Characteristics of adenosine binding sites in atrial sarcolemmal membranes

The studies reported here involve an exploration of the sites on atrial myocyte membranes with which adenosine interacts to produce its potent physiological effects in atrial muscle. Specific, high affinity binding of the stable adenosine analogs 2-chloro[3H]adenosine (2-ClAdo) and [3H]adenosine 5'-N-ethylcarboxamide (NECA) to atrial sarcolemmal membranes was measured in kinetic and equilibrium studies at 4 degrees C and 35 degrees C. Analysis of the [3H]2-ClAdo binding isotherm indicated the presence of two classes of binding site with equilibrium Kassoc values estimated to be 5.7 X 10(7) M-1 and 2.7 X 10(6) M-1. Displacement of bound [3H]2-ClAdo by adenosine 5'-N-cyclopropylcarboxamide (NCPCA) and by several N6-substituted adenosine analogs confirmed the presence of two classes of binding site. Analysis of the [3H]NECA binding also revealed the presence of two types of binding site for this ligand. The methylxanthines isobutylmethylxanthine and theophylline displaced bound [3H]2-ClAdo whereas adenosine uptake inhibitors and several other purines showed little activity. These atrial membrane binding sites exhibit many of the characteristics of the physiological adenosine receptors studied in intact atria. Furthermore, the [3H]2-ClAdo binding sites were sensitive to treatment with proteolytic enzymes, suggesting that these sites exist on sarcolemmal membrane proteins.