Aim: 

The aim of the present study was to give a parametric description of the most important potassium currents flowing during canine ventricular action potentials.

Methods: 

Our experiments were carried out on enzymatically isolated canine cardiomyocytes. Inward rectifier potassium current (I_K1), rapid delayed rectifier potassium current (I_Kr), and transient outward current (I_to) were measured under action potential clamp conditions using BaCl₂, E-4031, and 4-aminopyridine, respectively.

Results: 

The maximum amplitude of I_to was 3.0±0.23 pA/pF and its integral was 29.7±2.5 fC/pF. The current peaked 4.4±0.7 ms after the action potential upstroke and rapidly decayed to zero with a time constant of 7.4±0.6 ms. I_Kr gradually increased during the plateau, peaked 7 ms before the time of maximum rate of repolarization (V^-_max) at -54.2±1.7 mV, had peak amplitude of 0.62±0.08 pA/pF, and integral of 57.6±6.7 fC/pF. I_K1 began to rise from -22.4±0.8 mV, peaked 1 ms after the time of V^-_max at -58.3±0.6 mV, had peak amplitude of 1.8±0.1 pA/pF, and integral of 61.6±6.2 fC/pF. Good correlation was observed between peak I_K1 and V^-_max (r=0.93) but none between I_Kr and V^-_max. Neither I_K1 nor I_Kr was frequency-dependent between 0.2 and 1.66 Hz. Congruently, I_Kr failed to accumulate in canine myocytes at fast stimulating rates.

Conclusion: 

Terminal repolarization is dominated by I_K1, but action potential duration is influenced by several ion currents simultaneously. As Ito was not active during the plateau, and neither I_K1 nor I_Kr was frequency-dependent, other currents must play important role in the frequency dependence of action potential duration.