CaMKII is a serine/threonine kinase that is composed of two stacked hexamers assembled from 12 monomers (22,23). Each monomer is composed of an N-terminal catalytic region, an intermediate regulatory do-main and a C-terminal associated region (15,23). The catalytic region contains an ATP and target substrate binding site, which is responsible for the regulation of kinase activity (23). Under basic conditions, the function of the catalytic region is inhibited by interacting with the intermediate regulatory region (23). The intermediate regulatory region interacts with Ca²⁺/calmodulin (CaM) at a K_D of 10-50 nM, which not only activates CaMKII by preventing the inhibitory effect of the catalytic region, but also increases the activity of CaMKII by phosphorylating threonine 287 (Thr287) (18,23). The C-terminal associated domain is responsible for the oligomerization of individual CaMKII molecules to form a mature dodecameric-holoenzyme (Fig. 1) (18).

CaMKII has four subtypes (α, β, γ and δ), and each subtype has a different basic affinity for Ca²⁺/CaM (in order of highest to lowest, γ, β, δ and α) (15,18). The CaMKIIδ and CaMKIIγ subtypes are mainly present in myocardial tissue (18). CaMKIIδ has four splice variants (δA, δB, δC, and δ9), among which CaMKIIδB and CaMKIIδC are observed primarily expressed in the heart (18,23). CaMKIIδB contains an 11-amino acid nuclear localization sequence, which is preferentially localized in the nucleus, thereby exerting an important influence on the transcriptional activity of genes involved in cardiac hypertrophy (18,23). CaMKIIδC is the main cytoplasmic form, which is involved in membrane excitability and regulation of intracellular Ca²⁺ homeostasis (15,23). The ratio of δB to δC in the multimer can regulate the localization of holoenzymes, and stable hetero-oligomers are formed by these CaMKII subtypes (18,23,24).

In the presence of ATP, the pseudo-substrate section of the intermediate regulatory region of CaMKII can inhibit the func-tion of the N-terminal catalytic region, resulting in the inactivation of CaMKII (23). When Ca²⁺ content increases, Ca²⁺ combines with CaM (a ubiquitous intracellular Ca²⁺ binding protein) to form Ca²⁺/CaM (24). The intermediate regulatory region binds to Ca²⁺/CaM, which causes conformational changes in the pseudosubstrate region and releases the catalytic domain, exposing the substrate and ATP binding sites, further resulting in CaMKII activation (Fig. 1) (23,24).

In the presence of ATP, continuously increasing Ca²⁺/CaM sustainably combines with the intermediate regulatory region of CaMKII, which results in the autophosphorylation of Thr287. Thr287 autophosphorylation significantly increases the affinity of Ca²⁺/CaM to the intermediate regulatory region, slowing the release of Ca²⁺/CaM and retaining residual activity even after the dissociation of Ca²⁺/CaM, further resulting in CaMKII activation (3,16,24). A previous study by Erickson et al (25) showed that the methionine 281/282 (Met281/282) site is oxidized in the presence of reactive oxygen species (ROS). Oxidation of Met281/282 can not only lead to the autonomous activation of CaMKII by preventing the recombination of the catalytic domain and the intermediate regulatory region, but also promote CaMKII activation at low intracellular Ca²⁺ concentrations by increasing the capability of CaMKII to be activated by Ca²⁺/CaM (3,18). In addition, O-linked glycosylation at serine 280 (Ser280) and nitric oxide (NO)-dependent nitrosation at cysteine 290 (Cys290) can activate CaMKII. Ser280 O-linked-glycosylation of CaMKII has been demonstrated to promote Thr287 autophosphorylation (Fig. 1) (18,26).

Under normal conditions, Na_v channels rapidly activate and inactivate, resulting in a transient Na⁺ current (I_Na,T), which allows for AP depolarization (phase 0 of the AP). However, even under physiological conditions, a minor population of Na_v channels may fail to inactivate, giving rise to a late Na⁺ current (I_Na,L) that persists throughout the AP. Importantly, amplification of I_Na,L in disease set-tings has been demonstrated to increase arrhythmia susceptibility (27).

CaMKII has a VA-inducing effect by regulating Na_v channels. Previous studies have demonstrated that acute CaMKII overexpression may shift Na_v channel resting potential to more negative membrane potentials, enhancing in-termediate inactivation and slowing recovery from inactivation, thereby reducing the fraction of available Na⁺ channels. However, this also slows I_Na,T inactivation, enhances I_Na,L and increases intracellular Na⁺ concentrations. These effects increase susceptibility to arrhythmia (27,28) Additionally, serine 571 of Na_v1.5 is in the Na_v pore-forming subunit and is a key site of CaMKII phosphorylation. Na_v channels can be continuously opened or reopened to produce long-lasting I_Na,L via phosphorylation at this subunit (16,29). Increased I_Na,L can significantly prolong the AP duration (APD) and increase the Na⁺ load in cardiomyocytes, which can enhance the Na⁺-Ca²⁺ exchanger (NCX) activity in the reverse mode (3 Na⁺ extruded from the cell in exchange for 1 Ca²⁺), further increasing the Ca²⁺ load in cardiomyocytes (Fig. 2) (30-33). A prolonged APD in combination with an increased Ca²⁺ load can induce EADs and DADs, eventually lead-ing to VA. In addition, I_Na,L can enhance the Ca²⁺ regulation capacity through the feed-back regulation of CaMKII, thereby participating in the occurrence of VA (16).

The K⁺ current formed by the K⁺ channels of the heart is a key determinant of heart excitability. There are three types of K⁺ currents in the heart: Transient outward K⁺ current (I_to), inward rectifier K⁺ current (I_K1) and delayed rectifier K⁺ current (I_K). I_to is mainly generated by the activation of voltage-gated K⁺ channels with subunits that mainly consist of KV4.2, KV4.3 and KV1.4. I_to produced by KV4.3 is primarily involved in the formation of the first phase of the AP (the early stage of rapid repolarization) (34). I_K1 is primarily produced by activation of the inward rectifier K⁺ channel, which is important for maintaining the resting cell membrane potential and the third phase of the AP (end of rapid repolarization). The inward rectifier K⁺ channel pore-forming subunit is composed of Kir2.1 and Kir6.2. I_K1 is generally considered to be antiarrhythmic as it stabilizes the resting membrane potential (35). I_K is mainly produced by the activation of delayed rectifier K⁺ channel groups with pore-forming subu-nits consisting of K_v1.5, human ether-a-go-go-related gene and K_v7.1, participating in the formation of the second and third phases of the AP (36).

CaMKII induces VA by participating in the regulation of I_to, I_K1 and I_K. Chronic CaMKII activation reduces I_to intensity by reducing the mRNA and protein expression levels of the KV4.2 and KV4.3 subunits. In addition, de-creased expression of the KV4.3 subunit can cause feedback that activates CaMKII. The KV4.3 subunit can also bind to the Ca²⁺/CaM binding site of CaMKII (34). Activation of a large amount of CaMKII can increase its ability to regulate K⁺ channels. Chronic CaMKII activation also reduces the intensity of I_K1 by reducing the mRNA and protein expression levels of the Kir2.1 and Kir6.2 subunits (36,37). A slow change in I_K1 intensity causes the resting membrane potential to be unstable, such that the depolarization current can be transformed into larger DADs, leading to the occurrence of VA (37,38). Chronic activation of CaMKII can phosphorylate the serine 484 site of the KV7.1 subunit, leading to a decrease in I_K intensity (39). It has been suggested that reduction in I_to, I_K1 and I_K intensity can lead to prolongation of the APD, which promotes the occurrence of VA (Fig. 2) (36).

The excitation-contraction coupling of cardiomyocytes is a highly coordinated process that links electrical signals with mechanical contractions. LTCCs can produce an L-type Ca²⁺ current (I_Ca,L) that participates in the formation of the second phase of the AP. LTCCs coupled with Ca²⁺ induces Ca²⁺ release from RyR channels. Increased Ca²⁺ binds to troponin and triggers myofilament contraction. When ventricular myocytes enter the diastolic phase, Ca²⁺ in the cytoplasm is returned to the sarcoplasmic reticulum (SR) through sarco(endo)plasmic reticulum calcium ATPase 2 (SERCA2) (40,41).

CaMKII serves an important role in the regulation of Ca²⁺ homeostasis and has a VA-causing effect. CaMKII activation can increase LTCC phosphorylation, which generates a greater I_Ca,L (32). The serine 2814 site of RyR₂ is phosphorylated upon CaMKII activation, which occurs when the release of Ca²⁺ stored in the diastolic SR abnormally increases (31,42-44). Abnormally released Ca²⁺ propagates along adjacent RyR_S on the SR and activates them to trigger further Ca²⁺ release (8,12). Increased intracellular Ca²⁺ concentrations can participate in the regulation of Na_v channel function through CaMKII activation, thereby adjusting the flow of Na⁺ (45). Excess Ca²⁺ in the cytoplasm is extruded via the NCX, which produces an inward current (I_ti; Fig. 2). When I_ti is sufficient to depolarize the myocardial cell membrane, Na_v channels can be activated, which triggers additional APs and further results in DADs (8,29,31,40,43). When I_Ca,L or I_ti is greater than the outward current (mainly K⁺ current) during the later period of the AP, the APD can be prolonged, which leads to the occurrence of EADs (8). The occurrence of DADs and EADs will eventu-ally lead to VA. However, the threonine 17 (Thr17) site of PLB, which is mainly expressed in the SR to regulate SERCA2 activity, is a specific target of CaMKII phosphorylation. PLB phosphorylation at Thr17 helps to limit cytosolic Ca²⁺ overload by increasing SERCA2 activity and accelerating SR Ca²⁺ reuptake, which is beneficial for improving Ca²⁺ cycle dysfunction and reducing the risk of VA (46-48).