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Department of Physiology, Yale University School of Medicine, 333 Cedar Street, New Haven, CT 06510, U.S.A.

1. Calf cardiac Purkinje fibres were exposed briefly to the ionophore nystatin to promote exchange of caesium for intracellular potassium. The effects of Cs loading were stable for at least 30 min, but they could be reversed by nystatin-mediated K loading.

2. After Cs loading, the resting potential shifted to about -20 mV and the current—voltage relationship showed a strong inhibition of inwardly rectifying K channels.

3. Anodal break stimulation evoked normal action potential upstrokes and twitch contractions. The early repolarization (phase 1) was markedly slowed.

4. Cs loading simplified the pattern of current changes evoked by step depolarizations over the plateau range. Membrane current reached an inward peak and then declined monotonically.

5. The current signal showed no hint of the transient outward current found in untreated or K-loaded preparations. Furthermore, Cs loading abolished the outward tails associated with deactivation of transient outward current, and occluded the blocking effect of the K-channel inhibitor 4-aminopyridine.

6. Inhibition of transient outward current revealed a maximal inward current of about 5 µA/µF in 5·4 mM-Ca_o, which is considerably larger than the net inward current without Cs loading.

7. The inward current was attributed to Ca channels on the basis of its sensitivity to membrane potential, extracellular Ca, D600, Mn and Cd.

8. Cs loading also reduced slow current changes associated with delayed rectification and pace-maker depolarization.

9. The results support the hypothesis that the transient outward current is carried by K⁺ ions, while providing a method for unmasking inward Ca current.