105Contribution of small conductance K+ channels to sinoatrial node pacemaker activity of control and atrial-specific Na+/Ca2+ exchange knock out mice

Funding Acknowledgements: NIH R01HL04509 (JIG and KDP), NIH R01HL70828 (JIG). Introduction: Small K+ (SK) channels have been implicated as modulators of spontaneous repolarization as well as electrical conduction that may contribute to cardiac arrhythmias. Purpose: To investigate SK presence and role during sinoatrial node (SAN) pacemaker activity. Methods and results: Using quantitative PCR (q-PCR), immunostaining and patch clamp recordings of membrane current and voltage, we found all three SK isoforms (SK1, SK2 and SK3) in mouse SAN. Since SK channels are exquisitely sensitive to Ca2+, we compared their activity in wild-type (WT) and atrial-specific Na+/Ca2+ exchange (NCX) knockout (KO) mice, used as a model of diastolic Ca2+ accumulation in SAN cells. Spontaneous and induced action potentials (APs) in isolated WT SAN cells were prolonged by SK inhibition with the specific blocker, apamin (10 nM). Apamin also depolarized the maximal diastolic potential of WT SAN cells, reduced their slope of diastolic depolarization and decreased the spontaneous frequency of these cells. NCX KO SAN cells did not generate spontaneous APs, but when induced they showed prolongation of AP duration after apamin inhibition of SK channels. Confocal Ca2+ imaging of explanted SAN tissue revealed significant SAN rate slowing in WT tissue treated with apamin. Moreover, explanted NCX KO SAN tissues, which are characterized by a spontaneous arrhythmic burst pacing, showed regularization of pacing under apamin. Conclusions: SK channels have demonstrable effects on AP duration, diastolic depolarization and pacemaker rate in the mouse SAN. Their Ca2+-dependent activation translates intracellular Ca2+ into repolarizing current capable of modulating spontaneous diastolic depolarization and pacemaker rate. The Ca2+-dependent nature of SK channels can lead to arrhythmic pacemaker activity under abnormal Ca2+ handling, as shown in NCX KO mice. Therefore, these results suggest that SK channels may be a potential target to treat SAN dysfunction, particularly in the setting of Ca2+ overload. Abstract 105 Figure. Recovering of NCX KO SAN automaticity