Properties of the current if in the sino‐atrial node of the rabbit compared with those of the current iK, in Purkinje fibres.

Abstract

Properties of the pacemaker current if in rabbit sinoatrial node were investigated by voltage clamp of small preparations and compared with those of the iK2 current in the Purkinje fiber. Besides having a similar voltage range of activation and responding in a similar way to adrenaline [epinephrine], if resembles iK2 in other respects. When external Na is reduced, if decreases proportionally. In 25% Na the time-dependent current change due to if disappears. Cs (20 mM) completely abolishes if. During a hyperpolarizing voltage-clamp pulse if displays relatively high temperature dependence. In spite of the similarities between the 2 current systems, experiments in high K solutions (48 mM) rule out the possibility that the current change seen on a hyperpolarization reflects the decay of a pure K current. From conductance measurements during onset of if it is deduced that if behaves as an inward current activated by hyperpolarizations.