Analysis of cardiac pacemaker potential using a "voltage clamp" technique

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Abstract
Diastolic depolarization was studied by means of "voltage clamp" technique in short mammalian Purkinje fibers. Clamping the membrane potential at the maximal diastolic value resulted in an inward current rising with a time constant of 3 sec in 5.4 mM K, indicating that slow depolarization is essentially a time-dependent process. Membrane slope resistance during clamp increased as a function of time, suggesting a decrease in K conductance rather than an increase in Na conductance. Clamping the membrane to the calculated K equilibrium potential resulted in a steady inward current; at more negative clamps the inward current decreased with time. The latter findings were obtained also in Na-free solution, indicating that a fall in the activity of an electrogenic Na pump does not play a major role in the diastolic depolarization process. Diastolic depolarization can be initiated at any time during the plateau by clamping to the maximal diastolic potential. The threshold for all-or-nothing repolarization was determined during the plateau and found to follow the expected time course.

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