DEEXCITATION OF CARDIAC-CELLS

Excitation and deexcitation are fundamental phenomena in the electroph ysiology of excitable cells. Both of them can be induced by stimulatin g a cell with intracellularly injected currents. With extracellular st imulation, deexcitation was never observed; only cell excitation was f ound. Why? A generic model with two variables (FitzHugh) predicts that an extracellular stimulus can both excite the cell and terminate the action potential (AP). Our experiments with single mouse myocytes have shown that short (2-5 ms) extracellular pulses never terminated the A P. This result agrees with our numerical experiments with the Beeler-R euter model. To analyze the problem, we exploit the separation of time scales to derive simplified models with fewer equations. Our analysis has shown that the very specific form of the current-voltage (I-V) ch aracteristics of the time-independent potassium current (almost no dep endence on voltage for positive membrane potentials) is responsible he re. When the shape of the I-V characteristics of potassium currents wa s modified to resemble that in ischemic tissues, or when the external potassium concentration (K-0) is increased, the AP was terminated by e xtracellular pulses. These results may be important for understanding the mechanisms of defibrillation.