Beat-to-beat Ca2+-dependent regulation of sinoatrial nodal pacemaker cell rate and rhythm

Whether intracellular Ca2+ regulates sinoatrial node cell (SANC) action potential (AP) firing rate on a beat-to-beat basis is controversial. To directly test the hypothesis of beat-to-beat intracellular Ca2+ regulation of the rate and rhythm of SANC we loaded single isolated SANC with a caged Ca2+ buffer, NP-EGTA, and simultaneously recorded membrane potential and intracellular Ca2+. Prior to introduction of the caged Ca2+ buffer, spontaneous local Ca2+ releases (LCRs) during diastolic depolarization were tightly coupled to rhythmic APs (r2 = 0.9). The buffer markedly prolonged the decay time (T50) and moderately reduced the amplitude of the AP-induced Ca2+ transient and partially depleted the SR load, suppressed spontaneous diastolic LCRs and uncoupled them from AP generation, and caused AP firing to become markedly slower and dysrhythmic. When Ca2+ was acutely released from the caged compound by flash photolysis, intracellular Ca2+ dynamics were acutely restored and rhythmic APs resumed immediately at a normal rate. After a few rhythmic cycles, however, these effects of the flash waned as interference with Ca2+ dynamics by the caged buffer was reestablished. Our results directly support the hypothesis that intracellular Ca2+ regulates normal SANC automaticity on a beat-to-beat basis.