Key Elements of the Excitation–Contraction Coupling MachineryThe delicate balance that regulates Ca2+ fluxes between the intracellular compartment and the extracellular space in cardiomyocytes is critical to ensure cellular viability, preserve normal contractile function, and to provide a stable heart rhythm.
During the plateau phase of the cardiac action potential, a small amount of Ca2+
enters the cardiomyocytes through the voltage-dependent L-type Ca2+
channels, causing Ca2+
release into the cytosol through the RyR2 channel located in the SR membrane. This process of CICR is the basis of cardiac E–C coupling.
The attainment of higher concentrations of cytosolic Ca2+
causes activation of the contractile filaments of the cardiac sarcomere, which is followed by diminution of Ca2+
concentration to the diastolic level, thus causing relaxation. Ca2+
levels are lowered to diastolic values by means of three systems: 1) the sarcoplasmic endoplasmic reticulum calcium (SERCA2, the cardiac SERCA) which is responsible for reuptake of ∼75% of Ca2+
into the SR; 2) The sodium–calcium exchanger (NCX) which extrudes the remaining portion of cytosolic Ca2+
from the cytoplasm; and 3) a small portion of the Ca2+
is extruded to the extracellular space by means of an ATP-operated Ca2+
The Ca2+ Release Unit (CRU)
RyR2 is a large homotetrameric Ca2+
release channel located on the SR membrane. The RyR2 channels are composed of four pore-forming monomers, comprising a relatively small C-terminal transmembrane domain and a large N-terminal domain that protrudes into the cytosol. The cytoplasmic domain of RyR2 is stabilized by FKBP12.6 and is essential for channel closure during diastole.
The CASQ2 is the major Ca2+
storage protein in the SR and is capable of binding luminal Ca2+
ions/molecule) during diastole in order to prevent Ca2+
precipitation and to reduce the ionic Ca2+
On the luminal side, RyR2 binds junctin and triadin, which anchor the Ca2+
-buffering protein CASQ2,
collectively forming the SR Ca2+
release unit (CRU). The CRU is responsible for SR Ca2+
release, which is triggered by increased cytosolic Ca2+
resulting from opening of the L-type channel (CICR).
In addition, CASQ2 has been suggested to modulate the activity of RyR2 directly.
Under adrenergic stimulation, β-adrenergic receptors activate a GTP-binding protein that stimulates adenylyl cyclase to produce cAMP, which in turn activates protein kinase A (PKA). This kinase phosphorylates RyR2 and other central proteins related to E–C coupling, such as phospholamban and the L-type Ca2+
channels, thus causing gain of function of Ca2+
cycling in cardiomyocytes in response to adrenergic activation. FKBP12.6 stabilizes RyR2 in the closed state, and the hyperphosphorylation of RyR2 by PKA causes FKBP12.6 dissociation from RyR2, thereby increasing the open probability of RyR2.
Moreover, adrenergic stimulation also increases the activity of the SERCA pump via the phosphorylation of phospholamban by PKA which stabilizes SERCA.
The mechanism of CPVT
studies suggested that the RyR2 and CASQ2 mutations cause the CRU to open spontaneously without being triggered by voltage-gated Ca2+
influx, thereby leading to intracellular Ca2+
Increased intracellular Ca2+
can trigger early or delayed afterdepolarizations (oscillations of the membrane potential that occur during the plateau/ repolarization phase of the action potential or after its completion, respectively) that can reach the threshold potential and cause triggered activity.
overload leads to NCX activation which extrudes Ca2+
in exchange for Na+
with a stoichiometry of 1:3, thereby generating a net inward current (the so-called transient inward current, ITi
The transient inward current induces DADs which may reach threshold and trigger premature ventricular beats and ventricular arrhythmias (demonstrated in Figure 1
) by a mechanism called triggered activity.
Ca2+-induced Ca2+ release (CICR), store overload-induced Ca2+ release (SOICR), and triggered arrhythmia.