Pacing Dependence of the Slow Inward Current in Frog Atrial Myocardium

By means of an improved double sucrose gap technique the dependence of the slow inward current in small frog atrial trabeculae on the activation pattern was studied. Increasing the frequency of depolarizing voltage steps (pacing) the steady state slow inward current (TTX resistent and blocked by verapamil) was decreased. After a 300 s resting interval the slow inward current was increased but declined due to reactivation with frequencies greater than 1 Hz. If the duration of the first post pause depolarization was prolonged the slow inward current activated by a second depolarization was decreased. The voltage-steady state inactivation relationship of the slow inward current was shifted to more negative potentials due to increase in the pacing rate. In contrast to the onset of inactivation the recovery from inactivation was found to be insensitive to variation in the pacing pattern. The onset and extent of inactivation was pronounced by increasing the pacing rate. The strong pacing dependence of the slow inward current seems to be mediated by a pacing dependent inactivation.