Evidence of Direct Activation of Adenosine A₁ Receptor by 5'-Adenosine Monophosphate in Isolated Guinea Pig Atrial Myocytes

The electrophysiological effect of 5'-adenosine monophosphate (AMP) was examined in isolated atrial myocytes of guinea pig. Membrane potential and ionic currents were measured by the tight-seal, whole-cell patch-clamp technique. AMP caused the shortening of atrial action potential in a dose-dependent manner. In voltage-clamp experiments, AMP (3-10 μM) caused the activation of I_KACh as well as the decrease in basal I_Ca. Prolongation of action potential duration caused by isoproterenol (20nM) was antagonized by AMP (10 μM). Isoproterenol (20 nM) occasionally caused the sustained rhythmic activity and the subsequent application of AMP (10 fiM) terminated it. AOPCP (10 μM), which inhibits 5'-nucleotidase and hence prevents breakdown of AMP, did not significantly attenuate the effect of AMP on shortening action potential. However, the application of adenosine deaminase (1 U/ml), which deaminates adenosine to inosine, partially reversed the shortening of action potential caused by AMP (10 μM). These results indicate that (1) AMP could activate I_KACh and decrease basal I_Ca simultaneously, and antagonize the isoproterenol-stimulated action potential prolongation; and (2) the observed electrophysiological effect of AMP in whole-cell preparation is attributed to AMP per se as well as its degradative product, adenosine.