[Pacing-interval-dependent changes of action potentials in the heart atrium of the warm-blooded animal]

Investigated are the cellular post rest action potentials of the rabbit atrial trabeculae (length 2-4 mm, diameter 150-800 micrometers). The first action potential (A.P.) after a pacing pause develops a characteristic configuration: (1) a rapid early repolarization up to -30 to -60 mV within 4 to 8 ms, (2) an extremely decelerated repolarization phase or a secondary depolarization, (3) a prolongation of the A. P. duration at 90% repolarization. During Ca++ excess perfusion the secondary depolarization is increased, shifted to more positive potentials and an earlier maximum value. It disappears during zero Ca++ and verapamil perfusion. Elevating the Ca++ concentration and shortening the pacing intervals before the period of rest increases the A. P. duration at 90% repolarization. Prolonging the pacing pause and decreasing the external Ca++ concentration delays the restitution of the steady state A. P. configuration after the rest By Ca++ excess the post rest A. P. restitution is accelerated. To explain the experimental findings, the possible role of a Ca++-sensitive K+ conductivity a poststimulatory variation of the slow inward current and the time-dependent K+ currents as well as processes of accumulation are discussed.