Adenosine binding sites in pig ventricular sarcolemma and interaction with calcium channels

Two classes of adenosine binding sites were identified in pig ventricular sarcolemma using 5'-N-[3H]ethylcarboxamideadenosine ([3H]NECA) as radioligand (Bmax = 88 fmol/mg protein, Kd = 28 nM; Bmax = 7100 fmol/mg protein, Kd = 730 nM). Competition experiments indicated that the higher affinity group had the pharmacological characteristics of the A2 adenosine receptors. The lower affinity binding sites may correspond to the A3 adenosine receptors demonstrated in rat brain. Results with the calcium antagonist [3H]nitrendipine suggest that adenosine exerts some of its effects on the heart also through a direct binding to specific calcium-channel subtypes.