Transmission of information from cardiac dihydropyridine receptor to ryanodine receptor: evidence from BayK 8644 effects on resting Ca(2+) sparks

Peptide-Based Targeting of the L-Type Calcium Channel Corrects the Loss-of-Function Phenotype of Two Novel Mutations of the CACNA1 Gene Associated With Brugada Syndrome

Brugada syndrome (BrS) is an inherited arrhythmogenic disease that may lead to sudden cardiac death in young adults with structurally normal hearts. No pharmacological therapy is available for BrS patients. This situation highlights the urgent need to overcome current difficulties by developing novel groundbreaking curative strategies. BrS has been associated with mutations in 18 different genes of which loss-of-function (LoF) CACNA1C mutations constitute the second most common cause. Here we tested the hypothesis that BrS associated with mutations in the CACNA1C gene encoding the L-type calcium channel (LTCC) pore-forming unit (Ca_vα1.2) is functionally reverted by administration of a mimetic peptide (MP), which through binding to the LTCC chaperone beta subunit (Ca_vβ2) restores the physiological life cycle of aberrant LTCCs. Two novel Ca_vα1.2 mutations associated with BrS were identified in young individuals. Transient transfection in heterologous and cardiac cells showed LoF phenotypes with reduced Ca²⁺ current (I_Ca). In HEK293 cells overexpressing the two novel Ca_vα1.2 mutations, Western blot analysis and cell surface biotinylation assays revealed reduced Ca_vα1.2 protein levels at the plasma membrane for both mutants. Nano-BRET, Nano-Luciferase assays, and confocal microscopy analyses showed (i) reduced affinity of Ca_vα1.2 for its Ca_vβ2 chaperone, (ii) shortened Ca_vα1.2 half-life in the membrane, and (iii) impaired subcellular localization. Treatment of Ca_vα1.2 mutant-transfected cells with a cell permeant MP restored channel trafficking and physiologic channel half-life, thereby resulting in I_Ca similar to wild type. These results represent the first step towards the development of a gene-specific treatment for BrS due to defective trafficking of mutant LTCC.

Intracellular emetic signaling evoked by the L-type Ca2+ channel agonist FPL64176 in the least shrew (Cryptotis parva)

Ca²⁺ plays a major role in maintaining cellular homeostasis and regulates processes including apoptotic cell death and side-effects of cancer chemotherapy including vomiting. Currently we explored the emetic mechanisms of FPL64176, an L-type Ca²⁺ channel (LTCC) agonist with maximal emetogenic effect at its 10 mg/kg dose. FPL64176 evoked c-Fos immunoreactivity in shrew brainstem sections containing the vomit-associated nuclei, nucleus tractus solitarius (NTS) and dorsal motor nucleus of the vagus. FPL64176 also increased phosphorylation of proteins ERK1/2, PKCα/βII and Akt in the brainstem. Moreover, their corresponding inhibitors (PD98059, GF 109203X and LY294002, respectively) reduced FPL64176-evoked vomiting. A 30 min subcutaneous (s.c.) pretreatment with the LTCC antagonist nifedipine (10 mg/kg) abolished FPL64176-elicited vomiting, c-Fos expression, and emetic effector phosphorylation. Ryanodine receptors (RyRs) and inositol trisphosphate receptors (IP₃Rs) mediate intracellular Ca²⁺ release from the sarcoplasmic/endoplasmic reticulum. The RyR antagonist dantrolene (i.p.), or a combination of low doses of nifedipine and dantrolene, but not the IP₃R antagonist 2-APB, significantly attenuated FPL64176-induced vomiting. The serotonin type 3 receptor (5-HT₃R) antagonist palonosetron (s.c.), the neurokinin 1 receptor (NK₁R) antagonist netupitant (i.p.) or a combination of non-effective doses of netupitant and palonosetron showed antiemetic potential against FPL64176-evoked vomiting. Serotonin (5-HT) and substance P immunostaining revealed FPL64176-induced emesis was accompanied by an increase in 5-HT but not SP-immunoreactivity in the dorsomedial subdivision of the NTS. These findings demonstrate that Ca²⁺ mobilization through LTCCs and RyRs, and subsequent emetic effector phosphorylation and 5-HT release play important roles in FPL64176-induced emesis which can be prevented by 5-HT₃R and NK₁R antagonists.

Acute exposure to progesterone attenuates cardiac contraction by modifying myofilament calcium sensitivity in the female mouse heart

Acute application of progesterone attenuates cardiac contraction, although the underlying mechanisms are unclear. We investigated whether progesterone modified contraction in isolated ventricular myocytes and identified the Ca²⁺ handling mechanisms involved in female C57BL/6 mice (6-9 mo; sodium pentobarbital anesthesia). Cells were field-stimulated (4 Hz; 37°C) and exposed to progesterone (0.001-10.0 μM) or vehicle (35 min). Ca²⁺ transients (fura-2) and cell shortening were recorded simultaneously. Maximal concentrations of progesterone inhibited peak contraction by 71.4% (IC₅₀ = 160 ± 50 nM; n = 12) and slowed relaxation by 75.4%. By contrast, progesterone had no effect on amplitudes or time courses of underlying Ca²⁺ transients. Progesterone (1 µM) also abbreviated action potential duration. When the duration of depolarization was controlled by voltage-clamp, progesterone attenuated contraction and slowed relaxation but did not affect Ca²⁺ currents, Ca²⁺ transients, sarcoplasmic reticulum (SR) content, or fractional release of SR Ca²⁺ Actomyosin MgATPase activity was assayed in myofilaments from hearts perfused with progesterone (1 μM) or vehicle (35 min). While maximal responses to Ca²⁺ were not affected by progesterone, myofilament Ca²⁺ sensitivity was reduced (EC₅₀ = 0.94 ± 0.01 µM for control, n = 7 vs. 1.13 ± 0.05 μM for progesterone, n = 6; P < 0.05) and progesterone increased phosphorylation of myosin binding protein C. The effects on contraction were inhibited by lonaprisan (progesterone receptor antagonist) and levosimendan (Ca²⁺ sensitizer). Unlike results in females, progesterone had no effect on contraction or myofilament Ca²⁺ sensitivity in age-matched male mice. These data indicate that progesterone reduces myofilament Ca²⁺ sensitivity in female hearts, which may exacerbate manifestations of cardiovascular disease late in pregnancy when progesterone levels are high.

NEW & NOTEWORTHY

We investigated myocardial effects of acute application of progesterone. In females, but not males, progesterone attenuates and slows cardiomyocyte contraction with no effect on calcium transients. Progesterone also reduces myofilament calcium sensitivity in female hearts. This may adversely affect heart function, especially when serum progesterone levels are high in pregnancy.Listen to this article's corresponding podcast at https://ajpheart.podbean.com/e/acute-progesterone-modifies-cardiac-contraction/.

Serotonin 5-HT3 receptor-mediated vomiting occurs via the activation of Ca2+/CaMKII-dependent ERK1/2 signaling in the least shrew (Cryptotis parva)

Stimulation of 5-HT₃ receptors (5-HT₃Rs) by 2-methylserotonin (2-Me-5-HT), a selective 5-HT₃ receptor agonist, can induce vomiting. However, downstream signaling pathways for the induced emesis remain unknown. The 5-HT₃R channel has high permeability to extracellular calcium (Ca²⁺) and upon stimulation allows increased Ca²⁺ influx. We examined the contribution of Ca²⁺/calmodulin-dependent protein kinase IIα (Ca²⁺/CaMKIIα), interaction of 5-HT₃R with calmodulin, and extracellular signal-regulated kinase 1/2 (ERK1/2) signaling to 2-Me-5-HT-induced emesis in the least shrew. Using fluo-4 AM dye, we found that 2-Me-5-HT augments intracellular Ca²⁺ levels in brainstem slices and that the selective 5-HT₃R antagonist palonosetron, can abolish the induced Ca²⁺ signaling. Pre-treatment of shrews with either: i) amlodipine, an antagonist of L-type Ca²⁺ channels present on the cell membrane; ii) dantrolene, an inhibitor of ryanodine receptors (RyRs) Ca²⁺-release channels located on the endoplasmic reticulum (ER); iii) a combination of their less-effective doses; or iv) inhibitors of CaMKII (KN93) and ERK1/2 (PD98059); dose-dependently suppressed emesis caused by 2-Me-5-HT. Administration of 2-Me-5-HT also significantly: i) enhanced the interaction of 5-HT₃R with calmodulin in the brainstem as revealed by immunoprecipitation, as well as their colocalization in the area postrema (brainstem) and small intestine by immunohistochemistry; and ii) activated CaMKIIα in brainstem and in isolated enterochromaffin cells of the small intestine as shown by Western blot and immunocytochemistry. These effects were suppressed by palonosetron. 2-Me-5-HT also activated ERK1/2 in brainstem, which was abrogated by palonosetron, KN93, PD98059, amlodipine, dantrolene, or a combination of amlodipine plus dantrolene. However, blockade of ER inositol-1, 4, 5-triphosphate receptors by 2-APB, had no significant effect on the discussed behavioral and biochemical parameters. This study demonstrates that Ca²⁺ mobilization via extracellular Ca²⁺ influx through 5-HT₃Rs/L-type Ca²⁺ channels, and intracellular Ca²⁺ release via RyRs on ER, initiate Ca²⁺-dependent sequential activation of CaMKIIα and ERK1/2, which contribute to the 5-HT₃R-mediated, 2-Me-5-HT-evoked emesis.

The impact of ovariectomy on calcium homeostasis and myofilament calcium sensitivity in the aging mouse heart

This study determined whether deficiency of ovarian estrogen starting very early in life promoted age-associated Ca(2+) dysregulation and contractile dysfunction in isolated ventricular myocytes. Myocytes were isolated from anesthetized C57BL/6 female mice. Animals received an ovariectomy or sham-operation at one month and were aged to ~24 months. Excitation-contraction coupling parameters were compared in fura-2 loaded myocytes (37°C). While Ca(2+) transients were larger and faster in field-stimulated myocytes from ovariectomized mice, ovariectomy had no effect on peak fractional shortening. Similarly, ovariectomy had no effect on fractional shortening measured in vivo by echocardiography (values were 60.5 ± 2.9 vs. 60.3 ± 2.5% in sham and ovariectomized, respectively; n=5 mice/group). Ovariectomy did decrease myofilament Ca(2+) sensitivity, as evidenced by a 26% increase in the Ca(2+) required to activate actomyosin MgATPase in ovariectomized hearts. Larger Ca(2+) transients were attributable to a 48% increase in peak Ca(2+) current, along with an increase in the amplitude, width and frequency of Ca(2+) sparks measured in fluo-4 loaded myocytes. These changes in Ca(2+) handling were not due to increased expression of Ca(2+) channels (Cav1.2), sarcoplasmic reticulum Ca(2+) ATPase (SERCA2) or Na(+)-Ca(2+) exchanger in ovariectomized hearts. However, ovariectomy increased sarcoplasmic reticulum Ca(2+) stores by ~90% and promoted spontaneous Ca(2+) release from the sarcoplasmic reticulum when compared to sham controls. These observations demonstrate that long-term ovariectomy promotes intracellular Ca(2+) dysregulation, reduces myofilament Ca(2+) sensitivity and increases spontaneous Ca(2+) release in the aging female heart.

Reversibility of T-tubule remodelling in heart failure: mechanical load as a dynamic regulator of the T-tubules

The T-tubule system in ventricular cardiomyocytes is essential for synchronous Ca(2+) handling, and, therefore, efficient contraction. T-tubular remodelling is a common feature of heart disease. In this review, we discuss whether t-tubular remodelling can be reversed and which factors may be implicated in this process. In particular, we focus on the interaction between mechanical load variation and T-tubule structure and function. What is the evidence of this relationship? What is the role of different degrees and durations of mechanical load variation? In what settings might mechanical load variation have detrimental or beneficial effects on T-tubule structure and function? What are the molecular determinants of this interaction? Ultimately this discussion is used to address the question of whether mechanical load variation can provide an understanding to underpin attempts to induce recovery of the T-tubule system. In reviewing these questions, we define what remains to be discovered in understanding T-tubule recovery.

Voltage-driven Ca(2+) binding at the L-type Ca(2+) channel triggers cardiac excitation-contraction coupling prior to Ca(2+) influx

The activation of the ryanodine Ca(2+) release channels (RyR2) by the entry of Ca(2+) through the L-type Ca(2+) channels (Cav1.2) is believed to be the primary mechanism of excitation-contraction (EC) coupling in cardiac cells. This proposed mechanism of Ca(2+)-induced Ca(2+) release (CICR) cannot fully account for the lack of a termination signal for this positive feedback process. Using Cav1.2 channel mutants, we demonstrate that the Ca(2+)-impermeable α(1)1.2/L775P/T1066Y mutant introduced through lentiviral infection into neonate cardiomyocytes triggers Ca(2+) transients in a manner independent of Ca(2+) influx. In contrast, the α(1)1.2/L775P/T1066Y/4A mutant, in which the Ca(2+)-binding site of the channel was destroyed, supports neither the spontaneous nor the electrically evoked contractions. Ca(2+) bound at the channel selectivity filter appears to initiate a signal that is conveyed directly from the channel pore to RyR2, triggering contraction of cardiomyocytes prior to Ca(2+) influx. Thus, RyR2 is activated in response to a conformational change in the L-type channel during membrane depolarization and not through interaction with Ca(2+) ions diffusing in the junctional gap space. Accordingly, termination of the RyR2 activity is achieved when the signal stops upon the return of the L-channel to the resting state. We propose a new model in which the physical link between Cav1.2 and RyR2 allows propagation of a conformational change induced at the open pore of the channel to directly activate RyR2. These results highlight Cav1.2 as a signaling protein and provide a mechanism for terminating the release of Ca(2+) from RyR2 through protein-protein interactions. In this model, the L-type channel is a master regulator of both initiation and termination of EC coupling in neonate cardiomyocytes.

Subcellular heterogeneity of ryanodine receptor properties in ventricular myocytes with low T-tubule density

RATIONALE

In ventricular myocytes of large mammals, not all ryanodine receptor (RyR) clusters are associated with T-tubules (TTs); this fraction increases with cellular remodeling after myocardial infarction (MI).

OBJECTIVE

To characterize RyR functional properties in relation to TT proximity, at baseline and after MI.

METHODS

Myocytes were isolated from left ventricle of healthy pigs (CTRL) or from the area adjacent to a myocardial infarction (MI). Ca(2+) transients were measured under whole-cell voltage clamp during confocal linescan imaging (fluo-3) and segmented according to proximity of TTs (sites of early Ca(2+) release, F>F(50) within 20 ms) or their absence (delayed areas). Spontaneous Ca(2+) release events during diastole, Ca(2+) sparks, reflecting RyR activity and properties, were subsequently assigned to either category.

RESULTS

In CTRL, spark frequency was higher in proximity of TTs, but spark duration was significantly shorter. Block of Na(+)/Ca(2+) exchanger (NCX) prolonged spark duration selectively near TTs, while block of Ca(2+) influx via Ca(2+) channels did not affect sparks properties. In MI, total spark mass was increased in line with higher SR Ca(2+) content. Extremely long sparks (>47.6 ms) occurred more frequently. The fraction of near-TT sparks was reduced; frequency increased mainly in delayed sites. Increased duration was seen in near-TT sparks only; Ca(2+) removal by NCX at the membrane was significantly lower in MI.

CONCLUSION

TT proximity modulates RyR cluster properties resulting in intracellular heterogeneity of diastolic spark activity. Remodeling in the area adjacent to MI differentially affects these RyR subpopulations. Reduction of the number of sparks near TTs and reduced local NCX removal limit cellular Ca(2+) loss and raise SR Ca(2+) content, but may promote Ca(2+) waves.

Effect of 4-aryl-2-methyl-5-nitro-1,4-dihydropyridine-3-carboxylates on the guinea pig papillary muscle and isolated human vena saphena magna that is used for coronary artery bypass grafting

BACKGROUND

The goal of this study was to estimate: (i) the action of 5-nitro-substituted 1,4-dihydropyridines as well as Bay K 8644 (CAS [71145-03-4]) and CGP 28392 (CAS [89289-93-0]) on cardiac action potential duration (APD) and isometric contraction in the isolated guinea pig papillary muscles; (ii) whether the effects of 2-propoxyethyl 4-(2-difluoromethoxyphe-nyl)-2-methyl-5-nitro-1,4-dihydropyridine-3-carboxylate on the lengthening of cardiac APD were related to certain potassium channels (e.g., I(K1), K(ATP) and I(K)); and (iii) the modulation of the contraction-relaxation effects on isolated human vena saphena magna samples using three 5-nitro-substituted 1,4-dihydropyridine derivatives, displaying the positive inotropic and AP duration effects.

METHODS

The experiments were conducted on isolated human vena saphena magna samples and papillary muscles from adult guinea pigs. Isometric contractions and APs were recorded using a force transducer and microelectrode technique, respectively.

RESULTS

2-Propoxyethyl 4-(2-difluoromethoxyphenyl)-2-methyl-5-nitro-1,4-dihydropyridine-3-carboxylate significantly increased APD and isometric contractions in a concentration-dependent manner. Its effects were suppressed by dl-sotalol. Other derivatives tested, such as Bay K 8644 and CGP 28392, showed either negligible effects or increased the contraction force but did not influence the APD. Compounds possessing positive inotropic properties at a concentration of 10(-7) to 10(-4) M significantly relaxed the isolated vessel samples pre-contracted with phenylephrine (10(-4) M). The weakest response was shown by 2-propoxyethyl 4-(2-difluoromethoxyphenyl)-2-methyl-5-nitro-1,4-dihydropyridine-3-carboxylate.

CONCLUSION

These results show that 5-nitro-substituted 1,4-dihydropyridine derivatives with positive inotropic action significantly relaxed isolated vein samples that were pre-contracted with phenylephrine in a dose-dependent manner. 2-Propoxyethyl 4-(2-difluoromethoxyphenyl)-2-methyl-5-nitro-1,4-dihydropyridine-3-carboxylate prolongs the cardiac APD, which could be determined by the rapid component I(Kr) of the delayed potassium current I(K) blocker.

In vitro studies of early cardiac remodeling: impact on contraction and calcium handling

Cardiac remodeling, hypertrophy, and alterations in calcium signaling are changes of the heart that often lead to failure. After a hypertrophic stimulus, the heart progresses through a state of compensated hypertrophy which over time leads to decompensated hypertrophy or failure. It is at this point that a cardiac transplant is required for survival making early detection imperative. Current experimental systems used to study the remodeling of the heart include in vivo systems (the whole body), isolated organ and sub-organ tissue, and the individual cardiac muscle cells and organelles.. During pathological remodeling there is a derangement in the intracellular calcium handling processes. These derangements are thought to lead to a dysregulation of contractile output. Hence, understanding the mechanism between remodeling and dysregulation is of great interest in the cardiac field and will ultimately help in the development of future treatment and early detection. This review will center on changes in contraction and calcium handling in early cardiac remodeling, with a specific focus on findings in two different in vitro model systems: multicellular and individual cell preparations.

Spontaneous calcium oscillations during diastole in the whole heart: the influence of ryanodine reception function and gap junction coupling

Triggered arrhythmias due to spontaneous cytoplasmic calcium oscillations occur in a variety of disease conditions; however, their cellular mechanisms in tissue are not clear. We hypothesize that spontaneous calcium oscillations in the whole heart are due to calcium release from the sarcoplasmic reticulum and are facilitated by calcium diffusion through gap junctions. Optical mapping of cytoplasmic calcium from Langendorff perfused guinea pig hearts (n = 10) was performed using oxygenated Tyrode's solution (in mM): 140 NaCl, 0.7 MgCl, 4.5 KCl, 5.5 dextrose, 5 HEPES, and 5.5 CaCl₂ (pH 7.45, 34°C). Rapid pacing was used to induce diastolic calcium oscillations. In all preparations, pacing-induced multicellular diastolic calcium oscillations (m-SCR) occurred across most of the mapping field, at all pacing rates tested. Ryanodine (1 μM) eliminated all m-SCR activity. Low-dose caffeine (1 mM) increased m-SCR amplitude (+10.4 ± 4.4%, P < 0.05) and decreased m-SCR time-to-peak (-17.4 ± 6.7%, P < 0.05) and its temporal synchronization (i.e., range) across the mapping field (-26.9 ± 17.1%, P < 0.05). Surprisingly, carbenoxolone increased the amplitude of m-SCR activity (+14.8 ± 4.1%, P < 0.05) and decreased m-SCR time-to-peak (-11.3 ± 9.6%, P < 0.01) and its synchronization (-37.0 ± 19.1%, P < 0.05), similar to caffeine. In isolated myocytes, carbenoxolone (50 μM) had no effect on the frequency of aftercontractions, suggesting the effect of cell-to-cell uncoupling on m-SCR activity is tissue specific. Therefore, in the whole heart, overt m-SCR activity caused by calcium release from the SR can be induced over a broad range of pacing rates. Enhanced ryanodine receptor open probability and, surprisingly, decreased cell-to-cell coupling increased the amplitude and temporal synchronization of spontaneous calcium release in tissue.

Acute atrial arrhythmogenesis in murine hearts following enhanced extracellular Ca(2+) entry depends on intracellular Ca(2+) stores

Aim

To investigate the effect of increases in extracellular Ca²⁺ entry produced by the L-type Ca²⁺ channel agonist FPL-64176 (FPL) upon acute atrial arrhythmogenesis in intact Langendorff-perfused mouse hearts and its dependence upon diastolic Ca²⁺ release from sarcoplasmic reticular Ca²⁺ stores.

Methods

Confocal microscope studies of Fluo-3 fluorescence in isolated atrial myocytes were performed in parallel with electrophysiological examination of Langendorff-perfused mouse hearts.

Results

Atrial myocytes stimulated at 1 Hz and exposed to FPL (0.1 μm) initially showed (<10 min) frequent, often multiple, diastolic peaks following the evoked Ca²⁺ transients whose amplitudes remained close to control values. With continued pacing (>10 min) this reverted to a regular pattern of evoked transients with increased amplitudes but in which diastolic peaks were absent. Higher FPL concentrations (1.0 μm) produced sustained and irregular patterns of cytosolic Ca²⁺ activity, independent of pacing. Nifedipine (0.5 μm), and caffeine (1.0 mm) and cyclopiazonic acid (CPA) (0.15 μm) pre-treatments respectively produced immediate and gradual reductions in the F/F₀ peaks. Such nifedipine and caffeine, or CPA pre-treatments, abolished, or reduced, the effects of 0.1 and 1.0 μm FPL on cytosolic Ca²⁺ signals. FPL (1.0 μm) increased the incidence of atrial tachycardia and fibrillation in intact Langendorff-perfused hearts without altering atrial effective refractory periods. These effects were inhibited by nifedipine and caffeine, and reduced by CPA.

Conclusion

Enhanced extracellular Ca²⁺ entry exerts acute atrial arrhythmogenic effects that is nevertheless dependent upon diastolic Ca²⁺ release. These findings complement reports that associate established, chronic, atrial arrhythmogenesis with decreased overall inward Ca²⁺ current.

Cross-talk between L-type Ca2+ channels and mitochondria

1. Calcium is necessary for myocardial function, including contraction and maintenance of cardiac output. Calcium is also necessary for myocardial energetics and production of ATP by mitochondria, but the mechanisms for calcium regulation by mitochondria are still not fully resolved. 2. The cytoskeleton plays an important role in maintaining a cell's integrity. It is now recognized that cytoskeletal proteins can also assist in the transmission of signals from the plasma membrane to intracellular organelles. Cytoskeletal proteins can regulate the function of the L-type Ca(2+) channel and alter intracellular calcium homeostasis. 3. Recent evidence suggests that calcium influx through the L-type Ca(2+) channel is sufficient to alter a number of mitochondrial functional parameters, including superoxide production, NADH production and metabolic activity, assessed as the formation of formazan from tetrazolium salt. This occurs in a calcium-dependent manner. 4. Activation of the L-type Ca(2+) channel also alters mitochondrial membrane potential in a calcium-independent manner and this is assisted by movement of the auxiliary beta(2)-subunit through F-actin filaments. 5. Because the L-type Ca(2+) channel is the initiator of contraction, a functional coupling between the channels and mitochondria may assist in meeting myocardial energy demand on a beat-to-beat basis.

Evidence for regulation of mitochondrial function by the L-type Ca2+ channel in ventricular myocytes

The L-type Ca(2+) channel is responsible for initiating contraction in the heart. Mitochondria are responsible for meeting the cellular energy demands and calcium is required for the activity of metabolic intermediates. We examined whether activation of the L-type Ca(2+) channel alone is sufficient to alter mitochondrial function. The channel was activated directly with the dihydropyridine agonist BayK(-) or voltage-clamp of the plasma membrane and indirectly by depolarization of the membrane with high KCl. Activation of the channel increased superoxide production (assessed as changes in dihydroethidium fluorescence), NADH production and metabolic activity (assessed as formation of formazan from tetrazolium) in a calcium-dependent manner. Activation of the channel also increased mitochondrial membrane potential assessed as changes in JC-1 fluorescence. The response was reversible upon inactivation of the channel during voltage-clamp of the plasma membrane and did not appear to require calcium. We examined whether the response may be mediated through movement of cytoskeletal proteins. Depolymerization of actin or exposing cells to a peptide directed against the alpha-interacting domain of the alpha(1C)-subunit of the channel (thereby preventing movement of the beta-subunit) attenuated the increase in mitochondrial membrane potential. We conclude that activation of the L-type Ca(2+) channel can regulate mitochondrial function and the response appears to be modulated by movement through the cytoskeleton.

Calcium sparks

The calcium ion (Ca(2+)) is the simplest and most versatile intracellular messenger known. The discovery of Ca(2+) sparks and a related family of elementary Ca(2+) signaling events has revealed fundamental principles of the Ca(2+) signaling system. A newly appreciated "digital" subsystem consisting of brief, high Ca(2+) concentration over short distances (nanometers to microns) comingles with an "analog" global Ca(2+) signaling subsystem. Over the past 15 years, much has been learned about the theoretical and practical aspects of spark formation and detection. The quest for the spark mechanisms [the activation, coordination, and termination of Ca(2+) release units (CRUs)] has met unexpected challenges, however, and raised vexing questions about CRU operation in situ. Ample evidence shows that Ca(2+) sparks catalyze many high-threshold Ca(2+) processes involved in cardiac and skeletal muscle excitation-contraction coupling, vascular tone regulation, membrane excitability, and neuronal secretion. Investigation of Ca(2+) sparks in diseases has also begun to provide novel insights into hypertension, cardiac arrhythmias, heart failure, and muscular dystrophy. An emerging view is that spatially and temporally patterned activation of the digital subsystem confers on intracellular Ca(2+) signaling an exquisite architecture in space, time, and intensity, which underpins signaling efficiency, stability, specificity, and diversity. These recent advances in "sparkology" thus promise to unify the simplicity and complexity of Ca(2+) signaling in biology.

Hypothermia increases the gain of excitation-contraction coupling in guinea pig ventricular myocytes

Components of excitation-contraction (EC)-coupling were compared at 37°C and 22°C to determine whether hypothermia altered the gain of EC coupling in guinea pig ventricular myocytes. Ca2+concentration (fura-2) and cell shortening (edge detector) were measured simultaneously. Hypothermia increased fractional shortening (8.3 ± 1.7 vs. 2.6 ± 0.3% at 37°C), Ca2+transients (157 ± 33 vs. 35 ± 5 nM at 37°C), and diastolic Ca2+(100 ± 9 vs. 60 ± 6 nM at 37°C) in field-stimulated myocytes (2 Hz). In experiments with high-resistance microelectrodes, the increase in contractions and Ca2+transients was accompanied by a twofold increase in action potential duration (APD). When voltage-clamp steps eliminated changes in APD, cooling still increased contractions and Ca2+transients. Hypothermia increased sarcoplasmic reticulum (SR) Ca2+stores (83 ± 17 at 37°C to 212 ± 50 nM, assessed with caffeine) and increased fractional SR Ca2+release twofold. In contrast, peak Ca2+current was much smaller at 22°C than at 37°C (1.3 ± 0.4 and 3.5 ± 0.7 pA/pF, respectively). In cells dialyzed with sodium-free pipette solutions to inhibit Ca2+influx via reverse-mode Na+/Ca2+exchange, hypothermia still increased contractions, Ca2+transients, SR stores, and fractional release but decreased the amplitude of Ca2+current. The rate of SR Ca2+release per unit Ca2+current, a measure of EC-coupling gain, was increased sixfold by hypothermia. This increase in gain occurred regardless of whether cells were dialyzed with sodium-free solutions. Thus an increase in EC-coupling gain contributes importantly to positive inotropic effects of hypothermia in the heart.

Ryanoids and imperatoxin affect the modulation of cardiac ryanodine receptors by dihydropyridine receptor Peptide A

Ca(2+)-entry via L-type Ca(2+) channels (DHPR) is known to trigger ryanodine receptor (RyR)-mediated Ca(2+)-release from sarcoplasmic reticulum (SR). The mechanism that terminates SR Ca(2+) release is still unknown. Previous reports showed evidence of Ca(2+)-entry independent inhibition of Ca(2+) sparks by DHPR in cardiomyocytes. A peptide from the DHPR loop II-III (PepA) was reported to modulate isolated RyRs. We found that PepA induced voltage-dependent "flicker block" and transition to substates of fully-activated cardiac RyRs in planar bilayers. Substates had less voltage-dependence than block and did not represent occupancy of a ryanoid site. However, ryanoids stabilized PepA-induced events while PepA increased RyR2 affinity for ryanodol, which suggests cooperative interactions. Ryanodol stabilized Imperatoxin A (IpTx(A)) binding but when IpTx(A) bound first, it prevented ryanodol binding. Moreover, IpTx(A) and PepA excluded each other from their sites. This suggests that IpTx(A) generates a vestibular gate (either sterically or allosterically) that prevents access to the peptides and ryanodol binding sites. Inactivating gate moieties ("ball peptides") from K(+) and Na(+) channels (ShakerB and KIFMK, respectively) induced well resolved slow block and substates, which were sensitive to ryanoids and IpTx(A) and allowed, by comparison, better understanding of PepA action. The RyR2 appears to interact with PepA or ball peptides through a two-step mechanism, reminiscent of the inactivation of voltage-gated channels, which includes binding to outer (substates) and inner (block) vestibular regions in the channel conduction pathway. Our results open the possibility that "ball peptide-like" moieties in RyR2-interacting proteins could modulate SR Ca(2+) release in cells.

Muscarinic M(1) modulation of N and L types of calcium channels is mediated by protein kinase C in neostriatal neurons

In some neurons, muscarinic M(1)-class receptors control L-type (Ca(V)1) Ca(2+)-channels via protein kinase C (PKC) or calcineurin (phosphatase 2B; PP-2B) signaling pathways. Both PKC and PP-2B pathways start with phospholipase C (PLC) activation. In contrast, P/Q- and N-type (Ca(V)2.1, 2.2, respectively) Ca(2+)-channels are controlled by M(2)-class receptors via G proteins that may act, directly, to modulate these channels. The hypothesis of this work is that this description is not enough to explain muscarinic modulation of Ca(2+) channels in rat neostriatal projection neurons. Thus, we took advantage of the specific muscarinic toxin 3 (MT-3) to block M(4)-type receptors in neostriatal neurons, and leave in isolation the M(1)-type receptors to study them separately. We then asked what Ca(2+) channels are modulated by M(1)-type receptors only. We found that M(1)-receptors do modulate L, N and P/Q-types Ca(2+) channels. This modulation is blocked by the M(1)-class receptor antagonist (muscarinic toxin 7, MT-7) and is voltage-independent. Thereafter, we asked what signaling pathways, activated by M(1)-receptors would control these channels. We found that inactivation of PLC abolishes the modulation of all three channel types. PKC activators (phorbol esters) mimic muscarinic actions, whereas reduction of intracellular calcium virtually abolishes all modulation. As expected, PKC inhibitors prevented the muscarinic reduction of the afterhyperpolarizing potential (AHP), an event known to be dependent on Ca(2+) entry via N- and P/Q-type Ca(2+) channels. However, PKC inhibitors (bisindolylmaleimide I and PKC-1936) only block modulation of currents through N and L types Ca(2+) channels; while the modulation of P/Q-type Ca(2+) channels remains unaffected. These results show that different branches of the same signaling cascade can be used to modulate different Ca(2+) channels. Finally, we found no evidence of calcineurin modulating these Ca(2+) channels during M(1)-receptor activation, although, in the same cells, we demonstrate functional PP-2B by activating dopaminergic D(2)-receptor modulation.

A pharmacological model for calcium overload-induced tachycardia in isolated rat left atria

Few experimental models produce spontaneous tachycardia in normal left atria to allow the study of the cellular mechanisms underlying this contributor to atrial fibrillation. We reported 2-aminoethoxydiphenyl borate (2-APB) that provokes sporadic spontaneous mechanical activity and calcium leak in isolated rat left atria. Since sarcoplasmic reticulum calcium leak in the presence of high calcium load may trigger tachyarrhythmias, we tested how conditions that increase calcium load affect 2-APB-induced ectopic activity. Exposing superfused rat left atria to (i) 30 nM isoproterenol, (ii) 3 microM forskolin, (iii) 300 nM (-)BayK 8644 ((4S)-1,4-Dihydro-2,6-dimethyl-5-nitro-4-[2-(trifluormethyl)phenyl]-3-pyridinecarboxylic acid methyl ester), (iv) 300 nM FPL-64176 (2,5-Dimethyl-4-[2-(phenylmethyl)benzoyl]-1H-pyrrole-3-carboxylic acid methyl ester) or (v) 120 microM ouabain increases their force of contraction, evidence of calcium loading, but does not produce ectopic activity. Spontaneous mechanical activity occurs in left atria superfused with 20 microM 2-APB at 47+/-6 contractions/min in the absence of pacing. Any of these five agents increase rates of 2-APB-induced spontaneous mechanical activity to >200 contractions/min in the absence of pacing. Washing tachycardic left atria with superfusate lacking 2-APB restores normal function, demonstrating the reversibility of these effects. Decreasing superfusate sodium reverses this tachycardia and two hyperpolarization-activated current (I(f)) inhibitors blunt this ectopic activity. Thus conditions that increase atrial calcium load increase the frequency of spontaneous mechanical activity. Decreasing extracellular sodium and I(f) inhibitors suppress this spontaneous tachycardia suggesting forward-mode sodium-calcium exchange and I(f)-like activities underlie this activity. This model may help define cell pathways that trigger atrial tachyarrhythmias.

Ca2+ entry-independent effects of L-type Ca2+ channel modulators on Ca2+ sparks in ventricular myocytes

During the cardiac action potential, Ca(2+) entry through dyhidropyridine receptor L-type Ca(2+) channels (DHPRs) activates ryanodine receptors (RyRs) Ca(2+)-release channels, resulting in massive Ca(2+) mobilization from the sarcoplasmic reticulum (SR). This global Ca(2+) release arises from spatiotemporal summation of many localized elementary Ca(2+)-release events, Ca(2+) sparks. We tested whether DHPRs modulate Ca(2+)sparks in a Ca(2+) entry-independent manner. Negative modulation by DHPR of RyRs via physical interactions is accepted in resting skeletal muscle but remains controversial in the heart. Ca(2+) sparks were studied in cat cardiac myocytes permeabilized with saponin or internally perfused via a patch pipette. Bathing and pipette solutions contained low Ca(2+) (100 nM). Under these conditions, Ca(2+) sparks were detected with a stable frequency of 3-5 sparks.s(-1).100 microm(-1). The DHPR blockers nifedipine, nimodipine, FS-2, and calciseptine decreased spark frequency, whereas the DHPR agonists Bay-K8644 and FPL-64176 increased it. None of these agents altered the spatiotemporal characteristics of Ca(2+) sparks. The DHPR modulators were also without effect on SR Ca(2+) load (caffeine-induced Ca(2+) transients) or sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) activity (Ca(2+) loading rates of isolated SR microsomes) and did not change cardiac RyR channel gating (planar lipid bilayer experiments). In summary, DHPR modulators affected spark frequency in the absence of DHPR-mediated Ca(2+) entry. This action could not be attributed to a direct action of DHPR modulators on SERCA or RyRs. Our results suggest that the activity of RyR Ca(2+)-release units in ventricular myocytes is modulated by Ca(2+) entry-independent conformational changes in neighboring DHPRs.

Increases in diastolic [Ca2+] can contribute to positive inotropy in guinea pig ventricular myocytes in the absence of changes in amplitudes of Ca2+ transients

Increases in contraction amplitude following rest or in elevated extracellular Ca(2+) concentration ([Ca(2+)]) have been attributed to increased sarcoplasmic reticulum (SR) Ca(2+) stores and/or increased trigger Ca(2+). However, either manipulation also may elevate diastolic [Ca(2+)]. The objective of this study was to determine whether elevation of diastolic [Ca(2+)] could contribute to positive inotropy in isolated ventricular myocytes. Voltage-clamp experiments were conducted with high-resistance microelectrodes in isolated myocytes at 37 degrees C. Intracellular free [Ca(2+)] was measured with fura-2, and cell shortening was measured with an edge detector. SR Ca(2+) stores were assessed with 10 mM caffeine (0 mM Na(+), 0 mM Ca(2+)). Following a period of rest, cells were activated with trains of pulses, which generated contractions of increasing amplitude, called positive staircases. Positive staircases were accompanied by increasing diastolic [Ca(2+)] but no change in Ca(2+) transient amplitudes. When extracellular [Ca(2+)] was elevated from 2.0 to 5.0 mM, resting intracellular [Ca(2+)] increased and resting cell length decreased. Amplitudes of contractions and L-type Ca(2+) current increased in elevated extracellular [Ca(2+)], although SR Ca(2+) stores, assessed by rapid application of caffeine, did not increase. Although Ca(2+) transient amplitude did not increase in 5.0 mM extracellular [Ca(2+)], diastolic [Ca(2+)] continued to increase with increasing extracellular [Ca(2+)]. These data suggest that increased diastolic [Ca(2+)] contributes to positive inotropy following rest or with increasing extracellular [Ca(2+)] in guinea pig ventricular myocytes.

Calcium spark properties in ventricular myocytes are altered in aged mice

This study determined whether whole cell Ca2+ transients and unitary sarcoplasmic reticulum (SR) Ca2+ release events are constant throughout adult life or whether Ca2+ release is altered in aging ventricular myocytes. Myocytes were isolated from young adult (∼5 mo old) and aged (∼24 mo old) mice. Spontaneous Ca2+ sparks and Ca2+ transients initiated by field stimulation were detected with fluo-4. All experiments were conducted at 37°C. Ca2+ transient amplitudes were reduced, and Ca2+ transient rise times were abbreviated in aged cells stimulated at 8 Hz compared with young adult myocytes. Furthermore, the incidence and frequency of spontaneous Ca2+ sparks were markedly higher in aged myocytes compared with young adult cells. Spark amplitudes and spatial widths were similar in young adult and aged myocytes. However, spark half-rise times and half-decay times were abbreviated in aged cells compared with younger cells. Resting cytosolic Ca2+ levels and SR Ca2+ stores were assessed by rapid application of caffeine in fura-2-loaded cells. Neither resting Ca2+ levels nor SR Ca2+ content differed between young adult and aged cells. Thus increased spark frequency in aging cells was not attributable to increased SR Ca2+ stores. Furthermore, the decrease in Ca2+ transient amplitude was not due to a decrease in SR Ca2+ load. These results demonstrate that alterations in fundamental SR Ca2+ release units occur in aging ventricular myocytes and raise the possibility that alterations in Ca2+ release may reflect age-related changes in fundamental release events rather than changes in SR Ca2+ stores and diastolic Ca2+ levels.

Effects of L-type Ca2+ channel modulation on direct myocardial effects of diazepam and midazolam in adult rat ventricular myocytes

PURPOSE

Our objective was to determine whether an L-type Ca2+ channel modulation could alter myocardial depression induced by midazolam or diazepam in adult rat ventricular myocytes.

METHODS

Freshly isolated rat ventricular myocytes were loaded with fura-2/AM and field-stimulated (0.3 Hz) at 28 degrees C. Amplitude and timing of intracellular Ca2+ concentration ([Ca2+]i) and myocyte shortening were simultaneously monitored in individual cells.

RESULTS

Midazolam (3-100 microM) caused a decrease in both peak [Ca2+]i and shortening. Diazepam (30, 100 microM) increased myocyte shortening and peak [Ca2+]i; however, higher concentration of diazepam (300 microM) decreased shortening and peak [Ca2+]i. Bay K 8644 (0.01-10 microM), an L-type Ca2+ channel agonist, caused dose-dependent increases in peak [Ca2+]i and shortening. In contrast, verapamil (0.1-50 microM), an L-type Ca(2+) channel antagonist, caused dose-dependent decreases in peak [Ca2+]i and shortening. Dose-response curves to benzodiazepines on peak [Ca2+]i and shortening were not affected by pretreatment with Bay K 8644 (0.1 microM) or verapamil (1 microM). Diazepam (30, 100 microM), but not midazolam (3-30 microM), increased shortening and [Ca2+]i in the presence or absence of L-type Ca2+ channel modulators. Diazepam (30 microM) and midazolam (10 microM) had no effect on peak [Ca2+]i of a caffeine-induced [Ca2+]i transient, which was used as a measure of SR Ca2+ content.

CONCLUSION

Midazolam and diazepam have differential effects on cardiac E-C coupling. Diazepam, but not midazolam, enhances cardiac E-C coupling independent of L-type Ca2+ channel modulation.

The recombinant dihydropyridine receptor II-III loop and partly structured 'C' region peptides modify cardiac ryanodine receptor activity

A physical association between the II-III loop of the DHPR (dihydropryidine receptor) and the RyR (ryanodine receptor) is essential for excitation-contraction coupling in skeletal, but not cardiac, muscle. However, peptides corresponding to a part of the II-III loop interact with the cardiac RyR2 suggesting the possibility of a physical coupling between the proteins. Whether the full II-III loop and its functionally important 'C' region (cardiac DHPR residues 855-891 or skeletal 724-760) interact with cardiac RyR2 is not known and is examined in the present study. Both the cardiac DHPR II-III loop (CDCL) and cardiac peptide (C(c)) activated RyR2 channels at concentrations >10 nM. The skeletal DHPR II-III loop (SDCL) activated channels at < or =100 nM and weakly inhibited at > or =1 microM. In contrast, skeletal peptide (C(s)) inhibited channels at all concentrations when added alone, or was ineffective if added in the presence of C(c). Ca2+-induced Ca2+ release from cardiac sarcoplasmic reticulum was enhanced by CDCL, SDCL and the C peptides. The results indicate that the interaction between the II-III loop and RyR2 depends critically on the 'A' region (skeletal DHPR residues 671-690 or cardiac 793-812) and also involves the C region. Structure analysis indicated that (i) both C(s) and C(c) are random coil at room temperature, but, at 5 degrees C, have partial helical regions in their N-terminal and central parts, and (ii) secondary-structure profiles for CDCL and SDCL are similar. The data provide novel evidence that the DHPR II-III loop and its C region interact with cardiac RyR2, and that the ability to interact is not isoform-specific.

Temporal and spatial properties of cellular Ca2+flux in trout ventricular myocytes

Confocal microscopy was used to investigate the temporal and spatial properties of Ca2+transients and Ca2+sparks in ventricular myocytes of the rainbow trout ( Oncorhynchus mykiss). Confocal imaging confirmed the absence of T tubules and the long (∼160 μm), thin (∼8 μm) morphology of trout myocytes. Line scan imaging of Ca2+transients evoked by electrical stimulation in cells loaded with fluo 4 revealed spatial inhomogeneities in the temporal properties of Ca2+transients across the width of the myocytes. The Ca2+wavefront initiated faster, rose faster, and reached larger peak amplitudes in the periphery of the myocyte compared with the center. These differences were exacerbated by stimulation with the L-type Ca2+channel agonist (−)BAY K 8644 or by sarcoplasmic reticulum (SR) inhibition with ryanodine and thapsigargin. Results reveal that the shape of the trout myocyte allows for rapid diffusion of Ca2+from the cell periphery to the cell center, with SR Ca2+release contributing to the cytosolic Ca2+rise in a time-dependent manner. Spontaneous Ca2+sparks were exceedingly rare in trout myocytes under control conditions (1 sparking cell from 238 cells examined). This is in marked contrast to the rat where a total of 56 spontaneous Ca2+sparks were observed in 9 of 11 myocytes examined. Ca2+sparklike events were observed in a very small number of trout myocytes (15 sparks from 9 of 378 cells examined) after stimulation with either (−)BAY K 8644 or high Ca2+(6 mM). Reducing temperature to 15°C in intact myocytes or permeabilizing myocytes to adjust intracellular conditions to favor Ca2+spark detection was without significant effects. Possible reasons for the rarity of Ca2+sparks in a cardiac myocyte with an active SR are discussed.

Modification of sarcoplasmic reticulum (SR) Ca2+release by FK506 induces defective excitation-contraction coupling only when SR Ca2+recycling is disturbed

This study examined whether the effects of FK506-binding protein dissociation from sarcoplasmic reticulum (SR) Ca2+release channels on excitation-contraction (EC) coupling changed when SR Ca2+reuptake and (or) the trans-sarcolemmal Ca2+extrusion were altered. The steady-state twitch Ca2+transient (CaT), cell shortening, post-rest caffeine-induced CaT, and Ca2+sparks were measured in rat ventricular myocytes using laser-scanning confocal microscopy. In the normal condition, 50 µmol FK506/L significantly increased steady-state CaT, cell shortening, and post-rest caffeine-induced CaT. When the cells were solely perfused with thapsigargin, FK506 did not reduce any of the states, but when low [Ca2+]0(0.1 mmol/L) was perfused additionally, FK506 reduced CaT and cell shortening, and accelerated the reduction of post-rest caffeine-induced CaT. FK506 significantly increased Ca2+spark frequency in the normal condition, whereas it mainly prolonged duration of individual Ca2+sparks under the combination of thapsigargin and low [Ca2+]0perfusion. Modification of SR Ca2+release by FK506 impaired EC coupling only when released Ca2+could not be taken back into the SR and was readily extruded to the extracellular space. Our findings could partly explain the controversy regarding the contribution of FK506-binding protein dissociation to defective EC coupling.Key words: FK506, ryanodine receptor, sarcoplasmic reticulum Ca2+-ATPase, Na+/Ca2+exchange, excitation-contraction coupling

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.

Novel regulatory mechanism of cardiomyocyte contractility involving ICAM-1 and the cytoskeleton

ICAM-1 mediates interaction of cardiomyocytes with the extracellular matrix and leukocytes and may play a role in altering contractility. To investigate this possibility, rat ventricular cardiomyocytes were activated using TNF-α, IL-1β, or LPS, washed, cultured with quiescent rat polymorphonuclear leukocytes (PMNs) for 4 h, and electrically stimulated to determine fractional shortening. PMNs cultured with activated cardiomyocytes reduced control fractional shortening of 20.5 ± 0.7% by −2.8 ± 0.3% per adherent PMN ( P < 0.001). Fixing PMNs with paraformaldehyde or glutaraldehyde did not prevent PMN-mediated decreases in cardiomyocyte fractional shortening. However, PMN adherence and decreased fractional shortening were prevented by anti-ICAM-1 and anti-CD18 antibodies. Reduced fractional shortening was reproduced in the absence of PMNs by ICAM-1 binding using cross-linking antibodies (reduced by 36 ± 3% from control, P < 0.01). Immunofluorescent staining demonstrated increased cortical cytoskeleton-associated focal adhesion kinase expression after ICAM-1 cross-linking, suggesting involvement of the actin cytoskeleton. Indeed, disruption of F-actin filament assembly using cytochalasin D or latrunculin A did not prevent PMN adherence but prevented decreased fractional shortening. Inhibition of the cytoskeleton-associated Rho-kinase pathway with HA-1077 prevented ICAM-1-mediated decreases in cardiomyocyte contractility, further suggesting a central role of the actin cytoskeleton. Importantly, ICAM-1 cross-linking did not alter the total intracellular Ca2+transient during cardiomyocyte contraction but greatly increased heterogeneity of intracellular Ca2+release. Thus we have identified a novel regulatory mechanism of cardiomyocyte contractility involving the actin cytoskeleton as a central regulator of the normally highly coordinated pattern of sarcoplasmic Ca2+release. Cardiomyocyte ICAM-1 binding, by PMNs or other ligands, induces decreased cardiomyocyte contractility via this pathway.

Peptide fragments of the dihydropyridine receptor can modulate cardiac ryanodine receptor channel activity and sarcoplasmic reticulum Ca2+ release

We show that peptide fragments of the dihydropyridine receptor II-III loop alter cardiac RyR (ryanodine receptor) channel activity in a cytoplasmic Ca2+-dependent manner. The peptides were AC (Thr-793-Ala-812 of the cardiac dihydropyridine receptor), AS (Thr-671-Leu-690 of the skeletal dihydropyridine receptor), and a modified AS peptide [AS(D-R18)], with an extended helical structure. The peptides added to the cytoplasmic side of channels in lipid bilayers at > or = 10 nM activated channels when the cytoplasmic [Ca2+] was 100 nM, but either inhibited or did not affect channel activity when the cytoplasmic [Ca2+] was 10 or 100 microM. Both activation and inhibition were independent of bilayer potential. Activation by AS, but not by AC or AS(D-R18), was reduced at peptide concentrations >1 mM in a voltage-dependent manner (at +40 mV). In control experiments, channels were not activated by the scrambled AS sequence (ASS) or skeletal II-III loop peptide (NB). Resting Ca2+ release from cardiac sarcoplasmic reticulum was not altered by peptide AC, but Ca2+-induced Ca2+ release was depressed. Resting and Ca2+-induced Ca2+ release were enhanced by both the native and modified AS peptides. NMR revealed (i) that the structure of peptide AS(D-R18) is not influenced by [Ca2+] and (ii) that peptide AC adopts a helical structure, particularly in the region containing positively charged residues. This is the first report of specific functional interactions between dihydropyridine receptor A region peptides and cardiac RyR ion channels in lipid bilayers.

Overexpression of human beta2-adrenergic receptors increases gain of excitation-contraction coupling in mouse ventricular myocytes

This study investigated cardiac excitation-contraction coupling at 37 degrees C in transgenic mice with cardiac-specific overexpression of human beta2-adrenergic receptors (TG4 mice). In field-stimulated myocytes, contraction was significantly greater in TG4 compared with wild-type (WT) ventricular myocytes. In contrast, when duration of depolarization was controlled with rectangular voltage clamp steps, contraction amplitudes initiated by test steps were the same in WT and TG4 myocytes. When cells were voltage clamped with action potentials simulating TG4 and WT action potential configurations, contractions were greater with long TG4 action potentials and smaller with shorter WT action potentials, which suggests an important role for action potential configuration. Interestingly, peak amplitude of L-type Ca2+ current (I(Ca-L)) initiated by rectangular test steps was reduced, although the voltage dependencies of contractions and currents were not altered. To explore the basis for the altered relation between contraction and I(Ca-L), Ca2+ concentrations were measured in myocytes loaded with fura 2. Diastolic concentrations of free Ca2+ and amplitudes of Ca2+ transients were similar in voltage-clamped myocytes from WT and TG4 mice. However, sarcoplasmic reticulum (SR) Ca2+ content assessed with the rapid application of caffeine was elevated in TG4 cells. Increased SR Ca2+ was accompanied by increased frequency and amplitudes of spontaneous Ca2+ sparks measured at 37 degrees C with fluo 3. These observations suggest that the gain of Ca(2+)-induced Ca2+ release is increased in TG4 myocytes. Increased gain counteracts the effects of decreased amplitude of I(Ca-L) in voltage-clamped myocytes and likely contributes to increased contraction amplitudes in field-stimulated TG4 myocytes.

Removal of clustered positive charge from dihydropyridine receptor II-III loop peptide augments activation of ryanodine receptors

Peptides based on the skeletal muscle DHPR II-III loop have been shown to regulate ryanodine receptor channel activity. The N-terminal region of this cytoplasmic loop is predicted to adopt an alpha-helical conformation. We have selected a peptide sequence of 26 residues (Ala(667)-Asp(692)) as the minimum sequence to emulate the helical propensity of the corresponding protein sequence. The interaction of this control peptide with skeletal and cardiac RyR channels in planar lipid bilayers was then assessed and was found to lack isoform specificity. At low concentrations peptide A(667)-D(692) increased RyR open probability, whilst at higher concentrations open probability was reduced. By replacing a region of clustered positive charge with a neutral sequence with the same predisposition to helicity, the inhibitory effect was ablated and activation was enhanced. This novel finding demonstrates that activation does not derive from the presence of positively charged residues adjacent in the primary structure and, although it may be mediated by the alignment of basic residues down one face of an amphipathic helix, not all of these residues are essential.

Calcium sparks in mouse ventricular myocytes at physiological temperature

In cardiac muscle, Ca2+ is released from the sarcoplasmic reticulum (SR) in units called Ca2+ sparks. Ca2+ spark characteristics have been studied almost entirely at room temperature. This study compares characteristics of spontaneous sparks detected with fluo 3 in resting mouse ventricular myocytes at 22 and 37 degrees C. The incidence and frequency of Ca2+ sparks decreased dramatically at 37 degrees C compared with 22 degrees C. Also, spark amplitudes and times to peak were significantly reduced at 37 degrees C. In contrast, spatial width and decay times were unchanged. During field stimulation, peak spatially averaged transients were similar at 22 and 37 degrees C, and experiments with fura 2 demonstrated that diastolic and systolic Ca2+ concentrations were unchanged. However, SR Ca2+ content decreased significantly at 37 degrees C. Restoration of SR Ca2+ by superfusion with 5 mM Ca2+ increased spark frequency but did not reverse the effects of temperature on spark parameters. Thus effects of temperature on spark frequency may reflect changes in SR stores, whereas changes in spark amplitude and rise time may reflect known effects of temperature on ryanodine receptor function.

RyR1/RyR3 chimeras reveal that multiple domains of RyR1 are involved in skeletal-type E-C coupling

Skeletal-type E-C coupling is thought to require a direct interaction between RyR1 and the alpha(1S)-DHPR. Most available evidence suggests that the cytoplasmic II-III loop of the dihydropyridine receptor (DHPR) is the primary source of the orthograde signal. However, identification of the region(s) of RyR1 involved in bidirectional signaling with the alpha(1S)-DHPR remains elusive. To identify these regions we have designed a series of chimeric RyR cDNAs in which different segments of RyR1 were inserted into the corresponding region of RyR3 and expressed in dyspedic 1B5 myotubes. RyR3 provides a preferable background than RyR2 for defining domains essential for E-C coupling because it possesses less sequence homology to RyR1 than the RyR2 backbone used in previous studies. Our data show that two regions of RyR1 (chimera Ch-10 aa 1681-2641 and Ch-9 aa 2642-3770), were independently able to restore skeletal-type E-C coupling to RyR3. These two regions were further mapped and the critical RyR1 residues were 1924-2446 (Ch-21) and 2644-3223 (Ch-19). These results both support and refine the previous hypothesis that multiple domains of RyR1 combine to functionally interact with the DHPR during E-C coupling.

Functional properties of ryanodine receptors in hippocampal neurons change during early differentiation in culture

6-((4,4-difluoro-5,7-dimethyl-4-bora-3a,4a-diaza-s-indacene-3-propionyl)amino)hexanoic acid ryanodine (BODIPY-ryanodine) binding and Ca(2+) imaging were used to study the properties of ryanodine receptors (RyRs) and cytoplasmic Ca(2+) (Ca) changes in neurons cultured from the embryonic rat hippocampus during the earliest stages of differentiation. Baseline Ca levels declined from 164 +/- 5 (SD) nM at early stages to 70 +/- 4 nM in differentiated neurons. Fluorescent BODIPY-ryanodine binding signals identified activated RyRs in somata, which were eliminated by removal of external Ca(2+) or by blockage of Ca(2+) entry through L-type but not N-type Ca(2+) channels. The GABA synthesis inhibitor 3-mercaptopropionic acid completely abolished ryanodine binding. Caffeine or K(+)-depolarization inhibited the activity of RyRs at very early stages of differentiation but had stimulatory effects at later stages after a network of processes had formed. BayK-8644 stimulated RyRs throughout all regions of all differentiating cells. The results suggest that in differentiating embryonic hippocampal neurons the activity of RyRs is maintained via Ca(2+) entering through L-type Ca(2+) channels. The mode of activation of L-type voltage-gated Ca(2+) channels with either membrane depolarization or specific pharmacological agents affects the coupled activity of RyRs differently as neurons differentiate processes and networks.

Activation of cardiac ryanodine receptors by the calcium channel agonist FPL-64176

We investigated the possibility that the Ca(2+) channel agonist FPL-64176 (FPL) might also activate the cardiac sarcoplasmic reticulum (SR) Ca(2+) release channel ryanodine receptor (RyR). The effects of FPL were tested on single channel activity of purified and crude vesicular RyR (RyR2) isolated from human and dog hearts using the planar lipid bilayer technique. FPL (100-200 microM) increased single channel open probability (P(o)) when added to the cytoplasmic side of the channel (P(o) = 0.070 +/- 0.021 in control RyR2; 0.378 +/- 0.086 in 150 microM FPL, n = 9, P < 0.01) by prolonging open times and decreasing closed times without changing current magnitude. FPL had no effect on P(o) when added to the trans (luminal) side of the bilayer (P(o) = 0.079 +/- 0.036 in control and 0.103 +/- 0.066 in FPL, n = 4, no significant difference). The bell-shaped [Ca(2+)] dependence of [(3)H]ryanodine binding and of P(o) was altered by FPL, suggesting that the mechanism by which FPL increases channel activity is by an increase in Ca(2+)-induced activation at low [Ca(2+)] (without a change in threshold) and suppression of Ca(2+)-induced inactivation at high [Ca(2+)]. However, the fact that inactivation was restored at elevated [Ca(2+)] suggests a competitive interaction between Ca(2+) and FPL on inactivation. FPL had no effect on RyR skeletal channels (RyR1), where P(o) was 0.039 +/- 0.005 in control versus 0.030 +/- 0.006 in 150 microM FPL (no significant difference). These results suggest that, in addition to its ability to activate the L-type Ca(2+) channels, FPL activates cardiac RyR2 primarily by reducing the Ca(2+) sensitivity of inactivation.

Interactions between dihydropyridine receptors and ryanodine receptors in striated muscle

Excitation-contraction coupling in both skeletal and cardiac muscle depends on structural and functional interactions between the voltage-sensing dihydropyridine receptor L-type Ca(2+) channels in the surface/transverse tubular membrane and ryanodine receptor Ca(2+) release channels in the sarcoplasmic reticulum membrane. The channels are targeted to either side of a narrow junctional gap that separates the external and internal membrane systems and are arranged so that bi-directional structural and functional coupling can occur between the proteins. There is strong evidence for a physical interaction between the two types of channel protein in skeletal muscle. This evidence is derived from studies of excitation-contraction coupling in intact myocytes and from experiments in isolated systems where fragments of the dihydropyridine receptor can bind to the ryanodine receptors in sarcoplasmic reticulum vesicles or in lipid bilayers and alter channel activity. Although micro-regions that participate in the functional interactions have been identified in each protein, the role of these regions and the molecular nature of the protein-protein interaction remain unknown. The trigger for Ca(2+) release through ryanodine receptors in cardiac muscle is a Ca(2+) influx through the L-type Ca(2+) channel. The Ca(2+) entering through the surface membrane Ca(2+) channels flows directly onto underlying ryanodine receptors and activates the channels. This was thought to be a relatively simple system compared with that in skeletal muscle. However, complexities are emerging and evidence has now been obtained for a bi-directional physical coupling between the proteins in cardiac as well as skeletal muscle. The molecular nature of this coupling remains to be elucidated.

Effects of FPL 64176 on Ca transients in voltage-clamped rat ventricular myocytes

1. The L-type Ca channel agonist FPL 64176 increased the amplitude of both Ca currents and Ca transients elicited from isolated voltage clamped rat ventricular myocytes far more than it increased the rate of rise of the Ca transients. Consequently, the gain function relating the amplitude of peak Ca current to Ca transient rate of rise was greatly reduced at all potentials. 2. Furthermore, an increase in this gain function normally observed at negative potentials is abolished by FPL 64716. 3. Despite slowing the rate of decline of Ca transients, FPL 64176, at the concentration of 1 microM used throughout, had no direct effect on sarcoplasmic reticulum (SR) Ca uptake or release using isolated cardiac membranes. 4. Arguments based on results presented here and elsewhere suggest that decreased gain was not due to increased ryanodine receptor adaptation or inactivation, to decreased L-type single channel current, to decreased SR Ca content, or to decreased synchronization of release. Decreased gain instead appears to reflect a form of decrease in coupling efficiency due either to differential effects of long openings on whole cell currents as opposed to the Ca release the long openings trigger or to some compensatory mechanism activated by the increased Ca trigger or resting free [Ca(2+)](i). 5. Abolition by FPL 64176 of the increased gain normally seen at negative potentials rendered it impossible to confirm or refute the claim that a single Ca ion suffices to activate ryanodine receptors.

A cardiac dihydropyridine receptor II-III loop peptide inhibits resting Ca(2+) sparks in ferret ventricular myocytes

1. We studied the effect of a peptide (Ac-10C) on cardiac ryanodine receptor (RyR) opening. This decapeptide (KKERKLARTA) is a fragment of the cardiac dihydropyridine receptor (DHPR) from the cytosolic loop between the second and third transmembrane domains (II-III loop). Studies were carried out in ferret ventricular myocytes by simultaneously applying ruptured-patch voltage clamp and line-scan confocal microscopy with fluo-3 to measure intracellular [Ca(2+)] ([Ca(2+)](i)) and Ca(2+) sparks. 2. Inclusion of Ac-10C in the dialysing pipette solution inhibited resting Ca(2+) spark frequency (due to diastolic RyR openings) by > 50 %. This occurred without changing sarcoplasmic reticulum (SR) Ca(2+) content, which was measured via the caffeine-induced Ca(2+) transient amplitude and the caffeine-induced Na(+)-Ca(2+) exchange current (I(NCX)) integral. Ac-10C also reduced slightly the size of Ca(2+) sparks. 3. Ac-10C did not alter either resting [Ca(2+)](i) (assessed by indo-1 fluorescence) or DHPR gating (measured as L-type Ca(2+) current). 4. The SR Ca(2+) fractional release was depressed by Ac-10C at relatively low SR Ca(2+) content, but not at higher SR Ca(2+) content. 5. A control scrambled peptide (Ac-10CS) did not alter any of the measured parameters (notably Ca(2+) spark frequency or SR Ca(2+) fractional release). Thus, the Ac-10C effects may be sequence or charge distribution specific. 6. Our results suggest an inhibitory regulation of RyRs at rest via the cardiac DHPR II-III loop N-terminus region. The mechanism of the effect and whether this interaction is important in cardiac excitation-contraction coupling (E-C coupling) per se, requires further investigation.