Separation of the oscillatory current from other currents in cardiac Purkinje fibres

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Abstract
An oscillatory current (Ios) can be recorded after depolarising clamps of a given amplitude and duration. To determine whether Ios depends on other currents activated during the clamp, the Ios was investigated in the absence and in the presence of 4-aminopyridine (4-AP) and cesium (Cs). The following results were obtained. 4-AP, as expected, markedly reduced the early outward current Iqr, and this effect was fully reversible. The Ios persisted when Iqr was suppressed by 4-AP and in fact became greater. Also, several procedures (progressive increase in clamp duration, two step repolarisation, repetitive clamps) affected Ios similarly whether Iqr was present or not. Ios does not depend on the plateau current Ix1 since with 4-AP Ios increased while the tail of Ix1 were unaffected. Cesium abolished the pacemaker current but not Ios, demonstrating that Ios does not result from an oscillatory behaviour of the pacemaker current. Often Cs reduced the amplitude of Ios: prolonging the depolarising clamp increased Ios but did not restore the pacemaker current. It is concluded that the oscillatory current is independent of either the early outward current Iqr or the pacemaker current. Also, while the action of 4-AP and Cs on Iqr and the pacemaker current are confirmed, it appears doubtful that these two agents act specifically on these two currents alone.