Development of dyadic junctional complexes between sarcoplasmic reticulum and plasmalemma in rabbit left ventricular myocardial cells. Morphometric analysis

Integrins Increase Sarcoplasmic Reticulum Activity for Excitation-Contraction Coupling in Human Stem Cell-Derived Cardiomyocytes

Engagement of the sarcoplasmic reticulum (SR) Ca2+ stores for excitation-contraction (EC)-coupling is a fundamental feature of cardiac muscle cells. Extracellular matrix (ECM) proteins that form the extracellular scaffolding supporting cardiac contractile activity are thought to play an integral role in the modulation of EC-coupling. At baseline, human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) show poor utilisation of SR Ca2+ stores, leading to inefficient EC-coupling, like developing or human CMs in cardiac diseases such as heart failure. We hypothesised that integrin ligand-receptor interactions between ECM proteins and CMs recruit the SR to Ca2+ cycling during EC-coupling. hiPSC-CM monolayers were cultured on fibronectin-coated glass before 24 h treatment with fibril-forming peptides containing the integrin-binding tripeptide sequence arginine-glycine-aspartic acid (2 mM). Micropipette application of 40 mM caffeine in standard or Na+/Ca2+-free Tyrode's solutions was used to assess the Ca2+ removal mechanisms. Microelectrode recordings were conducted to analyse action potentials in current-clamp. Confocal images of labelled hiPSC-CMs were analysed to investigate hiPSC-CM morphology and ultrastructural arrangements in Ca2+ release units. This study demonstrates that peptides containing the integrin-binding sequence arginine-glycine-aspartic acid (1) abbreviate hiPSC-CM Ca2+ transient and action potential duration, (2) increase co-localisation between L-type Ca2+ channels and ryanodine receptors involved in EC-coupling, and (3) increase the rate of SR-mediated Ca2+ cycling. We conclude that integrin-binding peptides induce recruitment of the SR for Ca2+ cycling in EC-coupling through functional and structural improvements and demonstrate the importance of the ECM in modulating cardiomyocyte function in physiology.

Nitric Oxide and Mechano-Electrical Transduction in Cardiomyocytes

The ability§ of the heart to adapt to changes in the mechanical environment is critical for normal cardiac physiology. The role of nitric oxide is increasingly recognized as a mediator of mechanical signaling. Produced in the heart by nitric oxide synthases, nitric oxide affects almost all mechano-transduction pathways within the cardiomyocyte, with roles mediating mechano-sensing, mechano-electric feedback (via modulation of ion channel activity), and calcium handling. As more precise experimental techniques for applying mechanical stresses to cells are developed, the role of these forces in cardiomyocyte function can be further understood. Furthermore, specific inhibitors of different nitric oxide synthase isoforms are now available to elucidate the role of these enzymes in mediating mechano-electrical signaling. Understanding of the links between nitric oxide production and mechano-electrical signaling is incomplete, particularly whether mechanically sensitive ion channels are regulated by nitric oxide, and how this affects the cardiac action potential. This is of particular relevance to conditions such as atrial fibrillation and heart failure, in which nitric oxide production is reduced. Dysfunction of the nitric oxide/mechano-electrical signaling pathways are likely to be a feature of cardiac pathology (e.g., atrial fibrillation, cardiomyopathy, and heart failure) and a better understanding of the importance of nitric oxide signaling and its links to mechanical regulation of heart function may advance our understanding of these conditions.

The SLMAP/Striatin complex: An emerging regulator of normal and abnormal cardiac excitation-contraction coupling

The excitation-contraction (E-C) module involves a harmonized correspondence between the sarcolemma and the sarcoplasmic reticulum. This is provided by membrane proteins, which primarily shape the caveolae, the T-tubule/Sarcoplasmic reticulum (TT/SR) junction, and the intercalated discs (ICDs). Distortion of either one of these structures impairs myocardial contraction, and subsequently translates into cardiac failure. Thus, detailed studies on the molecular cues of the E-C module are becoming increasingly necessary to pharmacologically eradicate cardiac failure Herein we reviewed the organization of caveolae, TT/SR junctions, and the ICDs in the heart, with special attention to the Sarcolemma Membrane Associated Protein (SLMAP) and striatin (STRN) in cardiac membranes biology and cardiomyocyte contraction. We emphasized on their in vivo and in vitro signaling in cardiac function/dysfunction. SLMAP is a cardiac membrane protein that plays an important role in E-C coupling and the adrenergic response of the heart. Similarly, STRN is a dynamic protein that is also involved in cardiac E-C coupling and ICD-related cardiomyopathies. Both SLMAP and STRN are linked to cardiac conditions, including heart failure, and their role in cardiomyocyte function was elucidated in our laboratory. They interact together in a protein complex that holds therapeutic potentials for cardiac dysfunction. This review is the first of its kind to conceptualize the role of the SLMAP/STRN complex in cardiac function and failure. It provides in depth information on the signaling of these two proteins and projects their interaction as a novel therapeutic target for cardiac failure.

Controversies in the identification and management of acute pulmonary hypertension in preterm neonates

It is increasingly recognized that the abnormal physiologic consequences of pulmonary hypertension (PH) may contribute to poor cardiopulmonary health in premature babies. Conflicting literature has led to clinical uncertainty, pathological misinterpretation, and variability in treatment approaches among practitioners. There are several disorders with overlapping and interrelated presentations, and other disorders with a similar clinical phenotype but diverse pathophysiological contributors. In this review, we provide a diagnostic approach for acute hypoxemic respiratory failure in the preterm neonate, outline the pathophysiological conditions that may present as acute PH, and discuss the implications of high pulmonary vascular resistance (PVR) on the cardiovascular system. Although PVR and respiratory management are highly interrelated, there may be a population of preterm neonates in whom inhaled nitric oxide may improve illness severity and may relate to outcomes. A management approach based on physiology that considers common clinical conundrums is provided. A more comprehensive understanding of the physiology may help in informed decision-making in clinical situations where conclusive scientific evidence is lacking. Regardless, high-quality research is required, and appropriate definition of the target population is paramount. A thoughtful approach to cardiovascular therapy may also provide an avenue to improve neurodevelopmental outcomes while awaiting more clear answers.

A new twist in cardiac muscle: dislocated and helicoid arrangements of myofibrillar z-disks in mammalian ventricular myocytes

Using deconvolved confocal microscopy of fluorescently labeled markers for z-disks, t-tubules and ryanodine receptors, we have examined sarcomere organization in cardiac myocytes from rat, rabbit and human. We show that sarcomeres exhibit dislocations in registration and occasionally more complex helicoidal topology. This organization was present at both slack ( approximately 1.8 microm) and long sarcomere lengths ( approximately 2.2 microm). Misregistrations in z-disks persisted over 15-20 sarcomere lengths and appeared to arise primarily from variations in fiber direction; particularly as myofibrils pass around nuclei. In addition, myofibrils twist along the cell length. T-tubules generally follow the sarcomere z-disks although additional elements bridging adjacent myofibrils and along the length of the myofibril are present to varying degrees in all cells. Ryanodine receptors (the sarcoplasmic reticulum Ca(2+) release channel) are generally located within 250 nm of the local plane containing t-tubules and z-disks, but a small fraction ( approximately 2%) is found on longitudinal elements of the t-system between z-disks. The results are discussed with respect to the possible role(s) of such complex z-disk organization and z-disk dislocations in the maintenance of cell structure and sarcomere assembly. In addition, the non-planar organization of z-disks may be important in the propagation of local Ca(2+) waves which may have a useful role in helping maintain the uniformity of sarcomere activation in the presence of t-tubule remodeling.

Regulatory roles of junctin in sarcoplasmic reticulum calcium cycling and myocardial function

Junctin (JCN), a 26-kd sarcoplasmic reticulum (SR) transmembrane protein, forms a quaternary protein complex with the ryanodine receptor, calsequestrin, and triadin in the SR lumen of cardiac muscle. Within this complex, calsequestrin, triadin, and JCN appear to be critical for normal regulation of ryanodine receptor-mediated calcium (Ca) release. Junctin and triadin exhibit 60% to 70% amino acid homology in their transmembrane domains, including repeated KEKE motifs important for macromolecular protein-protein interactions within their SR luminal tails. Recent studies have uncovered functional roles of both JCN and triadin in the mouse heart, using transgenic overexpression strategies, which exhibit varying phenotypes including mild SR structural alterations, prolongation of Ca transient decay, impaired relaxation, and cardiac hypertrophy and/or heart failure. More specifically, both in vitro adenoviral gene transfer and in vivo gene-targeting techniques to manipulate JCN expression levels have shown that JCN is an essential factor in maintaining normal cardiac Ca handling and cardiac function. This article reviews the new findings on the regulatory roles of JCN in cardiac SR Ca cycling and contractility, with special emphasis on the effects of JCN ablation on delayed after depolarization-induced arrhythmias and premature mortality in mouse models.

SR Ca2+refilling upon depletion and SR Ca2+uptake rates during development in rabbit ventricular myocytes

While it has been reported that a sparse sarcoplasmic reticulum (SR) and a low SR Ca2+pump density exist at birth, we and others have recently shown that significant amounts of Ca2+are stored in the neonatal rabbit heart SR. Here we try to determine developmental changes in SR Ca2+loading mechanisms and Ca2+pump efficacy in rabbit ventricular myocytes. SR Ca2+loading (loadSR) and k0.5(Ca2+concentration at half-maximal SR Ca2+uptake) were higher and lower, respectively, in younger age groups. Inhibition of the L-type calcium current ( ICa) with 15 μM nifedipine dramatically reduced loadSRin older but not in younger age groups. In contrast, subsequent inhibition of the Na+/Ca2+exchanger (NCX) with 10 μM KB-R7943 strongly reduced loadSRin the younger but not the older age groups. Accordingly, the time integral of the inward NCX current (tail INCX) elicited on repolarization was highly sensitive to nifedipine in the older groups and sensitive to KB-R7943 in the younger groups. Interestingly, slow SR loading took place in the presence of both nifedipine and KB-R7943 in all age groups, although it was less prominent in the older groups. We conclude that the SR loading capacity at the earliest postnatal stages is at least as large as that of adult myocytes. However, reverse-mode NCX plays a prominent role in SR Ca2+loading at early postnatal stages while ICais the main source of SR Ca2+loading at late postnatal and adult stages.

T-tubule formation in cardiacmyocytes: two possible mechanisms?

We have followed the differentiation of transverse (T) tubules and of the associations between sarcoplasmic reticulum (SR) and either the plasmalemma (peripheral couplings) or the T tubules (dyads) in postnatal rat ventricular myocytes using electron microscopy. Dyads and peripheral couplings are collectively called Ca(2+) Release Units (CRUs) because they are the sites at which Ca(2+) is released from the SR. Profiles of T tubules, caveolae and dyads are mostly at the cell edge in early postnatal days and are found with increased frequency in the cell interior during the first two postnatal weeks. Using ferritin to trace continuity of T tubules lumen with the extracellular space, we find that some of T tubules (between approximately 6 and 25%), either singly or within dyads, lack ferritin in their lumen. The percentage of tubules that do not contain ferritin decreases slightly during postnatal differentiation and is not very different at the cells' edges and interior. We propose that T tubules form as invaginations of the plasmalemma that penetrate inward driven by accrual of membrane lipids and specific proteins. This occurs by a dual mechanism: either by the independent flow of SR and T tubule proteins into the two separate membranes or by the fusion of preformed vesicle tandems into the dyads. Most of the CRUs (approximately 86%) are constituted by peripheral couplings and ferritin containing dyads, thus constituting CRUs in which Ca(2+ )release from the SR is initiated by a membrane depolarization. In the remaining CRUs, activation of Ca(2+) release must be dependent on some other mechanisms.

L-type Ca2+channel function and expression in neonatal rabbit ventricular myocytes

L-type Ca2+channel-mediated, Ca2+-induced Ca2+release (CICR) is the dominant mode of excitation-contraction (E-C) coupling in the mature mammalian myocardium but is thought to be absent in the fetal and newborn mammalian myocardium. Furthermore, the characteristics and contributors of E-C coupling at the earliest developmental stages are poorly understood. In this study, we measured [3H](+)PN200-110 dihydropyridine binding capacity, functionality and expression of the L-type Ca2+channel, and cytosolic [Ca2+] ([Ca2+]i) at various developmental stages (3, 6, 10, 20, and 56 days old) to characterize ontogenetic changes in E-C coupling. We found that 1) the whole cell L-type Ca2+channel peak current ( ICa) density increased slightly in parallel with cell growth, but the current-voltage relationship, the steady-state activation, and the maximum DHP binding and binding affinity did not exhibit significant developmental changes; 2) sarcoplasmic reticulum Ca2+dependence of inactivation rates of L-type Ca2+channel and peak of ICadensity were only observed after 10 days of age, which temporally coincides with transverse (T)-tubule formation; 3) the relationship between [Ca2+]iand voltage changed from a linear relationship at the earliest developmental stages to a “bell-shaped” relationship at the later developmental stages, presumably corresponding to a switch from reverse-mode Na/Ca exchange-dependent to ICa-dependent E-C coupling; and 4) the expression of two different splice variants of CaV1.2, IVS3A and IVS3B, switched from predominantly IVS3A at the earliest stages to IVS3B at the later developmental stages. Our data suggest that whereas the density of functional dihydropyridine receptors (DHPRs) increases only slightly during ontogeny, the enhancement of functional coupling between DHPR and ryanodine receptor is dramatic between the second and third weeks after birth. Furthermore, we found that the differential expression of splice variants during development temporally correlated with the appearance of ICa-dependent E-C coupling and T-tubule formation.

Store-operated Ca2+ entry modulates sarcoplasmic reticulum Ca2+ loading in neonatal rabbit cardiac ventricular myocytes

Store-operated Ca2+ entry (SOCE), which is Ca2+ entry triggered by the depletion of intracellular Ca2+ stores, has been observed in many cell types, but only recently has it been suggested to occur in cardiomyocytes. In the present study, we have demonstrated SOCE-dependent sarcoplasmic reticulum (SR) Ca2+ loading (load(SR)) that was not altered by inhibition of L-type Ca2+ channels, reverse mode Na+/Ca2+ exchange (NCX), or nonselective cation channels. In contrast, lowering the extracellular [Ca2+] to 0 mM or adding either 0.5 mM Zn2+ or the putative store-operated channel (SOC) inhibitor SKF-96365 (100 microM) inhibited load(SR) at rest. Interestingly, inhibition of forward mode NCX with 30 microM KB-R7943 stimulated SOCE significantly and resulted in enhanced load(SR). In addition, manipulation of the extracellular and intracellular Na+ concentrations further demonstrated the modulatory role of NCX in SOCE-mediated SR Ca2+ loading. Although there is little knowledge of SOCE in cardiomyocytes, the present results suggest that this mechanism, together with NCX, may play an important role in SR Ca2+ homeostasis. The data reported herein also imply the presence of microdomains unique to the neonatal cardiomyocyte. These findings may be of particular importance during open heart surgery in neonates, in which uncontrolled SOCE could lead to SR Ca2+ overload and arrhythmogenesis.

Junctophilin type 2 is associated with caveolin-3 and is down-regulated in the hypertrophic and dilated cardiomyopathies

Functional coupling between the sarcolemmal membrane and the sarcoplasmic reticulum is based on distinct structures called junctional membrane complexes (JMCs). Recently, junctophilins are found to be responsible for normal formation of JMCs. In the present study, we found that junctophilin type 2 (JP-2), a unique isoform in the heart, was localized in caveolin-rich membranes, and that the expression of JP-2 was up-regulated during normal development and down-regulated in a hypertrophic or a dilated cardiomyopathic mouse model. The expression levels of JP-2 may be associated with the development of T-tubules and impaired Ca(2+)-induced Ca(2+) release in the heart.

Analysing cardiac excitation–contraction coupling with mathematical models of local control

Cardiac excitation-contraction (E-C) coupling describes the process that links sarcolemmal Ca2+ influx via L-type Ca2+ channels to Ca2+ release from the sarcoplasmic reticulum via ryanodine receptors (RyRs). This process has proven difficult to study experimentally, and complete descriptions of how the cell couples surface membrane and intracellular signal transduction proteins to achieve both stable and sensitive intracellular calcium release are still lacking. Mathematical models provide a framework to test our understanding of how this is achieved. While no single model is yet capable of describing all features of cardiac E-C coupling, models of increasing complexity are revealing unexpected subtlety in the process. In particular, modelling has established a general failure of 'common-pool' models and has emphasized the requirement for 'local control' so that microscopic sub-cellular domains can separate local behaviour from the whole-cell average (common-pool) behaviour. The micro-architecture of the narrow diadic cleft in which the local control takes place is a key factor in determining local Ca2+ dynamics. There is still considerable uncertainty about the number of Ca2+ ions required to open RyRs within the cleft and various gating models have been proposed, many of which are in reasonable agreement with available experimental data. However, not all models exhibit a realistic voltage dependence of E-C coupling gain. Furthermore, it is unclear which model features are essential to producing reasonable gain properties. Thus, despite the success of local-control models in explaining many features of cardiac E-C coupling, more work will be needed to provide a sound theoretical basis of cardiac E-C coupling.

Developmental changes of intracellular Ca2+ transients in beating rat hearts

Postnatal maturation of the rat heart is characterized by major changes in the mechanism of excitation-contraction (E-C) coupling. In the neonate, the t tubules and sarcoplasmic reticulum (SR) are not fully developed yet. Consequently, Ca(2+)-induced Ca(2+) release (CICR) does not play a central role in E-C coupling. In the neonate, most of the Ca(2+) that triggers contraction comes through the sarcolemma. In this work, we defined the contribution of the sarcolemmal Ca(2+) entry and the Ca(2+) released from the SR to the Ca(2+) transient during the first 3 wk of postnatal development. To this end, intracellular Ca(2+) transients were measured in whole hearts from neonate rats by using the pulsed local field fluorescence technique. To estimate the contribution of each Ca(2+) flux to the global intracellular Ca(2+) transient, different pharmacological agents were used. Ryanodine was applied to evaluate ryanodine receptor-mediated Ca(2+) release from the SR, nifedipine for dihydropyridine-sensitive L-type Ca(2+) current, Ni(2+) for the current resulting from the reverse-mode Na(+)/Ca(2+) exchange, and mibefradil for the T-type Ca(2+) current. Our results showed that the relative contribution of each Ca(2+) flux changes considerably during the first 3 wk of postnatal development. Early after birth (1-5 days), the sarcolemmal Ca(2+) flux predominates, whereas at 3 wk of age, CICR from the SR is the most important. This transition may reflect the progressive development of the t tube-SR units characteristic of mature myocytes. We have hence directly defined in the whole beating heart the developmental changes of E-C coupling previously evaluated in single (acutely isolated or cultured) cells and multicellular preparations.

Ontogeny of excitation-contraction coupling in the mammalian heart

The neonate mammalian heart is phenotypically different from the adult heart in many respects. Understanding these phenotypic differences are a fundamental component of understanding the mechanisms of congenital heart disease and its treatment. Differences in excitation-contraction (E-C) coupling of the neonatal heart from that of the adult include less reliance on intercellular sources of Ca(2+) such as that from sarcoplasmic reticulum (SR). Electron micrographs indicate that these immature cardiomyocytes lack transverse tubules and the SR is sparse. This paper focuses on the changes in the phenotype of E-C coupling during ontogeny in the mammalian heart and the molecular mechanisms underlying these changes.

Pathophysiology of pediatric heart failure

Our understanding of the syndrome of heart failure has undergone several revisions, most importantly in the second half of the 20th century. New insights into the mechanisms of diseases offer new, challenging, controversial and sometimes counterintuitive forms of therapy. The development and progression of heart failure results from a complex interplay of hemodynamic and neurohormonal, cellular and genetic factors, rather than simply changes in cardiac function. It is because of this reason that our therapeutic focus can no longer be solely based on supply and demand models. Since the description of the pulsatile nature of the heart function and the flow of blood around a circuit by W. Harvey, numerous new paradigms have been put forward to explain the nature of heart failure. However, no single new model thus far proposed has been able to displace previous ones and successfully dictate therapy. It is the purpose of this manuscript to review the overall current understanding of the heart failure syndrome and how these new ideas may affect our therapeutic approach.

Cellular basis for age-related differences in cardiac excitation-contraction coupling

Clinical experience indicates that infants and young children respond to a variety of cardiovascular pharmacological and physiological interventions differently than adults. What is less clear, however, are the cellular and molecular mechanisms that contribute to these age-related differences. Based largely upon results from animal models, it is apparent that developmental changes occur in numerous pathways and proteins involved in the regulation of contractile function and in the determinants of inotropic responsiveness. The purposes of this review are to provide a brief overview of cardiac excitation-contraction and to illustrate some of the important age-related differences in the mechanisms involved in calcium regulation in the heart. This scientific foundation may help to explain certain clinical observations in the very young. Furthermore, it is hoped that a better understanding of the fundamental processes involved in controlling cardiac contractile function will stimulate additional research in the search for more specific, rational and age-appropriate cardiovascular therapeutics.

The heart and development

The perspective from which the developing heart is viewed can lead to differing conclusions about the effects of development on cardiac function. The hearts of the embryo, fetus and adult, viewed from a global perspective, sustain the circulation through the same basic mechanisms of developing pressure and ejecting blood. The failure of the embryonic heart to perform these tasks results in growth failure, edema, and embryonic death, just as in the infant and adult such failure results in premature death. Furthermore, from the viewpoint of gross anatomy, following embryonic morphogenesis, the developing and adult hearts appear in general to be structurally similar, differing only in size and mass. However, a closer view shows, in the molecular and structural makeup of the myocardium, richly complex changes that can modulate the basic physiological properties of the cardiac myocyte. This article focuses on how these changes and the effects of birth and development alter ventricular function.

Effects of thapsigargin and cyclopiazonic acid on intracellular calcium activity in newborn rat cardiomyocytes during their development in primary culture

The effects of specific inhibitors of sarcoplasmic reticulum (SR) calcium ATPase, thapsigargin (TG), and cyclopiazonic acid (CPA) were investigated on the resting and transient levels of intracellular free calcium concentrations recorded in Indo-1-loaded ventricular myocytes of newborn rat heart in primary culture. The calcium transients were induced by caffeine (10 mM) or high potassium (100 mM) solutions. In 2 day- as in 7-day-old cultured cells, the calcium transients induced by 10 mM caffeine were blocked dose dependently by TG and CPA. The dose-response curves suggest that TG was more efficient than CPA and that both drugs were more efficient in 7-day- than in 2-day-old cells. The calcium transients induced by 100 mM K+ were also strongly inhibited by these agents. The lack of effect on sarcolemmal calcium currents, as shown by whole-cell patch-clamp experiments, suggests that these drugs affect only SR function. In cells exhibiting spontaneous activity, the associated calcium transients were not affected by TG or CPA at the beginning of the culture (2-day-old cells) but were fully blocked at the end (7-day-old cells). These results confirm that TG and CPA specifically inhibit the cardiac SR Ca2+ pump without affecting the sarcolemmal calcium current. Their blocking effect of the calcium transients as a function of the developmental stage of neonatal cardiac cells in culture suggests an increasing role of the SR in the regulation of intracellular calcium. This argues for developmental changes of the SR through the differentiation and maturation of newborn cardiomyocytes at the early stage of the postnatal life, leading to a predominant role of the SR in excitation-contraction coupling mechanisms in adult cells.

Intracellular calcium transients from newborn rat cardiomyocytes in primary culture

Resting and transient levels of intracellular free calcium concentrations were recorded in indo-1 loaded neonatal rat ventricular cardiomyocytes in primary culture by means of an interactive laser cytometer. The calcium transients were induced by high potassium and caffeine applications. The resting level of intracellular calcium remained constant (about 140 nM) throughout the culture (up to 7 days). The calcium transients induced by 100 mM K+ changed during culture from a low, cobalt sensitive response at 2 days, to a strong biphasic response at 7 days. At 2 days the response was fully blocked by cobalt. At 7 days the transient phase was abolished by cobalt and ryanodine, whereas the second sustained phase was only partially blocked. The calcium transient induced by caffeine was present as early as the first days, and increased with the age of the culture. This transient was blocked by ryanodine. The calcium influx through sarcolemmal calcium channels could be responsible for intracellular calcium transients in 2 day-old cells, whereas in 7 day-old cells, they seem to be only the trigger for sarcoplasmic reticulum calcium release via a mechanism such as 'calcium-induced calcium-release'. Other mechanisms, such as the sodium-calcium exchange mechanism activated by sarcolemmal depolarisation, seem to be implicated too and therefore could explain the sustained level of intracellular calcium during 100 mM K+ stimulation. The developmental changes through differentiation and maturation of myocytes in culture could account for the age dependent evolution of the responses obtained. From these results it is possible to conclude that calcium movements implicated in the excitation-contraction coupling mechanism in the development of rat neonatal cardiomyocytes are similar in primary culture and in the postnatal period in vivo.

Organization of calsequestrin-positive sarcoplasmic reticulum in rat cardiomyocytes in culture

The sarcoplasmic reticulum (SR) regulates the levels of cytoplasmic free Ca2+ ions in muscle cells. Calsequestrin is a major Ca(2+)-storing protein and is localized at special sites in the SR. To investigate the development of calsequestrin-positive SR and its interaction with the cytoskeleton, we examined the distribution of calsequestrin in cultured cardiomyocytes from newborn rats by immunofluorescence with anticalsequestrin and antitubulin antibodies and rhodamine-phalloidin. In frozen sections of neonatal rat heart, anticalsequestrin immunostaining was apparent as cross-striations at Z-lines. When newborn cardiomyocytes were isolated, calsequestrin-positive SR was disorganized and was apparent as small vesicles beneath the sarcolemma, whereas myofibrils accumulated in the center of the cells. As the cells spread in culture, calsequestrin-positive vesicles spread to the periphery of the cytoplasm, becoming associated with the developing myofibrils. In mature cells, calsequestrin was closely associated with myofibrils, showing cross-striations at the Z-lines. Double-labeling using anticalsequestrin and antitubulin antibodies demonstrated that the distribution of calsequestrin-positive structures was similar to that of the microtubular arrays. When the microtubules were depolymerized by nocodazole at an early stage, the extension of the SR to the cell periphery was inhibited. In mature cardiomyocytes, nocodazole appeared not to affect the distribution of the SR. These results indicate that the calsequestrin-positive SR in cardiomyocytes is organized at the proper sites of myofibrils during myofibrillogenesis and that the microtubules might serve as tracts for the transport of components of the SR.

Molecular cloning and characterization of a Ca2+ + Mg2+-dependent adenosine triphosphatase from rat cardiac sarcoplasmic reticulum. Regulation of its expression by pressure overload and developmental stage

To investigate the regulation of expression of cardiac Ca2+ + Mg2+-dependent ATPase (Ca2+-ATPase) in sarcoplasmic reticulum (SR), we isolated cDNA (pHA6) encoding a Ca2+-ATPase of rat cardiac SR. The clone consisted of 2,311 mRNA-derived nucleotides, which covered half the coding region and the entire 3'-untranslated regions. The nucleotides and deduced amino acid sequences of pHA6 showed striking homology, 89 and 98%, respectively, to those of rabbit Ca2+-ATPase of the slow-twitch form. Northern blot analyses revealed that the mRNA levels of Ca2+-ATPase were decreased by pressure overload and became 32% of sham in 1 mo. During the developmental stage the mRNA levels were very low in the fetal period and steeply increased around birth. These changes in mRNA levels were correlated with the corresponding protein levels. These results suggest that the expression of cardiac Ca2+-ATPase in SR is regulated by pressure overload and the developmental stage, at least in part, at the pretranslational level.

Development of the myocardial contractile system

Recent studies regarding developmental changes in the myocardial contractile system from fetal, newborn, and adult animals are reviewed. From the data obtained so far, we conclude that in the early fetus myocardial contraction is mainly dependent on Ca which enters via the sarcolemma. Ca release from the sarcoplasmic reticulum is minimal. The role of the sarcoplasmic reticulum as a source of contractile Ca increases and the role of Ca influx across the sarcolemma in contractile system decreases with development.

Changes in the calcium current of rat heart ventricular myocytes during development

1. Calcium current (ICa) was recorded in single rat heart cells at two periods during development: (1) at 2-7 days post-partum (neonatal), and (2) at 6-8 weeks (adult). 2. We measured both transient and steady-state components of ICa and could describe ICa in terms of the steady-state activation (d infinity) and inactivation (f infinity) parameters, the channel reversal potential (Echannel) and a relative conductance parameter, gr. 3. In adult single cells, the application of ryanodine (10 microM), an agent known to alter the function of the sarcoplasmic reticulum (SR), abolished contraction rapidly and increased ICa. Ryanodine also produced a 13 mV shift in f infinity towards more positive potentials and altered its slope, while producing a small increase in gr but no effect on d infinity. In neonatal single cells, ryanodine (10 microM) had no significant effect on contraction, ICa, d infinity, f infinity, or gr. Caffeine (10 mM), a less specific agent widely used to investigate sarcoplasmic reticulum function, had actions similar to those of ryanodine. 4. In adult myocytes, when EGTA (10 or 20 mM) or bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA, 10 mM) were included in the pipette solution, contractions were rapidly abolished, while a small (4 mV) shift of f infinity to more positive potentials was seen. A large additional shift of f infinity was observed when ryanodine (10 microM) was added to the superfusion solution in the continued presence of EGTA or BAPTA. The alterations of ICa in EGTA (or BAPTA) plus ryanodine were the same as those seen in ryanodine alone. In neonatal cells, in contrast, when EGTA or BAPTA were included in the pipette solution we observed only a small effect on f infinity and the application of ryanodine had no effect. 5. Electron micrographs of our preparations show that the dissociated adult cells have sharp sarcolemmal borders, fully developed sarcomeres with T-tubules and sarcoplasmic reticulum membranes. In contrast, the neonatal cells that we use have few of these intracellular structures. Our observations in these preparations are consistent with the work of others (e.g. Penefsky, 1974; Hirakow & Gotoh, 1975; Ishikawa & Yamada, 1975; Legato, 1975; Hoerter, Mazet & Vassort, 1981). 6. Our data suggest that fully developed sarcoplasmic reticulum in rat heart cells can affect ICa.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of isoproterenol on myocardial mechanical function and cyclic AMP content in the fetal rabbit

The effect of isoproterenol on mechanical function was studied in the isolated arterially perfused heart of the fetal (21st and 28th day of gestation) and newborn rabbits. The inotropic effect of isoproterenol in the fetus was less than in the newborn. In contrast, myocardial cyclic AMP levels after isoproterenol infusion in the fetus were greater than in the newborn. The inotropic effects of dibutyryl cyclic AMP and of calcium in the fetus were less than in the newborn. These data suggest that the process from beta-receptor to cyclic AMP in the fetus was equally or even more responsive to isoproterenol than in the newborn. The diminished inotropic effect of isoproterenol in the fetus may be due, at least in part, to the decreased inotropic effect of calcium.

Ontogenetic differences in cardiac sensitivity to verapamil in rats

The degree of a negative inotropic response of the isolated right ventricle to verapamil as well as the mortality rate were studied in rats during their postnatal development. Male Wistar rats aged 3, 15, 30, and 90 days were used. The isolated right ventricle was incubated in a glucose-free solution with a mixture of 95% O2 and 5% CO2 and electrically stimulated. The amplitude of isotonic contractions (AIC) was registered. In 90-day-old rats, AIC was 74.1 +/- 6.2% of initial amplitude 45 min after administration of verapamil; in 30-day-old, 41.1 +/- 6.4%; in 15-day-old, 38.2 +/- 4.1%; and in 3-day-old rats, only 2.6 +/- 1.5. The difference between the 3-day-old rats and all older groups was statistically highly significant. The mortality rate of verapamil-treated rats increased with decreasing age of animals. It is concluded that the sensitivity of the rat myocardium to verapamil is age dependent: the negative inotropic effect of this drug increases with decreasing age of the animal. This indicates a possible risk in the therapeutic use of verapamil when given to newborns and infants.

The sarcoplasmic reticulum of mouse heart: its divisions, configurations, and distribution

The sarcoplasmic reticulum (SR) is a prominent, highly ramified component of mouse myocardial cells. The use of ferrocyanide-reduced osmium tetroxide (OsFeCN) as a postfixative solution facilitates appreciation of both its extent and three-dimensional architecture. We have found that the individual volume fractions (Vv) of myofibrils, mitochondria, and SR are similar in cells of the right and left ventricular walls. Vv(total SR) is approximately 7%, a value considerably larger than previously reported. We attribute this disparity in large part to the recognition factor which comes into play with OsFeCN-treated tissue. Previous observations pertaining to the stereology of myocardial SR have likely substantially underestimated both volume fraction and surface density of this membrane system, since none to this point has utilized specific staining such as that conferred by the OsFeCN regimen. Our stereological measurements of different depths of the ventricular cell indicate that although considerable differences are found between SR configuration at peripheral and deep cell levels, no significant difference exists between the volume fractions of either the total SR or its individual constituents. Two different stereologic regimens gave close agreement on volume fractions of the various SR segments; the majority (approximately 92%) of the total SR is network SR, whereas the remainder is composed of the various categories of junctional SR (peripheral, apposed to the surface sarcolemma; interior, complexed with the transverse-axial tubular system; corbular, existing free of sarcolemmal contact). In the adult mouse, interior junctional SR greatly preponderates the other types of junctional SR; corbular SR is qualitively assessed to be a far more common component of atrial cells than of ventricular cardiomyocytes.

Myocellular calcium regulation by the sarcolemmal membrane in the adult and immature rabbit heart

Previous studies have suggested ontogenic differences in Ca-mediated excitation-contraction coupling in mammalian heart. Sarcolemmal (SL) Ca regulation may predominate prior to the development of the specialized Ca-regulatory properties of the sarcoplasmic reticulum (SR). The effect of development on selected Ca-regulatory properties of cardiac SL was evaluated utilizing membrane vesicles obtained from immature (14 to 21-day-old) and adult rabbit heart. Methods were adapted to comparably enrich SL membrane vesicles from immature and adult rabbit heart. The global fluidity characteristics were determined by the polarized fluorescence of diphenylhexatriene passively incorporated into enriched SL membrane vesicles. No age-related differences in either the membrane microviscosity of the lipid-order parameter between 10 degrees C and 37 degrees C were observed. The membrane characteristics of the voltage-gated Ca channel were determined by the membrane binding characteristics of 3[H]-nitrendipine. Scatchard analysis of high affinity specific nitrendipine binding demonstrated comparable binding affinity (KD; 511 +/- 40 vs 484 +/- 40 pM) and theoretical maximal binding site density (Bmax; 218 +/- 19 vs 240 +/- 40 fmoles/mg prot.) in immature and adult respectively. ATP-independent Ca binding to SL membrane vesicles was determined between 1.5 and 10 mM [Ca]. Ca binding was greater in the immature at 10 mM [Ca] as compared to the adult (840 +/- 120 vs 350 +/- 30 nmoles/mg). Ca bound to SL over this Ca concentration range is indicative of a "pool" of Ca for cellular influx across the SL by the Na-Ca exchange mechanism and the voltage-gated Ca channel. In view of electrophysiologic evidence also suggesting that Ca-channel-mediated Ca conductance is greater in the immature than the adult, it is proposed that the number of voltage-activatable Ca channels localized to the SL is greater in the immature than the adult. The larger transsarcolemmal Ca fluxes plays a larger role in the beat-to-beat- regulation of cardiac contraction in the developing mammalian heart prior to full expression of the specialized Ca regulatory properties of the SR.

Temporal appearance and distribution of the Ca2+ + Mg2+ ATPase of the sarcoplasmic reticulum in developing chick myocardium as determined by immunofluorescence labeling

The temporal appearance and distribution of the Ca2+ + Mg2+ ATPase of the sarcoplasmic reticulum were determined in the developing chick heart (stage 9 to stage 16) by indirect immunofluorescence labeling. The results obtained showed that the Ca2+ + Mg2+ ATPase was first observed in the bulbus ventricular region of the single tubular heart at stage 9 to 10 of development, when these myocardial cells first contract. As the atrial and later the sinus venosus tissues became incorporated into the single tubular heart the Ca2+ + Mg2+ ATPase was also observed in these regions, however, the highest density of Ca2+ + Mg2+ ATPase labeling was generally observed in the region of the heart most recently incorporated. These results suggest that the sarcoplasmic reticulum is present and perhaps functional in the regulation of the cytoplasmic Ca2+ concentration and thereby the contraction-relaxation cycle in myocardial cells when the first contraction occurs, as well as throughout all subsequent stages of development. Furthermore comparison between the relative density and intensity of the Ca2+ + Mg2+ ATPase labeling and the intrinsic rate of contraction of the myocardial cells in the various regions of the heart (A. Barry, 1942, J. Exp. Zool. 91, 119-130) supports the possibility that a positive correlation exists between these two characteristics of the myocardial cells.

Ultrastructure of terminally differentiated adult rat cardiac muscle cells in culture

Ventricular cardiac muscle cells isolated from adult rats were maintained in culture for 28 to 60 days and examined by transmission electron microscopy. In order to elucidate the detailed ultrastructure of the cultured myocytes, several different electron-dense stains were used. These included tannic acid, osmium ferrocyanide, osmium tetroxide (applied as a primary fixative), and lanthanum chloride, as well as more widely used stains such as osmium tetroxide, uranyl acetate, and lead citrate. Our results show that, compared to cultured neonatal rat myocytes, cultured myocytes derived from adult rats more closely resemble in vivo adult ventricular cells. The cultured adult myocytes contained typically distributed organelles such as nuclei, mitochondria, and elements of the sarcoplasmic reticulum. Myofilaments were well organized, and typical intercalated disks were observed between adjacent cells. Unlike cultured neonatal myocytes, the adult cells contained numerous residual bodies and a relatively well developed transverse tubular system. The transverse tubular system was identified by its continuity with the extracellular space (as indicated by the penetration of electron-dense extracellular tracers), location at or near the Z line, large lumenal diameter, and frequent participation in couplings with elements of the sarcoplasmic reticulum.