The Pharmacology of Autonomic Failure: From Hypotension to Hypertension

Primary neurodegenerative autonomic disorders are characterized clinically by loss of autonomic regulation of blood pressure. The clinical picture is dominated by orthostatic hypotension, but supine hypertension is also a significant problem. Autonomic failure can result from impairment of central autonomic pathways (multiple system atrophy) or neurodegeneration of peripheral postganglionic autonomic fibers (pure autonomic failure, Parkinson's disease). Pharmacologic probes such as the ganglionic blocker trimethaphan can help us in the understanding of the underlying pathophysiology and diagnosis of these disorders. Conversely, understanding the pathophysiology is crucial in the development of effective pharmacotherapy for these patients. Autonomic failure patients provide us with an unfortunate but unique research model characterized by loss of baroreflex buffering. This greatly magnifies the effect of stimuli that would not be apparent in normal subjects. An example of this is the discovery of the osmopressor reflex: ingestion of water increases blood pressure by 30-40 mm Hg in autonomic failure patients. Animal studies indicate that the trigger of this reflex is related to hypo-osmolality in the portal circulation involving transient receptor potential vanilloid 4 receptors. Studies in autonomic failure patients have also revealed that angiotensin II can be generated through noncanonical pathways independent of plasma renin activity to contribute to hypertension. Similarly, the mineralocorticoid receptor antagonist eplerenone produces acute hypotensive effects, highlighting the presence of non-nuclear mineralocorticoid receptor pathways. These are examples of careful clinical research that integrates pathophysiology and pharmacology to advance our knowledge of human disease.