The dependence of plateau currents in cardiac Purkinje fibres on the interval between action potentials.

1. The influence of diastolic interval on ionic currents that may determine the action potential duration in cardiac Purkinje fibres was investigated. As the diastolic interval is shortened from about 5 sec, the first effect on the action potential is to reduce and then abolish the notch at the beginning of the plateau.2. This effect corresponds to the influence of diastolic interval on the magnitude of a transient outward chloride current known as the ;dynamic current'.3. Further shortening of the diastolic interval produces a slight shortening of the action potential until intervals less than about 500 msec are used. The action potential then becomes considerably shorter. The ;time constant' of decay of this major influence of one action potential on the duration of the subsequent action potential is about 200 msec.4. This effect corresponds to the time course of decay of an outward (mainly K) current known as i(x1).5. It is shown that variations in the magnitude of i(x1) may be responsible for the alternation in action potential duration at the beginning of a train of stimuli known as ;electrical alternans'.6. The results in general are consistent with the view that i(x1) is the main current involved in determining the interval-duration relation although they cannot exclude the possibility that an inward current with a reavailability time course similar to the decay time course of i(x1) might also be involved.