The reverse mode of the Na/Ca exchanger contributes to the pacemaker mechanism in rabbit sinus node cells.

Sinus node (SN) pacemaking is based on a coupling between surface membrane ion-channels and intracellular Ca-handling. The fundamental role of the inward Na/Ca exchanger (NCX) is firmly established. However, little is known about the reverse mode exchange. A simulation study attributed important role to reverse NCX activity, however experimental evidence is still missing. Whole-cell and perforated patch-clamp experiments were performed on rabbit SN cells supplemented with fluorescent Ca-tracking. We established 2 and 8 mM pipette NaCl groups to suppress and enable reverse NCX. NCX was assessed by specific block with 1 μM ORM-10962. Mechanistic simulations were performed by Maltsev-Lakatta minimal computational SN model. Active reverse NCX resulted in larger Ca-transient amplitude with larger SR Ca-content. Spontaneous action potential (AP) frequency increased with 8 mM NaCl. When reverse NCX was facilitated by 1 μM strophantin the Ca and spontaneous rate increased. ORM-10962 applied prior to strophantin prevented Ca and AP cycle change. Computational simulations indicated gradually increasing reverse NCX current, Ca and heart rate with increasing Na. Our results provide further evidence for the role of reverse NCX in SN pacemaking. The reverse NCX activity may provide additional Ca-influx that could increase SR Ca-content, which consequently leads to enhanced pacemaking activity.