhibiting reuptake of catecholamines already released (eg, the mechanism of action of cocaine and tricyclic antidepressants) or (2b) preventing the enzymatic metabolism of norepinephrine ...