Mechanisms involved in generation of oscillatory afterpotentials in myocardium

We used ouabain-treated papillary muscles from guinea pig to study oscillatory afterpotentials (OAP) that follow an action potential. Two effects were investigated in more detail. 1. The membrane Na gradient was changed by isoomotical substitution of NaCl with sucrose. 50% Na substitution induced a nearly complete suppression of OAP. 2. Different K+ concentrations were used to compare OAP at high and low resting potentials. Depolarization of the ventricular preparations from about-85 mV (4mM KCl) to -55 mV (22 mM KCl) reversibly reduced OAP. Both effects can be predicted by a model of OAP induced by a transient inward current. This inward current is generated by an electrogenic Na/Ca exchanger. A supposed oscillatory changing of intracellular Ca concentration modifies the exchange current in agreement with the experimental data.