The L-type calcium channel in the heart: the beat goes on

Abstract

Sydney Ringer would be overwhelmed today by the implications of his simple experiment performed over 120 years ago showing that the heart would not beat in the absence of Ca2+. Fascination with the role of Ca2+ has proliferated into all aspects of our understanding of normal cardiac function and the progression of heart disease, including induction of cardiac hypertrophy, heart failure, and sudden death. This review examines the role of Ca2+ and the L-type voltage-dependent Ca2+ channels in cardiac disease.

Figure 1

A model illustrating the Ca²⁺ signaling pathways implicated in hypertrophy and heart failure. Stimulation of the β1-AR activates G_s (stimulatory G protein),, which activates adenylyl cyclase (AC), causing production of cAMP. This stimulates cAMP-dependent PKA, which phosphorylates (P) and alters the function of numerous substrates important for SR Ca²⁺ regulation, including the L-VDCC, RyR2, and phospholamban (PLN). β₂-ARs couple Gi/Ras/MEK1/2/ERK1/2 pathways to hypertrophy (G_i, inhibitory G protein; MEK1/2, mitogen-activated protein kinase kinase). Subsequently, the activated β1-AR is desensitized when it is phosphorylated by β-AR kinase-1 (βARK1). During hypertrophy, β₁-AR expression increases. In heart failure, while the levels of PLN protein expression remain unchanged (or decreased), the phosphorylation status at Ser16 and Thr17 is decreased, even though the levels of SR Ca²⁺-ATPase 2a (SERCA2a) are decreased. Cardiac SR-associated protein phosphatase-1 (PP-1) removes phosphate at Ser16 in PLN and is upregulated in heart failure. Calstabin2 (FKB12.6) plays a role in stabilizing RyR2 in order to help maintain the channel in a closed state during diastole. RyR2 is hyperphosphorylated in heart failure, and calstabin2 dissociates from RyR2. Elevated NCX is an adaptive change in heart failure that becomes maladaptive and may be responsible for both arrhythmogenesis and contractile dysfunction. PKC-α expression and activity are elevated in heart failure. Calcineurin (CN) is activated by sustained elevation of [Ca²⁺]_i. It dephosphorylates nuclear factor of activated T cells (NFAT), enabling its translocation to the nucleus, which is sufficient to induce hypertrophy. Hypertrophic stimuli, such as α1-adrenergic agonists, Ang II, and endothelin-1 (ET-1), all elevate [Ca²⁺]_i and activate the CN-NFAT, Ca²⁺/CaM-CaMKII, and PKC-MAPK-NFAT signaling systems through G protein–coupled receptors (GPCRs) and PLC-DAG-IP₃–dependent Ca²⁺ release [PLC, phospholipase C; DAG, diacylglycerol, IP₃, inositol (1,4,5)-trisphosphate]. Transcription factors, such as myocyte-enhancer factor 2 (MEF2) and GATA4 (cardiac zinc finger transcription factor) are located in the nucleus and serve as endpoints for hypertrophic-signaling pathways. AT₁, type 1 angiotensin II receptors; G_iβγ, βγ subunit of the activated Gi-binding protein; G_q, heterotrimeric GTP-binding protein, consisting of G_α and G_βγ, which dissociate upon receptor activation; NHE, Na⁺/H⁺ exchanger, regulates cytosolic pH; PIP₂, phosphatidylinositol 4,5-biphosphate; T tubule, transverse tubule.

Figure 2

Structural organization of L-VDCCs. The predicted membrane topological organization of the core subunits, their interactions, and structural domains of the auxiliary subunits, which are common to all VDCC types, are shown. The primary structure of the pore-forming α₁ subunit is composed of 4 homologous repeating motifs (I–IV), each of which consists of 6 putative transmembrane segments (S1–S6). The cytoplasmic loops are named according to the motifs they link. The α₂δ and γ (not shown in the Figure) subunits contain transmembrane domains whereas the β subunit is entirely intracellular. Sites of interaction between subunits are indicated. The numbers point to areas found to be important for specific channel functions. The EF hand, A, C, and IQ motifs represent specific peptide sequences involved in CaM binding. Key amino acids required for Ca²⁺ antagonist binding are represented in red letters. At least 5 consensus sites for phosphorylation by cAMP-dependent PKA have been discovered within the C terminal tail of α_1C. AKAP79 (79 kDa A-kinase–anchoring protein) helps to target PKA to its specific substrate. COOH, carboxyterminal.

Figure 3

Model for CDI and VDI. (A) Ca²⁺ channel at rest when no Ca²⁺ influx occurs. At rest, in the absence of Ca²⁺, the CaM binds to peptide A, located between the EF hand and the IQ motif of the C terminus of the L-VDCC α_1C subunit. In response to a depolarizing stimulus, Ca²⁺ enters through the L-VDCC and binds to CaM. In the open Ca²⁺ channel state, the EF hand prevents structural conformation of the I–II loop required to block Ca²⁺ entry through the channel pore (B). In addition, the hydrophobic I1654 in the IQ motif is a stabilizing factor preventing the occlusion of the pore. Upon elevation of [Ca²⁺]_i (depolarization), the Ca²⁺/CaM complex undergoes the Ca²⁺-dependent conformational change that relieves the inhibition of EF hand, permitting the I–II loop to interact with the pore and accelerate the fast inactivation process (C). The graph shows representative I_Ca traces evoked by depolarization from –50 mV to +40 mV, as labeled, using –60 mV as holding potential. (D) Involvement of CaM and CaMKII in the facilitation process. CaMKII enhances the I_Ca through phosphorylation of L-VDCC. We show murine whole-cell I_Ca generated from paired depolarizing pulses (–60 mV ± 10 mV at 0.5 Hz) representing Ca²⁺-dependent facilitation (graph).