Refractoriness of sarcoplasmic reticulum Ca2+ release determines Ca2+ alternans in atrial myocytes

Abstract

Cardiac alternans is a recognized risk factor for cardiac arrhythmia and sudden cardiac death. At the cellular level, Ca²⁺ alternans appears as cytosolic Ca²⁺ transients of alternating amplitude at regular beating frequency. Cardiac alternans is a multifactorial process but has been linked to disturbances in intracellular Ca²⁺ regulation. In atrial myocytes, we tested the role of voltage-gated Ca²⁺ current, sarcoplasmic reticulum (SR) Ca²⁺ load, and restitution properties of SR Ca²⁺ release for the occurrence of pacing-induced Ca²⁺ alternans. Voltage-clamp experiments revealed that peak Ca²⁺ current was not affected during alternans, and alternans of end-diastolic SR Ca²⁺ load, evaluated by application of caffeine or measured directly with an intra-SR fluorescent Ca²⁺ indicator (fluo-5N), were not a requirement for cytosolic Ca²⁺ alternans. Restitution properties and kinetics of refractoriness of Ca²⁺ release after activation during alternans were evaluated by four different approaches: measurements of 1) the delay (latency) of occurrence of spontaneous global Ca²⁺ releases and 2) Ca²⁺ spark frequency, both during rest after a large and small alternans Ca²⁺ transient; 3) the magnitude of premature action potential-induced Ca²⁺ transients after a large and small beat; and 4) the efficacy of a photolytically induced Ca²⁺ signal (Ca²⁺ uncaging from DM-nitrophen) to trigger additional Ca²⁺ release during alternans. The results showed that the latency of global spontaneous Ca²⁺ release was prolonged and Ca²⁺ spark frequency was decreased after the large Ca²⁺ transient during alternans. Furthermore, the restitution curve of the Ca²⁺ transient elicited by premature action potentials or by photolysis-induced Ca²⁺ release from the SR lagged behind after a large-amplitude transient during alternans compared with the small-amplitude transient. The data demonstrate that beat-to-beat alternation of the time-dependent restitution properties and refractory kinetics of the SR Ca²⁺ release mechanism represents a key mechanism underlying cardiac alternans.