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Effects of high frequency (HF) stimulation on membrane potential were examined in ventricular muscle and Purkinje fibers, which were isolated from canine and rabbit hearts, by using microelectrode techniques. When the stimulating frequency of preparations was increased abruptly from basic frequency (BF, 0-30/min) to high frequency (HF, 60-150/min), the maximum diastolic potential (MDP) of both ventricular muscle and Purkinje fibers decreased with each beat and reached a minimal value (initial depolarization). Then, MDP gradually increased beyond the steady state level at BF (hyperpolarization). On returning the stimulating frequency from HF to BF, a further terminal hyperpolarization ensued, and finally MDP decayed back (both depolarization and hyperpolarization) was apparently greater in ventricular muscle than in Purkinje fibers. A significant increase of threshold current for excitation and a slight increase of membrane input resistance were also observed at the hyperpolarized period after HF stimulation. Lowering the temperature of the perfusate from 37 degrees C (control) to 31 degrees C or 26 degrees C led to an augmentation of initial depolarization and also a decrease in the intensity of ensuing hyperpolarization. The decay time of the hyperpolarization after HF stimulation was much prolonged. In the presence of ouabain, HF stimulation caused a monotonic decline in MDP and no hyperpolarization was recorded during and after HF stimulation. These results suggest that an active ion transport through the sodium pump contributes significantly to the electrical properties of ventricular muscle as well as the Purkinje fibers when the firing rate of the fibers is abruptly changed.

Effects of high frequency stimulation on the membrane potential of isolated ventricular muscle and Purkinje fibers.