Regulation of calcium clock‐mediated pacemaking by inositol‐1,4,5‐trisphosphate receptors in mouse sinoatrial nodal cells

Key points Inositol‐1,4,5‐trisphosphate receptors (IP 3 Rs) modulate pacemaking in embryonic heart, but their role in adult sinoatrial node (SAN) pacemaking is uncertain. We found that stimulation of IP 3 Rs accelerates spontaneous pacing rate in isolated mouse SAN cells, whereas inhibition of IP 3 Rs slows pacing. In atrial‐specific sodium‐calcium exchanger (NCX) knockout (KO) SAN cells, where the Ca 2+ clock is uncoupled from the membrane clock, IP 3 R agonists and antagonists modulate the rate of spontaneous Ca 2+ waves, suggesting that IP 3 R‐mediated Ca 2+ release modulates the Ca 2+ clock. IP 3 R modulation also regulates Ca 2+ spark parameters, a reflection of ryanodine receptor open probability, consistent with the effect of IP 3 signalling on Ca 2+ clock frequency. Modulation of Ca 2+ clock frequency by IP 3 signalling in NCX KO SAN cells demonstrates that the effect is independent of NCX. These findings support development of IP 3 signalling modulators for regulation of heart rate, particularly in heart failure where IP 3 Rs are upregulated.Abstract Cardiac pacemaking initiated by the sinus node is attributable to the interplay of several membrane currents. These include the depolarizing ‘funny current’ ( I f ) and the sodium‐calcium exchanger current ( I NCX ). The latter is activated by ryanodine receptor (RyR)‐mediated calcium (Ca 2+ ) release from the sarcoplasmic reticulum (SR). Another SR Ca 2+ release channel, the inositol‐1,4,5‐triphosphate receptor (IP 3 R), has been implicated in the generation of spontaneous Ca 2+ release in atrial and ventricular cardiomyocytes. Whether IP 3 R‐mediated Ca 2+ release also influences SAN automaticity is controversial, in part due to the confounding influence of periodic Ca 2+ flux through the sarcolemma accompanying each beat. We took advantage of atrial‐specific sodium–calcium exchanger (NCX) knockout (KO) SAN cells to study the influence of IP 3 signalling on cardiac pacemaking in a system where periodic intracellular Ca 2+ cycling persists despite the absence of depolarization or Ca 2+ flux across the sarcolemma. We recorded confocal line scans of spontaneous Ca 2+ release in WT and NCX KO SAN cells in the presence or absence of an IP 3 R blocker (2‐aminoethoxydiphenyl borate, 2‐APB), or during block of IP 3 production by the phospholipase C inhibitor U73122. 2‐APB and U73122 decreased the frequency of spontaneous Ca 2+ transients and waves in WT and NCX KO cells, respectively. Alternatively, increased IP 3 production induced by phenylephrine increased Ca 2+ transient and wave frequency. We conclude that IP 3 R‐mediated SR Ca 2+ flux is crucial for initiating and modulating the RyR‐mediated Ca 2+ cycling that regulates SAN pacemaking. Our results in NCX KO SAN cells also demonstrate that RyRs, but not NCX, are required for IP 3 to modulate Ca 2+ clock frequency.