Species-specific difference in distribution of voltage-gated L-type Ca(2+) channels of cardiac myocytes

The Role of Junctophilin Proteins in Cellular Function

Junctophilins (JPHs) comprise a family of structural proteins that connect the plasma membrane to intracellular organelles such as the endo/sarcoplasmic reticulum. Tethering of these membrane structures results in the formation of highly organized subcellular junctions that play important signaling roles in all excitable cell types. There are four JPH isoforms, expressed primarily in muscle and neuronal cell types. Each JPH protein consists of 6 'membrane occupation and recognition nexus' (MORN) motifs, a joining region connecting these to another set of 2 MORN motifs, a putative alpha-helical region, a divergent region exhibiting low homology between JPH isoforms, and a carboxy-terminal transmembrane region anchoring into the ER/SR membrane. JPH isoforms play essential roles in developing and maintaining subcellular membrane junctions. Conversely, inherited mutations in JPH2 cause hypertrophic or dilated cardiomyopathy, while trinucleotide expansions in the JPH3 gene cause Huntington Disease-Like 2. Loss of JPH1 protein levels can cause skeletal myopathy, while loss of cardiac JPH2 levels causes heart failure and atrial fibrillation, among other disease. This review will provide a comprehensive overview of the JPH gene family, phylogeny, and evolutionary analysis of JPH genes and other MORN domain proteins. JPH biogenesis, membrane tethering, and binding partners will be discussed, as well as functional roles of JPH isoforms in excitable cells. Finally, potential roles of JPH isoform deficits in human disease pathogenesis will be reviewed.

The long and winding road of cardiomyocyte maturation

Knowledge about the molecular mechanisms regulating cardiomyocyte (CM) proliferation and differentiation has increased exponentially in recent years. Such insights together with the availability of more efficient protocols for generation of CMs from induced pluripotent stem cells (iPSCs) have raised expectations for new therapeutic strategies to treat congenital and non-congenital heart diseases. However, the poor regenerative potential of the postnatal heart and the incomplete maturation of iPSC-derived CMs represent important bottlenecks for such therapies in future years. CMs undergo dramatic changes at the doorstep between prenatal and postnatal life, including terminal cell cycle withdrawal, change in metabolism, and further specialization of the cellular machinery required for high-performance contraction. Here, we review recent insights into pre- and early postnatal developmental processes that regulate CM maturation, laying specific focus on genetic and metabolic pathways that control transition of CMs from the embryonic and perinatal to the fully mature adult CM state. We recapitulate the intrinsic features of CM maturation and highlight the importance of external factors, such as energy substrate availability and endocrine regulation in shaping postnatal CM development. We also address recent approaches to enhance maturation of iPSC-derived CMs in vitro, and summarize new discoveries that might provide useful tools for translational research on repair of the injured human heart.

Beyond Intracellular Signaling: The Ins and Outs of Second Messengers Microdomains

A typical characteristic of eukaryotic cells compared to prokaryotes is represented by the spatial heterogeneity of the different structural and functional components: for example, most of the genetic material is surrounded by a highly specific membrane structure (the nuclear membrane), continuous with, yet largely different from, the endoplasmic reticulum (ER); oxidative phosphorylation is carried out by organelles enclosed by a double membrane, the mitochondria; in addition, distinct domains, enriched in specific proteins, are present in the plasma membrane (PM) of most cells. Less obvious, but now generally accepted, is the notion that even the concentration of small molecules such as second messengers (Ca²⁺ and cAMP in particular) can be highly heterogeneous within cells. In the case of most organelles, the differences in the luminal levels of second messengers depend either on the existence on their membrane of proteins that allow the accumulation/release of the second messenger (e.g., in the case of Ca²⁺, pumps, exchangers or channels), or on the synthesis and degradation of the specific molecule within the lumen (the autonomous intramitochondrial cAMP system). It needs stressing that the existence of a surrounding membrane does not necessarily imply the existence of a gradient between the cytosol and the organelle lumen. For example, the nuclear membrane is highly permeable to both Ca²⁺ and cAMP (nuclear pores are permeable to solutes up to 50 kDa) and differences in [Ca²⁺] or [cAMP] between cytoplasm and nucleoplasm are not seen in steady state and only very transiently during cell activation. A similar situation has been observed, as far as Ca²⁺ is concerned, in peroxisomes.

Three-Dimensional Reconstruction of Cardiac Sarcoplasmic Reticulum Reveals a Continuous Network Linking Transverse-Tubules

Rationale:

The organization of the transverse-tubular (t-t) system and relationship to the sarcoplasmic reticulum (SR) underpins cardiac excitation–contraction coupling. The architecture of the SR, and relationship with the t-ts, is not well characterized at the whole-cell level. Furthermore, little is known regarding changes to SR ultrastructure in heart failure.

Objective:

The aim of this study was to unravel interspecies differences and commonalities between the relationship of SR and t-t networks within cardiac myocytes, as well as the modifications that occur in heart failure, using a novel high-resolution 3-dimensional (3D) imaging technique.

Methods and Results:

Using serial block face imaging coupled with scanning electron microscopy and image analysis, we have generated 3D reconstructions of whole cardiomyocytes from sheep and rat left ventricle, revealing that the SR forms a continuous network linking t-ts throughout the cell in both species. In sheep, but not rat, the SR has an intimate relationship with the sarcolemma forming junctional domains. 3D reconstructions also reveal details of the sheep t-t system. Using a model of tachypacing-induced heart failure, we show that there are populations of swollen and collapsed t-ts, patches of SR tangling, and disorder with rearrangement of the mitochondria.

Conclusions:

We provide the first high-resolution 3D structure of the SR network showing that it forms a cell-wide communication pipeline facilitating Ca 2+ diffusion, buffering, and synchronicity. The distribution of the SR within the cell is related to interspecies differences in excitation–contraction coupling, and we report the first detailed analysis of SR remodeling as a result of heart failure.

A quantitative comparison of regional myocardial motion in mice, rabbits and humans using in-vivo phase contrast CMR

BACKGROUND

Genetically manipulated animals like mice or rabbits play an important role in the exploration of human cardiovascular diseases. It is therefore important to identify animal models that closely mimic physiological and pathological human cardiac function.

METHODS

In-vivo phase contrast cardiovascular magnetic resonance (CMR) was used to measure regional three-directional left ventricular myocardial motion with high temporal resolution in mice (N=18), rabbits (N=8), and humans (N=20). Radial, long-axis, and rotational myocardial velocities were acquired in left ventricular basal, mid-ventricular, and apical short-axis locations.

RESULTS

Regional analysis revealed different patterns of motion: 1) In humans and rabbits, the apex showed slower radial velocities compared to the base. 2) Significant differences within species were seen in the pattern of long-axis motion. Long-axis velocities during systole were fairly homogeneously distributed in mice, whereas humans showed a dominant component in the lateral wall and rabbits in the base. 3) Rotational velocities and twist showed the most distinct patterns in both temporal evolution and relative contribution of base, mid-ventricle and apex, respectively. Interestingly, a marked difference in rotational behavior during early-systole was found in mice, which exhibited clockwise rotation in all slice locations compared to counter-clockwise rotation in rabbits and humans.

CONCLUSIONS

Phase contrast CMR revealed subtle, but significantly different regional myocardial motion patterns in mice, rabbits and humans. This finding has to be considered when investigating myocardial motion pattern in small animal models of heart disease.

Role of mixed ion channel effects in the cardiovascular safety assessment of the novel anti-MRSA fluoroquinolone JNJ-Q2

BACKGROUND AND PURPOSE

JNJ-Q2, a novel broad-spectrum fluoroquinolone with anti-methicillin-resistant Staphylococcus aureus activity, was evaluated in a comprehensive set of non-clinical and clinical cardiovascular safety studies. The effect of JNJ-Q2 on different cardiovascular parameters was compared with that of moxifloxacin, sparfloxacin and ofloxacin. Through comparisons with these well-known fluoroquinolones, the importance of effects on compensatory ion channels to the cardiovascular safety of JNJ-Q2 was investigated.

EXPERIMENTAL APPROACH

JNJ-Q2 and comparator fluoroquinolones were evaluated in the following models/test systems: hERG-transfected HEK293 cells sodium channel-transfected CHO cells, guinea pig right atria, arterially perfused rabbit left ventricular wedge preparations and in vivo studies in anaesthetized guinea pigs, anaesthetized and conscious telemetered dogs, and a thorough QT study in humans.

KEY RESULTS

The trend for effects of JNJ-Q2 on Tp-Te, QT, QRS and PR intervals in the non-clinical models and the plateau in QTc with increasing plasma concentration in humans are consistent with offsetting sodium and calcium channel activities that were observed in the non-clinical studies. These mixed ion channel activities result in the less pronounced or comparable increase in QTc interval for JNJ-Q2 compared with moxifloxacin and sparfloxacin despite its greater in vitro inhibition of I(Kr).

CONCLUSIONS AND IMPLICATIONS

Based on the non-clinical and clinical cardiovascular safety assessment, JNJ-Q2 has a safe cardiovascular profile for administration in humans with comparable or reduced potential to prolong QT intervals, compared with moxifloxacin. The results demonstrate the importance of compensatory sodium and calcium channel activity in offsetting potassium channel activity for compounds with a fluoroquinolone core.

Molecular Determinants of Cav1.2 Calcium Channel Inactivation

Voltage-gated L-type Ca_v1.2 calcium channels couple membrane depolarization to transient increase in cytoplasmic free Ca²⁺ concentration that initiates a number of essential cellular functions including cardiac and vascular muscle contraction, gene expression, neuronal plasticity, and exocytosis. Inactivation or spontaneous termination of the calcium current through Ca_v1.2 is a critical step in regulation of these processes. The pathophysiological significance of this process is manifested in hypertension, heart failure, arrhythmia, and a number of other diseases where acceleration of the calcium current decay should present a benefit function. The central issue of this paper is the inactivation of the Ca_v1.2 calcium channel mediated by multiple determinants.

Modeling effects of L-type ca(2+) current and na(+)-ca(2+) exchanger on ca(2+) trigger flux in rabbit myocytes with realistic T-tubule geometries

The transverse tubular system of rabbit ventricular myocytes consists of cell membrane invaginations (t-tubules) that are essential for efficient cardiac excitation-contraction coupling. In this study, we investigate how t-tubule micro-anatomy, L-type Ca²⁺ channel (LCC) clustering, and allosteric activation of Na⁺/Ca²⁺ exchanger by L-type Ca²⁺ current affects intracellular Ca²⁺ dynamics. Our model includes a realistic 3D geometry of a single t-tubule and its surrounding half-sarcomeres for rabbit ventricular myocytes. The effects of spatially distributed membrane ion-transporters (LCC, Na⁺/Ca²⁺ exchanger, sarcolemmal Ca²⁺ pump, and sarcolemmal Ca²⁺ leak), and stationary and mobile Ca²⁺ buffers (troponin C, ATP, calmodulin, and Fluo-3) are also considered. We used a coupled reaction-diffusion system to describe the spatio-temporal concentration profiles of free and buffered intracellular Ca²⁺. We obtained parameters from voltage-clamp protocols of L-type Ca²⁺ current and line-scan recordings of Ca²⁺ concentration profiles in rabbit cells, in which the sarcoplasmic reticulum is disabled. Our model results agree with experimental measurements of global Ca²⁺ transient in myocytes loaded with 50 μM Fluo-3. We found that local Ca²⁺ concentrations within the cytosol and sub-sarcolemma, as well as the local trigger fluxes of Ca²⁺ crossing the cell membrane, are sensitive to details of t-tubule micro-structure and membrane Ca²⁺ flux distribution. The model additionally predicts that local Ca²⁺ trigger fluxes are at least threefold to eightfold higher than the whole-cell Ca²⁺ trigger flux. We found also that the activation of allosteric Ca²⁺-binding sites on the Na⁺/Ca²⁺ exchanger could provide a mechanism for regulating global and local Ca²⁺ trigger fluxes in vivo. Our studies indicate that improved structural and functional models could improve our understanding of the contributions of L-type and Na⁺/Ca²⁺ exchanger fluxes to intracellular Ca²⁺ dynamics.

Different subcellular populations of L-type Ca2+ channels exhibit unique regulation and functional roles in cardiomyocytes

Influx of Ca(2+) through L-type Ca(2+) channels (LTCCs) contributes to numerous cellular processes in cardiomyocytes including excitation-contraction (EC) coupling, membrane excitability, and transcriptional regulation. Distinct subpopulations of LTCCs have been identified in cardiac myocytes, including those at dyadic junctions and within different plasma membrane microdomains such as lipid rafts and caveolae. These subpopulations of LTCCs exhibit regionally distinct functional properties and regulation, affording precise spatiotemporal modulation of L-type Ca(2+) current (I(Ca,L)). Different subcellular LTCC populations demonstrate variable rates of Ca(2+)-dependent inactivation and sometimes coupled gating of neighboring channels, which can lead to focal, persistent I(Ca,L). In addition, the assembly of spatially defined macromolecular signaling complexes permits compartmentalized regulation of I(Ca,L) by a variety of neurohormonal pathways. For example, β-adrenergic receptor subtypes signal to different LTCC subpopulations, with β(2)-adrenergic activation leading to enhanced I(Ca,L) through caveolar LTCCs and β(1)-adrenergic stimulation modulating LTCCs outside of caveolae. Disruptions in the normal subcellular targeting of LTCCs and associated signaling proteins may contribute to the pathophysiology of a variety of cardiac diseases including heart failure and certain arrhythmias. Further identifying the characteristic functional properties and array of regulatory molecules associated with specific LTCC subpopulations will provide a mechanistic framework to understand how LTCCs contribute to diverse cellular processes in normal and diseased myocardium. This article is part of a Special Issue entitled "Local Signaling in Myocytes".

Rate-dependent shortening of action potential duration increases ventricular vulnerability in failing rabbit heart

Congestive heart failure (CHF) predisposes to ventricular fibrillation (VF) in association with electrical remodeling of the ventricle. However, much remains unknown about the rate-dependent electrophysiological properties in a failing heart. Action potential properties in the left ventricular subepicardial muscles during dynamic pacing were examined with optical mapping in pacing-induced CHF ( n = 18) and control ( n = 17) rabbit hearts perfused in vitro. Action potential durations (APDs) in CHF were significantly longer than those observed for controls at basic cycle lengths (BCLs) >1,000 ms but significantly shorter at BCLs <400 ms. Spatial APD dispersions were significantly increased in CHF versus control (by 17–81%), and conduction velocity was significantly decreased in CHF (by 6–20%). In both groups, high-frequency stimulation (BCLs <150 ms) always caused spatial APD alternans; spatially concordant alternans and spatially discordant alternans (SDA) were induced at 60% and 40% in control, respectively, whereas 18% and 82% in CHF. SDA in CHF caused wavebreaks followed by reentrant excitations, giving rise to VF. Incidence of ventricular tachycardia/VFs elicited by high-frequency dynamic pacing (BCLs <150 ms) was significantly higher in CHF versus control (93% vs. 20%). In CHF, left ventricular subepicardial muscles show significant APD shortenings at short BCLs favoring reentry formations following wavebreaks in association with SDA. High-frequency excitation itself may increase the vulnerability to VF in CHF.

Membrane localization, Caveolin-3 association and rapid actions of vitamin D receptor in cardiac myocytes

The active form of vitamin D, 1alpha, 25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)), mediates both genomic and rapid non-genomic actions in heart cells. We have previously shown that the vitamin D receptor (VDR) is located in the t-tubular structure of cardiomyocytes. Here we show that VDR specifically interacts with Caveolin-3 in the t-tubules and sarcolemma of adult rat cardiac myocytes. Co-immunoprecipitation studies using VDR antibodies revealed that Caveolin-3 specifically co-precipitates with the VDR and similarly the VDR is co-precipitated with Caveolin-3 antibody. Confocal immuno-fluorescence microscopy analysis also showed co-localization of VDR and Caveolin-3 in t-tubules and sarcolemma. The non-genomic effects of the functional VDR were studied in electrically stimulated myocytes isolated from adult rat hearts. Sarcomere shortening and re-lengthening were measured in 1,25(OH)(2)D(3) treated cardiac myocytes. A 1nM treatment decreased peak shortening within minutes, suggesting a rapid effect through the membrane-bound VDR. This novel finding of the interaction between VDR and Caveolin-3 is fundamentally important in understanding 1,25(OH)(2)D(3) signal transduction in heart cells and provides further evidence that VDR plays a role in regulation of heart structure and function.

Interaction between vitamin D receptor with caveolin-3 and regulation by 1,25-dihydroxyvitamin D3 in adult rat cardiomyocytes

We show that 1alpha,25-dihydroxyvitamin D3 (1,25(OH)2D3) and a synthetic non-genotropic vitamin D analog agonist, 1a,25(OH)2-lumisterol (JN), exhibit similar rapid effects on sarcomere shortening (contraction) of isolated adult cardiomyocyte. We also report that the vitamin D receptor (VDR) specifically interacts with caveolin-3 in the t-tubules and sarcolemma of isolated adult rat cardiac myocytes. Confocal immunofluorescence microscopy analysis showed co-localization of VDR and caveolin-3 in the t-tubules and sarcolemma of cardiomyocytes. Co-immunoprecipitation studies using VDR antibodies revealed that caveolin-3 specifically co-precipitates with the VDR and similarly the VDR is co-precipitated with caveolin-3 antibody. VDR is also in association with Serca-2, the sarcoplasmic reticulum Ca2+-ATPase, as demonstrated by co-immunoprecipitation, suggesting a role of VDR in regulating cardiac contractility by direct interaction with Serca-2. Treatment of isolated adult rat cardiomyocytes with 10 nM 1,25(OH)2D3 for 1 h caused decreased association between VDR and caveolin-3. These discoveries of the association between VDR and caveolin-3 and the regulation of this interaction by 1,25(OH)2D3 are fundamentally important in understanding 1,25(OH)2D3 signal transduction in heart cells and suggest a novel mechanism for VDR in the regulation of heart structure and function.

Excitation-contraction coupling changes during postnatal cardiac development

Cardiac contraction is initiated by the release of Ca(2+) from intracellular stores in response to an action potential, in a process known as "excitation-contraction coupling" (ECC). Here we investigate the maturation of ECC in the rat heart during postnatal development. We provide new information on how proteins of the sarcoplasmic reticulum (SR) and the t-tubules (TTs) assemble to form the structures that support EC coupling during postnatal development. We show that the surface membrane protein, caveolin-3 (Cav3), is a good protein marker for TTs in ventricular myocytes and compared it quantitatively to junctophilin-2 (JP2), a protein found on the SR at sites of SR-TT junctions, or couplons. Although JP2 and Cav3 associate primarily with the SR and TTs, respectively, we found that they occupy the appropriate sites at maturing structures in synchrony, as visualized with high resolution, quantitative 3-dimensional imaging. We also found the surprising result that while both ryanodine receptor type 2, (RyR2) and JP2 proteins are localized to the same membrane and sub-compartments, they assume their positions at very different rates: RyR2 moves to the SR membrane at the Z-disc very early in development while JP2 only appears in the SR membrane as the TTs mature. Our data suggest that, although RyR2 appears to be prepositioned at the sites ultimately occupied by dyad junctions, JP2 arrives at these sites in synchrony with the development of the TTs at the Z-discs. Finally, we report that EC coupling efficiency changes with development, in concert with these structural changes. Thus we provide the first well-integrated information that links the developing organization of proteins underlying EC coupling (RyR2, DHPR, Cav3 and JP2) to the developing efficacy of EC coupling.

Effect of Ca(v)beta subunits on structural organization of Ca(v)1.2 calcium channels

BACKGROUND

Voltage-gated Ca(v)1.2 calcium channels play a crucial role in Ca(2+) signaling. The pore-forming alpha(1C) subunit is regulated by accessory Ca(v)beta subunits, cytoplasmic proteins of various size encoded by four different genes (Ca(v)beta(1)-beta(4)) and expressed in a tissue-specific manner.

METHODS AND RESULTS

Here we investigated the effect of three major Ca(v)beta types, beta(1b), beta(2d) and beta(3), on the structure of Ca(v)1.2 in the plasma membrane of live cells. Total internal reflection fluorescence microscopy showed that the tendency of Ca(v)1.2 to form clusters depends on the type of the Ca(v)beta subunit present. The highest density of Ca(v)1.2 clusters in the plasma membrane and the smallest cluster size were observed with neuronal/cardiac beta(1b) present. Ca(v)1.2 channels containing beta(3), the predominant Ca(v)beta subunit of vascular smooth muscle cells, were organized in a significantly smaller number of larger clusters. The inter- and intramolecular distances between alpha(1C) and Ca(v)beta in the plasma membrane of live cells were measured by three-color FRET microscopy. The results confirm that the proximity of Ca(v)1.2 channels in the plasma membrane depends on the Ca(v)beta type. The presence of different Ca(v)beta subunits does not result in significant differences in the intramolecular distance between the termini of alpha(1C), but significantly affects the distance between the termini of neighbor alpha(1C) subunits, which varies from 67 A with beta(1b) to 79 A with beta(3).

CONCLUSIONS

Thus, our results show that the structural organization of Ca(v)1.2 channels in the plasma membrane depends on the type of Ca(v)beta subunits present.

Calcium channel inactivation: possible role in signal transduction and Ca2+ signaling

Voltage gated Ca2+ channels are major routes for the entry of intracellular Ca2+ coupled to membrane depolarization that appear to vary greatly with respect to their voltage dependence and kinetics. Such variability maybe in part related to the attached signaling properties of the channel, in addition to the transport of calcium. In the present review we consider the possible role of calcium-dependent inactivation of Cav1.2 in Ca2+ signal transduction and signaling of calcium release from the cardiac sarcoplasmic reticulum. We explore the specific roles of Ca2+-sensing calmodulin-binding domains of the C-terminal tail (LA and K) of the channel in mediating Ca2+-induced Ca2+ release and signal transduction. Our experiments point to an intriguing possibility that the C-terminal tail of Cav1.2 may translocate the Ca2+ signal as a part of inactivation mechanism and the corresponding voltage-gated rearrangement of the C-terminus. We show how a dynamic and transient regulation, in a Ca2+-dependent manner, defines molecular events including Ca2+ release and signaling of cAMP-responsive element-binding protein (CREB)-dependent transcription. We propose that such Ca2+-dependent C-tail translocation that also initiates the channel inactivation, may have evolved specifically for the Cav1.2 channel.

Silencing of ZnT-1 expression enhances heavy metal influx and toxicity

ZnT-1 reduces intracellular zinc accumulation and confers resistance against cadmium toxicity by a mechanism which is still unresolved. A functional link between the L-type calcium channels (LTCC) and ZnT-1 has been suggested, indicating that ZnT-1 may regulate ion permeation through this pathway. In the present study, immunohistochemical analysis revealed a striking overlap of the expression pattern of LTCC and ZnT-1 in cardiac tissue and brain. Using siRNA to silence ZnT-1 expression, we then assessed the role of ZnT-1 in regulating cation permeation through the L-type Ca(2+) channels in cells that are vulnerable to heavy metal permeation. Transfection of cortical neurons with ZnT-1 siRNA resulted in about 70% reduction of ZnT-1 expression and increased Ca(2+) influx via LTCC by approximately fourfold. Moreover, ZnT-1 siRNA transfected neurons showed approximately 30% increase in synaptic release, monitored using the FM1-43 dye. An increased cation influx rate, through the LTCC, was also recorded for Zn(2+) and Cd(2+) in cells treated with the ZnT-1 siRNA. Furthermore, Cd(2+)-induced neuronal death increased by approximately twofold after transfection with ZnT-1 siRNA. In addition, ZnT-1 siRNA transfection of the ovarian granulosa cell line, POGRS1, resulted in a twofold increase in Cd(2+) influx rate via the LTCC. Finally, a robust nimodipine-sensitive Cd(2+) influx was observed using a low extracellular Cd(2+) concentration (5 muM) in neurons and testicular slice cultures, attesting to the relevance of the LTCC pathway to heavy metal toxicity. Taken together, our results indicate that endogenously-expressed ZnT-1, by modulating LTCC, has a dual role: regulating calcium influx, and attenuating Cd(2+) and Zn(2+) permeation and toxicity in neurons and other cell types.

Effects of low-calcium reperfusion and adenosine on diastolic behavior during the transitory systolic overshoot of the stunned myocardium in the rabbit

The aims of the present study were to determine whether the transitory systolic overshoot (TSO) that occurs in the early reperfusion (R) of the stunned myocardium is accompanied by diastolic alterations, and to determine whether the R with low Ca2+ Krebs–Henseleit’s solution or with adenosine modifies these alterations. Isolated–isovolumic rabbit hearts were divided in 3 groups (G). G1 (n = 11) was perfused with Krebs–Henseleit's solution, subjected to 15 min of global ischemia and 30 min R; G2 (n = 10) was reperfused during the first 10 min with Krebs–Henseleit's solution [Ca2+] = 1 mmol/L, which was increased in the perfusate to 1.5 mmol/L up to 20 min R and at 2.5 mmol/L from 20 to 30 min R. G3 (n = 12) was perfused with Krebs–Henseleit's solution with adenosine (0.03 μg·kg–1·min –1) from 10 min before ischemia and during all R. Left ventricular (LV) +dP/dtmax (mmHg/s), LV end diastolic pressure (LVEDP, mmHg), and 1 relaxation index (t1/2) were measured in preischemic state, at 30, 50, 60, 70, 90, and 120 s R, and then at 5 and 30 min R. The +dP/dtmax recovered to 621 ± 77 mmHg/s (p > 0.05), 346 ± 31 mmHg/s (p < 0.05 vs. G1), and 533 ± 76 mmHg/s (p > 0.05) from preischemic value of 730 ± 39, 690 ± 32, and 758 ± 57 in G1, G2, and G3, respectively. The LVEDP in G1 and G3 increased early in the R, and it was negatively correlated with the +dP/dtmax (r = –0.63, p = 0.0369; and r = –0.71, p = 0.0090, respectively). The R with low Ca2+ abolished this correlation and attenuated the TSO phase. The correlation between LVEDP and +dP/dtmax in G1 and G3 and the lack of correlation in G2 suggests there are common mechanisms for the systolic and diastolic alterations during the TSO phase that are possibly related to Ca2+ overload but not with the vascular tone.

Alterations in dihydropyridine receptors in dystrophin-deficient cardiac muscle

The deficiency of dystrophin, a critical membrane stabilizing protein, in the mdx mouse causes an elevation in intracellular calcium in myocytes. One mechanism that could elicit increases in intracellular calcium is enhanced influx via the L-type calcium channels. This study investigated the effects of the dihydropyridines BAY K 8644 and nifedipine and alterations in dihydropyridine receptors in dystrophin-deficient mdx hearts. A lower force of contraction and a reduced potency of extracellular calcium (P < 0.05) were evident in mdx left atria. The dihydropyridine agonist BAY K 8644 and antagonist nifedipine had 2.7- and 1.9-fold lower potencies in contracting left atria (P < 0.05). This corresponded with a 2.0-fold reduction in dihydropyridine receptor affinity evident from radioligand binding studies of mdx ventricular homogenates (P < 0.05). Increased ventricular dihydropyridine receptor protein was evident from both radioligand binding studies and Western blot analysis and was accompanied by increased mRNA levels (P < 0.05). Patch-clamp studies in isolated ventricular myocytes showed no change in L-type calcium current density but revealed delayed channel inactivation (P < 0.05). This study indicates that a deficiency of dystrophin leads to changes in dihydropyridine receptors and L-type calcium channel properties that may contribute to enhanced calcium influx. Increased influx is a potential mechanism for the calcium overload observed in dystrophin-deficient cardiac muscle.

Variability in couplon size in rabbit ventricular myocytes

Variation in couplon size is thought to be essential for graded Ca(2+) transients in cardiac myocytes. We examined this variation by investigating spark appearance in rabbit ventricular myocytes at various locations and at potentials from -20 to 0 mV. At 0 mV, sparks appeared at the beginning of the voltage step with a probability of unity. On the other hand, at -20 mV, sparks appeared later during the voltage step with a lower probability. The cumulative spark probabilities at various potentials were fitted with exponential functions of both time and voltage. Spark latency became longer as spark probability decreased at more negative potentials. At -20 mV, the cumulative spark probability and the mean spark latency were not only variable among locations but also inversely related. Under the assumption that a single opening of an L-type Ca(2+) channel triggers a spark, we suggest a simple mathematical explanation for the distribution of spark appearance. The variation in spark probability and latency with location suggests that the couplon size, and hence the number of L-type Ca(2+) channels in a couplon is variable.

Ca2+ currents in cardiac myocytes: Old story, new insights

Calcium is a ubiquitous second messenger which plays key roles in numerous physiological functions. In cardiac myocytes, Ca2+ crosses the plasma membrane via specialized voltage-gated Ca2+ channels which have two main functions: (i) carrying depolarizing current by allowing positively charged Ca2+ ions to move into the cell; (ii) triggering Ca2+ release from the sarcoplasmic reticulum. Recently, it has been suggested than Ca2+ channels also participate in excitation-transcription coupling. The purpose of this review is to discuss the physiological roles of Ca2+ currents in cardiac myocytes. Next, we describe local regulation of Ca2+ channels by cyclic nucleotides. We also provide an overview of recent studies investigating the structure-function relationship of Ca2+ channels in cardiac myocytes using heterologous system expression and transgenic mice, with descriptions of the recently discovered Ca2+ channels alpha(1D) and alpha(1E). We finally discuss the potential involvement of Ca2+ currents in cardiac pathologies, such as diseases with autoimmune components, and cardiac remodeling.

Expression of glutamate transporters in rat cardiomyocytes and their localization in the T-tubular system

Glutamate and aspartate play important roles in the intermediary metabolism of the myocardium and have been shown to improve cardiac recovery after hypoxia or ischemia. Limited data are available about the expression of glutamate transporters that are involved in the uptake of glutamate and aspartate in cardiomyocytes. In this study, non-radioactive in situ hybridization (ISH) using complementary RNA probes was applied to detect the glutamate transporters GLT1 variant (GLT1v) and EAAC1 mRNA in rat cardiomyocytes. The transporter proteins were demonstrated by Western blotting and immunocytochemistry using affinity-purified antibodies against transporter peptides. ISH and immunocytochemistry showed that both glutamate transporters are coexpressed in cardiomyocytes. The ISH labeling indicates the distribution of transporter mRNA throughout the cytoplasm of cardiomyocytes. GLT1v and EAAC1 proteins, which showed in Western blots a molecular mass of approximately 60 kD, are strongly enriched and colocalized in the transverse (T)-tubular system of cardiomyocytes. These results may indicate that glutamate/aspartate uptake into cardiomyocytes could be mediated by the high-affinity transporters GLT1v and EAAC1. A high efficiency of glutamate/aspartate transport into cardiomyocytes could be achieved by their localization in the T-tubular system, which consists of tubular invaginations of the sarcolemma extending deep into the cell.

Subcellular localization of the delayed rectifier K+channels KCNQ1 and ERG1 in the rat heart

In the heart, several K+channels are responsible for the repolarization of the cardiac action potential, including transient outward and delayed rectifier K+currents. In the present study, the cellular and subcellular localization of the two delayed rectifier K+channels, KCNQ1 and ether- a- go- go-related gene-1 (ERG1), was investigated in the adult rat heart. Confocal immunofluorescence microscopy of atrial and ventricular cells revealed that whereas KCNQ1 labeling was detected in both the peripheral sarcolemma and a structure transversing the myocytes, ERG1 immunoreactivity was confined to the latter. Immunoelectron microscopy of atrial and ventricular myocytes showed that the ERG1 channel was primarily expressed in the transverse tubular system and its entrance, whereas KCNQ1 was detected in both the peripheral sarcolemma and in the T tubules. Thus, whereas ERG1 displays a very restricted subcellular localization pattern, KCNQ1 is more widely distributed within the cardiac cells. The localization of these K+channels to the transverse tubular system close to the Ca2+channels renders them with maximal repolarizing effect.

T-tubule function in mammalian cardiac myocytes

The transverse tubules (t-tubules) of mammalian cardiac ventricular myocytes are invaginations of the surface membrane. Recent studies have suggested that the structure and function of the t-tubules are more complex than previously believed; in particular, many of the proteins involved in cellular Ca2+ cycling appear to be concentrated at the t-tubule. Thus, the t-tubules are an important determinant of cardiac cell function, especially as the main site of excitation-contraction coupling, ensuring spatially and temporally synchronous Ca2+ release throughout the cell. Changes in t-tubule structure and protein expression occur during development and in heart failure, so that changes in the t-tubules may contribute to the functional changes observed in these conditions. The purpose of this review is to provide an overview of recent studies of t-tubule structure and function in cardiac myocytes.

Ca2+ sparks in rabbit ventricular myocytes evoked by action potentials: involvement of clusters of L-type Ca2+ channels

It is not clear how many L-type Ca2+ channels (LCCs) are required to ensure that a Ca2+ spark is triggered during a normal mammalian action potential (AP). We investigated this in rabbit ventricular myocytes by examining both the properties of sparks evoked by APs and the activity of LCCs. We measured Ca2+ sparks evoked by repeated APs with pipettes containing 2 mmol/L EGTA and single LCC activity in cell-attached patches depolarized to +50 mV using pipettes containing 110 mmol/L Ba2+. With 2 mmol/L Ca2+ in the external solution, we observed sparks at the beginning of every evoked AP at numerous locations. Each spark was observed repeatedly at a fixed location and began during a limited interval after the AP peak. These sparks occurred with a probability of approximately unity. However, the chance that an LCC does not open during the interval when a spark is triggered is quite high ( approximately 0.13). Therefore, because single channels open with a probability significantly lower than 1, more than one LCC must be available to ensure that sparks are triggered with a probability of approximately unity. We conclude that it is likely that a cluster of LCCs is involved in gating a cluster of ryanodine receptors at the beginning of an AP.

Comparative immunohistochemical localizations of aquaporin-1 and aquaporin-4 in the cochleae of three different species of rodents

The species-specific difference of the immunohistochemical localization of aquaporin-1 (AQP1) and aquaporin-4 (AQP4) was investigated in the cochleae of the 3 different species of rodents, including guinea pig, mouse and Mongolian gerbil. In the guinea pig cochlea, intense AQP1-like immunoreactivity was present in the type III fibrocytes in the spiral ligament and the mesenchymal cells just below the basilar membrane. Immunostaining was also found in some type IV fibrocytes in the spiral ligament, fibrocytes in the spiral limbus and mesenchymal cells lining the perilymphatic space against the bony otic capsule. In contrast, no remarkable immunostaining was found in the basilar membrane of the mouse cochlea. The medial part of the Reissner's membrane was positively immunostained with anti-AQP1 antibody only in the mouse cochlea. In the gerbil cochlea, AQP1-like immunoreactivity was weak compared with the other 2 species. AQP4 was found in the cochlear supporting cells, including Claudius cells, Hensen's cells and inner sulcus cells of the 3 rodent species. AQP4 was also expressed in some interdental cells of the spiral limbus. Weak immunoreactivity was also found in the root cells only in the upper turns of the guinea pig cochlea. In contrast, no detectable immunoreactivity was found in the root cells of the other 2 species. The results obtained in the present study provide the first evidence for the existence of the species differences in the expression of the AQP1 and AQP4 proteins in the rodent cochlea.

Modulation of Ca(2+) release in cardiac myocytes by changes in repolarization rate: role of phase-1 action potential repolarization in excitation-contraction coupling

The early rate of action potential (AP) repolarization varies in the mammalian heart regionally, during development, and in disease. We used confocal microscopy to assess the effects of changes in repolarization rate on spatially resolved sarcoplasmic reticulum (SR) Ca(2+) release. The kinetics and peak amplitude of Ca(2+) transients were reduced, and the amplitude, frequency, and temporal synchronization of Ca(2+) spikes decreased as the rate of repolarization was slowed. The first latencies and temporal dispersion of Ca(2+) spikes tracked closely with the time to peak and the width of the L-type Ca(2+) current (I(Ca,L)), suggesting that the effects of repolarization on excitation-contraction coupling occur primarily via changes in I(Ca,L). Next, we examined the effect of changes in the rapid early repolarization rate (phase 1) of a model human AP on SR Ca(2+) release by varying the amount of transient outward K(+) current. Slowing of phase-1 repolarization also caused a loss of temporal synchrony and recruitment of Ca(2+)-release events, associated with a reduced amplitude and lengthened time to peak of I(Ca,L). Isoproterenol application enhanced and largely resynchronized SR Ca(2+) release, while it increased the magnitude and shortened the time to peak of I(Ca,L). Our data demonstrate that membrane repolarization modulates the recruitment and synchronization of SR Ca(2+) release via I(Ca,L) and illustrate a physiological role for the phase-1 notch of the AP in optimizing temporal summation and recruitment of Ca(2+)-release events. The effects of slowing phase-1 repolarization can be overcome by beta-adrenergic stimulation.