Caffeine alters A(2A) adenosine receptors and their function in human platelets

Background—Caffeine acts mainly via blockade of adenosine receptors, which have been classified into A1, A2A, A2B, and A3 subtypes. We determined whether repeated caffeine administration (750 mg/d for 1 week) upregulates the human platelet A2A adenosine receptor and is accompanied by sensitization of platelet responses (increase in cAMP accumulation and decrease in platelet aggregation) to selective stimulation of the A2A 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 A2A 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 Bmax of 9862 fmol/mg protein and a KD of 1.2960.05 nmol/L. At 12 and 60 hours after caffeine withdrawal, the radioligand bound with similar affinity (KD51.3660.06 and 1.2160.05 nmol/L, respectively), but the Bmax was increased (P,0.01) to 12863 and 13262 fmol/mg protein. The A2A receptor agonist 2-hexynyl-59-N-ethylcarboxamidoadenosine (HE-NECA) increased cAMP accumulation (EC5055963 nmol/L) and inhibited (IC5059066 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 (3163 and 2162 nmol/L, respectively). The IC50 values for HE-NECA to inhibit ADP-induced platelet aggregation were 5065 and 3062 nmol/L at 12 and 60 hours after caffeine withdrawal, respectively