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Calcium waves and sparks
Cardiomyopathies Links

Calcium sparks and quarks:

How do they start?

How do they stop?

Cardiac muscle cells use transient elevations of the intracellular Ca2+ concentration to activate the force producing proteins. While a transsarcolemmal influx of Ca2+ ions (as L-type Ca2+ current) initiates the process of excitation-contraction (EC) coupling, most of the Ca2+ is released from intracellular Ca2+ stores, the sarcoplasmic reticulum (SR). Recently, it has been recognized that these Ca2+ release signals are summations of elementary release events, Ca2+ sparks. Ca2+ sparks can either be observed to occur individually, or they take the shape of a homogeneous Ca2+ transient (i.e. if there are many simultaneous sparks) or they can occur as Ca2+ waves traveling along the cardiac muscle cell. Our lab has also observed Ca2+ release events smaller than Ca2+ sparks, termed Ca2+ quarks, that are thought to arise from openings of single SR Ca2+ release channels (Ryanodine receptors, RyRs).

During the last few years we have mainly addressed two open questions in our research:

1) What is or are the mechanisms terminating SR Ca2+ release?

We found experimental evidence to support the view that functional Ca2+ depletion of the SR contributes to release termination by deactivation of the RyRs. CICR then becomes refractory and the RyR less sensitive for Ca2+ triggers. Refilling of the SR via the SERCA then allows recovery of CICR after a whole-cell Ca2+ transient. Pharmacologically slowing SR refilling slows recovers, PLB knock-out mice have 5-fild faster receovery of CICR than wild-type littermates. After local Ca2+ release events, such as Ca2+ sparks or two-photon photolytically induced Ca2+ release, recovery from refractoriness appears to be much faster, presumably because of rapid Ca2+ redistribution within the SR network by diffusion.

2) Is the SR Ca2+ release itself a regulated process?

The sensitivity of the RyRs for Ca2+ on the cytosolic (trigger) side, as well as on the luminal SR side may change as a consequence of regulatory mechanisms, or as a result of cardiac diseases. We have examined global and local CICR after b-adrenergic stimulation of cardiac myocytes and found enhanced local CICR, consistent with the idea that RyRs are regulated by phosphorylation of the RyRs themselves, or of a closely associated regulatory protein. Hyperphosphorylation of RyRs has been proposed to represent a maladaptive process during cardiac diseases, such as congestive heart failure.

Below see some examples of experimental results. For details, please refer to the References page.

Two subcellular calcium (Ca2+) waves imaged with a laser-scanning confocal microscope in the line-scan mode and shown as surface plot (left, duration 3 seconds). A Ca2+ spark was triggerd with two-photon photolysis of caged Ca2+ (right, duration of plot 500 ms).

Computer animation of a linear (left) and spiral (right) Ca2+ waves in a Guinea pig cardiac myocyte recorded with a laser-scanning confocal microscope at 12 fps

 

Cardiac Ca2+ sparks and Ca2+ quarks operating in concert. At t = 1 ms an L-type Ca2+ channel located in the plasmalemma opens and a puff of Ca2+ enters the diadic cleft (15 nm wide). This almost immediately triggers Ca2+ release from a low-sensitivity RyR that generates a high Ca2+ flux from the SR into the cleft. Spreading of Ca2+ within the cleft triggers a number of Ca2+ quarks from high-sensitivity low-flux RyRs that also contribute Ca2+ quarks to the spark.

 

© E. Niggli