WILLIAM J. PARSONS, M.D.; GARY L. STILES, M.D.
To the editor: In the recent report concerning theophylline toxicity and the beta-adrenergic receptor system by Kearney and associates (1), theophylline and caffeine are referred to as "adrenergic agonists." We believe this assertion warrants clarification.
Methylxanthines, such as theophylline and caffeine, have long been known to inhibit phosphodiesterase activity. Although this was previously thought to be their primary mode of action, recent data suggest that the physiologic effects of methylxanthines are medicated principally through antagonism of adenosine receptors (2). Adenosine receptors are effectively blocked by caffeine at micromolar concentrations (observed clinically), whereas millimolar concentrations are required to inhibit phosphodiesterase
PARSONS WJ, STILES GL. Methylxanthines as Adenosine Receptor Antagonists. Ann Intern Med. ;103:643. doi: 10.7326/0003-4819-103-4-643_1
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Published: Ann Intern Med. 1985;103(4):643.
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