Ca2+ dependent regulation of sinoatrial node pacemaking

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Introduction: Normal heart rhythm depends on a precise and regular activity of the sinoatrial node (SAN) pacemaker cells. The exact electrophysiological mechanism of spontaneous cardiac pacemaking is not fully understood. Recent results suggest a tight cooperation between the intracellular Ca2+ handling (‘Ca2+ clock’) and surface membrane ion channels (‘membrane clock’) which is referred as the coupled clock mechanism. One of the suggested key players of this crosstalk is the Na+/Ca2+-exchanger (NCX), however direct evidence was unavailable so far, due to the lack of selective inhibitor. Purpose: Our aim was to investigate the robustness and flexibility of SAN automaticity via analysing the role of different Ca2+ dependent currents such as NCX and Ca2+ activated K+ current (IK(Ca)) in the mechanism of cardiac pacemaking. We also studied the effect of SAN frequency on the development of ventricular action potential (AP) and Ca2+ transient (CaT) alternans. Methods: APs were measured from both SAN and ventricular papillary muscle preparations by conventional microelectrode technique. Ion currents combined with fluorescent Ca2+ tracking were recorded by whole cell and perforated configuration of patch clamp technique on isolated rabbit SAN cells and dog ventricular cardiomyocytes. Results: The cycle length (CL) lengthening effect of NCX inhibition could be augmented when the funny current (If) was previously inhibited and vice-versa the effect of If reduction was facilitated when the Ca2+ handling was suppressed. Functioning reverse NCX resulted in larger Ca2+ transient amplitude with enhanced SR Ca2+ content. Spontaneous AP frequency increased as a consequence of active reverse NCX. Ca2+ activated K+ current had no role in SAN pacemaking. Ventricular action potential alternans showed strong rate-dependence having clear frequency threshold. Conclusion: Our results provide the first pharmacological evidence regarding the role of both forward and reverse NCX current in pacemaker mechanism. Experimental and modelling data support a close cooperation between If and NCX establishing a "pacemaker reserve" mechanism. The reverse NCX activity may provide additional Ca2+ influx that could increase SR Ca2+ content providing a „Ca2+ reserve” mechanism. This „dual” function of NCX may provide a safety margin of pacemaking and largely increases the robustness (i.e. fail-safe) of spontaneous automaticity. Excessive SAN pacemaking induces ventricular AP and Ca2+ transient alternans as a result of mutual crosstalk of surface membrane ion channels and intracellular Ca2+.

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