The overall aim of this proposal is to determine the mechanism whereby adenosine affects ventricular overdrive suppression. To establish a direct effect of adenosine on overdrive suppression, it is required to demonstrate that adenosine potentiates the pause following cessation of stimulation independent of slowing the spontaneous rate. Preliminary studies in isolated guinea pig hearts have shown that adenosine markedly increases the magnitude of the pause duration when compared to other interventions producing similar degrees of pre-drive rate slowing (e.g., hypothermia). To further elucidate the role of adenosine and its mechanism of action, the following experiments are planned. In isolated perfused guinea pig hearts, reversal of the adenosine effects will be attempted by pulse-pressure injection of specific adenosine antagonists. Since adenosine can cause pacemaker shift, the possibility that the increase in pause duration is due to such effect must be ruled out. Thus, in isolated rabbit Purkinje fibers, the effect of adenosine-induced shift in pacemaker dominance will be investigated with multiple intracellular recordings. Finally, in single isolated porcine Purkinje cells (a preparation that excludes pacemaker shift), the effect of adenosine on post-drive hyperpolarization, slope of phase 4 depolarization, threshold potential, and K+ conductance will be investigated by whole cell recording and voltage clamp with patch electrodes. Expertise in various techniques (cardiac cell isolation, pulse pressure injection, voltage clamp) will be gained while assisting the sponsor to determine the ionic conductances underlying the effect of adenosine in single AV nodal cells. Successful accomplishment of this project will provide further insight into the overall mechanism of ventricular overdrive suppression. Furthermore, the establishment of a role for adenosine may alter the clinical approach to ventricular standstill following ventricular fibrillation, complete heart block, sinus arrest, or demand pacemaker failure during myocardial ischemia where adenosine is produced in large amounts.
