Potentiation of slow action potentials with theophylline or "micro" adenosine deaminase

The effects of endogenous adenosine on rat atrial and ventricular slow action potentials (AP) were studied using theophylline, an adenosine receptor antagonist, or "micro" adenosine deaminase (mADA), small polypeptides having adenosine deaminase activity. Exogenous adenosine (10(-6) M) depressed slow APs at low and high isoproterenol concentrations and shifted the isoproterenol dose-response curve to the right in the atrium. In the ventricle, exogenous adenosine inhibited slow APs at low isoproterenol doses and only shifted the bottom of the dose-response relationship to the right. mADA (0.84 U) or theophylline (5 X 10(-5) M) potentiated the response to threshold concentrations of isoproterenol and caused a parallel shift of the curve to the left in the atrium but only shifted the bottom portion of the curve in the ventricle. This potentiation of slow APs in the presence of mADA or theophylline suggests that endogenous adenosine attenuates the response to isoproterenol in cardiac muscle.