Crosstalk between mitochondrial and sarcoplasmic reticulum Ca2+ cycling modulates cardiac pacemaker cell automaticity

Cardiac pacemaker cells regulate heart rate through spontaneous electrical activity. Sinoatrial node cells (SANC) rely on sarcoplasmic reticulum (SR) calcium (Ca²⁺) release to accelerate diastolic depolarization. Mitochondria in ventricular cells modulate cytosolic Ca²⁺ levels, which in turn affects SR Ca²⁺ load. However, the extent of interaction between mitochondrial Ca²⁺ cycling and SR Ca²⁺ dynamics in pacemaker cells remains unclear. Prior research has shown that mitochondrial Ca²⁺ flux influences SR Ca²⁺ handling in non-pacemaker cells, but its role in SANC automaticity is not fully understood. This gap motivated the current study to explore how mitochondrial Ca²⁺ cycling may influence SR Ca²⁺ release in SANC. The study aimed to determine if mitochondrial Ca²⁺ flux affects SR Ca²⁺ load and whether this crosstalk impacts pacemaker cell function. Establishing a connection between these two organelles could refine models of cardiac pacemaking. The study also sought to clarify how Ca²⁺ fluxes in mitochondria could modulate the timing of action potentials in SANC. By focusing on Ca²⁺ cycling in SANC, the research addresses a specific gap in understanding how intracellular Ca²⁺ dynamics contribute to heart rate regulation.

The study aimed to investigate whether mitochondrial Ca²⁺ cycling interacts with SR Ca²⁺ dynamics in sinoatrial node cells (SANC) and how this interaction might affect pacemaker cell automaticity. The researchers focused on how changes in mitochondrial Ca²⁺ influx or efflux influence SR Ca²⁺ load and spontaneous Ca²⁺ release events. They sought to determine if these changes could modulate the frequency of action potentials in SANC. The motivation stemmed from the observation that mitochondrial Ca²⁺ handling in ventricular cells affects SR function, but its role in pacemaker cells was unclear. The study aimed to clarify whether mitochondrial Ca²⁺ flux could alter SR Ca²⁺ release characteristics. The researchers hypothesized that mitochondrial Ca²⁺ cycling could influence SR Ca²⁺ load and thereby affect pacemaker cell activity. By using pharmacological tools to manipulate mitochondrial Ca²⁺ flux, the study aimed to observe how these changes would impact SR Ca²⁺ release patterns. The ultimate goal was to understand how mitochondrial Ca²⁺ cycling could modulate the intrinsic firing rate of pacemaker cells.

The researchers used isolated rabbit sinoatrial node cells to study mitochondrial and SR Ca²⁺ interactions. They employed confocal linescan imaging to visualize Ca²⁺ release events from the SR. Mitochondrial Ca²⁺ influx was inhibited using Ru360, while Ca²⁺ efflux was blocked using CGP-37157. These drugs altered mitochondrial Ca²⁺ levels, allowing the team to observe how these changes affected SR Ca²⁺ load and release dynamics. The study measured mitochondrial Ca²⁺ concentration ([Ca²⁺]m) and compared it with SR Ca²⁺ load and local Ca²⁺ release characteristics. The researchers analyzed the size, duration, amplitude, and period of SR Ca²⁺ release events (LCRs) in response to mitochondrial Ca²⁺ changes. They also recorded spontaneous action potentials (APs) to assess how mitochondrial Ca²⁺ flux influenced pacemaker cell firing rate. The study combined pharmacological manipulation with high-resolution imaging to track Ca²⁺ dynamics in real time.

Inhibition of mitochondrial Ca²⁺ influx with Ru360 reduced [Ca²⁺]m to 80% of control levels. This led to a significant increase in SR Ca²⁺ load, as well as in the size, duration, amplitude, and period of SR Ca²⁺ release events (LCRs). Conversely, blocking Ca²⁺ efflux with CGP-37157 increased [Ca²⁺]m to 119% of control levels, which decreased SR Ca²⁺ load and LCR characteristics. Changes in total LCR Ca²⁺ signal were strongly correlated with SR Ca²⁺ load (r² = 0.97). The study found that alterations in mitochondrial Ca²⁺ flux directly affected SR Ca²⁺ release dynamics. Spontaneous action potential (AP) cycle length also changed in response to mitochondrial Ca²⁺ flux modifications. With Ru360, AP cycle length increased to 111% of control, while with CGP-37157, it decreased to 89% of control. These changes in AP cycle length were predicted by changes in LCR period (r² = 0.84). The results suggest that mitochondrial Ca²⁺ cycling modulates SR Ca²⁺ release and thereby influences pacemaker cell automaticity.

The study suggests that mitochondrial Ca²⁺ cycling interacts with SR Ca²⁺ dynamics in sinoatrial node cells (SANC). Changes in mitochondrial Ca²⁺ flux correlate with changes in SR Ca²⁺ load and spontaneous Ca²⁺ release events. These findings indicate that mitochondrial Ca²⁺ handling may modulate pacemaker cell automaticity. The researchers observed that blocking mitochondrial Ca²⁺ influx or efflux altered SR Ca²⁺ release characteristics. These changes in SR Ca²⁺ release were associated with shifts in action potential (AP) cycle length. The study proposes that mitochondrial Ca²⁺ flux could influence pacemaker cell function through Ca²⁺ cycling effects. The results support the idea that mitochondrial Ca²⁺ cycling affects SR Ca²⁺ release in SANC. The authors suggest that this crosstalk between mitochondria and SR may be a key factor in regulating pacemaker cell activity.