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

摘要

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.