The reverse mode of the Na(+)/Ca(2+) 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(2+)-handling. The fundamental role of the inward Na(+)/Ca(2+) 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(2+)-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(2+)-transient amplitude with larger SR Ca(2+)-content. Spontaneous action potential (AP) frequency increased with 8 mM NaCl. When reverse NCX was facilitated by 1 μM strophantin the Ca(2+)(i) and spontaneous rate increased. ORM-10962 applied prior to strophantin prevented Ca(2+)(i) and AP cycle change. Computational simulations indicated gradually increasing reverse NCX current, Ca(2+)(i) and heart rate with increasing Na(+)(i). Our results provide further evidence for the role of reverse NCX in SN pacemaking. The reverse NCX activity may provide additional Ca(2+)-influx that could increase SR Ca(2+)-content, which consequently leads to enhanced pacemaking activity.