Induction of the oscillatory current by low concentrations of caffeine in sheep cardiac Purkinje fibres

Summary The actions of low concentrations of caffeine (0.5–2 mmol/1) on the transient inward oscillatory current (I_os) and the inward tail current (I_ex) were studied in sheep cardiac Purkinje fibres by means of a two microelectrode voltage clamp method. The following results were obtained. Caffeine: 1. induced an I_os when this current was not already present; 2. increased the amplitude (within limits) and consistently decreased the time to peak of an already present I_os; 3. increased I_ex upon which I_os may be superimposed; 4. shifted the depolarizing threshold for the appearance of I_os to more negative and the repolarizing threshold to less negative values; 5. increased the effects of strophanthidin of I_os and I_ex; 6. exaggerated the effects of high Ca]_o which by itself mimicked some of the actions of caffeine; 7. had a small effect of I_os and I_ex in low Ca]_o; 8. reversed the effect of norepinephrine on I_ex; 9. enhanced the effects of trains of clamps and of longer clamp steps on I_os and I_ex. It is concluded that low concentrations of caffeine facilitate the manifestations of calcium overload thereby inducing or exaggerating the oscillatory and tail currents and that these effects are modulated by the cellular calcium load but are not mediated through adrenergic mechanisms. Send offprint requests to M. Vassalle at the above address