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 A(1), A(2A), A(2B), and A(3) subtypes, We determi ned whether repeated caffeine administration (750 mg/d for 1 week) upregula tes the human platelet A(2A), adenosine receptor and is accompanied by sens itization 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) a nd at 12 and 60 hours after the last dose of caffeine (withdrawal), The A,, receptor radioligand [H-3]SCH 58261 {5-amino-7(phenylethyl)-2-(2-furyl)-py razolo[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 f mol/mg protein and a K-D of 1.29+/-0.05 nmol/L, At 12 and 60 hours after ca ffeine 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,, receptor agonist 2-he xynyl-5'-N-ethylcarboxamidoadenosine (HE-NECA) increased cAMP accumulation (EC50=59+/-3 nmol/L) and inhibited (IC50=90+/-6 nmol/L) aggregation of cont rol 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 withdrawa l (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 1 2 and 60 hours after caffeine withdrawal, respectively. Conclusions-Chronic caffeine intake leads to upregulation of A,, receptors and is accompanied by sensitization to the actions of the agonist HE-NECA.