Persistent Periodic Ca2+ Release in Sinoatrial Node of NA+-CA2+ Exchanger Knockout Mice

Atrial-specific Na+-Ca2+ exchanger (NCX1) KO mice have no P waves on their surface electrocardiograms; instead they rely upon a junctional escape rhythm. This suggests that the sinoatrial node (SAN) requires the “calcium clock” mechanism for normal pacemaker activity.To test this hypothesis, we examined Ca2+ handling in intact preparations of SAN and atrioventricular node (AVN) tissue, as well as in isolated SAN cells. Compared to WT, Ca2+ transients indicative of depolarization were slow and irregular in NCX1 KO SAN tissue. β-adrenergic stimulation with isoproterenol (ISO, 100nM) increased the rate and often promoted bursts of Ca2+ transients, while the pacing remained irregular. Surprisingly, we found no evidence of spontaneous activity in AVN tissue isolated from the NCX1 KO. However, ∼50% of NCX1 KO AVNs recovered spontaneous transients in ISO.These findings suggest that despite the lack of NCX1, abnormal pacemaker activity is maintained in the SAN and is still responsive to β-adrenergic stimulation.To study the underlying pacing mechanism in the KO, we enzymatically isolated SAN cells from WT and NCX1 KO mice. NCX1 KO SAN cells were ∼30% longer than WT and occasionally exhibited regular Ca2+ transients (∼15% of cells). However, periodic Ca2+ waves were often observed in NCX1 KO cells (but rarely in WT). These waves occurred at about the same rate as the spontaneous transients in WT cells. ISO (10nM) increased the frequency of waves by ∼80% in KO cells, similar to the increase in the rate of Ca2+ transients in WT cells.In summary, periodic Ca2+ waves persist in NCX1 KO SAN cells. However, only a minority of these cells generate Ca2+ transients consistent with action potentials. Therefore, the pacing clock can “tick” in the absence of NCX, but seems no longer coupled to membrane depolarization.