Beat-to-beat Ca ²⁺-dependent regulation of sinoatrial nodal pacemaker cell rate and rhythm

Whether intracellular Ca ²⁺ 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 Ca ²⁺ regulation of the rate and rhythm of SANC we loaded single isolated SANC with a caged Ca ²⁺ buffer, NP-EGTA, and simultaneously recorded membrane potential and intracellular Ca ²⁺. Prior to introduction of the caged Ca ²⁺ buffer, spontaneous local Ca ²⁺ releases (LCRs) during diastolic depolarization were tightly coupled to rhythmic APs (r ²=0.9). The buffer markedly prolonged the decay time (T ₅₀) and moderately reduced the amplitude of the AP-induced Ca ²⁺ 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 Ca ²⁺ was acutely released from the caged compound by flash photolysis, intracellular Ca ²⁺ 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 Ca ²⁺ dynamics by the caged buffer was reestablished. Our results directly support the hypothesis that intracellular Ca ²⁺ regulates normal SANC automaticity on a beat-to-beat basis.