Developmental changes in cardiac sarcoplasmic reticulum in sheep

Electroanatomical adaptations in the guinea pig heart from neonatal to adulthood

AIMS

Electroanatomical adaptations during the neonatal to adult phase have not been comprehensively studied in preclinical animal models. To explore the impact of age as a biological variable on cardiac electrophysiology, we employed neonatal and adult guinea pigs, which are a recognized animal model for developmental research.

METHODS AND RESULTS

Electrocardiogram recordings were collected in vivo from anaesthetized animals. A Langendorff-perfusion system was employed for the optical assessment of action potentials and calcium transients. Optical data sets were analysed using Kairosight 3.0 software. The allometric relationship between heart weight and body weight diminishes with age, it is strongest at the neonatal stage (R2 = 0.84) and abolished in older adults (R2 = 1E-06). Neonatal hearts exhibit circular activation, while adults show prototypical elliptical shapes. Neonatal conduction velocity (40.6 ± 4.0 cm/s) is slower than adults (younger: 61.6 ± 9.3 cm/s; older: 53.6 ± 9.2 cm/s). Neonatal hearts have a longer action potential duration (APD) and exhibit regional heterogeneity (left apex; APD30: 68.6 ± 5.6 ms, left basal; APD30: 62.8 ± 3.6), which was absent in adults. With dynamic pacing, neonatal hearts exhibit a flatter APD restitution slope (APD70: 0.29 ± 0.04) compared with older adults (0.49 ± 0.04). Similar restitution characteristics are observed with extrasystolic pacing, with a flatter slope in neonates (APD70: 0.54 ± 0.1) compared with adults (younger: 0.85 ± 0.4; older: 0.95 ± 0.7). Neonatal hearts display unidirectional excitation-contraction coupling, while adults exhibit bidirectionality.

CONCLUSION

Postnatal development is characterized by transient changes in electroanatomical properties. Age-specific patterns can influence cardiac physiology, pathology, and therapies for cardiovascular diseases. Understanding heart development is crucial to evaluating therapeutic eligibility, safety, and efficacy.

The halothane era in pediatric anesthesia: The convergence of a cardiac depressant anesthetic with the immature myocardium of infancy

Introduced in the late 1950s, halothane became the anesthetic of choice for inhalational induction of children for over 40 years. Halothane enjoyed a generally favorable safety record during its time, but its cardiac contractility depressant effect-well tolerated by most age groups-was profoundly heightened in neonates and infants, leading to increased incidences of hypotension and cardiac arrest. The neonatal myocardium is immature and is characterized by poor ventricular compliance, poor contractility due to fewer contractile elements, immature sympathetic innervation with decreased norepinephrine stores, and immature mechanisms for storage and exchange of calcium in the sarcoplasmic reticulum. In vitro studies of myocardial contractility of mammalian fetal and adult myocardium demonstrated that the fetal heart was twice as sensitive to halothane as the adult. Clinical studies demonstrated that most neonates and infants less than 6 months of age experienced hypotension during halothane induction of anesthesia and significantly (p < .01) greater decreases in blood pressure than older children at equipotent concentrations of halothane. Intraoperative cardiac arrest during the halothane era occurred over twice as frequently in neonates aged less than 1 month than in infants aged 1-12 months and nearly 10 times more frequently than children 1-5 years of age. Halothane was associated with 66% of intraoperative drug-related cardiac arrests in children. The halothane era began to close in the late 1990s with the introduction of sevoflurane, which had a more favorable hemodynamic profile. Shortly thereafter, halothane was completely displaced from pediatric anesthesia practice in North America.

Electroanatomical Adaptations in the Guinea Pig Heart from Neonatal to Adulthood

Background

Electroanatomical adaptations during the neonatal to adult phase have not been comprehensively studied in preclinical animal models. To explore the impact of age as a biological variable on cardiac electrophysiology, we employed neonatal and adult guinea pigs, which are a recognized animal model for developmental research.

Methods

Healthy guinea pigs were categorized into three age groups (neonates, n=10; younger adults, n=13; and older adults, n=26). Electrocardiogram (ECG) recordings were collected in vivo from anesthetized animals (2-3% isoflurane). A Langendorff-perfusion system was employed for optical assessment of epicardial action potentials and calcium transients, using intact excised heart preparations. Optical data sets were analyzed and metric maps were constructed using Kairosight 3.0.

Results

The allometric relationship between heart weight and body weight diminishes with age, as it is strongest at the neonatal stage (R 2 = 0.84) and completely abolished in older adults (R 2 = 1E-06). Neonatal hearts exhibit circular activation waveforms, while adults show prototypical elliptical shapes. Neonatal conduction velocity (40.6±4.0 cm/s) is slower than adults (younger adults: 61.6±9.3 cm/s; older adults: 53.6±9.2 cm/s). Neonatal hearts have a longer action potential duration (APD) and exhibit regional heterogeneity (left apex; APD30: 68.6±5.6 ms, left basal; APD30: 62.8±3.6), which was absent in adult epicardium. With dynamic pacing, neonatal hearts exhibit a flatter APD restitution slope (APD70: 0.29±0.04) compared to older adults (0.49±0.04). Similar restitution characteristics are observed with extrasystolic pacing, with a flatter slope in neonatal hearts (APD70: 0.54±0.1) compared to adults (Younger adults: 0.85±0.4; Older adults: 0.95±0.7). Finally, neonatal hearts display unidirectional excitation-contraction coupling, while adults exhibit bidirectionality.

Conclusion

The transition from neonatal to adulthood in guinea pig hearts is characterized by transient changes in electroanatomic properties. Age-specific patterns can influence cardiac physiology, pathology, and therapies for cardiovascular diseases. Understanding postnatal heart development is crucial to evaluating therapeutic eligibility, safety, and efficacy.

What is Known

Age-specific cardiac electroanatomical characteristics have been documented in humans and some preclinical animal models. These age-specific patterns can influence cardiac physiology, pathology, and therapies for cardiovascular diseases.

What the Study Adds

Cardiac electroanatomical characteristics are age-specific in guinea pigs, a well-known preclinical model for developmental studies. Age-dependent adaptations in cardiac electrophysiology are readily observed in the electrocardiogram recordings and via optical mapping of epicardial action potentials and calcium transients. Our findings reveal unique activation and repolarization characteristics between neonatal and adult animals.

The effect of cardioplegic supplementation with sildenafil on cardiac energetics in a piglet model of cardiopulmonary bypass and cardioplegic arrest with warm or cold cardioplegia

Cardioplegic cardioprotection strategies used during paediatric open-heart surgery remain suboptimal. Sildenafil, a phosphodiesterase 5 (PDE-5) inhibitor, has been shown to be cardioprotective against ischemia/reperfusion injury in a variety of experimental models and this study therefore tested the efficacy of supplementation of cardioplegia with sildenafil in a piglet model of cardiopulmonary bypass and arrest, using both cold and warm cardioplegia protocols. Piglets were anaesthetized and placed on coronary pulmonary bypass (CPB), the aorta cross-clamped and the hearts arrested for 60 min with cardioplegia with or without sildenafil (10 nM). Twenty minutes after removal of cross clamp (reperfusion), attempts were made to wean the pigs from CPB. Termination was carried out after 60 min reperfusion. Throughout the protocol blood and left ventricular tissue samples were taken for analysis of selected metabolites (using HPLC) and troponin I. In both the cold and warm cardioplegia protocols there was evidence that sildenafil supplementation resulted in faster recovery of ATP levels, improved energy charge (a measure of metabolic flux) and altered release of hypoxanthine and inosine, two purine catabolites. There was no effect on troponin release within the studied short timeframe. In conclusion, sildenafil supplementation of cardioplegia resulted in improved cardiac energetics in a translational animal model of paediatric CPB surgery.

Acute Cardiac Care for Neonatal Heart Disease

This manuscript is one component of a larger series of articles produced by the Neonatal Cardiac Care Collaborative that are published in this supplement of Pediatrics. In this review article, we summarize the contemporary physiologic principles, evaluation, and management of acute care issues for neonates with complex congenital heart disease. A multidisciplinary team of authors was created by the Collaborative's Executive Committee. The authors developed a detailed outline of the manuscript, and small teams of authors were assigned to draft specific sections. The authors reviewed the literature, with a focus on original manuscripts published in the last decade, and drafted preliminary content and recommendations. All authors subsequently reviewed and edited the entire manuscript until a consensus was achieved. Topics addressed include cardiopulmonary interactions, the pathophysiology of and strategies to minimize the development of ventilator-induced low cardiac output syndrome, common postoperative physiologies, perioperative bleeding and coagulation, and common postoperative complications.

Cardiopulmonary Bypass in Premature and Low Birth Weight Neonates - Implications for Postoperative Care From a Neonatologist/Intensivist Perspective

Prematurity and low weight remain significant risk factors for mortality after neonatal cardiac surgery despite steady gains in survival. Newer and lower weight thresholds for operability are constantly generated as surgeons gather proficiency, technical mastery, and experience in performing complex procedures on extremely small infants. Relationship between birth weight and survival after cardiac surgery is nonlinear with 2 kg being an inflection point below which marked decline in survival occurs. If strides toward improved survival in this weight category are to be made, understanding the inherent vulnerabilities of the premature and low birth weight infant is important in addition to acknowledging the vulnerabilities of the system in which care is delivered.

Neonatal cardiac care, a perspective

Every year in the United States approximately 40,000 infants are born with congenital heart disease. Several of these infants require corrective or palliative surgery in the neonatal period. Mortality rates after cardiac surgery are highest amongst neonates, particularly those born prematurely. There are several reasons for the increased surgical mortality risk in neonates. This review outlines these risks, with particular emphasis on the relative immaturity of the organ systems in the term and preterm neonate.

Changes in cardiac troponins with gestational age explain changes in cardiac muscle contractility in the sheep fetus

The development of the adult cardiac troponin complex in conjunction with changes in cardiac function and cardiomyocyte binucleation has not been systematically characterized during fetal life in a species where maturation of the cardiomyocytes occurs prenatally as it does in the human. The aim of this study was to correlate the expression of each of the major adult troponin isoforms (T, I, and C) during late gestation (term of 150 days) to changes in both Ca(2+) sensitivity and maximum Ca(2+)-activated force of the contractile apparatus and the maturation of cardiomyocytes. The percentage of mononucleated cardiomyocytes in the right ventricle decreased with gestational age to 46% by 137-142 days of gestation. The length of binucleated cardiomyocytes did not change with gestational age, but the length of binucleated cardiomyocytes relative to heart weight decreased with gestational age. There was no change in the expression of adult cardiac troponin T with increasing gestation. The contractile apparatus was significantly more sensitive to Ca(2+) at 90 days compared with either 132 or 139 days of gestation, consistent with an ∼30% increase in the expression of adult cardiac troponin I between 90 and 110 days of gestation. Maximum Ca(2+)-activated force significantly increased from 90 days compared with 130 days consistent with an increase of ∼40% in cardiac troponin C protein expression. These data show that increased adult cardiac troponin I and C protein expression across late gestation is consistent with reduced Ca(2+) sensitivity and increased maximum Ca(2+)-activated force. Furthermore, changes in cardiac troponin C, not I, protein expression track with the timing of cardiomyocyte binucleation.

Developmental aspects of cardiac Ca(2+) signaling: interplay between RyR- and IP(3)R-gated Ca(2+) stores

The dominant mode of intracellular Ca(2+) release in adult mammalian heart is gated by ryanodine receptors (RyRs), but it is less clear whether inositol 1,4,5-trisphosphate (IP(3))-gated Ca(2+) release channels (IP(3)Rs), which are important during embryogenesis, play a significant role during early postnatal development. To address this question, we measured confocal two-dimensional Ca(2+) dependent fluorescence images in acutely isolated neonatal (days 1 to 2) and juvenile (days 8-10) rat cardiomyocytes, either voltage-clamped or permeabilized, where rapid exchange of solution could be used to selectively activate the two types of Ca(2+) release channel. Targeting RyRs with caffeine produced large and rapid Ca(2+) signals throughout the cells. Application of ATP and endothelin-1 to voltage-clamped, or IP(3) to permeabilized, cells produced smaller and slower Ca(2+) signals that were most prominent in subsarcolemmal regions and were suppressed by either the IP(3)R-blocker 2-aminoethoxydiphenylborate or replacement of the biologically active form of IP(3) with its L-stereoisomer. Such IP(3)R-gated Ca(2+) releases were amplified by Ca(2+)-induced Ca(2+) release (CICR) via RyRs since they were also reduced by compounds that block the RyRs (tetracaine) or deplete the Ca(2+) pools they gate (caffeine, ryanodine). Spatial analysis revealed both subsarcolemmal and perinuclear origins for the IP(3)-mediated Ca(2+) release events RyR- and IP(3)R-gated Ca(2+) signals had larger magnitudes in juvenile than in neonatal cardiomyocytes. Ca(2+) signaling was generally quite similar in atrial and ventricular cardiomyocytes but showed divergent development of IP(3)-mediated regulation in juveniles. Our data suggest that an intermediate stage of Ca(2+) signaling may be present in developing cardiomyocytes, where, in addition to RyR-gated Ca(2+) pools, IP(3)-gated Ca(2+) release is sufficiently large in magnitude and duration to trigger or contribute to activation of CICR and cardiac contraction.

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.

Triadin is a critical determinant of cellular Ca cycling and contractility in the heart

Triadin is involved in the regulation of cardiac excitation-contraction coupling. However, the extent of its contribution to the regulation of sarcoplasmic reticulum (SR) Ca release remains unclear, because overexpression of triadin in single-transgenic mice was associated with the downregulation of its homologous protein, junctin. In the present study, this problem was circumvented by cross-breeding of mice with heart-directed overexpression of triadin and junctin (JxT). This resulted in a stable approximately threefold expression of total triadin but unchanged junctin protein. Transgenic mice exhibited cardiac hypertrophy and structural abnormalities of myofibrils. Measurement of cardiac function by echocardiography and edge detection in myocytes revealed an impaired relaxation in JxT mice. The stimulation of beta-adrenergic receptors resulted in a depressed contractility and an impaired relaxation in catheterized hearts and myocytes of JxT mice. The use of a maximum stimulation frequency (5 Hz) was associated with both a lower shortening and relengthening in isolated myocytes of JxT mice. The contractile effects in JxT myocytes were paralleled by similar changes of the intracellular Ca concentration ([Ca](i)) peak amplitude and Ca transient decay kinetics at basal conditions, under administration of isoproterenol, and with high-frequency stimulation. Finally, we found a higher caffeine-induced [Ca](i) peak amplitude in JxT myocytes. Our data show that the stable expression of triadin, independent of junctin expression, resulted in cardiac hypertrophy, prolonged basal relaxation, a depressed response to beta-adrenergic agonists, and altered Ca transients. Thus the maintenance of triadin expression is essential for normal SR Ca cycling and contractile function.

Contractile and Ca2+-handling properties of the right ventricular papillary muscle in the late-gestation sheep fetus

The force-generating capacity of cardiomyocytes rapidly changes during gestation and early postnatal life coinciding with a transition in cardiomyocyte nucleation in both mice and rats. Changes in nucleation, in turn, appear to coincide with important changes in the excitation-contraction coupling architecture. However, it is not clear whether similar changes are observed in other mammals in which this transition occurs prenatally, such as sheep. Using small (70–300 μM diameter) chemically skinned cardiomyocyte bundles from the right ventricular papillary muscle of sheep fetuses at 126–132 and 137–140 days (d) gestational age (GA), we aimed to examine whether changes in cardiomyocyte nucleation during late gestation coincided with developmental changes in excitation-contraction coupling parameters (e.g., Ca2+uptake, Ca2+release, and force development). All experiments were conducted at room temperature (23 ± 1°C). We found that the proportion of mononucleate cardiomyocytes decreased significantly with GA (126–132d, 45.7 ± 4.7%, n = 7; 137–140d, 32.8 ± 1.6%, n = 6; P < 0.05). When we then examined force development between the two groups, there was no significant difference in either the maximal Ca2+-activated force (6.73 ± 1.54 mN/mm2, n = 14 vs. 6.55 ± 1.25 mN/mm2, n = 7, respectively) or the Ca2+sensitivity of the contractile apparatus (pCa at 50% maximum Ca2+-activated force: 126–132d, 6.17 ± 0.06, n = 14; 137–140d, 6.24 ± 0.08, n = 7). However, sarcoplasmic reticulum (SR) Ca2+uptake rates (but not Ca2+release) increased with GA ( P < 0.05). These data reveal that during late gestation in sheep when there is a major transition in cardiomyocyte nucleation, SR Ca2+uptake rates increase, which would influence total SR Ca2+content and force production.

Developmental regulation of intracellular calcium transients during cardiomyocyte differentiation of mouse embryonic stem cells

AIM

To investigate the developmental regulation of intracellular Ca2+ transients, an essential event in excitation-contraction coupling, during cardiomyocyte differentiation.

METHODS

Using the embryonic stem (ES) cell in vitro differentiation system and pharmacological intervention, we investigated the molecular and functional regulation of Ca2+ handling proteins on the Ca2+ transients at early, intermediate and later differentiation stages of ES cell-derived cardiomyocytes (ESCM).

RESULTS

Nifedipine, a selective antagonist of L-type Ca2+ channels, totally blocked Ca2+ transients even in the condition of field-electric stimulation in ESCM at three differentiation stages. The Ca2+ transients of ESCM were also inhibited by both ryanodine [an inhibitor of ryanodine receptors (RyRs)] and 2-aminoethoxydipheylborate [2-APB, an inhibitor of inositol-1,4,5-trisphosphate receptors (IP3Rs)]. The inhibitory effect of ryanodine increased with the time of differentiation, while the effect of 2-APB decreased with the differentiation. Thapsigargin, an inhibitor of SR Ca2+-pump ATPase, inhibited Ca2+ transients equally at three differentiation stages that matched the expression profile. Na+ free solution, which inhibits Na+-Ca2+ exchanger (NCX) to extrude Ca2+ from cytosol, did not affect the amplitude of Ca2+ transients of ESCM until the latter differentiation stage, but it significantly enhanced the basal Ca2+ concentration.

CONCLUSION

The Ca2+ transients in ESCM depend on both the sarcolemmal Ca2+ entry via L-type Ca2+ channels and the SR Ca2+ release from RyRs and IP3Rs even at the early differentiation stage; but NCX seems not to regulate the peak of Ca2+ transients until the latter differentiation stage.

Clinical value of NT-ProBNP and BNP in pediatric cardiology

BACKGROUND

Although less common than in adults, heart disease is a significant cause for morbidity and death in infants and children. Congenital structural cardiac anomalies and acquired heart diseases may result in heart failure.

METHODS AND RESULTS

Available data suggest that the natriuretic peptide system has a similar role in health and disease in the pediatric age group as in adults. In healthy infants and children, levels of B-type natriuretic peptide (BNP) and the N-terminal segment of its pro-hormone (NT-proBNP) are elevated in the first few days after birth. Thereafter, their levels decrease and remain relatively constant throughout childhood. Infants and children with heart disease that causes significant pressure or volume overload of the right or the left ventricle have elevated BNP and NT-proBNP levels. In children with congestive heart failure, BNP and NT-proBNP levels correlate with functional capacity. Both peptides can differentiate cardiac from pulmonary causes in infants with respiratory distress. Limited data suggest that these peptides may also serve as markers in cyanotic, obstructive, and inflammatory heart diseases.

CONCLUSION

The present data suggest that both NT-proBNP and BNP are markers for heart disease in infants and children. Their use may improve clinical practice in pediatric cardiology.

Overexpression of junctin causes adaptive changes in cardiac myocyte Ca(2+) signaling

In cardiac muscle, junctin forms a quaternary protein complex with the ryanodine receptor (RyR), calsequestrin, and triadin 1 at the luminal face of the junctional sarcoplasmic reticulum (jSR). By binding directly the RyR and calsequestrin, junctin may mediate the Ca(2+)-dependent regulatory interactions between both proteins. To gain more insight into the underlying mechanisms of impaired contractile relaxation in transgenic mice with cardiac-specific overexpression of junctin (TG), we studied cellular Ca(2+) handling in these mice. We found that the SR Ca(2+) load was reduced by 22% in cardiomyocytes from TG mice. Consistent with this, the frequency of Ca(2+) sparks was diminished by 32%. The decay of spontaneous Ca(2+) sparks was prolonged by 117% in TG. This finding was associated with a lower Na(+)-Ca(2+) exchanger (NCX) protein expression (by 67%) and a higher basal RyR phosphorylation at Ser(2809) (by 64%) in TG. The shortening- and Delta[Ca](i)-frequency relationships (0.5-4 Hz) were flat in TG compared to wild-type (WT) which exhibited a positive staircase for both parameters. Furthermore, increasing stimulation frequencies hastened the time of relaxation and the decay of [Ca](i) by a higher percentage in TG. We conclude that the impaired relaxation in TG may result from a reduced NCX expression and/or a higher SR Ca(2+) leak. The altered shortening-frequency relationship in TG seems to be a consequence of an impaired excitation-contraction coupling with depressed SR Ca(2+) release at higher rates of stimulation. Our data suggest that the more prominent frequency-dependent hastening of relaxation in TG results from a stimulation of SR Ca(2+) transport reflected by corresponding changes of [Ca](i).

Phosphorylation-status of phospholamban and calsequestrin modifies their affinity towards commonly used antibodies

Phospholamban (PLB) and calsequestrin (CSQ) play important roles in sarcoplasmic reticulum Ca(2+) transport and storage in cardiac muscle. Specific antibodies have been frequently used to quantitate CSQ and PLB protein levels. Here we demonstrate that two of the commonly available anti-PLB antibodies, anti-PLB-2D12 and anti-PLB-A1, show lower reactivity to phosphorylated than dephosphorylated PLB. A custom anti-PLB antibody, generated using a peptide corresponding to amino acids 2-14, is not affected by the phosphorylation state of PLB. In contrast, anti-CSQ reacts less with dephosphorylated CSQ than with phosphorylated CSQ. All three commercially available antibodies tested in this study have been widely used to quantify PLB and CSQ expression, and the results are integrated in many publications. Our studies reveal that the phosphorylation status of PLB and CSQ can affect antibody reactivity and may lead to over- or underestimation of the relative protein content and erroneous interpretation of data.

Organization of Ca2+ release units in excitable smooth muscle of the guinea-pig urinary bladder

Ca(2+) release from internal stores (sarcoplasmic reticulum or SR) in smooth muscles is initiated either via pharmaco-mechanical coupling due to the action of an agonist and involving IP3 receptors, or via excitation-contraction coupling, mostly involving L-type calcium channels in the plasmalemma (DHPRs), and ryanodine receptors (RyRs), or Ca(2+) release channels of the SR. This work focuses attention on the structural basis for the coupling between DHPRs and RyRs in phasic smooth muscle cells of the guinea-pig urinary bladder. Immunolabeling shows that two proteins of the SR: calsequestrin and the RyR, and one protein the plasmalemma, the L-type channel or DHPR, are colocalized with each other within numerous, peripherally located sites located within the caveolar domains. Electron microscopy images from thin sections and freeze-fracture replicas identify feet in small peripherally located SR vesicles containing calsequestrin and distinctive large particles clustered within small membrane areas. Both feet and particle clusters are located within caveolar domains. Correspondence between the location of feet and particle clusters and of RyR- and DHPR-positive foci allows the conclusion that calsequestrin, RyRs, and L-type Ca(2+) channels are associated with peripheral couplings, or Ca(2+) release units, constituting the key machinery involved in excitation-contraction coupling. Structural analogies between smooth and cardiac muscle excitation-contraction coupling complexes suggest a common basic mechanism of action.

Ventricular fibrillation frequency characteristics and time evolution in piglets: a developmental study

BACKGROUND

Derived variables of ventricular fibrillation, such as the frequency distribution by fast Fourier transformation and its evolution over time, have been used to determine the optimum timing for defibrillation. We hypothesized that these frequency variables would differ among neonatal, young child and older child populations due to cardiac developmental and size differences. Such differences may have important implications for developing defibrillation algorithms for pediatric patients and for extrapolating adult defibrillation algorithms to children in VF.

METHODS

Ventricular fibrillation was induced and recorded for 6 min in 4 kg (n = 11), 14 kg (n = 10), and 24 kg (n = 16) piglets, corresponding to neonatal, young child and older children. Mean, median, and dominant frequencies were computed in 30 s intervals and compared among weight classes.

RESULTS

All frequency variables in all weight groups showed first a decline at 1.25-1.75 min, followed by a gradual rise and plateau. There were significant differences for mean, median and dominant frequencies among weight classes. Specifically, 14 kg piglets showed higher frequency variables overall with a time evolution that was different from that of 4 and 24 kg piglets. Mean frequency showed the most stable time evolution with the least moment-to-moment variability.

CONCLUSION

The frequency waveform characteristics and time course are somewhat different in 14 kg piglets compared with 4 and 24 kg piglets. If similar differences are demonstrable among children of different weights and ages, AEDs designed to determine optimal timing of defibrillation shocks in adults by frequency waveform characteristics may require modification for use in children with VF.

Age-dependent biochemical and contractile properties in atrium of transgenic mice overexpressing junctin

Junctin is a transmembrane protein of the cardiac junctional sarcoplasmic reticulum (SR) that binds to the ryanodine receptor, calsequestrin, and triadin 1. This quaternary protein complex is thought to facilitate SR Ca2+ release. To improve our understanding of the contribution of junctin to the regulation of SR function, we examined the age-dependent effects of junctin overexpression in the atrium of 3-, 6-, and 18-wk-old transgenic mice. The ratio of atrial weight and body weight was unchanged between junctin-overexpressing (JCN) and wild-type (WT) mice at all ages investigated (n=6-8). The protein expression of triadin 1 was decreased starting in 3-wk-old JCN atria (by 69%), whereas the expression of the ryanodine receptor was diminished in 6- (by 48%) and 18-wk-old (by 57%) JCN atria compared with age-matched WT atria. Force of contraction was decreased by 35% in 18-wk-old JCN compared with age-matched WT left atrial muscle strips, which was accompanied by a prolonged time of relaxation (48.1 +/- 0.9 vs. 44.2 +/- 0.8 ms, respectively, n=6-8, P <0.05). The spontaneous beating rate of isolated right atria was higher in 18-wk-old JCN mice compared with age-matched WT mice (389 +/- 10 vs. 357 +/- 6 beats/min, respectively, n=6-8, P <0.05). Heart rate was lower by 9% in telemetric ECG recordings in 18-wk-old JCN mice during stress tests. Three-week-old JCN atria exhibited a higher potentiation of force of contraction at rest pauses of 30 s (by 13%) and of 300 s (by 35%), suggesting increased SR Ca2+ content. This was consistent with the higher force of contraction in 3-wk-old JCN atria (by 29%) compared with age-matched WT atria (by 10%) under the administration of caffeine. We conclude that in 3-wk-old atria, junctin overexpression was associated with a reduced expression of triadin 1 resulting in a higher SR Ca2+ load without changes in contractility or heart rate. In 6-wk-old JCN atria, the compensatory downregulation of the ryanodine receptor may offset the effects of junctin overexpression. Finally, the progressive decrease in ryanodine receptor density may contribute to the decreased atrial contractility and lower heart rate during stress in 18-wk-old JCN mice.

Altered function in atrium of transgenic mice overexpressing triadin 1

Triadin 1 is a protein in the cardiac junctional sarcoplasmic reticulum (SR) that interacts with the ryanodine receptor, junctin, and calsequestrin, proteins that are important for Ca(2+) release. To better understand the role of triadin 1 in SR-Ca(2+) release, we studied the time-dependent expression of SR proteins and contractility in atria of 3-, 6-, and 18-wk-old transgenic mice overexpressing canine cardiac triadin 1 under control of the alpha-myosin heavy chain (MHC) promoter. Three-week-old transgenic atria exhibited mild hypertrophy. Finally, atrial weight was increased by 110% in 18-wk-old transgenic mice. Triadin 1 overexpression was accompanied by time-dependent changes in the protein expression of the ryanodine receptor, junctin, and cardiac/slow-twitch muscle SR Ca(2+)-ATPase isoform. Force of contraction was already decreased in 3-wk-old transgenic atria. The application of caffeine led to a positive inotropic effect in transgenic atria of 3-wk-old mice. Rest pauses resulted in an increased potentiation of force of contraction after restimulation in 3- and 6-wk-old mice and a reduced potentiation of force of contraction in 18-wk-old transgenic mice. Hence, triadin 1 overexpression triggered time-dependent alterations in SR protein expression, Ca(2+) homeostasis, and contractility, indicating for the first time an inhibitory function of triadin 1 on SR-Ca(2+) release in vivo.

Developmental changes in ryanodine- and IP(3)-sensitive Ca(2+) pools in ovine basilar artery

To explore the hypothesis that cerebrovascular maturation alters ryanodine- and inositol 1,4,5-trisphosphate (IP(3))-sensitive Ca(2+) pool sizes, we measured total intracellular Ca(2+) with (45)Ca and the fractions of intracellular Ca(2+) released by IP(3) and/or caffeine in furaptra-loaded permeabilized basilar arteries from nonpregnant adult and term fetal (139-141 days) sheep. Ca(2+) mass (nmol/mg dry weight) was similar in adult (1.60 +/- 0.18) and fetal (1.71 +/- 0.16) arteries in the pool sensitive to IP(3) alone but was significantly lower for adult (0.11 +/- 0.01) than for fetal (1.22 +/- 0.11) arteries in the pool sensitive to ryanodine alone. The pool sensitive to both ryanodine and IP(3) was also smaller in adult (0.14 +/- 0.01) than in fetal (0.85 +/- 0.08) arteries. Because the Ca(2+) fraction in the ryanodine-IP(3) pool was small in both adult (5 +/- 1%) and fetal (7 +/- 4%) arteries, the IP(3) and ryanodine pools appear to be separate in these arteries. However, the pool sensitive to neither IP(3) nor ryanodine was 10-fold smaller in adult (0.87 +/- 0.10) than in fetal (8.78 +/- 0.81) arteries, where it accounted for 72% of total intracellular membrane-bound Ca(2+). Thus, during basilar artery maturation, intracellular Ca(2+) mass plummets in noncontractile pools, decreases modestly in ryanodine-sensitive pools, and remains constant in IP(3)-sensitive pools. In addition, age-related increases in IP(3) efficacy must involve factors other than IP(3) pool size alone.

Gene expression of SERCA2a and L- and T-type Ca channels during human heart development

In this study we report, for the first time, on the gene expression of human cardiac SERCA2a, L-type (alpha(1C)) and T-type (alpha(1H)) Ca channels during development, using RNase protection assay, relative quantitative RT-PCR and Western blot. Human hearts during early gestation (8- to 20-wk gestation), neonatal (1- to 4-d-old) and adult (18- to 48-year-old) stages were used. The results show that T-type Ca channel alpha(1H) subunit mRNA decreased and that L-type Ca channel alpha(1C) subunit mRNA increased with development. While the levels of sarcoplasmic reticulum ATPase (SERCA2a) mRNA did not significantly change with development, its protein levels increased with development. In conclusion, SERCA2a, L-type and T-type Ca channel transcripts were detected as early as 8-wk gestation. Defining the profile of Ca handling proteins during development is important to the understanding of excitation-contraction (EC)-coupling of the developing human heart.

Altered SR protein expression associated with contractile dysfunction in diabetic rat hearts

The goal of this study was to examine whether alteration of sarcoplasmic reticulum (SR) protein levels is associated with early-onset diastolic and late-onset systolic dysfunction in streptozotocin (STZ)-induced diabetic rat hearts. Four-week diabetic rat hearts exhibited slow relaxation, whereas 6-wk diabetic rat hearts exhibited slow and depressed contraction. Total phospholamban level was increased, and phosphorylated level was decreased in 4- and 6-wk diabetic rat hearts. Sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA2) protein level was unchanged in 4-wk but decreased in 6-wk diabetic rat hearts. Only the apparent affinity of SR Ca2+ uptake for Ca2+ was decreased in 4-wk diabetic rat hearts, but the apparent affinity and the maximum rate was decreased in 6-wk diabetic rat hearts. Insulin treatment of the diabetic rats normalized SR protein expression and function. It was concluded that an increase in nonphosphorylated phospholamban and a decrease in the apparent affinity of SR Ca2+ pump for Ca2+ are associated with early-onset diastolic dysfunction and decreases in SERCA2 protein level and apparent affinity and maximum velocity of SR Ca2+ pump are associated with late-onset systolic dysfunction in diabetic rats.

Cardiac hypertrophy and impaired relaxation in transgenic mice overexpressing triadin 1

Triadin 1 is a major transmembrane protein in cardiac junctional sarcoplasmic reticulum (SR), which forms a quaternary complex with the ryanodine receptor (Ca(2+) release channel), junctin, and calsequestrin. To better understand the role of triadin 1 in excitation-contraction coupling in the heart, we generated transgenic mice with targeted overexpression of triadin 1 to mouse atrium and ventricle, employing the alpha-myosin heavy chain promoter to drive protein expression. The protein was overexpressed 5-fold in mouse ventricles, and overexpression was accompanied by cardiac hypertrophy. The levels of two other junctional SR proteins, the ryanodine receptor and junctin, were reduced by 55% and 73%, respectively, in association with triadin 1 overexpression, whereas the levels of calsequestrin, the Ca(2+)-binding protein of junctional SR, and of phospholamban and SERCA2a, Ca(2+)-handling proteins of the free SR, were unchanged. Cardiac myocytes from triadin 1-overexpressing mice exhibited depressed contractility; Ca(2+) transients decayed at a slower rate, and cell shortening and relengthening were diminished. The extent of depression of cell shortening of triadin 1-overexpressing cardiomyocytes was rate-dependent, being more depressed under low stimulation frequencies (0.5 Hz), but reaching comparable levels at higher frequencies of stimulation (5 Hz). Spontaneously beating, isolated work-performing heart preparations overexpressing triadin 1 also relaxed at a slower rate than control hearts, and failed to adapt to increased afterload appropriately. The fast time inactivation constant, tau(1), of the l-type Ca(2+) channel was prolonged in transgenic cardiomyocytes. Our results provide evidence for the coordinated regulation of junctional SR protein expression in heart independent of free SR protein expression, and furthermore suggest an important role for triadin 1 in regulating the contractile properties of the heart during excitation-contraction coupling.

Left ventricular alterations in a model of fetal left ventricular overload

Congenital aortic coarctation is well tolerated by the fetus because the foramen ovale and ductus arteriosus equalize intracardiac and great arteries pressures and shunts. The pathologic consequences only emerge after birth with closure of the foramen ovale and ductus arteriosus. There is, however, no documentation of myocardial effects in utero of the left ventricular (LV) pressure overload induced by aortic banding. We investigated whether prenatal aortic banding could be detrimental at the structural and/or functional level. The goal of the present study was to investigate the cardiac effects of LV pressure overload in a fetal lamb model. Nine fetal lambs underwent preductal banding of the aortic arch in utero at midgestation (CoA group), whereas their twins underwent sham surgery. All fetuses were studied between 27 and 37 d after surgery for LV pressure, anatomic and histologic anomalies, and steady state sarcoendoplasmic reticulum calcium ATPase (SERCA 2a) mRNA and protein levels and pump activity. Surgery resulted in severe aortic coarctation in all the animals in the CoA group and was associated with a 65% increase in the LV weight to body weight ratio relative to the sham-operated group (p < 0.001). Hemodynamic and histologic studies showed an evolutionary pattern depending on duration of the experimental coarctation with a shift occurring at 30 d of coarctation. The initial response of cardiomyocytes to ventricular overload was hypertrophy of the myocytes, followed by myocyte hyperplasia. Compared with sham, there was an apparent decrease in the percentage of binucleated cells in the CoA group after 30 d of coarctation. The earliest response to LV pressure overload appears to occur at the molecular level. Indeed, sarcoendoplasmic reticulum calcium ATPase (SERCA 2a) mRNA levels fell significantly to only 28.6% of the sham group value (p = 0.023), independently of the duration of coarctation. In the fetal lamb, the pressure overload-induced hypertrophy resulting from progressive aortic coarctation leads to hemodynamic and lesional abnormalities and slows ontogenic maturation.

Sarcoplasmic reticulum Ca(2+)ATPase and cell contraction in developing rabbit heart

The purpose of the present study was to determine whether age-related changes in the expression and function of the cardiac isoform of the sarcoplasmic reticulum Ca(2+)-ATPase (SERCA2a) play a role in SR Ca(2+)release and cell contraction. SERCA2a protein levels and subcellular localization were compared between fetal, neonatal, juvenile and adult New Zealand White rabbits. Studies of SERCA function in isolated myocytes were performed in situ by examining the rate of reloading of the SR Ca(2+)stores following caffeine-induced depletion. We found that significant quantities of SERCA2a were present early in immature heart and that SERCA2a expression reached adult levels within 15-30 days after birth. Furthermore, SERCA2a protein is present as a series of transverse striations within the cell as early as 1 day of age. In contrast to previous studies of SERCA in vitro, the SERCA protein function in situ was found to be comparable between neonatal and adult myocytes in maintaining SR Ca(2+)stores. These results indicate that the paucity of SR Ca(2+)release in immature ventricular cardiac myocytes is not the result of immaturity in SERCA2a expression.

The expression of SR calcium transport ATPase and the Na(+)/Ca(2+)Exchanger are antithetically regulated during mouse cardiac development and in Hypo/hyperthyroidism

The mouse has been used extensively for generating transgenic animal models to study cardiovascular disease. Recently, a number of transgenic mouse models have been created to investigate the importance of sarcoplasmic reticulum (SR) Ca(2+)transport proteins in cardiac pathophysiology. However, the expression and regulation of cardiac SR Ca(2+)ATPase and other Ca(2+)transport proteins have not been studied in detail in the mouse. In this study, we used multiplex RNase mapping analysis to determine SERCA2, phospholamban (PLB), and Na(+)/Ca(2+)-exchanger (NCX-1) gene expression throughout mouse heart development and in hypo/hyperthyroid animals. Our results demonstrate that the expression of SERCA2 and PLB mRNA increase eight-fold from fetal to adult stages, indicating that SR function increases with heart development. In contrast, the expression of the Na(+)/Ca(2+)-exchanger gene is two-fold higher in fetal heart compared to adult. Our study also makes the important observation that in hypothyroidic hearts the NCX-1 mRNA and protein levels were upregulated, whereas the SERCA2 mRNA/protein levels were downregulated. In hyperthyroidic hearts, however, an opposite response was identified. These findings are important and point out that the expression of NCX-1 is regulated antithetically to that of SERCA2 during heart development and in response to alterations in thyroid hormone levels.

Intracellular Ca2+ oscillations drive spontaneous contractions in cardiomyocytes during early development

Activity of cardiac pacemaker cells is caused by a balanced interplay of ion channels. However, it is not known how the rhythmic beating is initiated during early stages of cardiomyogenesis, when the expression of ion channels is still incomplete. Based on the observation that early-stage embryonic stem cell-derived cardiomyocytes continuously contracted in high extracellular K+ solution, here we provide experimental evidence that the spontaneous activity of these cells is not generated by transmembrane ion currents, but by intracellular [Ca2+]i oscillations. This early activity was clearly independent of voltage dependent L-type Ca2+ channels and the interplay between these and ryanodine sensitive Ca2+ stores. We also show that intracellular Ca2+ oscillations evoke small membrane depolarizations and that these can trigger L-type Ca2+ channel driven action potentials.

Extracellular calcium concentration affects susceptibility to global ischemic injury in newborn but not adult hearts

BACKGROUND

Whether immaturity in calcium handling, that persists for a time after birth, could increase sensitivity to extracellular calcium and affect the development of global ischemic injury in the newborn heart is unknown. To address this, the impact of alterations in extracellular calcium concentration on newborn vs. adult development of myocardial injury due to ischemia was studied.

METHODS

In Study 1, hearts of 3-day-old piglets and adult pigs were perfused with 1 of 3 different calcium concentrations: control (0.13 mmol/L); intermediate (2.23 mmol/L); high (4.44 mmol/L) before normothermic ischemia. In Study 2, newborn hearts were allocated to perfusion with or without the L-calcium channel antagonist verapamil before high (4.44 mmol/L) calcium exposure, followed by normothermic ischemia. Tolerance to ischemia was assessed by determining the time to irreversible injury in all hearts, and maximal intraventricular pressures at peak injury.

RESULTS

In adults, altering calcium did not significantly affect tolerance to ischemia. In newborns, increasing calcium exposure resulted in significantly greater intraventricular pressures at maximal injury when compared with the control (low) calcium group (p<.05). As well, newborns exposed to high calcium had a significantly shorter time to the development of ischemic injury compared with the other groups (p<.05). Those newborn hearts pretreated with an L-calcium channel antagonist before the high calcium exposure did not exhibit this increased susceptibility to ischemic injury (p<.05).

CONCLUSIONS

In contrast to adults, the development of ischemic injury in the newborn heart is affected by changes in extracellular calcium, that can be modified with an L-calcium channel antagonist. This information could be used to prolong the safe preservation time of newborn donor hearts harvested for transplantation, as well as to minimize postoperative ventricular dysfunction.

Age-Dependent Changes in the Effects of Amiodarone on Rabbit Cardiac Myocyte Contractions

BACKGROUND: Intravenous amiodarone has increasingly been used to control life-threatening atrial and ventricular arrhythmias. In addition to its four antiarrhythmic properties, amiodarone may have complex effects on intracellular Ca(2+) stores and myocyte contractility. METHODS AND RESULTS: Contraction amplitude was recorded for cardiac ventricular myocytes isolated from neonatal and adult rabbits. Sarcoplasmic reticulum (SR) Ca(2+) stores were loaded to steady-state levels by a train of eight electric field stimulations. The SR Ca(2+) load was quantified by recording the contraction amplitude resulting from the complete depletion of SR Ca(2+) stores by exposing the cell to a 1-second pulse of 10 mmol/L caffeine. After the cells were exposed to 1 µmol/L amiodarone for 10 minutes, electrically stimulated contraction amplitudes significantly decreased in both adult and neonatal cells. Caffeine-induced cell contraction amplitudes were not affected by amiodarone in adult ventricular myocytes. By contrast, amiodarone markedly inhibited caffeine-induced contractions in neonatal ventricular myocytes. The inhibitory effect of amiodarone on the caffeine-induced contractions was not replicated by Ca(2+) channel blockade with diltiazem. CONCLUSIONS: Amiodarone markedly inhibits caffeine-induced contraction in neonatal myocytes but has no significant effect on adult myocytes. Ca(2+) influx through amiodarone-sensitive Ca(2+) channels may play a primary role in maintaining SR Ca(2+) stores in neonatal heart.

Functional characteristics of ES cell-derived cardiac precursor cells identified by tissue-specific expression of the green fluorescent protein

In contrast to terminally differentiated cardiomyocytes, relatively little is known about the characteristics of mammalian cardiac cells before the initiation of spontaneous contractions (precursor cells). Functional studies on these cells have so far been impossible because murine embryos of the corresponding stage are very small, and cardiac precursor cells cannot be identified because of the lack of cross striation and spontaneous contractions. In the present study, we have used the murine embryonic stem (ES, D3 cell line) cell system for the in vitro differentiation of cardiomyocytes. To identify the cardiac precursor cells, we have generated stably transfected ES cells with a vector containing the gene of the green fluorescent protein (GFP) under control of the cardiac alpha-actin promoter. First, fluorescent areas in ES cell-derived cell aggregates (embryoid bodies [EBs]) were detected 2 d before the initiation of contractions. Since Ca2+ homeostasis plays a key role in cardiac function, we investigated how Ca2+ channels and Ca2+ release sites were built up in these GFP-labeled cardiac precursor cells and early stage cardiomyocytes. Patch clamp and Ca2+ imaging experiments proved the functional expression of the L-type Ca2+ current (ICa) starting from day 7 of EB development. On day 7, using 10 mM Ca2+ as charge carrier, ICa was expressed at very low densities 4 pA/pF. The biophysical and pharmacological properties of ICa proved similar to terminally differentiated cardiomyocytes. In cardiac precursor cells, ICa was found to be already under control of cAMP-dependent phosphorylation since intracellular infusion of the catalytic subunit of protein kinase A resulted in a 1.7-fold stimulation. The adenylyl cyclase activator forskolin was without effect. IP3-sensitive intracellular Ca2+ stores and Ca2+-ATPases are present during all stages of differentiation in both GFP-positive and GFP-negative cells. Functional ryanodine-sensitive Ca2+ stores, detected by caffeine-induced Ca2+ release, appeared in most GFP-positive cells 1-2 d after ICa. Coexpression of both ICa and ryanodine-sensitive Ca2+ stores at day 10 of development coincided with the beginning of spontaneous contractions in most EBs. Thus, the functional expression of voltage-dependent L-type Ca2+ channel (VDCC) is a hallmark of early cardiomyogenesis, whereas IP3 receptors and sarcoplasmic Ca2+-ATPases are expressed before the initiation of cardiomyogenesis. Interestingly, the functional expression of ryanodine receptors/sensitive stores is delayed as compared with VDCC.

Cardiac-specific overexpression of mouse cardiac calsequestrin is associated with depressed cardiovascular function and hypertrophy in transgenic mice

Calsequestrin is a high capacity Ca2+-binding protein in the sarcoplasmic reticulum (SR) lumen. To elucidate the functional role of calsequestrin in vivo, transgenic mice were generated that overexpressed mouse cardiac calsequestrin in the heart. Overexpression (20-fold) of calsequestrin was associated with cardiac hypertrophy and induction of a fetal gene expression program. Isolated transgenic cardiomyocytes exhibited diminished shortening fraction (46%), shortening rate (60%), and relengthening rate (60%). The Ca2+ transient amplitude was also depressed (45%), although the SR Ca2+ storage capacity was augmented, as suggested by caffeine application studies. These alterations were associated with a decrease in L-type Ca2+ current density and prolongation of this channel's inactivation kinetics without changes in Na+-Ca2+ exchanger current density. Furthermore, there were increases in protein levels of SR Ca2+-ATPase, phospholamban, and calreticulin and decreases in FKBP12, without alterations in ryanodine receptor, junctin, and triadin levels in transgenic hearts. Left ventricular function analysis in Langendorff perfused hearts and closed-chest anesthetized mice also indicated depressed rates of contraction and relaxation of transgenic hearts. These findings suggest that calsequestrin overexpression is associated with increases in SR Ca2+ capacity, but decreases in Ca2+-induced SR Ca2+ release, leading to depressed contractility in the mammalian heart.

Phospholamban: protein structure, mechanism of action, and role in cardiac function

A comprehensive discussion is presented of advances in understanding the structure and function of phospholamban (PLB), the principal regulator of the Ca2+-ATPase of cardiac sarcoplasmic reticulum. Extensive historical studies are reviewed to provide perspective on recent developments. Phospholamban gene structure, expression, and regulation are presented in addition to in vitro and in vivo studies of PLB protein structure and activity. Applications of breakthrough experimental technologies in identifying PLB structure-function relationships and in defining its interaction with the Ca2+-ATPase are also highlighted. The current leading viewpoint of PLB's mechanism of action emerges from a critical examination of alternative hypotheses and the most recent experimental evidence. The potential physiological relevance of PLB function in human heart failure is also covered. The interest in PLB across diverse biochemical disciplines portends its continued intense scrutiny and its potential exploitation as a therapeutic target.

Postnatal changes in contractile time parameters, calcium regulatory proteins, and phosphatases

Compared with isolated electrically driven neonatal ventricular preparations, the total time of contraction, the time to peak tension, and the time of relaxation were decreased to approximately 50% in adult ventricular preparations. The expression of sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) was increased to 133% at the protein level and to 154% at the mRNA level in adult vs. neonatal ventricular preparations, whereas phospholamban was unchanged at both the protein and mRNA levels. Moreover, Ca2+ uptake was increased to 180% in adult vs. neonatal ventricular preparations. Phospholamban phosphorylation was enhanced in adult vs. neonatal ventricular preparations. In adult ventricular preparations, phosphatase activity was reduced to 53% of neonatal preparations, the protein levels of the immunologically detectable catalytic subunits of protein phosphatase types 1 and 2A were reduced to 28 and 61% of neonatal preparations, respectively, and the mRNA levels of type 1alpha, 1beta, 1gamma, 2Aalpha, and 2Abeta phosphatase isoforms were decreased to 69, 68, 54, 67, and 63%, respectively. We conclude that in the adult rat heart, the shortened time parameters of contraction can be explained by an elevated expression of SERCA. In addition, an increased phosphorylation state of phospholamban due to reduced phosphatase activity may be involved.

Complex formation between junctin, triadin, calsequestrin, and the ryanodine receptor. Proteins of the cardiac junctional sarcoplasmic reticulum membrane

Several key proteins have been localized to junctional sarcoplasmic reticulum which are important for Ca2+ release. These include the ryanodine receptor, triadin, and calsequestrin, which may associate into a stable complex at the junctional membrane. We recently purified and cloned a fourth component of this complex, junctin, which exhibits homology with triadin and is the major 125I-calsequestrin-binding protein detected in cardiac sarcoplasmic reticulum vesicles (Jones, L. R., Zhang, L., Sanborn, K., Jorgensen, A. O., and Kelley, J. (1995) J. Biol. Chem. 270, 30787-30796). In the present study, we have examined the binding interactions between the cardiac forms of these four proteins with emphasis placed on the role of junctin. By a combination of approaches including calsequestrin-affinity chromatography, filter overlay, immunoprecipitation assays, and fusion protein binding analyses, we find that junctin binds directly to calsequestrin, triadin, and the ryanodine receptor. This binding interaction is localized to the lumenal domain of junctin, which is highly enriched in charged amino acids organized into "KEKE" motifs. KEKE repeats are also found in the common lumenal domain of triadin, which likewise is capable of binding to calsequestrin and the ryanodine receptor (Guo, W., and Campbell, K. P. (1995) J. Biol. Chem. 270, 9027-9030). It appears that junctin and triadin interact directly in the junctional sarcoplasmic reticulum membrane and stabilize a complex that anchors calsequestrin to the ryanodine receptor. Taken together, these results suggest that junctin, calsequestrin, triadin, and the ryanodine receptor form a quaternary complex that may be required for normal operation of Ca2+ release.

Caffeine-induced contractions in developing rabbit heart

Mature myocardium utilizes calcium released by the sarcoplasmic reticulum (SR) for cell contraction. Transient exposure of mature myocytes to caffeine is known to directly trigger Ca2+ release from the SR. In contrast, neonatal rabbit heart cells rely on transsarcolemmal Ca2+ influx for tension generation. SR function is decreased in immature heart and appears to play a minimal role as a calcium source. Accordingly, we hypothesized that neonatal rabbit myocytes would not respond to a caffeine pulse. Isolated neonatal and adult myocytes were paced to load the SR with calcium and then exposed to a 1-s pulse of 10 mM caffeine. As previously described, adult myocytes exhibited a brisk contraction in response to caffeine. Unexpectedly, neonatal myocytes also exhibited a similar, brisk response. These caffeine-induced contractions were not dependent on extracellular Ca2+ but were dependent upon the loading of SR Ca2+ stores. When SR Ca2+ stores were depleted by exposure to caffeine, mature myocytes exhibited only small, slow contractions in response to electrical field stimulation. Replenishing the SR Ca2+ stores resulted in normal, brisk contractions. In contrast, electrically stimulated contractions in immature myocytes were largely unaffected by caffeine-induced SR depletion. Thus, although neonatal myocytes are capable of loading and releasing calcium from the SR, such SR calcium release is not normally required for contraction in the developing heart. The minor role of SR Ca2+ release in immature rabbit heart may not result from immaturity of the SR, but rather from an inadequate mechanism to trigger SR calcium release.

Contribution of the sodium-calcium exchanger to contractions in immature rabbit ventricular myocytes

In immature cardiac myocytes, the sarcoplasmic reticulum is sparse. Thus, we hypothesized that sarcolemmal Ca2+ influx through Na(+)-Ca2+ exchange is the dominant mechanism for modulating intracellular Ca2+ during contractions in fetal and neonatal hearts. We measured Na(+)-Ca2+ exchange currents in neonatal and adult rabbit ventricular cells using a rapid solution switch into 0 mM external Na+. The current densities (mean +/- SEM) were larger in 8 neonatal cells than in 10 adult cells (5.4 +/- 1.38 versus 1.65 +/- 0.25 pA/pF). Intracellular Ca2+ transients after inhibiting the sarcoplasmic reticulum with ryanodine and thapsigargin were unchanged in 15 neonatal cells, but decreased in 15 adult cells to 78.9 +/- 5.6% of baseline. When the Ca2+ channels were also inhibited by adding nifedipine, Ca2+ transients from Na(+)-Ca2+ exchange were 30.0 +/- 3.5% of baseline in neonatal cells compared with 13.4 +/- 3.4% in adult cells. Simultaneous contractions were a larger percent of baseline in neonatal cells (85.7.6 +/- 6.4%) than in adult cells (78.9 +/- 5.6%) after inhibiting the sarcoplasmic reticulum, and were unmeasureable in many cells from both age groups after inhibiting the Ca2+ channels as well. The ratio of Na(+)-Ca2+ exchanger mRNA to sarcoplasmic reticulum Ca(2+)-ATPase mRNA levels decreased from 1.0 +/- 0.13 to 0.4 +/- 0.03 to 0.26 +/- 0.02 in fetal, neonatal and adult ventricles, respectively. These measurements were consistent with a dominant role for the Na(+)-Ca2+ exchanger in the immature heart.

Calcium homeostasis and control of contractility in the developing heart

The Ca2+ concentration within the myocyte is an important determinant of myocardial contractility. Substantial changes in the cellular processes responsible for transport of Ca2+ ions across the sarcolemmal and sarcoplasmic reticulum membranes occur during maturation of the heart. In this article, the mechanisms underlying these changes and their impact on myocardial performance are discussed in detail.

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.

Beta 2-adrenergic receptor actions in neonatal and adult rat ventricular myocytes

The physiological function of beta 2-adrenergic receptors in the neonatal and adult heart is incompletely understood, and possible age-dependent differences in beta 2-receptor actions have not been considered. We used isoproterenol (mixed beta 1- and beta 2-receptor agonist) and zinterol (beta 2-selective agonist) to compare beta-receptor subtype actions in neonatal and adult rat ventricular myocytes. When delivered as a bolus at a final concentration of 10(-7) mol/L, both isoproterenol and zinterol increased the amplitude and hastened the kinetics of the calcium and cell-shortening transients in neonatal myocytes. Under identical experimental conditions, isoproterenol increased the amplitude and accelerated the kinetics of the calcium transient and the twitch in adult myocytes, whereas zinterol did not. In the presence of CGP 20712A (beta 1-receptor blocker), a 100-fold higher concentration of zinterol increased the amplitude but prolonged the duration of the twitch in adult myocytes. To probe the mechanism for this age-dependent difference in beta 2-receptor responsiveness, we compared beta-receptor expression and stimulation of cAMP accumulation in neonatal and adult myocytes. beta-Receptor density was 44,339 +/- 5178 sites per cell in neonatal myocytes and 186,346 +/- 13,356 sites per cell in adult myocytes; the relative proportion of beta 2-receptors was comparable in each (16.7 +/- 2.3% and 16.9 +/- 0.9%, respectively). Isoproterenol induced a large increase in cAMP accumulation in neonatal and adult myocytes (20.0 +/- 1.0- and 20.6 +/- 2.6-fold over basal). In contrast, zinterol evoked a substantial increase in cAMP accumulation in neonatal myocytes but only a minor increase in adult myocytes. These studies provide evidence that at low agonist concentrations, beta 2-receptor activation contributes to the positive inotropic response by increasing cAMP and increasing the amplitude and hastening the kinetics of the twitch in neonatal, but not adult, myocytes. Moreover, these results suggest that age-dependent differences in beta 2-receptor coupling to more distal elements in the signaling cascade can influence myocyte beta 2-receptor responsiveness.

Developmental and tissue-specific regulation of rabbit skeletal and cardiac muscle calcium channels involved in excitation-contraction coupling

Two types of calcium channels signal excitation-contraction (E-C) coupling in striated muscle: dihydropyridine receptors (DHPRs, voltage-gated L-type calcium channels on the transverse tubule) and ryanodine receptors (RyRs, calcium release channels on the sarcoplasmic reticulum). Sarcolemmal depolarization activates the DHPR; subsequently, the RyR is activated and releases calcium that activates muscle contraction. We show in the present study that expression of the E-C coupling calcium channels is upregulated during myogenic development in the rabbit. Skeletal and cardiac muscle isoforms of the following genes were examined: the DHPR alpha 1, alpha 2, beta, and gamma subunits and the RyR. Distinct cardiac and skeletal muscle-specific cDNAs were isolated, encoding each of the DHPR subunits and the RyR. The skeletal muscle DHPR alpha 1, alpha 2, beta, and gamma subunits and the cardiac DHPR alpha 1 subunit mRNA levels increased on the day of birth and at the adult stage compared with fetal levels. The skeletal and cardiac RyR mRNA levels increased on the day of birth and at adult stages compared with fetal levels. Ryanodine binding sites increased in both skeletal and cardiac muscle. We now provide a molecular explanation for the physiological "maturation" of the E-C coupling apparatus observed at the day of birth and during early postnatal development in both skeletal and cardiac muscles. Low levels of calcium channel expression in fetal cardiac and skeletal muscle make these tissues more sensitive to pharmacological therapy with calcium channel blockers, a phenomenon that has been reported in human neonates.

Ca(2+)-transporting ATPase, phospholamban, and calsequestrin levels in nonfailing and failing human myocardium

BACKGROUND

Observations of abnormalities in the diastolic components of intracellular Ca2+ transients in failing human left ventricular myocardium have raised the possibility that reductions in the level or function of sarcoplasmic reticulum proteins involved in Ca2+ transport contribute to the pathophysiology of dilated cardiomyopathy in humans. Functional assays, however, have revealed no differences in ATP-dependent Ca2+ transport or its modulation by phospholamban in sarcoplasmic reticulum-enriched microsomes prepared from nonfailing and failing human left ventricular myocardium. The purpose of the present study was to quantify protein levels of Ca(2+)-transporting ATPase, phospholamban, and calsequestrin directly in nonfailing and failing human left ventricular myocardium.

METHOD AND RESULTS

Total protein extracts were prepared from nonfailing left ventricular myocardium from the hearts of unmatched organ donors with normal left ventricular contractility (n = 6) and from failing left ventricular myocardium from the excised hearts of transplant recipients with class IV heart failure resulting from idiopathic dilated cardiomyopathy (n = 6). Ca(2+)-transporting ATPase, phospholamban, and calsequestrin contents were determined by quantitative immunoblotting with monoclonal and affinity-purified polyclonal antibodies. The levels of the three proteins were identical in nonfailing and failing human left ventricular myocardium.

CONCLUSIONS

These results indicate that protein levels of Ca(2+)-transporting ATPase, phospholamban, and calsequestrin are not diminished in failing human left ventricular myocardium and that downregulation of the Ca(2+)-transporting ATPase and phospholamban is not part of the molecular pathophysiology of dilated cardiomyopathy in humans.

Sarcoplasmic reticulum calsequestrins: structural and functional properties

Calsequestrin is the major Ca(2+)-binding protein localized in the terminal cisternae of the sarcoplasmic reticulum (SR) of skeletal and cardiac muscle cells. Calsequestrin has been purified and cloned from both skeletal and cardiac muscle in mammalian, amphibian, and avian species. Two different calsequestrin gene products namely cardiac and fast have been identified. Fast and cardiac calsequestrin isoforms have a highly acidic amino acid composition. The amino acid composition of the cardiac form is very similar to the skeletal form except for the carboxyl terminal region of the protein which possess variable length of acidic residues and two phosphorylation sites. Circular dichroism and NMR studies have shown that calsequestrin increases its alpha-helical content and the intrinsic fluorescence upon binding of Ca2+. Calsequestrin binds Ca2+ with high-capacity and with moderate affinity and it functions as a Ca2+ storage protein in the lumen of the SR. Calsequestrin has been found to be associated with the Ca2+ release channel protein complex of the SR through protein-protein interactions. The human and rabbit fast calsequestrin genes have been cloned. The fast gene is skeletal muscle specific and transcribed at different rates in fast and slow skeletal muscle but not in cardiac muscle. We have recently cloned the rabbit cardiac calsequestrin gene. Heart expresses exclusively the cardiac calsequestrin gene. This gene is also expressed in slow skeletal muscle. No change in calsequestrin mRNA expression has been detected in animal models of cardiac hypertrophy and in failing human heart.

Contractile arrest increases sarcoplasmic reticulum calcium uptake and SERCA2 gene expression in cultured neonatal rat heart cells

We developed protocols with intact cultured neonatal rat myocytes to directly evaluate the function of the sarcoplasmic reticulum (SR) Ca-ATPase (or SERCA2), Na-Ca exchange (Na-CaX), and slow Ca transport systems (mitochondria and sarcolemmal Ca-ATPase). Spontaneously beating control cells were compared with cells cultured for 2 days in the presence of verapamil (verapamil-arrested cells, VA). Intracellular calcium (Cai) transients were measured by use of indo-1 during (1) spontaneous twitches, (2) contractures induced by rapid application of caffeine (CafC, with and without Nao), and (3) twitches induced by brief depolarizations with high [K]o solution (K-twitches). We also measured mRNA levels for the SR Ca-ATPase and Na-CaX in the same experimental preparations. The t1/2 for [Ca]i decline when both the SR Ca uptake and Na-CaX were prevented was the same for control and VA cells (approximately 20 seconds), indicating unaltered slow Ca transport systems. Similarly, there was no significant difference in the t1/2 of CafC when Na-CaX was the main mechanism responsible for [Ca]i decline (t1/2 approximately 1.5 seconds), indicating unaltered Na-CaX. Conversely, we found nearly a twofold increase in the rate of [Ca]i decline during K-twitches (control t1/2, 0.84 +/- 0.05 seconds; VA t1/2, 0.48 +/- 0.06 second; P < .001), indicating an increase in SR Ca-pumping activity in VA cells. This was also reflected by a 56% increase in the peak [Ca]i reached during CafC used to assess maximal SR Ca content (427 +/- 49 nmol/L in control versus 665 +/- 75 nmol/L in VA cells).(ABSTRACT TRUNCATED AT 250 WORDS)