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Key points

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    Cytosolic calcium (Ca2+) waves result from spontaneous release of Ca2+ from the sarcoplasmic reticulum (SR) Ca2+ store that occurs under Ca2+ overload conditions and can give rise to arrhythmias in the heart. The prevailing paradigm of Ca2+ wave propagation involves cytosolic Ca2+-induced Ca2+ release.
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    A recent challenge to this paradigm proposed the requirement for an intra-SR ‘sensitization’ Ca2+ wave that primes release activation due to the luminal Ca2+ sensitivity of the release mechanism.
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    We tested this hypothesis in cardiac myocytes with direct simultaneous high-resolution measurements of cytosolic and intra-SR Ca2+ using fluorescence confocal microscopy.
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    We found that the increase in cytosolic Ca2+ at the wave front preceded release and depletion of SR Ca2+ in time, and during this latency period a transient increase of SR Ca2+ was observed at individual release sites that gave rise to a propagating intra-SR Ca2+ sensitization wave.
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    The intra-SR sensitization wave depended on the activity of the sarco-endoplasmic reticulum Ca2+-ATPase (SERCA) and occurred by a mechanism where Ca2+ uptake by SERCA at the wave front facilitates propagation of cytosolic Ca2+ waves via luminal sensitization of the release mechanism, thus supporting a novel paradigm of a ‘fire-diffuse-uptake-fire’ mechanism for Ca2+ wave propagation.

Abstract  The widely accepted paradigm for cytosolic Ca2+ wave propagation postulates a ‘fire-diffuse-fire’ mechanism where local Ca2+-induced Ca2+ release (CICR) from the sarcoplasmic reticulum (SR) via ryanodine receptor (RyR) Ca2+ release channels diffuses towards and activates neighbouring release sites, resulting in a propagating Ca2+ wave. A recent challenge to this paradigm proposed the requirement for an intra-SR ‘sensitization’ Ca2+ wave that precedes the cytosolic Ca2+ wave and primes RyRs from the luminal side to CICR. Here, we tested this hypothesis experimentally with direct simultaneous measurements of cytosolic ([Ca2+]i; rhod-2) and intra-SR ([Ca2+]SR; fluo-5N) calcium signals during wave propagation in rabbit ventricular myocytes, using high resolution fluorescence confocal imaging. The increase in [Ca2+]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 [Ca2+]SR was observed. This transient elevation of [Ca2+]SR could be identified at individual release junctions and depended on the activity of the sarco-endoplasmic reticulum Ca2+-ATPase (SERCA). Increased SERCA activity (β-adrenergic stimulation with 1 μm isoproterenol (isoprenaline)) decreased the latency period and increased the amplitude of the transient elevation of [Ca2+]SR, whereas inhibition of SERCA (3 μm cyclopiazonic acid) had the opposite effect. In conclusion, the data provide experimental evidence that local Ca2+ uptake by SERCA into the SR facilitates the propagation of cytosolic Ca2+ waves via luminal sensitization of the RyR, and supports a novel paradigm of a ‘fire-diffuse-uptake-fire’ mechanism for Ca2+ wave propagation in cardiac myocytes.