Murine Sinoatrial Node Isolation

Mice were anesthetized by sodium pentobarbital (50 mg/kg intraperitoneal). The heart was excised and the supraventricular tissue was rapidly placed into Tyrode solution (36°C) oxygenated to saturation (mmol/L: NaCl 140, KCl 5.4, CaCl₂ 1.8, MgCl₂ 1, Hepes, 5, and glucose 5.5, pH 7.4 titrated with NaOH). The SAN was identified as being bordered by the crista terminalis, the superior and inferior vena cava and the interatrial septum (Supplemental Figure 1). SAN and some surrounding atrial tissue were dissected and pinned down with the endocardial side up in custom-made optical chambers bathed with Tyrode solution. Images were taken from the primary pacemaker region. Experiments and recordings were performed at room temperature (22–23°C). See online supplement for extended methods.