[Properties of the premature extrasystolic action potentials in the rabbit atrial myocardium]

Using a conventional microelectrode technique premature action potentials of left auricular trabeculae of the rabbit were studied. Varying the delay between a regular stimulus and the following premature stimulus the duration of the premature action potential (A. P.) is decreased during the relative refractory period and increased after this time at each level of the repolarization. Verapamil suppresses completely this augmentation of the A. P. duration. Turning to account the A. P. measurements an estimation of a potassium conductivity gK yields an enlargement of gK during the premature A. P. elevating the Ca+ concentration or prolonging the steady state pacing interval the increase of gK is more accentuated. Low Ca++ solutions and verapamil suppress the augmentation of gK during the premature A.P. lead to a membrane current responsible for the premature changes of the A. P. This current is inward directed at potentials more positive than --30 mV and outward directed at more negative potentials. By application of high-calcium solutions and in slow pacing both the inward and outward component are increased. The findings suggest an augmented Ca++ inward current during the premature action related to the postextrasystolic potentiation.