The neuromuscular and multisystem features of RYR1-related malignant hyperthermia and rhabdomyolysis: A study protocol

INTRODUCTION

Malignant hyperthermia (MH) and exertional rhabdomyolysis (ERM) have long been considered episodic phenotypes occurring in response to external triggers in otherwise healthy individuals with variants in RYR1. However, recent studies have demonstrated a clinical and histopathological continuum between patients with RYR1-related congenital myopathies and those with ERM or MH susceptibility. Furthermore, animal studies have shown non-neuromuscular features such as a mild bleeding disorder and an immunological gain-of-function associated with MH/ERM related RYR1 variants raising important questions for further research. Awareness of the neuromuscular disease spectrum and potential multisystem involvement in RYR1-related MH and ERM is essential to optimize the diagnostic work-up, improve counselling and and future treatment strategies for patients affected by these conditions. This study will examine in detail the nature and severity of continuous disease manifestations and their effect on daily life in patients with RYR1-related MH and ERM.

METHODS

The study protocol consists of four parts; an online questionnaire study, a clinical observational study, muscle imaging, and specific immunological studies. Patients with RYR1-related MH susceptibility and ERM will be included. The imaging, immunological and clinical studies will have a cross-sectional design, while the questionnaire study will be performed three times during a year to assess disease impact, daily living activities, fatigue and pain. The imaging study consists of muscle ultrasound and whole-body magnetic resonance imaging studies. For the immunological studies, peripheral mononuclear blood cells will be isolated for in vitro stimulation with toll-like receptor ligands, to examine the role of the immune system in the pathophysiology of RYR1-related MH and ERM.

DISCUSSION

This study will increase knowledge of the full spectrum of neuromuscular and multisystem features of RYR1-related MH and ERM and will establish a well-characterized baseline cohort for future studies on RYR1-related disorders. The results of this study are expected to improve recognition of RYR1-related symptoms, counselling and a more personalized approach to patients affected by these conditions. Furthermore, results will create new insights in the role of the immune system in the pathophysiology of MH and ERM.

TRIAL REGISTRATION

This study was pre-registered at ClinicalTrials.gov (ID: NCT04610619).

Super-Resolution Imaging Using a Novel High-Fidelity Antibody Reveals Close Association of the Neuronal Sodium Channel NaV1.6 with Ryanodine Receptors in Cardiac Muscle

The voltage-gated sodium channel [pore-forming subunit of the neuronal voltage-gated sodium channel (NaV1.6)] has recently been found in cardiac myocytes. Emerging studies indicate a role for NaV1.6 in ionic homeostasis as well as arrhythmogenesis. Little is known about the spatial organization of these channels in cardiac muscle, mainly due to the lack of high-fidelity antibodies. Therefore, we developed and rigorously validated a novel rabbit polyclonal NaV1.6 antibody and undertook super-resolution microscopy studies of NaV1.6 localization in cardiac muscle. We developed and validated a novel rabbit polyclonal antibody against a C-terminal epitope on the neuronal sodium channel 1.6 (NaV1.6). Raw sera showed high affinity in immuno-fluorescence studies, which was improved with affinity purification. The antibody was rigorously validated for specificity via multiple approaches. Lastly, we used this antibody in proximity ligation assay (PLA) and super-resolution STochastic Optical Reconstruction Microscopy (STORM) studies, which revealed enrichment of NaV1.6 in close proximity to ryanodine receptor (RyR2), a key calcium (Ca2+) cycling protein, in cardiac myocytes. In summary, our novel NaV1.6 antibody demonstrates high degrees of specificity and fidelity in multiple preparations. It enabled multimodal microscopic studies and revealed that over half of the NaV1.6 channels in cardiac myocytes are located within 100 nm of ryanodine receptor Ca2+ release channels.

Abnormal Expression of Genes Associated with Development and Inflammation in the Heart of Mouse Maternal Phenylketonuria Offspring

This study descibes gene expression in the fetus hearts obtained from mouse model for Phenylketonuria. These hearts have cardiovascular disease (CVD). Therefore genes involved in CVD were examined. Several genes associated with heart development and inflammation were found to be altered. In order to investigate whether the abnormal gene expression alters transcription and translation, the levels of troponin mRNA and protein were determined. One step real time RT-PCR showed a reduction in cardiac troponin I, troponin T2 and ryanodine receptor 2. Determination of troponin I and T protein levels showed reduced levels of these proteins. Our results suggest that altered gene expression affects protein production. These changes are likely involved in the cardiovascular defects seen in the mouse.

Calcium concentration response to uterine ischemia: a comparison of uterine fibroid cells and adjacent normal myometrial cells

OBJECTIVE

Uterine artery occlusion by laparoscopy (UAOL) has been used for the treatment of uterine fibroids and beneficial effects to patients have been shown in clinical studies since 2000. Fibroid cells are more susceptible to apoptosis than myometrial cells under hypoxic conditions, but the molecular mechanisms underlying this effect remain unclear. The aim of this study was to investigate the role of intracellular calcium (Ca(2+)) release mediated by Ca(2+) channel inositol 1,4,5 trisphosphate receptor1 (IP3R1)/ryanodine receptor1 (RYR1) in the apoptosis of uterine fibroid cells under hypoxia.

STUDY DESIGN

We compared the expressions of IP3R1 and RYR1 in fibroid and surrounding myometrial tissue from 20 patients before UAOL. After 6h treatment under hypoxia (1% O2) with or without Ca(2+) channel blockers (heparin or/and ruthenium red), the intracellular Ca(2+) concentration, cytochrome c (Cytc) protein and cell apoptosis were determined.

RESULTS

IP3R1 and RYR1 mRNA and protein levels were significantly higher in fibroid than in myometrial tissues. Under hypoxic conditions, Ca(2+) concentration in fibroid cells was significantly higher than in myometrial cells (Ca(2+): 82.69±16.92nmol/L vs 46.14±9.11nmol/L, P<0.05), and Cytc increased similarly in fibroid cells. These increases in Ca(2+) concentration, Cytc and cell apoptosis were significantly reversed by calcium blocker in fibroid cells.

CONCLUSION

This study demonstrated that intracellular calcium release mediated by IP3R1/RYR1 could induce apoptosis in uterine fibroid cells under hypoxic conditions, and was responsible for the susceptibility to apoptosis of fibroid cells under UAOL.

Quantitative analysis of calcium spikes in noisy fluorescent background

Intracellular calcium signals are studied by laser-scanning confocal fluorescence microscopy. The required spatio-temporal resolution makes description of calcium signals difficult because of the low signal-to-noise ratio. We designed a new procedure of calcium spike analysis based on their fitting with a model. The accuracy and precision of calcium spike description were tested on synthetic datasets generated either with randomly varied spike parameters and Gaussian noise of constant amplitude, or with constant spike parameters and Gaussian noise of various amplitudes. Statistical analysis was used to evaluate the performance of spike fitting algorithms. The procedure was optimized for reliable estimation of calcium spike parameters and for dismissal of false events. A new algorithm was introduced that corrects the acquisition time of pixels in line-scan images that is in error due to sequential acquisition of individual pixels along the space coordinate. New software was developed in Matlab and provided for general use. It allows interactive dissection of temporal profiles of calcium spikes from x-t images, their fitting with predefined function(s) and acceptance of results on statistical grounds, thus allowing efficient analysis and reliable description of calcium signaling in cardiac myocytes down to the in situ function of ryanodine receptors.

Subtype identification and functional characterization of ryanodine receptors in rat cerebral artery myocytes

Ryanodine receptors (RyRs) regulate contractility in resistance-size cerebral artery smooth muscle, yet their molecular identity, subcellular location, and phenotype in this tissue remain unknown. Following rat resistance-size cerebral artery myocyte sarcoplasmic reticulum (SR) purification and incorporation into POPE-POPS-POPC (5:3:2; wt/wt) bilayers, unitary conductances of 110 +/- 8, 334 +/- 15, and 441 +/- 27 pS in symmetric 300 mM Cs(+) were usually detected. The most frequent (34/40 bilayers) conductance (334 pS) decreased to Collapse

Key Words

• calcium-release channels
• vascular smooth muscle

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MESH Headings

• Animals
• Cells, Cultured
• Cerebral Arteries/chemistry
• Cerebral Arteries/cytology
• Cerebral Arteries/physiology
• Female
• Male
• Muscle Cells/chemistry
• Muscle Cells/physiology
• Protein Subunits/analysis
• Protein Subunits/physiology
• Rats
• Rats, Sprague-Dawley
• Ryanodine Receptor Calcium Release Channel/analysis
• Ryanodine Receptor Calcium Release Channel/physiology

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Grants

• AA11560 NIAAA NIH HHS
• R01 HL094378 NHLBI NIH HHS
• HL77424 NHLBI NIH HHS
• HL67061 NHLBI NIH HHS
• HL94378 NHLBI NIH HHS

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Affiliation(s)

Thirumalini Vaithianathan Department Pharmacology, University of Tennessee Health Science Center, Memphis, Tennessee 38163, USA
Damodaran Narayanan
Maria T Asuncion-Chin
Loice H Jeyakumar
Jianxi Liu
Sidney Fleischer
Jonathan H Jaggar
Alejandro M Dopico

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Analysis of ryanodine receptor clusters in rat and human cardiac myocytes

Single rat ventricular myocytes and human ventricle tissue sections were labeled with antibodies against the ryanodine receptor (RyR) and alpha-actinin to examine the 3D distribution of RyRs with confocal microscopy. Image contrast was maximized by refractive index matching and deconvolution. The RyR label formed discrete puncta representing clusters of RyRs or "couplons" around the edges of the myofilaments with a nearest-neighbor spacing of 0.66 +/- 0.06 microm in rat and 0.78 +/- 0.07 microm in human. Each bundle of myofibrils was served by approximately six couplons, which supplied a cross-sectional area of approximately 0.6 microm(2) in rat and approximately 0.8 microm(2) in human. Although the couplons were in reasonable registration with z-lines, there were discontinuities in the longitudinal position of sarcomeres so that dislocations in the order of RyR clusters occurred. There was approximately 53% longitudinal registration of RyR clusters, suggesting a nonrandom placement of couplons around the sarcomere. These data can explain the spherical propagation of Ca(2+) waves and provide quantitative 3D data sets needed for accurate modeling of cardiac Ca(2+)-induced Ca(2+) release. By quantifying labeling intensity in rat ventricular myocytes, a lower limit of 78 RyRs per cluster (on average) was obtained. By modeling the couplon as a disk wrapping around a t-tubule and fitting cluster images, 95% of couplons contained between 120 and 260 RyRs (assuming that RyRs are tight packed with a spacing of 29 nm). Assuming similar labeling efficiency in human, from the fluorescence intensity alone we estimate that human ventricular myocytes contain approximately 30% fewer RyRs per couplon than rat.

Control of IsAHP in mouse hippocampus CA1 pyramidal neurons by RyR3-mediated calcium-induced calcium release

In several neuronal preparations, the ryanodine-sensitive calcium store was reported to participate in the generation of slow afterhyperpolarization currents (IsAHP) involved in spike frequency adaptation. We show that calcium release from the ryanodine-sensitive calcium store is a major determinant of the triggering of IsAHP in mouse CA1 pyramidal neurons. Whole-cell patch clamp recordings in hippocampus slices show that the intracellular calcium stores depletion using an inhibitor of the endoplasmic reticulum Ca2+-ATPase (5 microM cyclopiazonic acid), as well as the specific blockade of ryanodine receptors (100 microM ryanodine) both reduced the IsAHP by about 70%. Immunohistology, using an anti-RyR3 specific antibody, indicates that RyR3 expression is particularly enriched in the CA1 apical dendrites (considered as the most important site for sAHP generation). We show that our anti-RyR3 antibody acts as a functional RyR3 antagonist and induced a reduction in IsAHP by about 70%. The additional ryanodine application (100 micro M) did not further affect IsAHP, thus excluding RyR2 in IsAHP activation. Our results argue in favor of a specialized function of RyR3 in CA1 pyramidal cells in triggering IsAHP due to their localization in the apical dendrite.

Carbonic anhydrase XIV in skeletal muscle: subcellular localization and function from wild-type and knockout mice

The expression of carbonic anhydrase (CA) XIV was investigated in mouse skeletal muscles. Sarcoplasmic reticulum (SR) and sarcolemmal (SL) membrane fractions were isolated from wild-type (WT) and CA XIV knockout (KO) mice. The CA XIV protein of 54 kDa was present in SR and SL membrane fractions as shown by Western blot analysis. CA activity measurements of WT and KO membrane fractions showed that CA XIV accounts for approximately 50% and 66% of the total CA activities determined in the SR and SL fractions, respectively. This indicates the presence of at least one other membrane-associated CA isoform in these membranes, e.g., CA IV, CA IX, or CA XII. Muscle fibers of the extensor digitorum longus (EDL) muscle were immunostained with anti-CA XIV/FITC and anti-sarco(endo)plasmic reticulum Ca(2+)-ATPase 1/TRITC, with anti-CA XIV/FITC and anti-ryanodine receptor/TRITC, or with anti-CA XIV/FITC and anti-monocarboxylate transporter-4/TRITC. CA XIV was expressed in the plasma membrane and in the longitudinal SR but not in the terminal SR. Isometric contraction measurements of single twitches and tetani and a fatigue protocol applied to fiber bundles of the fast-twitch EDL and of the slow-twitch soleus muscle from WT and KO mice showed that the lack of SR membrane-associated CA XIV did not affect maximum force, rise and relaxation times, and fatigue behavior. Thus, it is concluded that a reduction of the total SR CA activity by approximately 50% in CA XIV KO mice does not lead to an impairment of SR function.

Peptide probe study of the role of interaction between the cytoplasmic and transmembrane domains of the ryanodine receptor in the channel regulation mechanism

Ryanodine receptor (RyR) mutations linked with some congenital skeletal and cardiac diseases are localized to three easily definable regions: region 1 (N-terminal domain), region 2 (central domain), and a rather broad region 3 containing the channel pore. As shown in our recent studies, the interdomain interaction between regions 1 and 2 plays a critical role in channel regulation and pathogenesis. Here we present evidence that within region 3 there is a similar channel regulation mechanism mediated by an interdomain interaction. DP15, a peptide corresponding to RyR1 residues 4820-4841, produced significant activation of [3H]ryanodine binding above threshold Ca2+ concentrations (>or=0.3 microM), but MH mutations (L4823P or L4837V) made in DP15 almost completely abolished its channel activating function. To identify the DP15 binding site(s) within RyR1, DP15 (labeled with a fluorescent probe Alexa Fluor 680 and a photoaffinity cross-linker APG) was cross-linked to RyR1, and the site of cross-linking was identified by gel analysis of fluorescently labeled proteolytic fragments with the aid of Western blotting with site-specific antibodies. The shortest fluorescently labeled band was a 96 kDa fragment which was stained with an antibody directed to the region of residues 4114-4142 of RyR1, indicating that the interaction between the region of residues 4820-4841 adjacent to the channel pore and the 96 kDa segment containing the region of residues 4114-4142 is involved in the mechanism of Ca2+-dependent channel regulation. In further support of this concept, anti-DP15 antibody and cardiac counterpart of DP15 produced channel activation similar to that of DP15.

CaMKII-mediated increased lusitropic responses to β-adrenoreceptor stimulation in ANP-receptor deficient mice

OBJECTIVE

Mice with genetic disruption of the guanylyl cyclase-A (GC-A) receptor for atrial natriuretic peptide (ANP), have chronic arterial hypertension and marked cardiac hypertrophy. Intriguingly, despite pronounced remodeling, cardiac contractile functions and cardiomyocyte Ca(2+)-handling are preserved and even enhanced. The present study aimed to characterize the specific molecular mechanisms preventing cardiac failure.

METHODS AND RESULTS

Contractile function and expression as well as phosphorylation of regulatory proteins were evaluated in isolated perfused working hearts from wild-type and GC-A KO mice under baseline conditions and during beta(1)-adrenergic stimulation. Ca(i)(2+)-transients were monitored in Indo-1 loaded isolated adult cardiomyocytes. Cardiac contractile, especially lusitropic responsiveness to beta-adrenergic stimulation was significantly increased in GC-A KO mice. This was concomitant to enhanced expression and activation of Ca(2+)/calmodulin-dependent protein kinase II (CaMKII), increased dual-site phosphorylation of phospholamban (PLB) at Ser(16) and Thr(17), enhanced amplitude of Ca(i)(2+) transients, and accelerated Ca(i)(2+) decay. In contrast, the expression of cardiac ryanodine receptors and phosphorylation at Ser(2809) and Ser(2815) was not altered. Pharmacological inhibition of CaMKII-but not of protein kinase A-mediated PLB phosphorylation totally abolished the increased effects of beta-adrenergic stimulation on cardiac contractility and Ca(i)(2+)-handling. Thus, acceleration of sarcoplasmic reticulum Ca(2+)-uptake and increased availability of Ca(2+) for contraction, both secondary to increased CaMKII-mediated PLB phosphorylation, seem to mediate the augmented responsiveness of GC-A KO hearts to catecholamines.

CONCLUSION

Our observations show that increased CaMKII activity enhances the contractile relaxation response of hypertrophic GC-A KO hearts to beta-adrenergic stimulation and emphasize the critical role of CaMKII-dependent pathways in beta(1)-adrenoreceptor modulation of myocardial Ca(2+)-homeostasis and contractility.

TRPC3 channels colocalize with Na+/Ca2+exchanger and Na+pump in axial component of transverse-axial tubular system of rat ventricle

Transient receptor potential canonical (TRPC) proteins form Ca2+-permeable, nonselective cation channels activated after stimulation of G protein-coupled membrane receptors linked to phospholipase C (PLC). Although the PLC/inositol phosphate signaling pathway is known to exist in heart, expression and subcellular distribution of TRPC channel proteins in ventricular myocardium have not been evaluated. Of the six members of the TRPC channel family examined here, only TRPC3 was found by Western blot analysis of membrane proteins from rodent or canine ventricle. Likewise, only TRPC3 was observed in immunofluorescence analysis of thin sections from rat ventricle. TRPC3 was also the only family member observed in neonatal rat ventricular myocytes in culture. In longitudinal sections of rat ventricle, TRPC3 was predominantly localized to the intercalated disk region of the myocyte. However, transverse sections through heart muscle or single isolated adult myocytes revealed TRPC3-specific labeling in a vast network of intracellular membranes, where it colocalized with the Na+-K+-ATPase (NKA) pump and the Na+/Ca2+exchanger (NCX) but not with the ryanodine receptor or the sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) pump. Reciprocal immunoprecipitation assays from rat or canine ventricle showed that TRPC3 associates with NKA and NCX but not with the plasmalemmal Ca2+-ATPase pump. Immunoprecipitations from Sf9 insect cells heterologously expressing TRPC3, NKA, and NCX in various combinations revealed that NKA and NCX interact and that TRPC3 and NCX interact, but that TRPC3 does not directly associate with NKA. Together, these results suggest that TRPC3 is localized in the ventricular myocyte to the axial component of the transverse-axial tubular system, where it exists in a signaling complex that includes NCX and NKA.

The inotropic adaptation during late preconditioning against myocardial stunning is associated with an increase in FKBP12.6

The inotropic adaptation during late preconditioning against myocardial stunning is associated with an increase in FKBP12.6. by Laurence Lucats, Laurent Vinet, Alain Bizé, Xavier Monnet, Didier Morin, Jin Bo Su, Patricia Rouet-Benzineb, Olivier Cazorla, Jean-Jacques Mercadier, Luc Hittinger, Alain Berdeaux, Bijan Ghaleh.

OBJECTIVES

Late preconditioning reduces contractile dysfunction during myocardial stunning. Mechanisms involving adaptation of calcium handling during excitation-contraction coupling to late preconditioning remain to be established. Thus, we investigated whether the late preconditioned myocardium is associated with contractile adaptation and changes in the cardiac ryanodine receptor (RyR2) and its regulatory protein FKBP12.6.

METHODS

Chronically instrumented conscious dogs (coronary occluder, ultrasonic crystals for sonomicrometry) underwent a 10-min coronary artery occlusion followed by reperfusion. They were studied 24 h later in the late preconditioned state (day 1).

RESULTS

Maximal velocity of wall thickening at day 1 was increased as compared to corresponding baseline at day 0 (39+/-4 vs. 30+/-3 mm/s, p < 0.05) although systolic wall thickening was similar (2.8+/-0.2 vs. 2.9+/-0.2 mm), demonstrating a significant change in left ventricular inotropic state. Intracoronary infusion of ryanodine (0.5-6 microg) induced a dose-dependent decrease in wall thickening. In the late preconditioned state, this negative inotropic response was significantly reduced vs. control state, suggesting changes in sarcoplasmic reticulum (SR) Ca2+-release through RyR2. Immunoquantification of FKBP12.6 revealed a 2.8 fold ventricular increase after late preconditioning as compared to the control state. The amount of RyR2 and its phosphorylated state were similar and binding experiments did not reveal any alterations in B(max) or K(D) for RyR2. Calsequestrin, SERCA2a and phospholamban levels were not altered by late preconditioning.

CONCLUSIONS

The late preconditioned myocardium is characterized by an adaptation of regional function associated with an increased expression of FKBP12.6. This demonstrates an adaptation of the SR Ca2+-release through RyR2 during late preconditioning.

Occurrence of atypical Ca2+ transients in triadin-binding deficient-RYR1 mutants

Triadin in the junctional sarcoplasmic reticulum (SR) of skeletal muscle cells has been suggested to interact with ryanodine receptor 1 (RYR1) via its KEKE motifs. Recently, we showed that amino acid residues D4878, D4907, and E4908 in RYR1 are critical for triadin-binding in vitro [J.M. Lee, S.H. Rho, D.W. Shin, C. Cho, W.J. Park, S.H. Eom, J. Ma, D.H. Kim, Negatively charged amino acids within the intraluminal loop of ryanodine receptor are involved in the interaction with triadin, J. Biol. Chem. 279 (2004) 6994-7000]. In order to test whether a disruption of the triadin-binding site(s) in RYR1 affects SR Ca(2+) release, alanine-substituted single (D4878A, D4907A, and E4908A) and triple (RYR1-TM) mutants of D4878, D4907, and E4908 were expressed in RYR1-null myotubes. Co-immunoprecipitation experiments showed a 50-60% decrease of triadin brought down in the D4907A and RYR1-TM complexes compared to the triadin-wtRYR1 complex. Ca(2+) imaging experiments using Fluo-4-AM showed atypical caffeine responses in myotubes expressing D4907A and RYR1-TM characterized by either a lack of or slower activation and faster inactivation of Ca(2+) transients. The results suggest that disruption of interaction between triadin and RYR1 impairs RYR1 function and SR Ca(2+) release.

Combining Fluorescence Detection and Mass Spectrometric Analysis for Comprehensive and Quantitative Analysis of Redox-Sensitive Cysteines in Native Membrane Proteins

Monobromobimane (MBB) is a lipophilic reagent that selectively modifies free cysteine residues in proteins. Because of its lipophilic character, MBB is capable of labeling cysteine residues in membrane proteins under native conditions. Reaction of MBB with the sulfhydryl groups of free cysteines leads to formation of highly fluorescent derivatives. Here we describe a procedure for the detection and relative quantitation of MBB-labeled cysteines using fluorescence and mass spectrometric analyses, which allow determination of free cysteine content and unambiguous identification of MBB-modified cysteine residues. We have applied this approach to the analysis of the free and redox-sensitive cysteine residues of a large membrane protein, the sarcoplasmic reticulum Ca2+ release channel with a molecular mass of 2.2 million Da. Labeling was performed under physiologic conditions where the channel complex is in its native environment and is functionally active. The purified MBB-labeled channel complex was enzymatically digested, and the resulting peptides were separated by reversed-phase high-performance chromatography. MBB-labeled peptides were detected by fluorescence and identified by matrix-assisted laser desorption/ionization (MALDI) mass spectrometry. Under MALDI conditions, partial photolytic fragmentation of the MBB-peptide bound occurred, thus allowing convenient screening for the MBB-modified peptides in the MS spectrum by detection of the specific mass increment of 190.07 Da for MBB-modified cysteine residues. Modification of the peptides was further confirmed by tandem mass spectrometric analysis, utilizing sequencing information and the presence of the specific immonium ion for the MBB-modified cysteine residues at m/z 266.6. Quantitative information was obtained by comparison of both fluorescence and MS signal intensities of MBB-modified peptides. Combination of fluorescence with MS detection and analysis of MBB-labeled peptides supported by a customized software program provides a convenient method for identifying and quantifying redox-sensitive cysteines in membrane proteins of native biological systems. Identification of one redox-sensitive cysteine (2327) in the native membrane-bound sarcoplasmic reticulum Ca2+ release channel is described.

Molecular Determinants of Altered Ca 2+ Handling in Human Chronic Atrial Fibrillation

Background— Abnormal Ca 2+ handling may contribute to impaired atrial contractility and arrhythmogenesis in human chronic atrial fibrillation (cAF). Here, we assessed the phosphorylation levels of key proteins involved in altered Ca 2+ handling and contractility in cAF patients.

Methods and Results— Total and phosphorylation levels of Ca 2+ -handling and myofilament proteins were analyzed by Western blotting in right atrial appendages of 49 patients in sinus rhythm and 52 cAF patients. We found a higher total activity of type 1 (PP1) and type 2A phosphatases in cAF, which was associated with inhomogeneous changes of protein phosphorylation in the cellular compartments, ie, lower protein kinase A (PKA) phosphorylation of myosin binding protein-C (Ser-282 site) at the thick myofilaments but preserved PKA phosphorylation of troponin I at the thin myofilaments and enhanced PKA (Ser-16 site) and Ca 2+ -calmodulin protein kinase (Thr-17 site) phosphorylation of phospholamban. PP1 activity at sarcoplasmic reticulum is controlled by inhibitor-1 (I-1), which blocks PP1 in its PKA-phosphorylated form only. In cAF, the ratio of Thr-35–phosphorylated to total I-1 was 10-fold higher, which suggests that the enhanced phosphorylation of phospholamban may result from a stronger PP1 inhibition by PKA-hyperphosphorylated (activated) I-1.

Conclusions— Altered Ca 2+ handling in cAF is associated with impaired phosphorylation of myosin binding protein-C, which may contribute to the contractile dysfunction after cardioversion. The hyperphosphorylation of phospholamban probably results from enhanced inhibition of sarcoplasmic PP1 by hyperphosphorylated I-1 and may reinforce the leakiness of ryanodine channels in cAF. Restoration of sarcoplasmic reticulum–associated PP1 function may represent a new therapeutic option for treatment of atrial fibrillation.

Over-expression of Microspan, a novel component of the sarcoplasmic reticulum, causes severe muscle pathology with triad abnormalities

Sarcospan (SSPN) is a core component of the dystrophin-glycoprotein complex (DGC). Multiple SSPN transcripts are ubiquitously expressed and SSPN splicing is disrupted in many lung tumors, suggesting the importance of SSPN-related mRNAs. We describe the isolation of an alternatively spliced isoform of SSPN, which we designate 'microspan' based on its small size relative to SSPN. Microspan has two transmembrane domains and a novel C-terminus. We demonstrate that microspan is not an integral component of the DGC and is not perturbed by the loss of dystrophin. Microspan protein is detected at the sarcoplasmic reticulum (SR) using indirect immunofluorescence and immunoelectron microscopy. Furthermore, microspan purifies with skeletal muscle SR membranes and not transverse tubules. Mice engineered to over-express microspan display severe kyphosis and die at approximately 8 weeks of age. Levels of ryanodine receptor, dihydropyridine receptor, and SERCA-1 are greatly reduced in microspan transgenic muscle. Furthermore, electron microscopy reveals that microspan over-expression causes a dramatic perturbation in triad structure. Our findings suggest that microspan is an important component of the SR and may contribute to excitation-contraction coupling.

Role of intracellular stores in the regulation of rhythmical [Ca2+]i changes in interstitial cells of Cajal from rabbit portal vein

Interstitial cells of Cajal (ICCs) freshly isolated from rabbit portal vein and loaded with the Ca(2+)-sensitive indicator fluo-3 revealed rhythmical [Ca(2+)](i) changes occurring at 0.02-0.1 Hz. Each increase in [Ca(2+)](i) originated from a discrete central region of the ICC and propagated as a [Ca(2+)](i) wave towards the cell periphery, but usually became attenuated before reaching the ends of the cell. In about 40% of ICCs each rhythmical change in [Ca(2+)](i) consisted of an initial [Ca(2+)](i) increase (phase 1) followed by a faster rise in [Ca(2+)](i) (phase 2) and then a decrease in [Ca(2+)](i) (phase 3); the frequency correlated with the rate of rise of [Ca(2+)](i) during phase 1, but not with the peak amplitude. Rhythmical [Ca(2+)](i) changes persisted in nicardipine, but were abolished in Ca(2+)-free solution as well as by SK&F96365, cyclopiazonic acid, thapsigargin, 2-APB, xestospongin C or ryanodine. Intracellular Ca(2+) stores visualised with the low-affinity Ca(2+) indicator fluo-3FF were found to be enriched with ryanodine receptors (RyRs) detected with BODIPY TR-X ryanodine. Rhythmical [Ca(2+)](i) changes originated from a perinuclear S/ER element showing the highest RyR density. Immunostaining with anti-TRPC3,6,7 antibodies revealed the expression of these channel proteins in the ICC plasmalemma. This suggests that these rhythmical [Ca(2+)](i) changes, a key element of ICC pacemaking activity, result from S/ER Ca(2+) release which is mediated via RyRs and IP(3) receptors and is modulated by the activity of S/ER-Ca(2+)-ATPase and TRP channels but not by L-type Ca(2+) channels.

Altered calcium handling is an early sign of streptozotocin-induced diabetic cardiomyopathy

The main objective of the present study was to determine alterations of calcium handling in the diabetic rat heart during the transition from adaptive to maladaptive phase of cardiomyopathy. By inhibiting the nuclear enzyme poly(ADP-ribose) polymerase (PARP), we also investigated the possible role of this enzyme in the sequence of pathological events. Six weeks after induction of type I diabetes by injection of streptozotocin in rats, the hearts were perfused according to Langendorff. Intracellular-free calcium (Ca(2+)(i)) levels were measured by surface fluorometry using Indo-1 AM. Cyclic changes in Ca(2+)(i) concentrations and hemodynamic parameters were measured simultaneously. The hearts were challenged by infusion of isoproterenol. Six weeks of diabetes resulted in reduced inotropy and lusitropy. The diabetic hearts (DM) expressed a significantly elevated end-diastolic Ca(2+)(i) level (control, 111-/+20 vs DM, 221-/+35 nM). The maximal transport capacity of SERCA2a and conductance of RyR2 were reduced. These changes were not accompanied by major alterations in the tissue content of SERCA2a, RyR2, phospholamban and Na(+)/Ca(2+) exchanger. In response to beta-adrenergic activation, SERCA2a transport capacity and RyR2 conductance were stunted in the DM hearts. Inhibition of PARP induced minor changes in the mechanical function and calcium handling of the DM hearts. In conclusion, the observed changes in contractility and in Ca(2+)(i) handling are most likely attributable to functional disturbances of SERCA2a and RyR2 in this transitional phase of diabetes. At this stage of diabetes, PARP does not appear to play a significant pathogenetic role in the alterations in contractile function and calcium handling.

Essential role of Ca2+ release channels in angiotensin II-induced Ca2+ oscillations and mesangial cell contraction

The increased resistance of the glomerulus as a result of contractile dysfunction of mesangial cells (MCs) is associated with reduction of glomerular filtration rate and development of glomerulosclerosis. Evidences show MCs contraction changes with intracellular Ca(2+) concentration ([Ca(2+)](i)). Here, we explore the mechanism of angiotensin II (AngII)-induced Ca(2+) oscillations and MCs contraction. Primary MCs from 3-month-old and 28-month-old rats were used for detection of Ca(2+) oscillations and MC planar area with confocal microscopy. AngII could induce typical Ca(2+) oscillations and contraction of MCs. This process was abolished by thapsigargin, 2-aminoethoxydiphenyl borate, or 1-O-octadecyl-2-O-methyl-sn-glycero-3-phosphorylcholine, and partially inhibited by ryanodine, but could not be inhibited in the absence of extracellular Ca(2+). Ryanodine receptors (RyRs) and inositol 1,4,5-trisphosphate (InsP(3)) receptors displayed a strong colocalization, which may contribute to the amplification of Ca(2+) response. MLC(20) phosphorylation and MC planar area were associated with AngII-induced Ca(2+) oscillations. The frequency of Ca(2+) oscillations was dependent on the AngII concentration and correlated with the MCs' contractive extent, which could be attenuated by KN-93. The amplitude reduction of oscillations correlated with the decrease in aging-related contraction. In conclusion, [Ca(2+)](i) response of MCs to AngII is characterized by repetitive spikes through the following repetitive cycles: Ca(2+) release by phospholipase C -InsP(3) pathway, Ca(2+) amplification by Ca(2+)-activated RyRs and Ca(2+) reuptake by the endoplasmic reticulum. MCs contraction can be modulated by oscillations not only in an AngII-induced frequency-dependent mode but also in an aging-related, amplitude-dependent mode.

Cryo-EM and single particles

Cryoelectronmicroscopy is a method for the imaging of macromolecules in the electron microscope. It was originally developed to determine membrane protein structures from two-dimensional crystals, but more recently "single-particle" techniques have become powerful and popular. Three-dimensional reconstructions are obtained from sets of single-particle images by extensive computer processing; the methods are being applied to many macromolecular assemblies.

New cardioprotective agent K201 is natriuretic and glomerular filtration rate enhancing

BACKGROUND

K201 (JTV519) is a newly developed 1,4-benzothiazepine drug with antiarrhythmic and cardioprotective properties. It functions via stabilization of the ryanodine receptor-calcium release channel in the heart (RyR2). This receptor has been identified in the kidney, and in vitro studies suggest a role in the control of renal hemodynamics. To date, the in vivo function of this receptor is undefined. We hypothesized that this new drug, which is being developed for the treatment of heart failure for its myocardial actions, also possesses renal hemodynamic enhancing and excretory properties. We also used immunohistochemistry to identify RyR2 in the normal canine kidney.

METHODS AND RESULTS

We investigated the renal actions of K201 during intrarenal infusion in normal anesthetized dogs. K201 was infused after baseline measurements at 2 doses (0.1 and 0.5 mg.kg(-1).min(-1)). Immunohistochemistry was used to identify RyR2 presence in the kidney not exposed to K201. K201 was potently natriuretic and diuretic, with glomerular filtration rate- and renal blood flow-enhancing actions. The excretory responses to K201 administration were associated with decreases in distal tubular reabsorption of sodium despite a mild decrease in mean arterial pressure, which returned to baseline levels after K201 discontinuation. Immunohistochemistry of the normal canine kidney revealed the presence of RyR2 in the medullary collecting duct cells.

CONCLUSIONS

We report for the first time that the newly developed cardioprotective drug K201 possesses natriuretic, diuretic, glomerular filtration rate-enhancing, and vasodilating properties that go beyond myocardial actions and may support its therapeutic role in treatment of heart failure.

High Ambient Pressure Produces Hypertrophy and Up-Regulates Cardiac Sarcoplasmic Reticulum Ca2+ Regulatory Proteins in Cultured Rat Cardiomyocytes

Previously, we demonstrated in vivo that the nature of the alterations in sarcoplasmic reticulum (SR) function and SR Ca2+ regulatory proteins depends both on the type of mechanical overload imposed and on the duration of the heart disorder. The purpose of the present study was to determine in vitro whether an extrinsic mechanical overload (in the form of high ambient pressure) would cause an up-regulation of ryanodine receptor (RyR) and Ca2+-ATPase, as we previously reported mildly pressure-overloaded, hypertrophied rat hearts. Primary cultures of neonatal rat cardiomyocytes were prepared and high ambient pressure was produced using an incubator and pressure-overloading apparatus. Cells were exposed to one of two conditions for 72 h: atmospheric pressure conditions (APC) or high pressure conditions (HPC; HPC=APC+200 mmHg). The expression levels of RyR and Ca2+-ATPase were quantified and functional characteristics were monitored. The cell area was significantly greater under HPC. After 6 h exposure, the physiological properties of cardiomyocytes were impaired, but they returned to the baseline level within 24 h. After 24 h exposure, the expression level of RyR was significantly higher under HPC, and for Ca2+-ATPase, the expression level was significantly higher under HPC after 6 h exposure. HPC caused hypertrophy and up-regulated the expression of Ca2+ regulatory proteins and their genes. We suggest that this in vitro pressure-overloading model may prove useful, as is a stretch-overloading model, for investigation of the intracellular Ca2+ regulatory pathways responsible for the development of cardiac hypertrophy.

Calcium release from ryanodine receptors in the nucleoplasmic reticulum

Ca(2+) signals control DNA synthesis and repair, gene transcription, and other cell functions that occur within the nucleus. The nuclear envelope can store Ca(2+) and release it into the nucleus via either the inositol 1,4,5-trisphosphate receptor (InsP3R) or the ryanodine receptor (RyR). Furthermore, many cell types have a reticular network within their nuclei and InsP3Rs on this nucleoplasmic reticulum permit local subnuclear control of Ca(2+) signals and Ca(2+)-dependent intranuclear events. However, it is unknown whether RyR similarly is expressed on the nucleoplasmic reticulum and can control subnuclear Ca(2+) signals. Here we report that the type 1 RyR is expressed on intranuclear extensions of the sarcoplasmic reticulum of C2C12 cells, a skeletal muscle derived cell line. In addition, two-photon photorelease of caged Ca(2+) in the region of the nucleoplasmic reticulum evoked Ca(2+)-induced Ca(2+) release (CICR) within the nucleus, which could be suppressed by the RyR inhibitor dantrolene. These results show that intranuclear extensions of the nuclear envelope have functional RyR and provide a possible mechanism whereby cells expressing RyR can regulate Ca(2+) signals in discrete regions within the nucleus.

Comparison of contraction and calcium handling between right and left ventricular myocytes from adult mouse heart: a role for repolarization waveform

In the mammalian heart, the right ventricle (RV) has a distinct structural and electrophysiological profile compared to the left ventricle (LV). However, the possibility that myocytes from the RV and LV have different contractile properties has not been established. In this study, sarcomere shortening, [Ca2+]i transients and Ca2+ and K+ currents in unloaded myocytes isolated from the RV, LV epicardium (LVepi) and LV endocardium (LVendo) of adult mice were evaluated. Maximum sarcomere shortening elicited by field stimulation was graded in the order: LVendo > LVepi > RV. Systolic [Ca2+]i was higher in LVendo myocytes than in RV myocytes. Voltage-clamp experiments in which action potential (AP) waveforms from RV and LVendo were used as the command signal, demonstrated that total Ca2+ influx and myocyte shortening were larger in response to the LVendo AP, independent of myocyte subtypes. Evaluation of possible regional differences in myocyte Ca2+ handling was based on: (i) the current-voltage relation of the Ca2+ current; (ii) sarcoplasmic reticulum Ca2+ uptake; and (iii) mRNA expression of important components of the Ca2+ handling system. None of these were significantly different between RV and LVendo. In contrast, the Ca2+-independent K+ current, which modulates AP repolarization, was significantly different between RV, LVepi and LVendo. These results suggest that these differences in K+ currents can alter AP duration and modulate the [Ca2+]i transient and corresponding contraction. In summary, these findings provide an initial description of regional differences in excitation-contraction coupling in the adult mouse heart [corrected]

Molecular correlates of repolarization alternans in cardiac myocytes

Arrhythmogenic action potential alternans (APD-ALT) is thought to arise from beat to beat alteration in cellular Ca(2+) cycling. Previously, we found that spatial heterogeneity in APD-ALT between ventricular myocytes is key to the mechanism linking APD-ALT to cardiac arrhythmogenesis. However, the cellular and molecular basis for APD-ALT is poorly understood. To test the hypothesis that spatial heterogeneities in expression and function of calcium cycling proteins underlies heterogeneities in APD-ALT, endocardial and epicardial myocytes were isolated from left ventricular free wall of 20 guinea pig hearts. APD-ALT and Ca(2+) transient alternans (Ca-ALT) were measured simultaneously as stimulus rate was increased progressively. Endocardial myocytes exhibited greater susceptibility to cellular alternans than epicardial myocytes as evidenced by a significantly lower pacing rate threshold for APD-ALT (113 +/ -9 bpm vs. 151 +/- 8 bpm, respectively, P < 0.05) and for Ca-ALT (110 +/- 8 bpm vs. 149 +/- 8 bpm, respectively, P < 0.05). APD-ALT never occurred without Ca-ALT, whereas Ca-ALT was readily induced in the absence of APD-ALT by repetitive constant action potential waveform, suggesting that Ca-ALT was not secondary to APD-ALT. Importantly, there were significant voltage-independent differences in Ca(2+) cycling between endocardial and epicardial myocytes as evidenced by weaker Ca(2+) release (32% lower Ca(2+) amplitude, and 16% longer rise time), and slower Ca(2+) reuptake (24% larger Ca(2+) decay time constant, and 9% longer Ca(2+) transient duration) in endocardial compared to epicardial myocytes. Taken together these data indicate that myocytes that are most susceptible to APD-ALT exhibit impaired Ca(2+) release and reuptake. Moreover, transmural differences in Ca(2+) cycling function was associated with significantly reduced endocardial expression of ryanodine release channel (by 22%) and SERCA2 (by 40%), suggesting a potential molecular basis for spatially heterogeneous APD-ALT. Moreover, transmural differences in expression and function of key SR Ca(2+) cycling proteins may underlie spatial heterogeneity of APD-ALT that has been closely linked to cardiac arrhythmogenesis.

Ca2+ sparks and waves in canine purkinje cells: a triple layered system of Ca2+ activation

We have investigated the subcellular spontaneous Ca2+ events in canine Purkinje cells using laser scanning confocal microscopy. Three types of Ca2+ transient were found: (1) nonpropagating Ca2+ transients that originate directly under the sarcolemma and lead to (2) small Ca2+ wavelets in a region limited to 6-microm depth under the sarcolemma causing (3) large Ca2+ waves that travel throughout the cell (CWWs). Immunocytochemical studies revealed 3 layers of Ca2+ channels: (1) channels associated with type 1 IP3 receptors (IP3R1) and type 3 ryanodine receptors (RyR3) are prominent directly under the sarcolemma; (2) type 2 ryanodine receptors (RyR2s) are present throughout the cell but virtually absent in a layer between 2 and 4 microm below the sarcolemma (Sub-SL); (3) type 3 ryanodine receptors (RyR3) is the dominant Ca2+ release channel in the Sub-SL. Simulations of both nonpropagating and propagating transients show that the generators of Ca2+ wavelets differ from those of the CWWs with the threshold of the former being less than that of the latter. Thus, Purkinje cells contain a functional and structural Ca2+ system responsible for the mechanism that translates Ca2+ release occurring directly under the sarcolemma into rapid Ca2+ release in the Sub-SL, which then initiates large-amplitude long lasting Ca2+ releases underlying CWWs. The sequence of spontaneous diastolic Ca2+ transients that starts directly under the sarcolemma and leads to Ca2+ wavelets and CWWs is important because CWWs have been shown to cause nondriven electrical activity.

Role of the second-messenger cyclic-adenosine 5'-diphosphate-ribose on adrenocorticotropin secretion from pituitary cells

We examined the role of the second-messenger cyclic-ADP-ribose (cADPR) on the regulation of ACTH secretion using AtT20 corticotroph tumor cell line. We found that the cADPR antagonist, 8-Br-cADPR, substantially diminished the secretion of ACTH induced by CRH and potassium in these cells, whereas xestospongin C, an inositol 1,4,5-triphosphate receptor antagonist, had no effect. In addition, the cADPR agonist, 3-deaza-cADPR, augmented ACTH secretion. The presence of the components of the cADPR system, namely ryanodine receptor, CD38, and cADPR itself, was determined in AtT20 cells. Furthermore, we observed that antagonists of the ryanodine channel and cADPR system can decrease the potassium-induced Ca2+ transients in these cells. These results suggest that cADPR is a second messenger in pituitary cells and regulates ACTH secretion by a mechanism dependent on activation of the ryanodine channel by extracellular Ca2+.

Effect of gadolinium on the ryanodine receptor/sarcoplasmic reticulum calcium release channel of skeletal muscle

The effect of gadolinium ions on the sarcoplasmic reticulum (SR) calcium release channel/ryanodine receptor (RyR1) was studied using heavy SR (HSR) vesicles and RyR1 isolated from rabbit fast twitch muscle. In the [(3)H]ryanodine binding assay, 5 microM Gd(3+) increased the K(d) of the [(3)H]ryanodine binding of the vesicles from 33.8 nM to 45.6 nM while B(max), referring to the binding capacity, was not affected significantly. In the presence of 18 nM[(3)H]ryanodine and 100 microM free Ca(2+), Gd(3+) inhibited the binding of the radiolabeled ryanodine with an apparent K(d) value of 14.7 microM and a Hill coefficient of 3.17. In (45)Ca(2+) experiments the time constant of (45)Ca(2+) efflux from HSR vesicles increased from 90.9 (+/- 11.1) ms to 187.7 (+/- 24.9) ms in the presence of 20 microM gadolinium. In single channel experiments gadolinium inhibited the channel activity from both the cytoplasmic (cis) (IC(50) = 5.65 +/- 0.33 microM, n(Hill) = 4.71) and the luminal (trans) side (IC(50) = 5.47 +/- 0.24 microM, n(Hill) = 4.31). The degree of inhibition on the cis side didn't show calcium dependency in the 100 microM to 1 mM Ca(2+) concentration range which indicates no competition with calcium on its regulatory binding sites. When Gd(3+) was applied at the trans side, EGTA was present at the cis side to prevent the binding of Gd(+3) to the cytoplasmic calcium binding regulatory sites of the RyR1 if Gd(3+) accidentally passed through the channel. The inhibition of the channel did not show any voltage dependence, which would be the case if Gd(3+) exerted its effect after getting to the cis side. Our results suggest the presence of inhibitory binding sites for Gd(3+) on both sides of the RyR1 with similar Hill coefficients and IC(50) values.

Lysosome-sarcoplasmic reticulum junctions. A trigger zone for calcium signaling by nicotinic acid adenine dinucleotide phosphate and endothelin-1

Previous studies on pulmonary arterial smooth muscle cells have shown that nicotinic acid adenine dinucleotide phosphate (NAADP) evokes highly localized intracellular Ca(2+) signals by mobilizing thapsigargin-insensitive stores. Such localized Ca(2+) signals may initiate global Ca(2+) waves and contraction of the myocytes through the recruitment of ryanodine receptors on the sarcoplasmic reticulum via Ca(2+)-induced Ca(2+) release. Here we show that NAADP evokes localized Ca(2+) signals by mobilizing a bafilomycin A1-sensitive, lysosome-related Ca(2+) store. These lysosomal stores facilitate this process by co-localizing with a portion of the sarcoplasmic reticulum expressing ryanodine receptors to comprise a highly specialized trigger zone for NAADP-dependent Ca(2+) signaling by the vasoconstrictor hormone, endothelin-1. These findings further advance our understanding of how the spatial organization of discrete, organellar Ca(2+) stores may underpin the generation of differential Ca(2+) signaling patterns by different Ca(2+)-mobilizing messengers.

The role of cyclic-ADP-ribose-signaling pathway in oxytocin-induced Ca2+ transients in human myometrium cells

Human myometrial contraction plays a fundamental role in labor. Dysfunction of uterine contraction is an important cause of labor progression failure. Although the mechanisms controlling uterine contraction are not completely understood, intracellular Ca2+ mobilization plays an important role during uterine contraction. Several mechanisms of intracellular Ca2+ mobilization are present in smooth muscle, but in the human uterus, only 1,4,5-trisphosphate-induced Ca2+ release has been studied extensively. Ryanodine receptor channels are present in myometrium. We determined the role of the cyclic ADP-ribose (cADPR)-signaling pathway in oxytocin-induced intracellular Ca2+ [(Ca2+)i] transients in human myometrial cells. We found that oxytocin-induced Ca2+ transient is dependent on several sources of Ca2+, including extracellular Ca2+ and intracellular Ca2+ stores. In addition, we found that both the 1,4,5-trisphosphate- and the cADPR-induced Ca2+ releasing systems are important for the induction of [Ca2+]i transients by oxytocin in human myometrial cells. Furthermore, we investigated TNFalpha regulation of oxytocin-induced [Ca2+]i transients, CD38 cyclase activity, and CD38 expression in human myometrial cells. We found that oxytocin-induced [Ca2+]i transients were significantly increased by 50 ng/ml TNF. Similarly, CD38 mRNA levels, CD38 expression, and cyclase activity were increased by TNFalpha, thus increasing cADPR levels. We propose that a complex interaction between multiple signaling pathways is important for the development of intracellular Ca2+ transients induced by oxytocin and that TNFalpha may contribute for the myometrium preparation for labor by regulating the cADPR-signaling pathway. The observation that the cADPR-signaling pathway is important for the development of intracellular Ca2+ transients in human myometrial cells raises the possibility that this signaling pathway could serve as a target for the development of new therapeutic strategies for abnormal myometrial contraction observed during pregnancy.

Genetic analysis of ryanodine receptor function in Caenorhabditis elegans based on unc-68 revertants

The Caenorhabditis elegans ryanodine receptor is encoded by the unc-68 gene, and functions as a Ca2+-induced Ca2+ release channel during muscle contraction. To investigate the factors that suppress calcium release and identify molecules that interact with the ryanodine receptor, we isolated revertants from two unc-68 mutants. Three of the revertants obtained from the null allele unc-68(e540), which displayed normal motility, had intragenic mutations that resulted in failure to splice out intron 21. The other two, kh53 and kh55, had amino acid insertions in the third of the four RyR domains. The brood size and the egg laying rate remain abnormal in these revertants. This suggests the third RyR domain may be required for egg laying and embryogenesis, although we can not determine a molecular mechanism. Five ketamine sensitive revertants recovered from the missense mutant unc-68(kh30) showed altered responses to caffeine, ryanodine, levamisole and ouabain relative to those of the unc-68(kh30) animals. These may carry second-site suppressor mutations, which may define genes for proteins that regulate the Ca2+ concentration in body-wall muscle. One of these mutants, kh52, shows lower motility and higher sensitivity to drugs, and this mutation was mapped to chromosome X. These observations provide a basis for the study of ryanodine receptor functions in embryogenesis and in calcium-mediated regulation of muscle contraction in C. elegans. This is the first study to show that the conserved RyR domain of the receptor acts in egg laying and embryogenesis.

Subcellular distribution of ryanodine receptors in the cardiac muscle of carp (Cyprinus carpio)

We examined the subcellular localization of ryanodine receptors (RyR) in the cardiac muscle of carp using biochemical, immunohistochemical, and electron microscopic methods and compared it with those of rats and guinea pigs. To achieve this goal, an anti-RyR antibody was newly raised against a synthetic peptide corresponding to an amino acid sequence that was conserved among all sequenced RyRs. Western blot analysis using this antibody detected a single RyR band following the SDS-PAGE of sarcoplasmic reticulum (SR) membranes from carp atrium and ventricle as well as from mammalian hearts and skeletal muscles. The carp heart band had slightly greater mobility than those of mammalian hearts. Although immunohistochemical staining showed evident striations corresponding to the Z lines in longitudinal sections of mammalian hearts, clusters of punctate staining, in contrast, were distributed ubiquitously throughout carp atrium and ventricle. Electron microscopic images of the carp myocardium showed that the SR was observed largely as the subsarcolemmal cisternae and the reticular SR, suggesting that the RyR is localized in the junctional and corbular SR.

Sorcin regulates excitation-contraction coupling in the heart

Sorcin is a penta-EF hand Ca2+-binding protein that associates with both cardiac ryanodine receptors and L-type Ca2+ channels and has been implicated in the regulation of intracellular Ca2+ cycling. To better define the function of sorcin, we characterized transgenic mice in which sorcin was overexpressed in the heart. Transgenic mice developed normally with no evidence of cardiac hypertrophy and no change in expression of other calcium regulatory proteins. In vivo hemodynamics revealed significant reductions in global indices of contraction and relaxation. Contractile abnormalities were also observed in isolated adult transgenic myocytes, along with significant depression of Ca2+ transient amplitudes. Whole cell ICa density and the time course of activation were normal in transgenic myocytes, but the rate of inactivation was significantly accelerated. These effects of sorcin on L-type Ca2+ currents were confirmed in Xenopus oocyte expression studies. Finally, we examined the expression of sorcin in normal and failing hearts from spontaneous hypertensive heart failure rats. In normal myocardium, sorcin extensively co-localized with ryanodine receptors at the Z-lines, whereas in myopathic hearts the degree of co-localization was markedly disrupted. Together, these data indicate that sorcin modulates intracellular Ca2+ cycling and Ca2+ influx pathways in the heart.

Glutathione transport in the endo/sarcoplasmic reticulum

Glutathione transport through the endo/sarcoplasmic reticulum (ER/SR) membrane might play a role in the maintenance of the thiol redox potential difference between the lumen and the cytosol. The transport of glutathione (both GSH and glutathione disulfide, GSSG) is entirely different in the ER and SR membranes. The transport measurements based on either rapid filtration or light scattering techniques revealed that the SR membrane transports glutathione much faster than the hepatic ER membrane or microsomal membranes prepared from heart or brain. The fastest transport has been measured in the membrane of muscle terminal cisternae, which is enriched in ryanodine receptor type 1 (RyR1). All the studied membranes have been found to be equally impermeable to various hydrophilic substances of similar size to glutathione, thus the glutathione transport in muscle microsomes and terminal cysternae as well as the correlation between the rate of glutathione transport and the abundance of RyR1 are specific. In both muscle microsomes and terminal cysternae, glutathione influx can be either inhibited or activated by antagonists and agonists of the ryanodine receptor, respectively, while these agents do not influence the transport of other small permeant molecules. These findings strongly suggest that the ryanodine receptor channel activity is directly associated with glutathione transport activity in the skeletal muscle sarcoplasmic reticulum membrane.

Immunohistochemical identification of components of the chemoattractant signal transduction pathway in vomeronasal bipolar neurons of garter snakes

The chemosignal transduction pathway in the vomeronasal sensory epithelium of garter snakes involves activation of G-protein-coupled receptors and subsequent generation of second messengers leading to production of an electrical signal. Calcium imaging experiments demonstrate that ligand binding to the receptor leads to an increase in intracellular calcium and that the phosphatidylinositol-turnover pathway plays a major role in this Ca(2+) increase. Here, we demonstrate, using immunohistochemistry, that IP(3) receptors are largely distributed in dendritic regions of the epithelium, ryanodine receptors are confined to the somata region, and Na(+)/Ca(2+) exchanger protein is expressed throughout the vomeronasal (VN) sensory epithelium.

A pivotal role for cADPR-mediated Ca2+ signaling: regulation of endothelin-induced contraction in peritubular smooth muscle cells

cADPR, a potent calcium-mobilizing intracellular messenger synthesized by ADP-ribosyl cyclases regulates openings of ryanodine receptors (RyR). Here we report that in the rat testis, a functional cADPR Ca2+ release system is essential for the contractile response of peritubular smooth muscle cells (PSMC) to endothelin (ET). We previously showed that this potent smooth muscle agonist elicits intracellular Ca2+ release in PSMC and seminiferous tubule contraction via activation of ETA and ETB receptors. ETB-R induces the mobilization of a thapsigargin-sensitive but IP3-independent intracellular Ca2+ pool. Stimulation of permeabilized PSMC with cADPR was found to elicit large Ca2+ releases blocked by either a selective antagonist of cADPR or a RyR blocker, but not by heparin. Western blotting and confocal fluorescence microscopy indicated the specific expression of type 2 RyR in perinuclear localization. ET was found to stimulate the activity of ADP-ribosyl cyclase. Microinjection of the selective cADPR antagonist 8NH2-cADPR completely abolished subsequent stimulation of Ca2+ signaling via ETA and ETB receptors. cADPR therefore appears to have an obligatory role for ETA-R and ETB-R-mediated calcium signaling in PSMC. However, ETB-R seem to be coupled exclusively to cADPR whereas ETA-R activation may be linked to IP3 and cADPR signaling pathways.

Heterogeneous expression of Ca(2+) handling proteins in rabbit sinoatrial node

We investigated the densities of the L-type Ca(2+) current, i(Ca,L), and various Ca(2+) handling proteins in rabbit sinoatrial (SA) node. The density of i(Ca,L), recorded with the whole-cell patch-clamp technique, varied widely in sinoatrial node cells. The density of i(Ca,L) was significantly (p<0.001) correlated with cell capacitance (measure of cell size) and the density was greater in larger cells (likely to be from the periphery of the SA node) than in smaller cells (likely to be from the center of the SA node). Immunocytochemical labeling of the L-type Ca(2+) channel, Na(+)-Ca(2+) exchanger, sarcoplasmic reticulum Ca(2+) release channel (RYR2), and sarcoplasmic reticulum Ca(2+) pump (SERCA2) also varied widely in SA node cells. In all cases there was significantly (p<0.05) denser labeling of cells from the periphery of the SA node than of cells from the center. In contrast, immunocytochemical labeling of the Na(+)-K(+) pump was similar in peripheral and central cells. We conclude that Ca(2+) handling proteins are sparse and poorly organized in the center of the SA node (normally the leading pacemaker site), whereas they are more abundant in the periphery (at the border of the SA node with the surrounding atrial muscle).

A novel mechanism for coupling cellular intermediary metabolism to cytosolic Ca2+ signaling via CD38/ADP-ribosyl cyclase, a putative intracellular NAD+ sensor

CD38 is an ectocyclase that converts NAD+ to the Ca2+-releasing second messenger cyclic ADP-ribose (cADPr). Here we report that in addition to CD38 ecto-catalysis, intracellularly expressed CD38 may catalyze NAD+-->cADPr conversion to cause cytosolic Ca2+ release. High levels of CD38 were found in the plasma membranes, endoplasmic reticulum, and nuclear membranes of osteoblastic MC3T3-E1 cells. More important, intracellular CD38 was colocalized with target ryanodine receptors. The cyclase also converted a NAD+ surrogate, NGD+, to its fluorescent product, cGDPr (Km approximately 5.13 microM). NAD+ also triggered a cytosolic Ca2+ signal. Similar results were obtained with NIH3T3 cells, which overexpressed a CD38-EGFP fusion protein. The Delta(-49)-CD38-EGFP mutant with a deleted amino-terminal tail and transmembrane domain appeared mainly in the mitochondria with an expected loss of its membrane localization, but the NAD+-induced cytosolic Ca2+ signal was preserved. Likewise, Ca2+ release persisted in cells transfected with the Myr-Delta(-49)-CD38-EGFP or Delta(-49)-CD38-EGFP-Fan mutants, both directed to the plasma membrane but in an opposite topology to the full-length CD38-EGFP. Finally, ryanodine inhibited Ca2+ signaling, indicating the downstream activation of ryanodine receptors by cADPr. We conclude that intracellularly expressed CD38 might link cellular NAD+ production to cytosolic Ca2+ signaling.

Type 3 ryanodine receptors of skeletal muscle are segregated in a parajunctional position

A key event in skeletal muscle activation is the rapid release of Ca(2+) from the sarcoplasmic reticulum (SR), the Ca(2+) storage organelle in the muscle cell. The surface membrane/transverse tubules and the SR form functional units (calcium release units containing one or two couplons or junctions), where the voltage-sensing dihydropyridine receptor of the surface membrane interacts with the SR Ca(2+) release channel [ryanodine receptor (RyR)] and depolarization of the cell membrane is converted into Ca(2+) release from the SR. Although RyR1 is the most important isoform in skeletal muscle, some muscles also express high levels of RyR3, an isoform with a wide tissue distribution. The cytoplasmic domains of RyRs are visible in the electron microscope as periodically disposed feet. We find that, in muscles containing only RyR1, feet are exclusively located over the junctional SR surface facing the surface membrane/transverse tubule. In muscles containing RyR1 as well as RyR3, additional feet are located in lateral parajunctional regions immediately adjacent to junctional SR. Biochemical content of RyR3 and content of parajunctional feet are highly correlated in different muscles and the disposition of parajunctional versus junctional feet are notably different. On the basis of these two observations, we postulate that RyR3s are restricted to the parajunctional region, and thus their activation must be indirect and derivative during excitation-contraction coupling.

Calcium-dependent translocation of synaptotagmin to the plasma membrane in the dendrites of developing neurones

In neurones, the morphological complexity of the dendritic tree requires regulated growth and the appropriate targeting of membrane components. Accurate delivery of specific supplies depends on the translocation and fusion of transport vesicles. Vesicle SNAREs (soluble N-ethylmaleimide sensitive factor attachment protein receptors) and target membrane SNAREs play a central role in the correct execution of fusion events, and mediate interactions with molecules that endow the system with appropriate regulation. Synaptotagmins, a family of Ca(2+)-sensor proteins that includes neurone-specific members involved in regulating neurotransmitter exocytosis, are among the molecules that can tune the fusion mechanism. Using immunocytochemistry, confocal and electron microscopy, the localisation of synaptotagmin I in the dendrites of cultured rat hypothalamic neurones was demonstrated. Synaptotagmin labelling is concentrated at dendritic branch points, and in microprocesses. Following depolarisation, the N-terminal domain of synaptotagmin was detected at the extracellular surface of the dendritic plasma membrane. The insertion of synaptotagmin in the plasma membrane was elicited by L-type Ca(2+) channel activation and by mobilisation of the internal ryanodine-sensitive Ca(2+)stores. Furthermore, the localisation of L-type Ca(2+) channels and of ryanodine receptors, relative to the localisation of synaptotagmin in dendrites, suggests that both Ca(2+) entry and intracellular Ca(2+) stores may contribute to the fusion of dendritic transport vesicles with the membrane. Fusion is likely to involve SNAP-25 and syntaxin 1 as both proteins were also identified in dendrites. Taken together these results suggest a putative regulatory role of synaptotagmins in the membrane fusion events that contribute to shaping the dendritic tree during development.

Defective mitochondrial oxidative phosphorylation in myopathies with tubular aggregates originating from sarcoplasmic reticulum

Abnormalities of the sarcotubular system presenting as tubular aggregates (TAs) have been described in a variety of neuromuscular disorders. Here, we report on immunohistochemical and biochemical findings in 7 patients (2 familial and 5 sporadic cases) suffering from myopathies with TAs. In muscle biopsy specimens from 5 of the 7 patients, TAs were immunopositive for the ryanodine receptor (RYR 1) of the sarcoplasmic reticulum (SR), the SR Ca2+ pump (SERCA2-ATPase), and the intraluminal SR Ca2+ binding protein calsequestrin, indicating an SR origin of these aggregates. Furthermore, these 5 cases showed decreased respiratory chain enzyme activities (NADH:CoQ oxidoreductase. complex I and cytochrome c oxidase [COX], complex IV), while the remaining 2 patients exhibited normal values. Our findings indicate a functional link between mitochondrial dysfunction and the presence of TAs originating from the sarcoplasmic reticulum.

A ryanodine fluorescent derivative reveals the presence of high-affinity ryanodine binding sites in the Golgi complex of rat sympathetic neurons, with possible functional roles in intracellular Ca(2+) signaling

The plant alkaloid ryanodine (Ry) is a high-affinity modulator of ryanodine receptor (RyR) Ca(2+) release channels. Although these channels are present in a variety of cell types, their functional role in nerve cells is still puzzling. Here, a monosubstituted fluorescent Ry analogue, B-FL-X Ry, was used to reveal the distribution of RyRs in cultured rat sympathetic neurons. B-FL-X Ry competitively inhibited the binding of [3H]Ry to rabbit skeletal muscle SR membranes, with an IC(50) of 150 nM, compared to 7 nM of unlabeled Ry. Binding of B-FL-X Ry to the cytoplasm of sympathetic neurons is saturable, reversible and of high affinity. The pharmacology of B-FL-X Ry showed marked differences with unlabeled Ry, which are partially explained by its lower affinity: (1) use-dependent reversible inhibition of caffeine-induced intracellular Ca(2+) release; (2) diminished voltage-gated Ca(2+) influx, due to a positive shift in the activation of voltage gated Ca(2+) currents. B-FL-X Ry-stained sympathetic neurons, viewed under confocal microscopy, showed conspicuous labeling of crescent-shaped structures pertaining to the Golgi complex, a conclusion supported by experiments showing co-localization with Golgi-specific fluorescent probes and the breaking up of crescent-shaped staining after treatment with drugs that disassemble Golgi complex. The presence of RyRs to the Golgi could be confirmed with specific anti-RyR(2) antibodies, but evidence of caffeine-induced Ca(2+) release from this organelle could not be obtained using fast confocal microscopy. Rather, an apparent decrease of the cytosolic Ca(2+) signal was detected close to this organelle. In spite of that, short-term incubation with brefeldin A (BFA) suppressed the fast component of caffeine-induced Ca(2+) release, and the Ca(2+) release process lasted longer and appeared less organized. These observations, which suggest a possible role of the Golgi complex in Ca(2+) homeostasis and signaling in nerve cells, could be relevant to reports involving derangement of the Golgi complex as a probable cause of some forms of progressive neuronal degeneration, such as Alzheimer's disease and amyotrophic lateral sclerosis.

Local Ca(2+) transients and distribution of BK channels and ryanodine receptors in smooth muscle cells of guinea-pig vas deferens and urinary bladder

1. The relationship between Ca(2+) sparks spontaneously occurring at rest and local Ca(2+) transients elicited by depolarization was analysed using two-dimensional confocal Ca(2+) images of single smooth muscle cells isolated from guinea-pig vas deferens and urinary bladder. The current activation by these Ca(2+) events was also recorded simultaneously under whole-cell voltage clamp. 2. Spontaneous transient outward currents (STOCs) and Ca(2+) sparks were simultaneously detected at -40 mV in approximately 50 % of myocytes of either type. Ca(2+) sparks and corresponding STOCs occurred repetitively in several discrete sites in the subplasmalemmal area. Large conductance Ca(2+)-dependent K(+) (BK) channel density in the plasmalemma near the Ca(2+) spark sites generating STOCs was calculated to be 21 channels microm(-2). 3. When myocytes were depolarized from -60 to 0 mV, several local Ca(2+) transients were elicited within 20 ms in exactly the same peripheral sites where sparks occurred at rest. The local Ca(2+) transients often lasted over 300 ms and spread into other areas. The appearance of local Ca(2+) transients occurred synchronously with the activation of Ca(2+)-dependent K(+) current (I(K,Ca)). 4. Immunofluorescence staining of the BK channel alpha-subunit (BKalpha) revealed a spot-like pattern on the plasmalemma, in contrast to the uniform staining of voltage-dependent Ca(2+) channel alpha1C subunits along the plasmalemma. Ryanodine receptor (RyR) immunostaining also suggested punctate localization predominantly in the periphery. Double staining of BKalpha and RyRs revealed spot-like co-localization on/beneath the plasmalemma. 5. Using pipettes of relatively low resistance, inside-out patches that included both clustered BK channels at a density of over 20 channels microm(-2) and functional Ca(2+) storage sites were obtained at a low probability of approximately 5%. The averaged BK channel density was 3-4 channels microm(-2) in both types of myocyte. 6. These results support the idea that a limited number of discrete sarcoplasmic reticulum (SR) fragments in the subplasmalemmal area play key roles in the control of BK channel activity in two ways: (i) by generating Ca(2+) sparks at rest to activate STOCs and (ii) by generating Ca(2+) transients presumably triggered by sparks during an action potential to activate a large I(K,Ca) and also induce a contraction. BK channels and RyRs may co-localize densely at the junctional areas of plasmalemma and SR fragments, where Ca(2+) sparks occur to elicit STOCs.

Expression of ryanodine receptor type 3 and TRP channels in endothelial cells: comparison of in situ and cultured human endothelial cells

OBJECTIVE

Ca(2+) mobilization plays an important role in endothelial function by stimulating Ca(2+)-dependent synthesis of vasodilating factors. In addition to inositol-1,4,5-trisphosphate (InsP(3)) mediated Ca(2+) mobilization, Ca(2+) release from ryanodine-sensitive pools and Ca(2+)-influx through TRP channels have been suggested to be important in endothelial Ca(2+)-signaling. However, the function and molecular identity of TRP channels and ryanodine receptors in human endothelium in situ are still elusive. We hypothesized that expression of ryanodine-receptors (RyR) and TRP channels differs between human endothelium in situ and in cultured cells.

METHODS

By combining single-cell RT-PCR and patch-clamp techniques, expression of RyR and TRP channels was determined in situ in endothelial cells of human mesenteric artery (HMAECs) obtained from patients undergoing bowel resection and in the endothelial cell line EA.hy926.

RESULTS

At the single cell level, expression of RyR 3 was detected in 25 and 5% of HMAECs and EA.hy926 samples, respectively. Expression of the RyR 1 and 2 was not detected in either HMAECs or EA.hy926. In patch-clamp experiments in HMAECs, applications of caffeine (0.5 mM) induced sustained hyperpolarization mediated by activation of Ca(2+)-activated K channels. In EA.hy926, caffeine-induced hyperpolarization was not detected. Single HMAECs expressed the TRP genes, TRP1 and TRP3, but not TRP 4 and 6. The TRP1 was the predominantly expressed TRP gene in HMAECs in situ whereas TRP3 expression was rarely detected. EA.hy926 expressed only TRP1. In patch clamp experiments in HMAECs, Ca(2+)-store depletion activated non-selective cation currents leading to Ca(2+) entry.

CONCLUSIONS

Our findings suggest that, in addition to InsP(3) mediated Ca(2+) release, Ca(2+) release from ryanodine-sensitive stores mediated by RyR3 and Ca(2+) entry through TRP1 might represent important components of endothelial Ca(2+) signaling in situ and thereby of endothelial function in intact human blood vessels.

Identification of a ryanodine receptor in rat heart mitochondria

Recent studies have shown that, in a wide variety of cells, mitochondria respond dynamically to physiological changes in cytosolic Ca(2+) concentrations ([Ca(2+)](c)). Mitochondrial Ca(2+) uptake occurs via a ruthenium red-sensitive calcium uniporter and a rapid mode of Ca(2+) uptake. Surprisingly, the molecular identity of these Ca(2+) transport proteins is still unknown. Using electron microscopy and Western blotting, we identified a ryanodine receptor in the inner mitochondrial membrane with a molecular mass of approximately 600 kDa in mitochondria isolated from the rat heart. [(3)H]Ryanodine binds to this mitochondrial ryanodine receptor with high affinity. This binding is modulated by Ca(2+) but not caffeine and is inhibited by Mg(2+) and ruthenium red in the assay medium. In the presence of ryanodine, Ca(2+) uptake into isolated heart mitochondria is suppressed. In addition, ryanodine inhibited mitochondrial swelling induced by Ca(2+) overload. This swelling effect was not observed when Ca(2+) was applied to the cytosolic fraction containing sarcoplasmic reticulum. These results are the first to identify a mitochondrial Ca(2+) transport protein that has characteristics similar to the ryanodine receptor. This mitochondrial ryanodine receptor is likely to play an essential role in the dynamic uptake of Ca(2+) into mitochondria during Ca(2+) oscillations.

Caveolin-3 null mice show a loss of caveolae, changes in the microdomain distribution of the dystrophin-glycoprotein complex, and t-tubule abnormalities

Caveolin-3, a muscle-specific caveolin-related protein, is the principal structural protein of caveolae membrane domains in striated muscle cells. Recently, we identified a novel autosomal dominant form of limb-girdle muscular dystrophy (LGMD-1C) in humans that is due to mutations within the coding sequence of the human caveolin-3 gene (3p25). These LGMD-1C mutations lead to an approximately 95% reduction in caveolin-3 protein expression, i.e. a caveolin-3 deficiency. Here, we created a caveolin-3 null (CAV3 -/-) mouse model, using standard homologous recombination techniques, to mimic a caveolin-3 deficiency. We show that these mice lack caveolin-3 protein expression and sarcolemmal caveolae membranes. In addition, analysis of skeletal muscle tissue from these caveolin-3 null mice reveals: (i) mild myopathic changes; (ii) an exclusion of the dystrophin-glycoprotein complex from lipid raft domains; and (iii) abnormalities in the organization of the T-tubule system, with dilated and longitudinally oriented T-tubules. These results have clear mechanistic implications for understanding the pathogenesis of LGMD-1C at a molecular level.

Effects of cardioplegic arrest and reperfusion on rabbit cardiac ryanodine receptors

Calcium overload is considered to be a primary contributor to ischemia-reperfusion injury. Cardiac sarcoplasmic reticulum (SR), the main regulator of intracellular Ca2+ concentration under normal conditions, is a target for ischemic myocardial injury. The ryanodine receptor (RyR) is the SR Ca2+ release channel. Previous reports have shown that a reduction in RyR activity during global myocardial ischemia correlates with concomitant myocardial dysfunction. Crystalloid cardioplegia, a technique for myocardial protection during heart operations, reduces Ca2+ accumulation during global ischemia. Hence, the effects of cardioplegia on RyR in isolated rabbit hearts was investigated. The study also compared [3H] ryanodine binding before ischemia (control group), after 30 min of ischemia (either global ischemia (GI group) or cardioplegic arrest (CA group)), and after 20 min of reperfusion. The GI group, but not the CA group, showed a significant reduction in the maximum number of binding sites (Bmax) for RyR compared with the control group (Control vs GI group: after ischemia, 1.33+/-0.27 vs 0.83+/-0.12 pmol/mg protein, p<0.05; after reperfusion, 1.33+/-0.27 vs 0.80+/-0.08 pmol/mg protein; p<0.05). CA group: after ischemia, 1.22+/-0.20 pmol/mg protein; after reperfusion, 1.15+/-0.28 pmol/mg protein). The affinity (Kd) values for [3H] ryanodine binding were not different among the 3 groups at any point. The preservation of RyR numbers during cardioplegia correlated with the concomitant preservation of cardiac functions. The results indicate that number of functional RyR was much better preserved during cardioplegia than during global ischemia. It is postulated that cardioplegia-induced protection of cardiac RyR may result in the protection of SR function during ischemia-reperfusion.

Localisation and functional significance of ryanodine receptors during beta-adrenoceptor stimulation in the guinea-pig sino-atrial node

OBJECTIVE

Recent evidence shows that calcium released from the sarcoplasmic reticulum (SR) plays an important role in the regulation of heart rate. The aim of this study was to investigate the subcellular distribution of ryanodine receptors in the guinea-pig sino-atrial (SA) node and to determine their functional role in the regulation of pacemaker frequency in response to beta-adrenoceptor stimulation.

METHODS

Monoclonal antibodies raised against the cardiac ryanodine receptor were used with confocal microscopy to investigate ryanodine receptor distribution in single guinea-pig SA node cells. The functional role of ryanodine receptors was investigated in both multicellular SA node/atrial preparations and in single SA node cells.

RESULTS

Ryanodine receptor labelling was observed in all SA node cells studied and showed both subsarcolemmal and intracellular staining. In the latter, labelling appeared as transverse bands with a regular periodicity of approximately 2 microm. This interval resembled that of the expected sarcomere spacing but did not, however, depend on the presence of transverse tubules. The bands of ryanodine receptors appeared to be located in the region of the Z lines, based on co-distribution studies with antibodies to alpha-actinin, myomesin and binding sites for phalloidin. Functional studies on single SA node cells showed that application of ryanodine (2 micromol/l) reduced the rate of firing of spontaneous action potentials (measured using the perforated patch clamp technique) and this was associated with changes in action potential characteristics. Ryanodine also significantly decreased the positive chronotropic actions of isoprenaline in both multicellular and single cell preparations. In single cells exposed to 100 nmol/l isoprenaline, ryanodine caused a decrease in the rate of firing and this was associated with a decrease in the amplitude of the measured calcium transients.

CONCLUSIONS

These findings are the first to show immunocytochemical evidence for the presence and organisation of ryanodine receptor calcium release channels in mammalian SA node cells. This study also provides evidence of a role for ryanodine sensitive sites in the beta-adrenergic modulation of heart rate in this species.

Coordinate expression of Ca2+-ATPase slow-twitch isoform and of beta calmodulin-dependent protein kinase in phospholamban-deficient sarcoplasmic reticulum of rabbit masseter muscle

Modulation of sarcoplasmic reticulum (SR) Ca(2+) transport by endogenous calmodulin-dependent protein kinase II (CaM K II) involves covalent changes of regulatory protein phospholamban (PLB), as a common, but not the only mechanism, in limb slow-twitch muscles of certain mammalian species, such as the rabbit. Here, using immunofluorescent techniques in situ, and biochemical and immunological methods on the isolated SR, we have demonstrated that rabbit masseter, a muscle with a distinct embryological origin, lacks PLB. Accommodating embryological heterogeneity in the paradigm of neural-dependent expression of specific isogenes in skeletal muscle fibers, our results provide novel evidence for the differential expression in the SR of 72 kDa beta components of CaM K II, together with the expression of a slow-twitch sarcoendoplasmic reticulum Ca(2+)-ATPase isoform, both in limb muscle and in the masseter.