BURSTING BEHAVIOR DURING FIXED-DELAY STIMULATION OF SPONTANEOUSLY BEATING CHICK HEART CELL AGGREGATES

Spontaneously beating embryonic chick atrial heart cell aggregates wer e stimulated with depolarizing current pulses delivered at a fixed del ay after each action potential. This preparation is an experimental mo del of a reentrant tachycardia. During fixed-delay stimulation, bursti ng behavior was typically observed for a wide range of delays. Episode s of bursting at a rate faster (slower) than control were followed by overdrive suppression (underdrive acceleration). We use a simple nonli near model, based on the interaction between excitability and overdriv e suppression, to describe these dynamics. A modified version of the S hrier-Clay ionic model of electrical activity of the embryonic chick h eart cell aggregates that includes a simplified Na+ pump term is also presented. We show that the complex patterns during fixed-delay stimul ation arise as a result of delicate interactions between overdrive sup pression and phase resetting, which can be described in terms of the u nderlying ionic mechanisms. This study may provide a basis for underst anding incessant tachycardias in the intact heart, as well as an alter native mechanism for the emergence of bursting activity in other biolo gic tissue.