The role of adenosine action in modulating the action potential duration (APD) of single isolated guinea-pig atrial myocytes during β-adrenergic stimulation with isoproterenol was investigated. Membrane potential and ionic currents were measured by the tight-seal, whole-cell patch-clamp technique. Adenosine (10μM) antagonized isoproterenol (20 nM)-stimulated L-type calcium inward current (ICa). In the presence of isoproterenol, adenosine (10μM) also increased holding and late outward currents. Isoproterenol (20 nM) caused a significant prolongation in atrial action potential at the level of 0 mV, rather than at 90% repolarization. Adenosine shortened atrial action potential dose-dependently in the presence of isoproterenol. The respective values for EC50 and maximally shortened action potential duration at 0mV of adenosine with or without isoproterenol (20 nM) were 3.0 vs. 2.2μM and 42.9 vs. 30.4 ms. In summary, during β-adrenergic stimulation with isoproterenol, in guinea-pig atrial myocytes, adenosine elicited the specific K+-outward current and concomitantly attenuated the isoproterenol-stimulated ICa. These phenomena may account for the modulation of atrial action potenital by adenosine with or without isoproterenol effect.
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