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(Circulation. 1999;99:2499-2502.)
© 1999 American Heart Association, Inc.

Brief Rapid Communications

Caffeine Alters A_2A Adenosine Receptors and Their Function in Human Platelets

Katia Varani, PhD; Francesco Portaluppi, MD; Stefania Merighi, MSc; Ennio Ongini, PhD; Luiz Belardinelli, MD; Pier Andrea Borea, PhD

From the Department of Clinical and Experimental Medicine, University of Ferrara (K.V., F.P., S.M., P.A.B.), and Schering-Plough Research Institute, San Raffaele Science Park, Milan (E.O.), Italy; and the Department of Medicine, Division of Cardiology, University of Florida, Gainesville (L.B.).

Correspondence to Prof Pier Andrea Borea, Department of Clinical and Experimental Medicine, University of Ferrara, Via Fossato di Mortara 17-19, 44100 Ferrara, Italy. E-mail bpa{at}dns.unife.it

Abstract

Background—Caffeine acts mainly via blockade of adenosine receptors, which have been classified into A₁, A_2A, A_2B, and A₃ subtypes. We determined whether repeated caffeine administration (750 mg/d for 1 week) upregulates the human platelet A_2A adenosine receptor and is accompanied by sensitization of platelet responses (increase in cAMP accumulation and decrease in platelet aggregation) to selective stimulation of the A_2A receptors.

Methods and Results—Platelets were obtained from peripheral venous blood of 9 human volunteers at the end of 1 week of caffeine abstinence (control) and at 12 and 60 hours after the last dose of caffeine (withdrawal). The A_2A receptor radioligand [³H]SCH 58261 {5-amino-7(phenylethyl)-2-(2-furyl)-pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c]-pyrimidine} bound to a single affinity class of sites in platelet membranes from controls with a B_max of 98±2 fmol/mg protein and a K_D of 1.29±0.05 nmol/L. At 12 and 60 hours after caffeine withdrawal, the radioligand bound with similar affinity (K_D=1.36±0.06 and 1.21±0.05 nmol/L, respectively), but the B_max was increased (P<0.01) to 128±3 and 132±2 fmol/mg protein. The A_2A receptor agonist 2-hexynyl-5'-N-ethylcarboxamidoadenosine (HE-NECA) increased cAMP accumulation (EC₅₀=59±3 nmol/L) and inhibited (IC₅₀=90±6 nmol/L) aggregation of control platelets. The EC₅₀ values for HE-NECA to increase cAMP accumulation of platelets were reduced (P<0.01) at 12 and 60 hours after caffeine withdrawal (31±3 and 21±2 nmol/L, respectively). The IC₅₀ values for HE-NECA to inhibit ADP-induced platelet aggregation were 50±5 and 30±2 nmol/L at 12 and 60 hours after caffeine withdrawal, respectively.

Conclusions—Chronic caffeine intake leads to upregulation of A_2A receptors and is accompanied by sensitization to the actions of the agonist HE-NECA.

Key Words: adenosine • receptors • caffeine • platelets • platelet aggregation inhibitors

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