Effects of nystatin‐mediated intracellular ion substitution on membrane currents in calf Purkinje fibres

Abstract

Calf cardiac Purkinje fibers were briefly exposed to the ionophore nystatin to promote exchange of Cs for intracellular K. The effects of Cs loading were stable for at least 30 min, but they could be reversed by nystatin-mediated K loading. After Cs loading, the resting potential shifted to .apprx. -20 mV and the current-voltage relationship showed a strong inhibition of inwardly rectifying K channels. Anodal break stimulation evoked normal action potential upstrokes and twitch contractions. The early repolarization (phase 1) was markedly slowed. 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. The current signal showed no hint of the transient outward current found in untreated or K-loaded preparations. Cs loading abolished the outward blocking effect of the K-channel inhibitor 4-aminopyridine. Inhibition of transient outward current revealed a maximal inward current of .apprx. 5 .mu.A/.mu.F [faraday] in 5.4 mM-CaO, which is considerably larger than the net inward current without Cs loading. The inward current was attributed to Ca channels on the basis of its sensitivity to membrane potential, extracellular Ca, D600 [methoxy verapamil], Mn and Cd. Cs loading also reduced slow current changes associated with delayed rectification and pace-maker depolarization. Apparently, the transient outward current is carried by K+ ions, while providing a method for unmasking inward Ca current.