Does a calcium-mediated outward current exist in the atrial myocardium?

Using a microelectrode technique the influence of experimental conditions which are known to alter the intracellular free Ca concentration upon the repolarization phase in the rabbit atrial myocardium was studied. Under all the tested conditions (increased external Ca concentration, application of ouabain, decreased external K concentration) a positive inotropic effect was observed. The action potentials showed a rapid initial repolarization. The positive inotropic effect was found to be well correlated with the effect of the action potential's shortening at 25% repolarization. After pacing pauses a positive correlation between the velocity of the initial rapid repolarization and the potentiation of the atrial contraction was established. It is discussed that all the conditions were able to increase the internal Ca concentration and that in consequence of it a CA-sensitive outward current at potentials about 40 mV more positive than the resting potential is activated.