Abstract
1. The membrane potential of Purkinje fibers excised from the left ventricle of the dog heart was changed to various potential levels using the partition chamber method.
2. The existence of pacemaker activity of two kinds which are due to different mechanisms was suggested in Purkinje fibers: one is a natural pacemaker activity at high membrane potentials and the other, oscillations at membrane potential below about-60mV.
3. The natural pacemaker activity is strongly dependent on extracellular Na ions but little affected by changes in extracellular Ca ions except for an indirect action resulting from a change in the threshold potential.
4. On the other hand, the spontaneous repetitive discharges (oscillations) at low membrane potential depend markedly on extracellular Ca concentration but only slightly on Na ions.
5. It is postulated that background inward currents, responsible for generating the low voltage oscillation, are carried by both Ca and Na ions which pass through a probable “slow channel, ” which is activated at a membrane potential of about-50mV in normal Tyrode solution.
