Facilitation of cytosolic calcium wave propagation by local calcium uptake into the sarcoplasmic reticulum in cardiac myocytes

Key points•  Cytosolic calcium (Ca 2+ ) waves result from spontaneous release of Ca 2+ from the sarcoplasmic reticulum (SR) Ca 2+ store that occurs under Ca 2+ overload conditions and can give rise to arrhythmias in the heart. The prevailing paradigm of Ca 2+ wave propagation involves cytosolic Ca 2+ ‐induced Ca 2+ release. •  A recent challenge to this paradigm proposed the requirement for an intra‐SR ‘sensitization’ Ca 2+ wave that primes release activation due to the luminal Ca 2+ sensitivity of the release mechanism. •  We tested this hypothesis in cardiac myocytes with direct simultaneous high‐resolution measurements of cytosolic and intra‐SR Ca 2+ using fluorescence confocal microscopy. •  We found that the increase in cytosolic Ca 2+ at the wave front preceded release and depletion of SR Ca 2+ in time, and during this latency period a transient increase of SR Ca 2+ was observed at individual release sites that gave rise to a propagating intra‐SR Ca 2+ sensitization wave. •  The intra‐SR sensitization wave depended on the activity of the sarco‐endoplasmic reticulum Ca 2+ ‐ATPase (SERCA) and occurred by a mechanism where Ca 2+ uptake by SERCA at the wave front facilitates propagation of cytosolic Ca 2+ waves via luminal sensitization of the release mechanism, thus supporting a novel paradigm of a ‘fire‐diffuse‐uptake‐fire’ mechanism for Ca 2+ wave propagation.Abstract  The widely accepted paradigm for cytosolic Ca 2+ wave propagation postulates a ‘fire‐diffuse‐fire’ mechanism where local Ca 2+ ‐induced Ca 2+ release (CICR) from the sarcoplasmic reticulum (SR) via ryanodine receptor (RyR) Ca 2+ release channels diffuses towards and activates neighbouring release sites, resulting in a propagating Ca 2+ wave. A recent challenge to this paradigm proposed the requirement for an intra‐SR ‘sensitization’ Ca 2+ wave that precedes the cytosolic Ca 2+ wave and primes RyRs from the luminal side to CICR. Here, we tested this hypothesis experimentally with direct simultaneous measurements of cytosolic ([Ca 2+ ] i ; rhod‐2) and intra‐SR ([Ca 2+ ] SR ; fluo‐5N) calcium signals during wave propagation in rabbit ventricular myocytes, using high resolution fluorescence confocal imaging. The increase in [Ca 2+ ] i at the wave front preceded depletion of the SR at each point along the calcium wave front, while during this latency period a transient increase of [Ca 2+ ] SR was observed. This transient elevation of [Ca 2+ ] SR could be identified at individual release junctions and depended on the activity of the sarco‐endoplasmic reticulum Ca 2+ ‐ATPase (SERCA). Increased SERCA activity (β‐adrenergic stimulation with 1 μ m isoproterenol (isoprenaline)) decreased the latency period and increased the amplitude of the transient elevation of [Ca 2+ ] SR , whereas inhibition of SERCA (3 μ m cyclopiazonic acid) had the opposite effect. In conclusion, the data provide experimental evidence that local Ca 2+ uptake by SERCA into the SR facilitates the propagation of cytosolic Ca 2+ waves via luminal sensitization of the RyR, and supports a novel paradigm of a ‘fire‐diffuse‐uptake‐fire’ mechanism for Ca 2+ wave propagation in cardiac myocytes.