External pH dependency of delayed afterdepolarization in rabbit myocardium

Effects of changing external pH on delayed afterdepolarization (DAD) induced by a train of electrical stimuli in the rabbit ventricular papillary muscle fibers were examined using a conventional microelectrode technique in vitro. Preparations were superfused with Tyrode solution containing norepinephrine (3 X 10(-6) M), and the external pH was decreased. After a train of driven action potentials, DAD was induced at pH 7.0 in the presence of norepinephrine. The amplitude of the DAD induced by norepinephrine at pH 7.0 increased as the stimulus cycle length was shortened and as the number of applied stimuli increased. The increase in the amplitude of DAD resulted in yielding one or multiple nondriven action potentials. DAD, thus initiated, was completely suppressed by a further decrease of external pH to 6.7-6.0. Lidocaine and adriamycin depressed the amplitude of DAD, but verapamil did not. The results imply that only slight lowering of the external pH induced by ischemic conditions or by other means would provoke DAD which, in turn, would result in triggered activity in situ.