A novel mechanism of tandem activation of ryanodine receptors by cytosolic and SR luminal Ca2+ during excitation-contraction coupling in atrial myocytes

Key points: In atrial myocytes excitation-contraction coupling is strikingly different from ventricle because atrial myocytes lack a transverse tubule membrane system: CA²⁺ release starts in the cell periphery and propagates towards the cell centre by CA²⁺ -induced CA²⁺ release from the sarcoplasmic reticulum (SR) CA²⁺ store. The cytosolic CA²⁺ sensitivity of the ryanodine receptor (RyRs) CA²⁺ release channel is low and it is unclear how CA²⁺ release can be activated in the interior of atrial cells. Simultaneous confocal imaging of cytosolic and intra-SR calcium revealed a transient elevation of store CA²⁺ that we termed 'Ca²⁺ sensitization signal'. We propose a novel paradigm of atrial ECC that is based on tandem activation of the RyRs by cytosolic and luminal CA²⁺ through a 'fire-diffuse-uptake-fire' (or FDUF) mechanism: CA²⁺ uptake by SR CA²⁺ pumps at the propagation front elevates CA²⁺ inside the SR locally, leading to luminal RyR sensitization and lowering of the cytosolic CA²⁺ activation threshold.

Abstract: In atrial myocytes CA²⁺ release during excitation-contraction coupling (ECC) is strikingly different from ventricular myocytes. In many species atrial myocytes lack a transverse tubule system, dividing the sarcoplasmic reticulum (SR) CA²⁺ store into the peripheral subsarcolemmnal junctional (j-SR) and the much more abundant central non-junctional (nj-SR) SR. Action potential (AP)-induced CA²⁺ entry activates CA²⁺ -induced CA²⁺ release (CICR) from j-SR ryanodine receptor (RyR) CA²⁺ release channels. Peripheral elevation of [CA²⁺ ]_i initiates CICR from nj-SR and sustains propagation of CICR to the cell centre. Simultaneous confocal measurements of cytosolic ([CA²⁺ ]_i ; with the fluorescent CA²⁺ indicator rhod-2) and intra-SR ([CA²⁺ ]_SR ; fluo-5N) CA²⁺ in rabbit atrial myocytes revealed that CA²⁺ release from j-SR resulted in a cytosolic CA²⁺ transient of higher amplitude compared to release from nj-SR; however, the degree of depletion of j-SR [CA²⁺ ]_SR was smaller than nj-SR [CA²⁺ ]_SR . Similarly, CA²⁺ signals from individual release sites of the j-SR showed a larger cytosolic amplitude (CA²⁺ sparks) but smaller depletion (CA²⁺ blinks) than release from nj-SR. During AP-induced CA²⁺ release the rise of [CA²⁺ ]_i detected at individual release sites of the nj-SR preceded the depletion of [CA²⁺ ]_SR , and during this latency period a transient elevation of [CA²⁺ ]_SR occurred. We propose that CA²⁺ release from nj-SR is activated by cytosolic and luminal CA²⁺ (tandem RyR activation) via a novel 'fire-diffuse-uptake-fire' (FDUF) mechanism. This novel paradigm of atrial ECC predicts that CA²⁺ uptake by sarco-endoplasmic reticulum CA²⁺ -ATPase (SERCA) at the propagation front elevates local [CA²⁺ ]_SR , leading to luminal RyR sensitization and lowering of the activation threshold for cytosolic CICR.

SR Ca release; atrial myocytes; excitation-contraction coupling.