Quantitative analysis of cardiac electrical restitution.

Electrical restitution (ER) of cardiac cells is an aggregate of events that rhythmically restore the initial conditions of electric signal (action potential) generation. Its analysis represents an important insight into cardiac arrhythmogenesis. The aim of this work is to theoretically substantiate and verify a novel approach allowing for the quantification of the individual ionic current components of ER. A method of analysis of the primary, initial conditions-setting restitution processes (apart from the secondary, test pulse-affected ones) is proposed. Both processes are described as sums of their measurable constituents. It is demonstrated that the optimum parameter of ER is the electric charge that is transferred through ionic channels and carriers during the test impulse. The theory was tested by using voltage-clamped canine ventricular preparations and by computer simulations. The experimental ER curve of canine ventricular muscle was constructed using action potential (AP) plateau voltage and half-repolarization time as parameters. At 30 degrees C and 0.5 Hz stimulation, the ER curve peaked, on average, after 400 ms with a 10% overshoot. Of this plateau elevation, 50% was due to 4-aminopyridine-sensitive transient outward current and 44% was due to verapamil-sensitive current. The delayed outward current antagonized the overshoot by about 6%. It was found that the initial conditions (i.e. the primary restitution processes) tend to strongly alter the plateau voltage of the premature AP. However, the final deviation is by about one order less. It is concluded that the voltage-dependent secondary processes counteract the effect of the primary processes, thereby suggesting strong negative feedback control of natural APs.