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$. ... mehrOur results provide further evidence for the role of reverse NCX in SN pacemaking. The reverse NCX activity may provide additional Ca$^{2+}$-influx that could increase SR Ca$^{2+}$-content, which consequently leads to enhanced pacemaking activity.