Molecular Composition

LTCCs are multimeric proteins consisting of an α₁-subunit that constitutes the pore of the channel and several accessory subunits denoted β, α₂-δ, and γ2,5,6 (Figure 2-1). Currently, four α₁-subunits for L-type Ca²⁺ channels are known, and two of these, Ca_V1.2 (α1C encoded by the CACNA1C gene) and Ca_V1.3 (α1D encoded by the CACNA1D gene), are expressed in the heart^3,5,7 (see Table 2-1). These α₁-subunits form the channel pore and contain the voltage sensor that controls channel gating, as well as drug binding sites and regulatory sites targeted by second messengers. Ca²⁺ channel α₁-subunits have a similar structure to voltage-gated Na⁺ and K⁺ channels, whereby they are organized into four domains (I through IV), each containing six transmembrane segments (S1 through S6). The voltage sensor is located in the S4 transmembrane segment of each domain, and the pore loops are located between S5 and S6. Alternative splice variants of Ca_V1.2 have been documented and can impact regulation by second messengers and drug binding. Recently, alternative splice variants of Ca_V1.3 have also been described.^8,9 These variants result in the expression of long and short forms of Ca_V1.3, which are characterized by differences in biophysical properties such as the activation midpoint; however, not all of the short forms of Ca_V1.3 are expressed in the heart.⁸

Ca²⁺ channel β-subunits (Ca_Vβ₁ through Ca_Vβ₄) are encoded by four genes (CACNB1-4) and, like the α₁-subunits, can be alternatively spliced to generate a number of isoforms.10,11 Crystal structures of LTCC β-subunits demonstrate that these subunits contain conserved Src homology 3 (SH3) and guanylate kinase domains, as seen in scaffolding proteins in the membrane-associated guanylate kinase (MAGUK) family.^12,13 Ca_Vβ-subunits are intracellular proteins (see Figure 2-1) that bind to a single site, called the α interaction domain (AID), on the loop connecting domains I and II on the α₁-subunit of the LTCC. Ca_Vβ acts as a chaperone protein that facilitates the trafficking of LTCCs to the plasma membrane and also modulates the biophysical properties of these channels. Specifically, coexpression of Ca_Vβ2 with Ca_V1.2 increases I_Ca,L amplitude, accelerates activation and inactivation kinetics, and shifts the steady inactivation curve to more negative membrane potentials. Ca_Vβ also profoundly enhances the affinity of dihydropyridines (blockers of LTCCs) for the α₁-subunit by decreasing their dissociation from the channel.¹⁴

Four genes (CACNA2D1-4), which can be alternatively spliced, encode the α₂-δ-subunit of the LTCC.15,16 The α₂-δ₁ and α₂-δ₃ isoforms are expressed in the heart. The protein is cleaved post-translationally and then is relinked via disulfide interactions. This subunit is primarily extracellular, with the δ-subunit portion anchored in the plasma membrane (see Figure 2-1). CaVα₂-δ-subunits facilitate the targeting of LTCCs to the plasma membrane and increase I_Ca,L by shifting the voltage dependence of activation and inactivation to hyperpolarized membrane potentials.

Ca²⁺ channel γ-subunits are membrane-bound proteins encoded by eight genes (CACNG1-8) with several isoforms, including γ₄, γ₆, γ₇, and γ₈, which are present in cardiac muscle. These isoforms associate with Ca_V1.2 and alter activation and inactivation properties of the channel.¹⁷

Biophysical Properties of I_Ca,L

As their name suggests, LTCCs are highly selective for Ca²⁺ over monovalent cations. The channel pore is approximately 6 Å in diameter at its narrowest point,¹⁸ indicating that size is not the main factor in determining selectivity. Rather, a series of four glutamate residues (EEEE motif) is responsible for conferring ion selectivity through the ability of this motif to bind divalent cations (Ca²⁺ and Mg²⁺), which block the passage of monovalent cations.^19,20 As additional divalent ions enter the channel, bound Ca²⁺ ions are displaced from the EEEE motif and are pushed through the pore, which generates I_Ca,L.

At the single-channel level, I_Ca,L conductance in ventricular myocytes (i.e., Ca_V1.2 dependent) has been reported to be approximately 5 pS in 2 mM Ca²⁺ and approximately 15 pS with Ba²⁺ as the charge carrier.²¹ Similarly, single-channel conductance for Ca_V1.3 has been determined in heterologous expression systems and has been found to be approximately 15 pS with Ba²⁺ as the charge carrier.⁸

I_Ca,L is mediated by Ca_V1.2 and Ca_V1.3 in the heart, and these two channel isoforms are distinguished by their unique biophysical properties7,22 (Figure 2-2). Ca_V1.2-mediated I_Ca,L, which is expressed throughout the myocardium (atrium, ventricles, and conduction system), has a bell-shaped current-voltage (I-V) relationship in which the current activates at membrane potentials positive to −40 mV and peaks between 0 and +10 mV. The V_1/2 of channel activation (V_1/2(act) ) is typically between −10 and −15 mV. In contrast, recombinant Ca_V1.3–dependent I_Ca,L activates at more negative membrane potentials (i.e., the V_1/2(act) is hyperpolarized) and displays slower inactivation kinetics.^8,23,24 Ca_V1.3 is also less sensitive to dihydropyridines than is Ca_V1.2.

Ca_V1.2 is the primary determinant of I_Ca,L in the ventricular myocardium, where it plays a prominent role in excitation-contraction coupling and the Ca²⁺ induced–Ca²⁺ release process.¹ Approximately 75% of these LTCCs are located in dyads, in close proximity to the ryanodine receptors located in the junctional sarcoplasmic reticulum, within the t-tubular system (see Figure 2-4). Consistent with the critical role of Ca_V1.2 in the ventricles, Ca_V1.2 knockout mice die before birth in association with cardiac failure.²⁵ Ca_V1.2 is also expressed in the SAN, AVN, and atrial myocardium, along with Ca_V1.3.^22,26 As a result, total I_Ca,L in these supraventricular tissues is dependent on both α1C and α1D pore-forming subunits and demonstrates biophysical characteristics that are different from those of the ventricles. The generation of Ca_V1.3 knockout mice has been important in determining the biophysical properties of these channels and in distinguishing them from Ca_V1.2-mediated I_Ca,L (and I_Ca,T).^23,27 Specifically, these Ca_V1.3 knockouts have been used to show that I_Ca,L in SAN and atrial myocytes activates between −60 and −50 mV in association with a left shift in the V_1/2(act) and peaks at membrane potentials around −10 mV. The steady state I_Ca,L inactivation curve is also shifted to the left when Ca_V1.3 contributes to total I_Ca,L. Together, available data show that Ca_V1.3-dependent I_Ca,L has intermediate biophysical properties compared with Ca_V1.2-dependent I_Ca,L and I_Ca,T.

The unique biophysical properties of Ca_V1.3 enable I_Ca,L to play a prominent role in pacemaker activity in the SAN and in the electrical conduction in the atrial myocardium. In the SAN, Ca_V1.3-mediated I_Ca,L contributes to the diastolic depolarization phase of the action potential and thus is a major determinant of heart rate in vivo. Ca_V1.2, on the other hand, contributes more prominently to the action potential upstroke in SAN myocytes. Consistent with this, Ca_V1.3 knockout mice display sinus bradycardia in association with a reduced diastolic depolarization slope.26–28 Ca_V1.3 knockout mice also show increased susceptibility to atrial fibrillation,^24,29 a very common cardiac arrhythmia, confirming that these Ca²⁺ channels play an important role in atrial conduction.

I_Ca,L Inactivation

I_Ca,L undergoes decay during sustained depolarization, which is termed inactivation (Figure 2-3, A). This inactivation process is time, voltage, and calcium dependent.^30–32 Voltage-dependent activation is relatively slow, as is indicated by the degree of inactivation with Ba²⁺ as the charge carrier, or when monovalent cations permeate LTCCs in the absence of divalent cations.³³ In the presence of extracellular Ca²⁺, inactivation is much faster, and this Ca²⁺-dependent inactivation is further accelerated in the presence of functional SR Ca²⁺ release (i.e., during normal excitation-contraction coupling).³⁴ This indicates that both the Ca²⁺ entering the myocytes through the LTCC and the Ca²⁺ released from the SR during the Ca²⁺ transient contribute to LTCC inactivation.

Ca²⁺-dependent inactivation of I_Ca,L is a calmodulin (CaM) dependent process.35,36 CaM is bound to the C-terminus of the channel via the IQ domain, and when local Ca²⁺ is elevated, an increase in Ca²⁺ binding to CaM occurs. This enhances the interaction of CaM with the IQ domain, thereby causing inactivation of the channel. Ca²⁺-dependent inactivation may serve as a protective negative feedback mechanism to prevent Ca²⁺ overload in cardiomyocytes.

Steady state inactivation (i.e., channel availability), like voltage-dependent activation, follows a sigmoidal relationship, with a V_1/2(inact) of ≈−35 mV for Ca_V1.2-dependent I_Ca,L and −45 mV for Ca_V1.3-dependent I_Ca,L.²² The activation and inactivation curves for I_Ca,L can overlap, resulting in a “window current,” which has been postulated to play a role in the generation of arrhythmogenic early afterdepolarizations (EADs; see Figure 2-3, B).^37,38

LTCCs are also known to undergo a process called facilitation, whereby a progressive increase in I_Ca,L amplitude and the time constant of inactivation can occur during increases in pacing frequency.34,39 The facilitation process occurs as the result of a reduction in Ca²⁺-dependent inactivation and is mediated by calmodulin-dependent kinase II (CaMKII) phosphorylation.⁴⁰

Organization and Localization of LTCCs

It is now appreciated that LTCC can be organized into distinct subcellular compartments through the actions of scaffolding proteins, including A-kinase anchoring proteins (AKAPs),41,42 within cardiomyocytes⁴³ (Figure 2-4). Examples of subpopulations of LTCCs include those found in dyads within the T-tubules and those in separate plasma membrane domains such as caveolae and lipid rafts.² This pattern of organization is critical for the precise spatio-temporal modulation and regulation of LTCCs in different physiological functions such as excitation-contraction coupling, transcriptional regulation, and responses to β-adrenergic (β-AR) receptor activation.

Within dyads, LTCCs from a complex with β-ARs, adenylyl cyclase (AC), protein kinase A (PKA), and AKAPs.2,44,45 These proteins area also in close apposition to a complex of proteins associated with the SR, including ryanodine receptors, PKA, protein phosphatase 2A (PP2A), phosphodiesterases, and AKAPs (AKAP5 and AKAP15).^41,42 It is the AKAPs that are thought to be responsible for the organization of these signaling complexes. Within caveolae, LTCCs associate with β₂-ARs, AC, PKA, PP2A, and caveolin 3, all of which enable LTCCs in caveolae to be locally stimulated by β₂-AR activation.⁴⁶

LTCC also demonstrate a property called coupled gating, which occurs when LTCCs are grouped together into clusters in the plasma membrane, resulting in localized regions with significantly elevated intracellular Ca²⁺ concentrations. Coupled gating is enhanced by the scaffolding protein AKAP5 and by protein kinase C (PKC), which form the signaling complex with the clusters of LTCCs.43,47

Molecular Composition and Biophysical Properties

Although L-type Ca²⁺ channels get the lion’s share of the attention in both cardiac and vascular electrophysiology, low voltage-activated T-type Ca²⁺ channels also play important roles in normal and diseased hearts. Three T-type Ca²⁺ channels exist in mammals: Cav3.1 (α1G), Cav3.2 (α1H), and Cav3.3 (α1I), with Cav3.1 and Cav3.2 channels being the major channels in heart48,49 (see Table 2-1). Although L-type Ca²⁺ channels are complex oligomeric proteins that are heavily regulated by many distinct signal transduction mechanisms, much less is known about the auxiliary subunit structure for T-type Ca²⁺ channels.⁵⁰ The electrophysiological properties of T-type α1-subunits expressed alone are very similar to those observed in native channels, suggesting that T-type Ca²⁺ channels do not require auxiliary subunits, as do L-type Ca²⁺ channels, for proper function. Consistent with this, antisense depletion of Ca_Vβ-subunits has no effect on I_Ca,T (in neurons),^51,52 and coexpression of cloned Ca_Vβ-subunits has no major effect on I_Ca,T in heterologous expression systems.⁵³ Ca_Vγ-subunits have been found to have little or no effect on Ca_V3.3 currents⁵⁴; however, they have been found to accelerate inactivation and negatively shift steady state inactivation of the Ca_V3.1 current.⁵⁵ Coexpression of α₂δ-subunits with α₁G doubled the size of I_Ca,T, possibly through increased expression of α₁-α₂δ complexes at the plasma membrane.^53,56 T-type Ca²⁺ channels have been shown to interact with, and be modulated by, other proteins such as Kelch-like 1 protein⁵⁷ and caveolin-3.⁵⁸ In summary, available data indicate that Ca_V3 α₁-subunits can interact with some auxiliary subunits, but additional studies are needed to determine the physiological role for these interactions, particularly in the heart.

The biophysical properties of T-type Ca²⁺ channels in heart are distinct from those of L-type Ca²⁺ channels.⁵⁹ For example, T-type Ca²⁺ channels activate and inactivate at lower (more negative) membrane potentials with thresholds for activation observed at about −70 mV while inactivation begins to occur at approximately −90 mV⁵⁰ (see Figure 2-2). T-type Ca²⁺ channels also show much faster entry and exit from inactivation^50,60 that is independent of Ca²⁺. An analysis of the steady state activation and inactivation curves reveals that T-type Ca²⁺ channels have relatively large steady state window currents at membrane voltages between −80 and −40 mV,⁶¹ which are expected to facilitate depolarization in myocytes of the SAN and the conducting system, thereby promoting spontaneous action potential firing. Although the permeation properties of T-type Ca²⁺ channels have been challenging to quantify, the single-channel conductance is reported to be 2.5-fold smaller than that of L-type Ca²⁺ channels (when Ba²⁺ is the charge carrier).⁶²

T-type Ca²⁺ channels have distinct pharmacologic properties compared with L-type Ca²⁺ channels, and these differences have been exploited for dissecting the physiologic role of I_Ca,T in the myocardium. Relative to L-type channels, T-type channels have a very high sensitivity to block by Ni²⁺, and this sensitivity varies between different T-type channel isoforms (half maximal inhibitory concentration [IC₅₀] for Cav3.2 is nearly 10-fold lower than that for Cav3.1 channels).50,61,63 Putative T-type channel blockers include pimozide,⁶⁴ efonidipine,⁶⁵ and mibefradil,⁶⁶ although closer scrutiny has revealed that efonidipine and mibefradil also block L-type Ca²⁺ channels at higher concentrations. The only known specific blocker of T-type Ca²⁺ currents (generated by the Cav1.3 or Cav3.2 homologues) is kurtoxin⁶⁷ from scorpions. On the other hand, many putative selective L-type Ca²⁺ channel blockers (such as dihydropyridines and verapamil) also inhibit T-channels.⁶⁸ The development of more selective agents will clearly help to improve our understanding of the role of T-type Ca²⁺ channels in physiological and pathophysiological processes.⁴⁹

Expression Patterns of T-type Ca²⁺ Channels

T-type Ca²⁺ channels show marked developmental and regional differences in expression that have been linked to the unique functions of these channels. For example, T-type Ca²⁺ currents are present in the fetal heart with the expression of Cav3.1 predominating in mouse and Cav3.2 prevailing in rats.69,70 After birth, I_Ca,T levels decline progressively in ventricles and become undetectable in adults while remaining at measurable levels in SAN (≈3 pA/pF), conducting system (≈3 pA/pF), and atria.^49,71–75 In adult hearts of humans⁷⁶ and mice,⁷⁵ Cav3.1 is the more prevalent isoform, and Cav3.2 seems to be more abundant in canine Purkinje fibers,⁷¹ although this topic is controversial.⁷⁷ It is interesting to note that I_Ca,T appear to vary inversely with body size^48,78 in mammals, consistent with the conclusion that these channels influence basal heart rates. Indeed, mice lacking Cav3.1 channels showed bradycardia combined with slowed conduction through the AVN, and no electrical changes were seen in Cav3.2-knockout mice.^79–81 A common finding in diseased ventricles is the elevated expression of Cav3.1 and Cav3.2, along with the reappearance of I_Ca,T,^82–85 possibly resulting from fetal gene reactivation. These observations are particularly intriguing because elevations in I_Ca,T are linked to hypertrophy, disease progression, and arrhythmias.^70,78 In addition, I_CaT is reported to be preferentially expressed in small mononucleated myocytes of adolescent feline hearts,⁶³ possibly representing nascent myocytes originating from cardiac progenitors. In this regard, T-type Ca²⁺ channels, along with hyperpolarization activated cyclic nucleotide gated (HCN) channels, are expressed at high levels in embryonic stem cells and undergo a subtype switch during maturation from Cav3.2 to Cav3.1 α-subunits as they differentiate into cardiac myocytes.⁸⁶ Control of the developmental and regional expression patterns of T-type Ca²⁺ channels is poorly understood. Tbx3 is reported to be important in SAN development, along with expression of Cav3.1 channels and other channel genes associated with SAN.^87,88

T-type Ca²⁺ Channels in Pacemaking, Cardiac Conduction, and Heart Disease

Unlike L-type Ca²⁺ channels, the functional effects of T-type Ca²⁺ channel activity have been more challenging to demonstrate. Pharmacologic and genetic studies support the conclusion that T-type Ca²⁺ channels contribute to pacemaker activity by providing inward current during the diastolic depolarization,22,62,89 although these effects are relatively modest compared with the contributions of other depolarizing currents in SAN myocytes.^70,78 I_Ca,T also contributes to spontaneous firing and conductance properties of the AVN.^22,80,90 Mice with Cav3.1 ablation have reduced heart rates and slowed AVN conduction,⁷⁹ while Cav3.2-deficient mice are indistinguishable from wild type mice.⁹¹ These observations are consistent with the prevalence of Cav3.1 channels, compared with Cav3.2 channels, in the SAN of adult mice, as has been mentioned. A major complication of dissecting the role of T-type Ca²⁺ channels in pacemaker function is the presence of Cav1.3-dependent L-type Ca²⁺ channels,²³ along with Cav1.2 channels, in SAN and AVN. Cav1.3 channels activate at voltages that partially overlap with T-type Ca²⁺ channels, and no pharmacologic agent is sufficiently selective to unequivocally dissect the functional contributions of these channels. Moreover, it is conceivable that T-type channels are expressed in distinct regions of the SAN and AVN from Cav1.3 channels, thereby complicating further our understanding of the relative contributions of T-channels to pacemaker activity. A recent report in mice established that Cav1.3 and Cav3.1 contribute synergistically to pacemaker activity in the SAN and conduction in the AVN under baseline conditions and in response to β-adrenergic receptor stimulation.⁸⁰ These results also confirmed that Cav1.3-mediated I_Ca,L plays a relatively greater role in pacemaker activity and AVN conduction than Cav3.1-mediated I_Ca,T. Given that depolarizing current from Na⁺/Ca²⁺ exchanger secondary to Ca²⁺ release from the sarcoplasmic reticulum contributes to diastolic depolarization^92,93 (later called the Ca²⁺ clock⁹⁴), it remains to be determined to what extent the action of the T-type Ca²⁺ channel currents on pacemaker activity relies on the activation of RYR2 channels in the SR membrane. In this regard, conditional overexpression of Cav3.1 using the α-MHC promoter suggests that T-type Ca²⁺ channels are expressed on the surface sarcolemma in regions distinct from the L-type Ca²⁺ channels and remote from RYR2 channels.⁹⁵

Many studies have concluded, based on pharmacologic interventions, that re-expression of I_Ca,T plays a deleterious role in cardiac remodeling and arrhythmogenesis seen in diseased hearts.83,96 For example, treatment with nickel and mibefradil reduced cell proliferation of neonatal cardiomyocytes induced by hyperglycemia.⁹⁷ Mibefradil also reduced infarct sizes and improved ventricular function in rats subjected to myocardial infarction (MI)⁹⁸ while protecting dog ventricles from partial coronary artery occlusions⁹⁹ and pacing-induced arrhythmias.^100,101 Both efonidipine and mibefradil were also more effective than nitrendipine in reducing sudden death in mice with heart failure.¹⁰² T-type Ca²⁺ channels have been shown to initiate spontaneous activity in pulmonary veins, thereby potentially inducing paroxysmal atrial fibrillation.¹⁰³ These findings are consistent with the ability of efonidipine to prevent atrial remodeling in paced dogs.¹⁰⁴ By contrast, mibefradil has been shown to cause deleterious effects in animal models,^105,106 and treatment of cardiovascular disease in humans with mibefradil has terminated, presumably as a result of off-target effects on the liver.¹⁰⁷ The rationale for targeting T-type Ca²⁺ channels in treating heart disease has come into focus with several recent studies showing that not only do increases in I_CaT (via Cav3.1 overexpression in mice) not induce adverse cardiac remodeling,⁹⁵ they also abrogate cardiac hypertrophy induced by pressure overload, isoproterenol, and exercise.¹⁰⁸ Moreover, Cav3.1-deficient mice show enhanced adverse remodeling in these models and also show accelerated functional deterioration and enhanced arrhythmias following MI, in contrast to mice lacking Cav3.2, which responded similarly to wild type mice. This consensus, however, is at odds with a study showing that Cav3.2 ablation, not Cav3.1 deletion, is protective against pressure overload and angiotensin-induced hypertrophy.⁶⁰

General Structure and Biophysical Properties of TRP Channels

TRP channels are predicted to have a similar membrane topology to other voltage-gated (Na⁺, Ca²⁺, K⁺) ion channels in the heart in which the channel is formed by six transmembrane-spanning domains (S1-S6) with a pore between S5 and S6 (Figure 2-5, A). The N and C termini of these proteins are intracellular and bind a number of regulatory proteins. Similar to the other six transmembrane domain channels, TRP channels are thought to form a tetrameric structure in which each subunit contributes to the selectivity filter and channel pore.¹¹⁰ Most TRP channels, with the exception of TRPA and TRPP, contain a stretch of ≈25 intracellular residues on the C-terminal side of the S6 transmembrane domain (TRP box), from which these channels derive their name.¹²⁰

TRP channels do not contain the series of arginine residues in the S4 transmembrane segment that are typical of other voltage-gated channels, and the location and/or structure of specific gate(s) that control the passage of ions through the channel pore are not well resolved.¹¹⁰ Nevertheless, TRP channel activity is often voltage dependent and characterized by conductance-voltage relationships with relatively shallow slope factors. Furthermore, many TRP channels exhibit the voltage-dependent relaxation seen in tail currents following depolarizing voltage steps.¹²² TRP channel voltage sensitivity is modulated by a number of factors, including ligand binding, temperature, osmolarity, and mechanical perturbations.

TRPC Channels and Cardiac Hypertrophy

Increases in calcium influx have been implicated in pathological hypertrophic signaling in the heart due at least in part to the activation of calcineurin-nuclear factor of activated T cells (NFAT) signaling.¹²³ Although LTCCs have been shown to contribute to this increase in Ca²⁺ influx,^4,124,125 a number of studies have demonstrated that TRPC channel expression and activity are also upregulated in cardiac hypertrophy and heart failure. For example, pressure overload in rodents results in upregulation of TRPC1 and TRPC3.^126,127 Similarly, TRPC6 was upregulated in cardiac hypertrophy and in heart failure.¹²⁸ TRPC5 has been shown to be elevated in human heart failure patients.¹²⁶ Consistent with these findings, pro-hypertrophic agents such as endothelin, phenylephrine, and angiotensin II cause the upregulation of TRPC1 and TRPC3 in cultured cardiomyocytes.^129,130

Recent studies suggest that Ca²⁺ influx through TRPC channels initiates signaling pathways associated with hypertrophy and pathological cardiac remodeling.¹¹⁴ For example, activation of TRPC channels leads to activation of fetal genes (including atrial natriuretic peptide and skeletal α-actin), cell enlargement, and apoptosis. These responses have been shown to be dependent on enhanced calcineurin-NFAT signaling. Consistent with a role for TRPC channels in hypertrophic signaling, the TRPC3 blocker Pyr3 (a Bis-trifluoromethyl pyrazole compound) has been shown to prevent cardiac hypertrophy in pressure-overloaded mice,¹³¹ and TRPC1 knockout mice are less susceptible to cardiac hypertrophy and heart failure.¹²⁷ Similar protective responses have been demonstrated in TRPC3-, TRPC4-, and TRPC6-dominant/negative mutant mice.¹³² Together, these findings suggest an important role for Ca²⁺ entry through TRP channels in hypertrophy and heart failure.

TRP Channels in Cardiac Fibroblasts

Cardiac fibroblasts account for ≈80% of the nonmyocyte population in the heart and are responsible for the synthesis and secretion of extracellular matrix proteins, including collagen types I and II, as well as matrix metalloproteinases.133,134 In the setting of pathological conditions such as hypertension, cardiac hypertrophy, and heart failure, there is an increase in the proliferation of cardiac fibroblasts, which differentiate into myofibroblasts that contribute to the deposition of extracellular matrix and cardiac fibrosis.^133,134 This results in conduction abnormalities, which may be due to both structural remodeling and possible fibroblast-myocyte electrical interactions.¹³⁵

Several TRP channels are expressed in cardiac fibroblasts. In rat ventricular fibroblasts, mRNA for TRPC2, TRPC3, TRPV2, TRPV6, TRPM4, and TRPM7 has been detected,¹¹² while human atrial fibroblasts were shown to express mRNA for TRPC1, TRPC6, TRPC2, TRPC4, and TRPM7.¹¹⁶ These TRP channels facilitate the influx of Ca²⁺ into fibroblasts and may serve as a primary source of Ca²⁺ for these nonexcitable cells, which do not typically express voltage-gated Ca²⁺ channels.¹¹²

Recent data indicate that Ca²⁺ influx into fibroblasts through TRP channels is a significant contributor to fibroblast proliferation and fibrosis in atrial fibrillation. Du et al¹¹⁶ demonstrated that TRPM7 (Figure 2-5, B) is a major pathway for Ca²⁺ entry in human atrial fibroblasts, and that this Ca²⁺ activates transforming growth factor (TGF)-β1, a critical signaling molecule in atrial fibrosis. TRPC channels also constitute a major pathway for Ca²⁺ entry into cardiac fibroblasts via nonselective cation currents¹¹² (Figure 2-5, C), and a recent study¹¹⁵ suggests that Ca²⁺ entry in atrial fibroblasts via TRPC3 channels controls fibroblast proliferation and differentiation in an extracellular signal–related kinase (ERK)-dependent fashion in atrial fibrillation. This study further demonstrates a role for microRNA-26 and NFAT in the TRPC3-dependent enhancement of fibroblast proliferation and fibrosis in atrial fibrillation. The pathophysiological role of TRP channels in cardiac arrhythmias is an emerging area of interest, and the aforementioned studies strongly indicate the need to better understand the relationships between these ion channels and conduction disturbances in the heart.¹³⁶

In conclusion, several ion channels in the heart are responsible for mediating Ca²⁺ influx into myocytes and fibroblasts. Ca_V1.2-mediated I_Ca,L represents a major route for Ca²⁺ entry in all cardiomyocytes throughout the heart, while Ca_V1.3-mediated I_Ca,L is restricted to the SAN, the cardiac conduction system, and the atrial myocardium. T-type Ca²⁺ channels primarily contribute to Ca²⁺ influx in the specialized pacemaker cells of the SAN and AVN in normal physiology, but may also be expressed in ventricular myocytes in the diseased heart. Recently, a number of TRP channels have emerged as important contributors to Ca²⁺ influx in myocytes and fibroblasts. Each of the ion channels contributes importantly to the transport of Ca²⁺ into cells in the heart in normal physiology and in the setting of cardiovascular disease. These channels are importantly regulated by a variety of neurohumoral signaling molecules, and interest remains high in how these channels are or may be targeted for therapeutic interventions.

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