EXISTENCE AND ROLE OF A SLOW INWARD CURRENT DURING FROG ATRIAL ACTION POTENTIAL

Voltage clamp experiments performed on frog atrial heart muscle fibers show: 1. a rapid inward current, time and voltage dependent and inhibited by TTX, consequently a sodium current. This current is responsible for the first part of the ascending phase of the action potential. 2. a slow inward current, time and voltage dependent, not influenced by TTX but inhibited by Mn. In sodium-free solution this current is carried by calcium ions and in calcium-free solution it is a sodium current. In normal conditions this slow current is carried by both sodium and calcium ions. We propose to call this pathway for slow current slow channel". We think that it is probably at this level that the competition between sodium and calcium occurs. The slow current is responsible for the second part of the ascending phase and plays an important role during the plateau and the repolarization of the action potential. These results can explain the time course of the frog atrial action potential. © 1969 Springer-Verlag."