MECHANISM OF OSCILLATORY ACTIVITY AT LOW MEMBRANE POTENTIALS IN CARDIAC PURKINJE FIBRES

1. The mechanism of oscillations at low membrane potentials in Purkinje fibres has been investigated using voltage clamp experiments. 2. The oscillations are generated by time‐dependent variations in an outward current component, ix1, that is activated over the voltage range ‐40 to 10 mV. During normal activity, this current is responsible for initiating full repolarization to the resting potential (Noble & Tsien, 1969b) so that the oscillations represent a failure of the normal repolarization process, probably as a consequence of a small change in background (leakage) current. 3. These oscillations are distinct from the normal pacemaker activity of Purkinje fibres which is generated by a separate time‐dependent current, iK2 (Noble & Tsien, 1968). iK2 shows no time‐dependence when the membrane potential variations are entirely positive to ‐65 mV and cannot, therefore, be involved in the oscillatory activity apart from contributing a background outward current. 4. The amplitude and frequency of the oscillations are very sensitive to applied currents less than 1 μA/cm2. Larger currents abolish the oscillatory activity. 5. The mechanism of the oscillations is discussed in relation to the possible mechanisms underlying the natural pacemaker activity of the sino‐atrial (SA) node. © 1969 The Physiological Society