Regulation of Calcium Clock-Mediated Pacemaking by Inositol-1,4,5-Trisphosphate Receptors in Mouse Sinoatrial Nodal Cells

Sinoatrial node (SAN) automaticity is attributable to the interplay of several membrane currents such as funny current (If) and the Na-Ca exchanger (NCX) current activated in response to ryanodine receptor (RyR) mediated Ca release from the sarcoplasmic reticulum (SR). Whether another SR Ca release channel, the inositol-1,4,5-triphosphate receptor (IP3R), can influence automaticity in SAN is controversial, in part due to the confounding influence of periodic Ca flux through the sarcolemma accompanying each beat. We used atrial-specific NCX knockout (KO) SAN cells to study IP3 signaling in a system where periodic [Ca]i cycling persists despite the absence of depolarization or Ca flux across the sarcolemma. We recorded confocal line scans of spontaneous Ca release in WT and NCX KO SAN cells, in the presence or absence of an IP3R blocker (2-APB) or during inhibition of phospholipase C by U73122. We found that superfusion with 2APB (2 µM) decreased the frequency of Ca transients in WT by 82.7% (n=9, p 0.05) and Ca wave frequency in KO cells (n=9, p <0.05) that was reversed by 2-APB. To determine if IP3Rs exert their modulatory effect on pacemaking via RyRs, we recorded Ca transients during application of PE in the continued presence of ryanodine at a blocking concentration (100 µm) that does not deplete SR stores. Under these conditions PE was unable to restore Ca transients. Thus, Ca release from IP3Rs can modulate the “Ca clock” processes that regulate pacemaker frequency in the murine SAN via Ca-induced Ca release through the RyRs.