Control of maximum sarcoplasmic reticulum Ca load in intact ferret ventricular myocytes. Effects Of thapsigargin and isoproterenol

Oxidative stress in early metabolic syndrome impairs cardiac RyR2 and SERCA2a activity and modifies the interplay of these proteins during Ca2+ waves

We investigated how oxidative stress (OS) alters Ca2+ handling in ventricular myocytes in early metabolic syndrome (MetS) in sucrose-fed rats. The effects of N-acetyl cysteine (NAC) or dl-Dithiothreitol (DTT) on systolic Ca2+ transients (SCaTs), diastolic Ca2+ sparks (CaS) and Ca2+ waves (CaW), recorded by confocal techniques, and L-type Ca2+ current (ICa), assessed by whole-cell patch clamp, were evaluated in MetS and Control cells. MetS myocytes exhibited decreased SCaTs and CaS frequency but unaffected CaW propagation. In Control cells, NAC/DTT reduced RyR2/SERCA2a activity blunting SCaTs, CaS frequency and CaW propagation, suggesting that basal ROS optimised Ca2+ signalling by maintaining RyR2/SERCA2a function and that these proteins facilitate CaW propagation. Conversely, NAC/DTT in MetS recovered RyR2/SERCA2a function, improving SCaTs and CaS frequency, but unexpectedly decreasing CaW propagation. We hypothesised that OS decreases RyR2/SERCA2a activity at early MetS, and while decreased SERCA2a favours CaW propagation, diminished RyR2 restrains it.

Relative role of T-tubules disruption and decreased SERCA2 on contractile dynamics of isolated rat ventricular myocytes

AIMS

Ventricular myocytes (VM) depolarization activates L-type Ca2+ channels (LCC) allowing Ca2+ influx (ICa) to synchronize sarcoplasmic reticulum (SR) Ca2+ release, via Ca2+-release channels (RyR2). The resulting whole-cell Ca2+ transient triggers contraction, while cytosolic Ca2+ removal by SR Ca2+ pump (SERCA2) and sarcolemmal Na+/Ca2+ exchanger (NCX) allows relaxation. In diseased hearts, extensive VM remodeling causes heterogeneous, blunted and slow Ca2+ transients. Among remodeling changes are: A) T-tubules disorganization. B) Diminished SERCA2 and low SR Ca2+. However, those often overlap, hindering their relative contribution to contractile dysfunction (CD). Furthermore, few studies have assessed their specific impact on the spatiotemporal Ca2+ transient properties and contractile dynamics simultaneously. Therefore, we sought to perform a quantitative comparison of how heterogeneous and slow Ca2+ transients, with different underlying determinants, affect contractile performance.

METHODS

We used two experimental models: A) formamide-induced acute "detubulation", where VM retain functional RyR2 and SERCA2, but lack T-tubules-associated LCC and NCX. B) Intact VM from hypothyroid rats, presenting decreased SERCA2 and SR Ca2+, but maintained T-tubules. By confocal imaging of Fluo-4-loaded VM, under field-stimulation, simultaneously acquired Ca2+ transients and shortening, allowing direct correlations.

KEY FINDINGS

We found near-linear correlations among key parameters of altered Ca2+ transients, caused independently by T-tubules disruption or decreased SR Ca2+, and shortening and relaxation, SIGNIFICANCE: Unrelated structural and molecular alterations converge in similarly abnormal Ca2+ transients and CD, highlighting the importance of independently reproduce disease-specific alterations, to quantitatively assess their impact on Ca2+ signaling and contractility, which would be valuable to determine potential disease-specific therapeutic targets.

Warmer, faster, stronger: Ca2+ cycling in avian myocardium

Birds occupy a unique position in the evolution of cardiac design. Their hearts are capable of cardiac performance on par with, or exceeding that of mammals, and yet the structure of their cardiomyocytes resembles those of reptiles. It has been suggested that birds use intracellular Ca2+ stored within the sarcoplasmic reticulum (SR) to power contractile function, but neither SR Ca2+ content nor the cross-talk between channels underlying Ca2+-induced Ca2+ release (CICR) have been studied in adult birds. Here we used voltage clamp to investigate the Ca2+ storage and refilling capacities of the SR and the degree of trans-sarcolemmal and intracellular Ca2+ channel interplay in freshly isolated atrial and ventricular myocytes from the heart of the Japanese quail (Coturnix japonica). A trans-sarcolemmal Ca2+ current (ICa) was detectable in both quail atrial and ventricular myocytes, and was mediated only by L-type Ca2+ channels. The peak density of ICa was larger in ventricular cells than in atrial cells, and exceeded that reported for mammalian myocardium recorded under similar conditions. Steady-state SR Ca2+ content of quail myocardium was also larger than that reported for mammals, and reached 750.6±128.2 μmol l-1 in atrial cells and 423.3±47.2 μmol l-1 in ventricular cells at 24°C. We observed SR Ca2+-dependent inactivation of ICa in ventricular myocytes, indicating cross-talk between sarcolemmal Ca2+ channels and ryanodine receptors in the SR. However, this phenomenon was not observed in atrial myocytes. Taken together, these findings help to explain the high-efficiency avian myocyte excitation-contraction coupling with regard to their reptilian-like cellular ultrastructure.

Simultaneous assessment of calcium handling and contractility dynamics in isolated ventricular myocytes of a rat model of post-acute isoproterenol-induced cardiomyopathy

Stress-induced cardiomyopathy (SIC) results from a profound catecholaminergic surge during strong emotional or physical stress. SIC is characterized by acute left ventricular apex hypokinesia, in the absence of coronary arteries occlusion, and can lead to arrhythmias and acute heart failure. Although, most SIC patients recover, the process could be slow, and recurrence or death may occur. Despite that the SIC common denominator is a large catecholamine discharge, the pathophysiological mechanism is incompletely understood. It is thought that catecholamines have direct cytotoxicity on apical ventricular myocytes (VM), which have the highest β-adrenergic receptors density, and whose overstimulation might cause acute Ca²⁺ overload and oxidative stress, causing death in some VM and stunning others. Rodents receiving acute isoproterenol (ISO) overdose (OV) mimic SIC development, however, they have not been used to simultaneously assess Ca²⁺ handling and contractility status in isolated VM, which might explain ventricular hypokinesia. Therefore, treating rats with a single ISO-OV (67 mg/kg body weight), we sought out to characterize, with confocal imaging, Ca²⁺ and shortening dynamics in Fluo-4-loaded VM, during the early (1-5 days) and late post-acute phases (15 days). We found that ISO-OV VM showed contractile dysfunction; blunted shortening with slower force development and relaxation. These correlated with Ca²⁺ mishandling; blunted Ca²⁺ transient, with slower time to peak and SR Ca²⁺ recovery. SR Ca²⁺ content was low, nevertheless, diastolic Ca²⁺ sparks were more frequent, and their duration increased. Contractility and Ca²⁺ dysfunction aggravated or remained altered over time, explaining slow recovery. We conclude that diminished VM contractility is the main determinant of ISO-OV hypokinesia and is mostly related to Ca²⁺ mishandling.

Unique Ca2+-Cycling Protein Abundance and Regulation Sustains Local Ca2+ Releases and Spontaneous Firing of Rabbit Sinoatrial Node Cells

Spontaneous beating of the heart pacemaker, the sinoatrial node, is generated by sinoatrial node cells (SANC) and caused by gradual change of the membrane potential called diastolic depolarization (DD). Submembrane local Ca²⁺ releases (LCR) from sarcoplasmic reticulum (SR) occur during late DD and activate an inward Na⁺/Ca²⁺ exchange current, which accelerates the DD rate leading to earlier occurrence of an action potential. A comparison of intrinsic SR Ca²⁺ cycling revealed that, at similar physiological Ca²⁺ concentrations, LCRs are large and rhythmic in permeabilized SANC, but small and random in permeabilized ventricular myocytes (VM). Permeabilized SANC spontaneously released more Ca²⁺ from SR than VM, despite comparable SR Ca²⁺ content in both cell types. In this review we discuss specific patterns of expression and distribution of SR Ca²⁺ cycling proteins (SR Ca²⁺ ATPase (SERCA2), phospholamban (PLB) and ryanodine receptors (RyR)) in SANC and ventricular myocytes. We link ability of SANC to generate larger and rhythmic LCRs with increased abundance of SERCA2, reduced abundance of the SERCA inhibitor PLB. In addition, an increase in intracellular [Ca²⁺] increases phosphorylation of both PLB and RyR exclusively in SANC. The differences in SR Ca²⁺ cycling protein expression between SANC and VM provide insights into diverse regulation of intrinsic SR Ca²⁺ cycling that drives automaticity of SANC.

Differences in the regulation of RyR2 from human, sheep, and rat by Ca²⁺ and Mg²⁺ in the cytoplasm and in the lumen of the sarcoplasmic reticulum

Cardiac ryanodine receptors (RyR2) from humans, rats, and sheep show differential sensitivity to calcium and magnesium, with regulation of human RyR2 resembling that of sheep more than that of rat.

Regulation of the cardiac ryanodine receptor (RyR2) by intracellular Ca²⁺ and Mg²⁺ plays a key role in determining cardiac contraction and rhythmicity, but their role in regulating the human RyR2 remains poorly defined. The Ca²⁺- and Mg²⁺-dependent regulation of human RyR2 was recorded in artificial lipid bilayers in the presence of 2 mM ATP and compared with that in two commonly used animal models for RyR2 function (rat and sheep). Human RyR2 displayed cytoplasmic Ca²⁺ activation (K_a = 4 µM) and inhibition by cytoplasmic Mg²⁺ (K_i = 10 µM at 100 nM Ca²⁺) that was similar to RyR2 from rat and sheep obtained under the same experimental conditions. However, in the presence of 0.1 mM Ca²⁺, RyR2s from human were 3.5-fold less sensitive to cytoplasmic Mg²⁺ inhibition than those from sheep and rat. The K_a values for luminal Ca²⁺ activation were similar in the three species (35 µM for human, 12 µM for sheep, and 10 µM for rat). From the relationship between open probability and luminal [Ca²⁺], the peak open probability for the human RyR2 was approximately the same as that for sheep, and both were ∼10-fold greater than that for rat RyR2. Human RyR2 also showed the same sensitivity to luminal Mg²⁺ as that from sheep, whereas rat RyR2 was 10-fold more sensitive. In all species, modulation of RyR2 gating by luminal Ca²⁺ and Mg²⁺ only occurred when cytoplasmic [Ca²⁺] was <3 µM. The activation response of RyR2 to luminal and cytoplasmic Ca²⁺ was strongly dependent on the Mg²⁺ concentration. Addition of physiological levels (1 mM) of Mg²⁺ raised the K_a for cytoplasmic Ca²⁺ to 30 µM (human and sheep) or 90 µM (rat) and raised the K_a for luminal Ca²⁺ to ∼1 mM in all species. This is the first report of the regulation by Ca²⁺ and Mg²⁺ of native RyR2 receptor activity from healthy human hearts.

The Sarcoplasmic Reticulum and the Evolution of the Vertebrate Heart

The sarcoplasmic reticulum (SR) is crucial for contraction and relaxation of the mammalian cardiomyocyte, but its role in other vertebrate classes is equivocal. Recent evidence suggests differences in SR function across species may have an underlying structural basis. Here, we discuss how SR recruitment relates to the structural organization of the cardiomyocyte to provide new insight into the evolution of cardiac design and function in vertebrates.

Cardiac sarcoplasmic reticulum calcium leak: basis and roles in cardiac dysfunction

Synchronized SR calcium (Ca) release is critical to normal cardiac myocyte excitation-contraction coupling, and ideally this release shuts off completely between heartbeats. However, other SR Ca release events are referred to collectively as SR Ca leak (which includes Ca sparks and waves as well as smaller events not detectable as Ca sparks). Much, but not all, of the SR Ca leak occurs via ryanodine receptors and can be exacerbated in pathological states such as heart failure. The extent of SR Ca leak is important because it can (a) reduce SR Ca available for release, causing systolic dysfunction; (b) elevate diastolic [Ca]i, contributing to diastolic dysfunction; (c) cause triggered arrhythmias; and (d) be energetically costly because of extra ATP used to repump Ca. This review addresses quantitative aspects and manifestations of SR Ca leak and its measurement, and how leak is modulated by Ca, associated proteins, and posttranslational modifications in health and disease.

Proteins within the intracellular calcium store determine cardiac RyR channel activity and cardiac output

SUMMARY

The contractile function of the heart requires the release of Ca(2+) from intracellular Ca(2+) stores in the sarcoplasmic reticulum (SR) of cardiac muscle cells. The efficacy of Ca(2+) release depends on the amount of Ca(2+) loaded into the Ca(2+) store and the way in which this 'Ca(2+) load' influences the activity of the cardiac ryanodine receptor Ca(2+) release channel (RyR2). The effects of the Ca(2+) load on Ca(2+) release through RyR2 are facilitated by: (i) the sensitivity of RyR2 itself to luminal Ca(2+) concentrations; and (ii) interactions between the cardiac Ca(2+) -binding protein calsequestrin (CSQ) 2 and RyR2, transmitted through the 'anchoring' proteins junctin and/or triadin. Mutations in RyR2 are linked to catecholaminergic polymorphic ventricular tachycardia (CPVT) and sudden cardiac death. The tachycardia is associated with changes in the sensitivity of RyR2 to luminal Ca(2+) . Triadin-, junctin- or CSQ-null animals survive, but their longevity and ability to tolerate stress is compromised. These studies reveal the importance of the proteins in normal muscle function, but do not reveal the molecular nature of their functional interactions, which must be defined before changes in the proteins leading to CPVT and heart disease can be understood. Herein, we discuss known interactions between the RyR, triadin, junctin and CSQ with emphasis on the cardiac isoforms of the proteins. Where there is little known about the cardiac isoforms, we discuss evidence from skeletal isoforms.

Role of inositol 1,4,5-trisphosphate in the regulation of ventricular Ca(2+) signaling in intact mouse heart

Inositol 1,4,5-trisphosphate (InsP(3)R)-mediated Ca(2+) signaling is a major pathway regulating multiple cellular functions in excitable and non-excitable cells. Although InsP(3)-mediated Ca(2+) signaling has been extensively described, its influence on ventricular myocardium activity has not been addressed in contracting hearts at the whole-organ level. In this work, InsP(3)-sensitive intracellular Ca(2+) signals were studied in intact hearts using laser scanning confocal microscopy and pulsed local-field fluorescence microscopy. Intracellular [InsP(3)] was rapidly increased by UV flash photolysis of membrane-permeant caged InsP(3). Our results indicate that the basal [Ca(2+)] increased after the flash photolysis of caged InsP(3) without affecting the action potential (AP)-induced Ca(2+) transients. The amplitude of the basal [Ca(2+)] elevation depended on the intracellular [InsP(3)] reached after the UV flash. Pretreatment with ryanodine failed to abolish the InsP(3)-induced Ca(2+) release (IICR), indicating that this response was not mediated by ryanodine receptors (RyR). Thapsigargin prevented Ca(2+) release from both RyR- and InsP(3)R-containing Ca(2+) stores, suggesting that these pools have similar Ca(2+) reuptake mechanisms. These results were reproduced in acutely isolated cells where photorelease of InsP(3) was able to induce changes in endothelial cells but not in AP-induced transients from cardiomyocytes. Taken together, these results suggest that IICR does not directly regulate cardiac excitation-contraction coupling. To our knowledge, this is the first demonstration of IICR in intact hearts. Consequently, our work provides a reference framework of the spatiotemporal attributes of the IICR under physiological conditions.

CaMKIIδC slows [Ca]i decline in cardiac myocytes by promoting Ca sparks

Acute activation of calcium/calmodulin-dependent protein kinase (CaMKII) in permeabilized phospholamban knockout (PLN-KO) mouse myocytes phosphorylates ryanodine receptors (RyRs) and activates spontaneous local sarcoplasmic reticulum (SR) Ca release events (Ca sparks) even at constant SR Ca load. To assess how CaMKII regulates SR Ca release in intact myocytes (independent of SR Ca content changes or PLN effects), we compared Ca sparks in PLN-KO versus mice, which also have transgenic cardiac overexpression of CaMKIIδC in the PLN-KO background (KO/TG). Compared with PLN-KO mice, these KO/TG cardiomyocytes exhibited 1), increased twitch Ca transient and fractional release (both by ∼35%), but unaltered SR Ca load; 2), increased resting Ca spark frequency (300%) despite a lower diastolic [Ca]i, which also slowed twitch [Ca]i decline (suggesting CaMKII-dependent RyR Ca sensitization); 3), elevated Ca spark amplitude and rate of Ca release (which might indicate that more RyR channels participate in a single spark); 4), prolonged Ca spark rise time (which implies that CaMKII either delays RyR closure or prolongs the time when openings can occur); 5), more frequent repetitive sparks at single release sites. Analysis of repetitive sparks from individual Ca release sites indicates that CaMKII enhanced RyR Ca sensitivity, but did not change the time course of SR Ca refilling. These results demonstrate that there are dramatic CaMKII-mediated effects on RyR Ca release that occur via regulation of both RyR activation and termination processes.

Changes of SERCA activity have only modest effects on sarcoplasmic reticulum Ca2+ content in rat ventricular myocytes

Changes of the activity of the sarco-endoplasmic reticulum Ca(2+)-ATPase (SERCA) affect the amplitude of the systolic Ca(2+) transient and thence cardiac contractility. This is thought to be due to alterations of SR Ca(2+) content. Recent work on mice in which the expression of SERCA is decreased found that a large reduction of SERCA expression resulted in a proportionately much smaller decrease of SR Ca(2+) content. The aim of the current work was to investigate the quantitative nature of the dependence of both the amplitude of the systolic Ca(2+) transient and SR Ca(2+) content on SERCA activity during acute partial inhibition of SERCA. Experiments were performed on rat ventricular myocytes. Brief application of thapsigargin (1 μm) resulted in a decrease of SERCA activity as measured from the rate of decay of the systolic Ca(2+) transient. This was accompanied by a decrease in the amplitude of the systolic Ca(2+) transient which was linearly related to that of SERCA activity. However, the fractional decrease in the SR Ca(2+) content was much less than that of SERCA activity. On average SR Ca(2+) content was proportional to SERCA activity raised to the 0.38 ± 0.07 power. This shallow dependence of SR content on SERCA activity arises because Ca(2+) release is a steep function of SR Ca(2+) content. In contrast SR Ca(2+) content was increased 4.59 ± 0.40 (n = 8)-fold by decreasing ryanodine receptor opening with tetracaine (1 mm). Therefore a modest decrease of SR Ca(2+) content results in a proportionately larger fall of Ca(2+) release from the SR which can balance a larger initiating decrease of SERCA. In conclusion, the shallow dependence of SR Ca(2+) content on SERCA activity is expected for a system in which small changes of SR Ca(2+) content produce larger effects on the amplitude of the systolic Ca(2+) transient.

Luminal Mg2+, a key factor controlling RYR2-mediated Ca2+ release: cytoplasmic and luminal regulation modeled in a tetrameric channel

In cardiac muscle, intracellular Ca(2+) and Mg(2+) are potent regulators of calcium release from the sarcoplasmic reticulum (SR). It is well known that the free [Ca(2+)] in the SR ([Ca(2+)](L)) stimulates the Ca(2+) release channels (ryanodine receptor [RYR]2). However, little is known about the action of luminal Mg(2+), which has not been regarded as an important regulator of Ca(2+) release. The effects of luminal Ca(2+) and Mg(2+) on sheep RYR2 were measured in lipid bilayers. Cytoplasmic and luminal Ca(2+) produced a synergistic increase in the opening rate of RYRs. A novel, high affinity inhibition of RYR2 by luminal Mg(2+) was observed, pointing to an important physiological role for luminal Mg(2+) in cardiac muscle. At diastolic [Ca(2+)](C), luminal Mg(2+) inhibition was voltage independent, with K(i) = 45 microM at luminal [Ca(2+)] ([Ca(2+)](L)) = 100 microM. Luminal and cytoplasmic Mg(2+) inhibition was alleviated by increasing [Ca(2+)](L) or [Ca(2+)](C). Ca(2+) and Mg(2+) on opposite sides of the bilayer exhibited competitive effects on RYRs, indicating that they can compete via the pore for common sites. The data were accurately fitted by a model based on a tetrameric RYR structure with four Ca(2+)-sensing mechanisms on each subunit: activating luminal L-site (40-microM affinity for Mg(2+) and Ca(2+)), cytoplasmic A-site (1.2 microM for Ca(2+) and 60 microM for Mg(2+)), inactivating cytoplasmic I(1)-site (approximately 10 mM for Ca(2+) and Mg(2+)), and I(2)-site (1.2 microM for Ca(2+)). Activation of three or more subunits will cause channel opening. Mg(2+) inhibition occurs primarily by Mg(2+) displacing Ca(2+) from the L- and A-sites, and Mg(2+) fails to open the channel. The model predicts that under physiological conditions, SR load-dependent Ca(2+) release (1) is mainly determined by Ca(2+) displacement of Mg(2+) from the L-site as SR loading increases, and (2) depends on the properties of both luminal and cytoplasmic activation mechanisms.

Ca2+ stores regulate ryanodine receptor Ca2+ release channels via luminal and cytosolic Ca2+ sites

1. In muscle, intracellular calcium concentration, hence skeletal muscle force and cardiac output, is regulated by uptake and release of calcium from the sarcoplasmic reticulum. The ryanodine receptor (RyR) forms the calcium release channel in the sarcoplasmic reticulum. 2. The free [Ca2+] in the sarcoplasmic reticulum regulates the excitability of this store by stimulating the Ca2+ release channels in its membrane. This process involves Ca2+-sensing mechanisms on both the luminal and cytoplasmic sides of the RyR. In the cardiac RyR, these have been shown to be a luminal Ca2+ activation site (L-site; 60 micromol/L affinity), a cytoplasmic activation site (A-site; 0.9 micromol/L affinity) and a cytoplasmic Ca2+ inactivation site (I2-site; 1.2 micromol/L affinity). 3. Cardiac RyR activation by luminal Ca2+ occurs by a multistep process dubbed 'luminal-triggered Ca2+ feed-through'. Binding of Ca2+ to the L-site initiates brief (1 msec) openings at a rate of up to 10/s. Once the pore is open, luminal Ca2+ has access to the A-site (producing up to 30-fold prolongation of openings) and to the I2-site (causing inactivation at high levels of Ca2+ feed-through). 4. The present paper reviews the evidence for the principal aspects of the 'luminal-triggered Ca2+ feed-through' model, the properties of the various Ca2+-dependent gating mechanisms and their likely role in controlling sarcoplasmic reticulum (SR) Ca2+ release in cardiac muscle. 5. The model makes the following important predictions: (i) there will be a close link between luminal and cytoplasmic regulation of RyRs and any cofactor that prolongs channel openings triggered by cytoplasmic Ca2+ will also promote RyR activation by luminal Ca2+; (ii) luminal Mg2+ (1 mmol/L) is essential for the control of SR excitability in cardiac muscle by luminal Ca2+; and (iii) the different RyR isoforms in skeletal and cardiac muscle will be controlled quite differently by the luminal milieu. For example, Mg2+ in the SR lumen (approximately 1 mmol/L) can strongly inhibit RyR2 by competing with Ca2+ for the L-site, whereas RyR1 is not affected by luminal Mg2+.

Identification of a switching model of calcium cycling in isolated rat hearts

So far, the processes involved in regulation of intracellular calcium (Ca/sub i//sup 2+/) in cardiomyocytes have been mainly studied through biochemical and isolated cell analysis. Here, we present a novel technique to model and identify cardiac Ca/sub i//sup 2+/-cycling under physiologically relevant conditions in the intact beating heart. Ca/sub i//sup 2+/ was measured using fluorescence techniques in ex vivo perfused rat hearts. For analysis, we developed a parametric mathematical model, switching between active and inactive calcium release. The kinetic parameters of the two submodes of the model were computed using a recently developed technique from hybrid system identification. Application of the method to control and isoproterenol-stimulated hearts resulted in parameter values within a physiologically reliable range.

Frequency-dependent contractile strength in mice over- and underexpressing the sarco(endo)plasmic reticulum calcium-ATPase

One of the prominent markers of end-stage heart failure at the molecular level is a decrease in function and/or expression of the sarcoplasmic reticulum ATPase protein [sarco(endo)plasmic reticulum calcium-ATPase, SERCA]. It has been often postulated that a decrease in SERCA pump activity can contribute in a major way to decreased cardiac function. To establish a functional relationship, we assessed how alterations in SERCA activity level affect basic contractile function in healthy myocardium devoid of other significant molecular changes. We investigated baseline contractile function, frequency-dependent activation, and beta-adrenergic response in ultrathin trabeculae isolated from hearts of mice overexpressing SERCA (transgenic, TG), underexpressing SERCA2a (heterozygous knockout, Het), and their respective wild-type (WT) littermates. At physiological temperature and frequency, compared with their respective WT littermates, SERCA1a mice displayed increased developed force at frequencies of 4-8 Hz ( approximately 90% increase at 4 Hz) and force equal to WT mice at 10-14 Hz. Force development at 4 Hz in presence of 1 muM isoproterenol was similar in TG and WT mice. In Het mice, developed force was nearly identical at the lower end of the frequency range (4-8 Hz) but slightly depressed at higher frequency (P < 0.05 at 14 Hz). In presence of 1 muM isoproterenol, developed force at 4 Hz was equal to that in WT mice. Compared with normal levels, increased SERCA activity enhanced force development only at subphysiological frequencies. A reduction in SERCA activity only showed a depression of force at the higher frequency range. Thus generalizations regarding the correlation between SERCA activity and contractility can be highly ambiguous, because this relationship is critically dependent on other factors including stimulation frequency.

Sarcoplasmic reticulum calcium overloading in junctin deficiency enhances cardiac contractility but increases ventricular automaticity

BACKGROUND

Abnormal sarcoplasmic reticulum calcium (Ca) cycling is increasingly recognized as an important mechanism for increased ventricular automaticity that leads to lethal ventricular arrhythmias. Previous studies have linked lethal familial arrhythmogenic disorders to mutations in the ryanodine receptor and calsequestrin genes, which interact with junctin and triadin to form a macromolecular Ca-signaling complex. The essential physiological effects of junctin and its potential regulatory roles in sarcoplasmic reticulum Ca cycling and Ca-dependent cardiac functions, such as myocyte contractility and automaticity, are unknown.

METHODS AND RESULTS

The junctin gene was targeted in embryonic stem cells, and a junctin-deficient mouse was generated. Ablation of junctin was associated with enhanced cardiac function in vivo, and junctin-deficient cardiomyocytes exhibited increased contractile and Ca-cycling parameters. Short-term isoproterenol stimulation elicited arrhythmias, including premature ventricular contractions, atrioventricular heart block, and ventricular tachycardia. Long-term isoproterenol infusion also induced premature ventricular contractions and atrioventricular heart block in junctin-null mice. Further examination of the electrical activity revealed a significant increase in the occurrence of delayed afterdepolarizations. Consistently, 25% of the junctin-null mice died by 3 months of age with structurally normal hearts.

CONCLUSIONS

Junctin is an essential regulator of sarcoplasmic reticulum Ca release and contractility in normal hearts. Ablation of junctin is associated with aberrant Ca homeostasis, which leads to fatal arrhythmias. Thus, normal intracellular Ca cycling relies on maintenance of junctin levels and an intricate balance among the components in the sarcoplasmic reticulum quaternary Ca-signaling complex.

Enhanced calcium mobilization in rat ventricular myocytes during the onset of pressure overload-induced hypertrophy

Early cardiovascular changes evoked by pressure overload (PO) may reveal adaptive strategies that allow immediate survival to the increased hemodynamic load. In this study, systolic and diastolic Ca2+ cycling was analyzed in left ventricular rat myocytes before ( day 2, PO-2d group) and after ( day 7, PO-7d group) development of hypertrophy subsequent to aortic constriction, as well as in myocytes from time-matched sham-operated rats (sham group). Ca2+ transient amplitude was significantly augmented in the PO-2d group. In the PO-7d group, intracellular Ca2+ concentration ([Ca2+]i) was reduced during diastole, and mechanical twitch relaxation (but not [Ca2+]i decline) was slowed. In PO groups, fractional sarcoplasmic reticulum (SR) Ca2+ release at a twitch, SR Ca2+ content, SR Ca2+ loss during diastole, and SR-dependent integrated Ca2+ flux during twitch relaxation were significantly greater than in sham-operated groups, whereas the relaxation-associated Ca2+ flux carried by the Na+/Ca2+ exchanger was not significantly changed. In the PO-7d group, mRNA levels of cardiac isoforms of SR Ca2+-ATPase (SERCA2a), phospholamban, calsequestrin, ryanodine receptor, and NCX were not significantly altered, but the SERCA2a-to-phospholamban ratio was increased 2.5-fold. Moreover, greater sensitivity to the inotropic effects of the β-adrenoceptor agonist isoproterenol was observed in the PO-7d group. The results indicate enhanced Ca2+ cycling between SR and cytosol early after PO imposition, even before hypertrophy development. Increase in SR Ca2+ uptake may contribute to enhancement of excitation-contraction coupling (augmented SR Ca2+ content and release) and protection against arrhythmogenesis due to buildup of [Ca2+]i during diastole.

The inotropic effect of cardioactive glycosides in ventricular myocytes requires Na+-Ca2+ exchanger function

Glycoside-induced cardiac inotropy has traditionally been attributed to direct Na(+)-K(+)-ATPase inhibition, causing increased intracellular [Na(+)] and consequent Ca(2+) gain via the Na(+)-Ca(2+) exchanger (NCX). However, recent studies suggested alternative mechanisms of glycoside-induced inotropy: (1) direct activation of sarcoplasmic reticulum Ca(2+) release channels (ryanodine receptors; RyRs); (2) increased Ca(2+) selectivity of Na(+) channels (slip-mode conductance); and (3) other signal transduction pathways. None of these proposed mechanisms requires NCX or an altered [Na(+)] gradient. Here we tested the ability of ouabain (OUA, 3 microm), digoxin (DIG, 20 microm) or acetylstrophanthidin (ACS, 4 microm) to alter Ca(2+) transients in completely Na(+)-free conditions in intact ferret and cat ventricular myocytes. We also tested whether OUA directly activates RyRs in permeabilized cat myocytes (measuring Ca(2+) sparks by confocal microscopy). In intact ferret myocytes (stimulated at 0.2 Hz), DIG and ACS enhanced Ca(2+) transients and cell shortening during twitches, as expected. However, prior depletion of [Na(+)](i) (in Na(+)-free, Ca(2+)-free solution) and in Na(+)-free solution (replaced by Li(+)) the inotropic effects of DIG and ACS were completely prevented. In voltage-clamped cat myocytes, OUA increased Ca(2+) transients by 48 +/- 4% but OUA had no effect in Na(+)-depleted cells (replaced by N-methyl-d-glucamine). In permeabilized cat myocytes, OUA did not change Ca(2+) spark frequency, amplitude or spatial spread (although spark duration was slightly prolonged). We conclude that the acute inotropic effects of DIG, ACS and OUA (and the effects on RyRs) depend on the presence of Na(+) and a functional NCX in ferret and cat myocytes (rather than alternate Na(+)-independent mechanisms).

Regulation of cardiac sarcoplasmic reticulum Ca release by luminal [Ca] and altered gating assessed with a mathematical model

Cardiac excitation-contraction coupling is initialized by the release of Ca from the sarcoplasmic reticulum (SR) in response to a sudden increase in local cytosolic [Ca] ([Ca]i) within the junctional cleft. We have tested the hypothesis that functional ryanodine receptor (RyR) regulation plays a major role in the regulation of myocyte Ca. A mathematical model with unique characteristics was used to simulate Ca homeostasis. Specifically, the model was designed to accurately represent the SR [Ca]-dependence of release from a variety of experimentally produced data sets. The simulated data for altered RyR Ca sensitivity demonstrated a regulatory feedback loop that resulted in the same release at lower [Ca]SR. This suggests that the primary role of myocyte RyR regulation may be to decrease SR [Ca] without decreasing the size of the [Ca]i transient. The model results suggest that this action moderates the increased SR [Ca] observed with adrenergic stimulation and may keep the [Ca]SR below the threshold for delayed afterdepolarizations and arrhythmia. However, increased Ca affinity of the RyR increased the probability of delayed afterdepolarizations when heart failure was simulated. We conclude that RyR regulation may play a role in preventing arrhythmias in healthy myocytes but that the same regulation may have the opposite effect in chronic heart failure.

Difference in propagation of Ca2+ release in atrial and ventricular myocytes

Intracellular [Ca2+] ([Ca2+]i) was imaged in atrial and ventricular rat myocytes by means of a high-speed Nipkow confocal microscope. Atrial myocytes with an absent t-tubule system on 8-di- ANEPPS staining showed an initial rise in Ca2+ at the periphery of the cell, which propagated to the interior of the cell. Ventricular myocytes showed a uniform rise in [Ca2+]i after electrical stimulation, consistent with a prominent t-tubular network. In atrial myocytes, there was a much shorter time between the peak of the [Ca2+]i transient and the peak contraction as compared to ventricular myocytes. A regional release of Ca2+ induced by an exposure of one end of the myocyte to caffeine with a rapid solution switcher resulted in a uniform propagation of Ca2+ down the length of the cell in atrial myocytes, but we found no propagation in ventricular myocytes. A staining with rhodamine 123 indicated a much greater density of mitochondria in ventricular myocytes than in atrial myocytes. Thus the atrial myocytes display a lack of "local control" of Ca2+ release, with propagation after the Ca2+ release at the periphery induced by stimulation or at one end of the cell induced by exposure to caffeine. Ventricular myocytes showed the presence of local control, as indicated by an absence of the propagation of a local caffeine-induced Ca2+ transient. We suggest that this finding, as well as a reduced delay between the peak of the [Ca2+]i transient and the peak shortening in atrial myocytes, could be due in part to reduced Ca2+ buffering provided by mitochondria in atrial myocytes as opposed to ventricular myocytes.

Excessive sarcoplasmic/endoplasmic reticulum Ca2+-ATPase expression causes increased sarcoplasmic reticulum Ca2+ uptake but decreases myocyte shortening

BACKGROUND

Increasing sarcoplasmic/endoplasmic reticulum (SR) Ca2+-ATPase (SERCA) uptake activity is a promising therapeutic approach for heart failure. We investigated the effects of different levels of SERCA1a expression on contractility and Ca2+ cycling. We tested whether increased SERCA1a expression levels enhance myocyte contractility in a gene-dose-dependent manner.

METHODS AND RESULTS

Rabbit isolated cardiomyocytes were transfected at different multiplicities of infection (MOIs) with adenoviruses encoding SERCA1a (or beta-galactosidase as control). Myocyte relaxation half-time was decreased by 10% (P=0.052) at SERCA1a MOI 10 and by 28% at MOI 50 (P<0.05). Myocyte fractional shortening was increased by 12% at MOI 10 (P<0.05) but surprisingly decreased at MOI 50 (-22%, P<0.05) versus control. SR Ca2+ uptake (in permeabilized myocytes) demonstrated a gene-dose-dependent decrease in K(m) by 29% and 46% and an increase in Vmax by 37% and 72% at MOI 10 and MOI 50, respectively (all P<0.05 versus control). Ca2+ transient amplitude was increased in Ad-SERCA1a-infected myocytes at MOI 10 (by 121%, P<0.05), but at MOI 50, the Ca2+ transient amplitude was not significantly changed. Caffeine-induced Ca2+ transients indicated significantly increased SR Ca2+ content in Ad-SERCA1a-infected cells, by 72% at MOI 10 and by 87% at MOI 50. Mathematical simulations demonstrate that the functional increase in SR Ca2+-ATPase uptake activity at MOI 50 (and increased cytosolic Ca2+ buffering) is sufficient to curtail the Ca2+ transient amplitude and explain the reduced contraction.

CONCLUSIONS

Moderate SERCA1a gene transfer and expression improve contractility and Ca(2+) cycling. However, higher SERCA1a expression levels can impair myocyte shortening because of higher SERCA activity and Ca2+ buffering.

Integrated Ca2+ management in cardiac myocytes

Cardiac myocyte excitation-contraction coupling is complex. There are many systems involved that interact to form varied, but well-tuned, effects that are essential to contractile regulation. Nearly all of these systems are Ca-dependent, and Ca homeostasis within the myocyte is carefully controlled. Contractile activation results from Ca entry via Ca current, and Ca release from the sarcoplasmic reticulum (SR). Ca extrusion from the cytosol is controlled by Ca transport by (1) the Na-Ca exchanger, (2) the SR Ca-pump (which is balanced by a Ca leak out of the SR), and (3) slower systems (including Ca transport by mitochondria and the sarcolemmal Ca-pump). These systems interact to regulate the amount of Ca within the cell at rest, most of which is stored within the SR. The amount of Ca released from the SR depends nonlinearly upon SR [Ca], specifically the free SR [Ca] ([Ca](SR)). The relationship is particularly steep at high [Ca](SR), where spontaneous release can take place, resulting in electrical arrhythmias. In many models of heart failure, SR [Ca] is reduced, which may cause decreased Ca release and contractile dysfunction. In summary, the varied processes responsible for Ca regulation within the myocyte are critical to normal heart function, and disruption of the normal operation of these proteins can cause widely varied pathological effects, in large part due to dysfunctional Ca handling.

Over-expression of FK506-binding protein FKBP12.6 alters excitation-contraction coupling in adult rabbit cardiomyocytes

This study investigated the function of FK506-binding protein (FKBP12.6) using adenoviral-mediated gene transfer to over-express FKBP12.6 (Ad-FKBP12.6) in adult rabbit ventricular cardiomyocytes. Infection with a beta-galactosidase-expressing adenovirus (Ad-LacZ) was used as a control. Peak-systolic intracellular [Ca(2+)] (measured with Fura-2) was higher in the Ad-FKBP12.6 group compared to Ad-LacZ (1 Hz field stimulation at 37 degrees C). The amplitude of caffeine-induced Ca(2+) release was also greater, indicating a higher SR Ca(2+) content in the Ad-FKBP12.6 group. Voltage clamp experiments indicated that FKBP12.6 over-expression did not change L-type Ca(2+) current amplitude or Ca(2+) efflux rates via the Na(+)-Ca(2+) exchanger. Ca(2+) transients comparable to those after Ad-FKBP12.6 transfection could be obtained by enhancing SR Ca(2+) content of Ad-LacZ infected cells with periods of high frequency stimulation. Line-scan confocal microscopy (Fluo-3 fluorescence) of intact cardiomyocytes stimulated at 0.5 Hz (20-21 degrees C) revealed a higher degree of synchronicity of SR Ca(2+) release and fewer non-responsive Ca(2+) release sites in the Ad-FKBP12.6 group compared to control. Ca(2+) spark morphology was measured in beta-escin-permeabilized cardiomyocytes at a free [Ca(2+)](i) of 150 nm. The average values of the spark parameters (amplitude, duration, width and frequency) were reduced in the Ad-FKBP12.6 group. Increasing [Ca(2+)](i) to 400 nm caused coherent propagating Ca(2+) waves in the Ad-FKBP12.6 group but only limited Ca(2+) release events were recorded in the control group. These data indicate that FKBP12.6 over-expression enhances Ca(2+) transient amplitude predominately by increasing SR Ca(2+) content. Moreover, there is also evidence that FKBP12.6 can enhance the coupling between SR Ca(2+) release sites independently of SR content.

Modulation of excitation-contraction coupling by isoproterenol in cardiomyocytes with controlled SR Ca2+ load and Ca2+ current trigger

Cardiac Ca(2+) transients are enhanced by cAMP-dependent protein kinase (PKA). However, PKA-dependent modulation of ryanodine receptor (RyR) function in intact cells is difficult to measure, because PKA simultaneously increases Ca(2+) current (I(Ca)), SR Ca(2+) uptake and SR Ca(2+) loading (which independently increase SR Ca(2+) release). We measured I(Ca) and SR Ca(2+) release +/- 1 microm isoproterenol (ISO; isoprenaline) in voltage-clamped ventricular myocytes of rabbits and transgenic mice (expressing only non-phosphorylatable phospholamban). This mouse model helps control for any effect of ISO-enhanced SR uptake on observed release, but the two species produced essentially identical results. SR Ca(2+) load and I(Ca) were adjusted by conditioning. We thus evaluated PKA effects on SR Ca(2+) release at constant SR Ca(2+) load and I(Ca) trigger (with constant unitary I(Ca)). The amount of SR Ca(2+) release increased as a function of either I(Ca) or SR Ca(2+) load, but ISO did not alter the relationships (measured as gain or fractional release). This was true over a wide range of SR Ca(2+) load and I(Ca). However, the maximal rate of SR Ca(2+) release was approximately 50% faster with ISO (at most loads and I(Ca) levels). We conclude that the isolated effect of PKA on SR Ca(2+) release is an increase in maximal rate of release and faster turn-off of release (such that integrated SR Ca(2+) release is unchanged). The increased amount of SR Ca(2+) release normally seen with ISO depends primarily on increased I(Ca) trigger and SR Ca(2+) load, whereas faster release kinetics may be the main result of RyR phosphorylation.

Quantitative assessment of the SR Ca2+ leak-load relationship

Increased diastolic SR Ca2+ leak (J(leak)) could depress contractility in heart failure, but there are conflicting reports regarding the J(leak) magnitude even in normal, intact myocytes. We have developed a novel approach to measure SR Ca2+ leak in intact, isolated ventricular myocytes. After stimulation, myocytes were exposed to 0 Na+, 0 Ca2+ solution +/-1 mmol/L tetracaine (to block resting leak). Total cell [Ca2+] does not change under these conditions with Na+-Ca2+ exchange inhibited. Resting [Ca2+]i declined 25% after tetracaine addition (126+/-6 versus 94+/-6 nmol/L; P<0.05). At the same time, SR [Ca2+] ([Ca2+](SRT)) increased 20% (93+/-8 versus 108+/-6 micromol/L). From this Ca2+ shift, we calculate J(leak) to be 12 micromol/L per second or 30% of the SR diastolic efflux. The remaining 70% is SR pump unidirectional reverse flux (backflux). The sum of these Ca2+ effluxes is counterbalanced by unidirectional forward Ca2+ pump flux. J(leak) also increased nonlinearly with [Ca2+](SRT) with a steeper increase at higher load. We conclude that J(leak) is 4 to 15 micromol/L cytosol per second at physiological [Ca2+](SRT). The data suggest that the leak is steeply [Ca2+](SRT)-dependent, perhaps because of increased [Ca2+]i sensitivity of the ryanodine receptor at higher [Ca2+](SRT). Key factors that determine [Ca2+](SRT) in intact ventricular myocytes include (1) the thermodynamically limited Ca2+ gradient that the SR can develop (which depends on forward flux and backflux through the SR Ca2+ ATPase) and (2) diastolic SR Ca2+ leak (ryanodine receptor mediated).

Sustained high O2 use for Ca2+ handling in rat ventricular slices under decreased free shortening after ryanodine

We hypothesized that O2 wasting of Ca2+ handling in the excitation-contraction coupling in ryanodine-treated failing hearts might derive from an increased external Ca2+ extrusion via Na+/Ca2+ exchanger and futile Ca2+ cycling via sarcoplasmic reticulum (SR) Ca2+-ATPase. We tested this hypothesis by mechanoenergetic studies using rat left ventricular slices. After the slices were treated with ryanodine (0.1 microM), 1-Hz free shortening significantly decreased by 78-85%, whereas the observed O2 consumption (VO2) required for total Ca2+ handling, increased from 0.79 to 1.13 ml O2 x min(-1) x 100 g x LV(-1) (155.6% of control). We reconfirmed that cyclopiazonic acid (10 microM), a blocker of SR Ca2+-ATPase, decreased VO2 by 75-80% in normal slices. However, 100 microM of cyclopiazonic acid was needed to inhibit the VO2 by 80% after ryanodine treatment. Blockade of a sarcolemmal Na+/Ca2+ exchanger by KB-R7943 (10 microM) significantly decreased VO2 by 45% after ryanodine treatment without significant effects on normal slices. Our results indicated that the VO2 increase following ryanodine treatment was derived from a net change of an increased external Ca2+ extrusion via Na+/Ca2+ exchanger and futile Ca2+ cycling via SR Ca2+-ATPase.

Dynamic regulation of sarcoplasmic reticulum Ca(2+) content and release by luminal Ca(2+)-sensitive leak in rat ventricular myocytes

In cardiac muscle, excitation-contraction (E-C) coupling is determined by the ability of the sarcoplasmic reticulum (SR) to store and release Ca(2+). It has been hypothesized that the Ca(2+) sequestration and release mechanisms might be functionally linked to optimize the E-C coupling process. To explore the relationships between the loading status of the SR and functional state of the Ca(2+) release mechanism, we examined the effects of changes in SR Ca(2+) content on spontaneous Ca(2+) sparks in saponin-permeabilized and patch-clamped rat ventricular myocytes. SR Ca(2+) content was manipulated by pharmacologically altering the capacities of either Ca(2+) uptake or leak. Ca(2+) sparks were recorded using a confocal microscope and Fluo-3 and were quantified considering missed events. SR Ca(2+) content was assessed by application of caffeine. Exposure of permeabilized cells to anti-phospholamban antibodies elevated the SR Ca(2+) content and increased the frequency of sparks. Suppression of the SR Ca(2+) pump by thapsigargin lowered [Ca(2+)](SR) and reduced the frequency of sparks. The ryanodine receptor (RyR) blockers tetracaine and Mg(2+) transiently suppressed the frequency of sparks. Upon washout of the drugs, sparking activity transiently overshot control levels. Low doses of caffeine transiently potentiated sparking activity upon application and transiently depressed the sparks upon removal. In patch-clamped cardiac myocytes, exposure to caffeine produced only a transient increase in the probability of sparks induced by depolarization. We interpret these results in terms of a novel dynamic control scheme for SR Ca(2+) cycling. A central element of this scheme is a luminal Ca(2+) sensor that links the functional activity of RyRs to the loading state of the SR, allowing cells to auto-regulate the size and functional state of their SR Ca(2+) pool. These results are important for understanding the regulation of intracellular Ca(2+) release and contractility in cardiac muscle.

Phospholamban decreases the energetic efficiency of the sarcoplasmic reticulum Ca pump

We tested the hypothesis that increased Sarcoplasmic reticulum (SR) Ca content ([Ca](SRT)) in phospholamban knockout mice (PLB-KO) is because of increased SR Ca pump efficiency defined by the steady-state SR [Ca] gradient. The time course of thapsigargin-sensitive ATP-dependent (45)Ca influx into and efflux out of cardiac SR vesicles from PLB-KO and wild-type (WT) mice was measured at 100 nm free [Ca]. We found that PLB decreased the initial SR Ca uptake rate (0.13 versus 0.31 nmol/mg/s) and decreased steady-state (45)Ca content (0.9 versus 4.1 nmol/mg protein). Furthermore, at similar total SR [Ca], the pump-mediated Ca efflux rate was higher in WT (0.065 versus 0.037 nmol/mg/s). The pump-independent leak rate constant (k(leak)) was also measured at 100 nm free [Ca]. The results indicate that k(leak) was < 1% of pump-mediated backflux and was not different among nonpentameric mutant PLB (PLB-C41F), WT pentameric PLB (same expression level), and PLB-KO. Therefore differences in passive SR Ca leak cannot be the cause of the higher thapsigargin-sensitive Ca efflux from the WT membranes. We conclude that the decreased total SR [Ca] in WT mice is caused by decreased SR Ca influx rate, an increased Ca-pump backflux, and unaltered leak. Based upon both thermodynamic and kinetic analysis, we conclude that PLB decreases the energetic efficiency of the SR Ca pump.

Allosteric regulation of Na/Ca exchange current by cytosolic Ca in intact cardiac myocytes

The cardiac sarcolemmal Na-Ca exchanger (NCX) is allosterically regulated by [Ca](i) such that when [Ca](i) is low, NCX current (I(NCX)) deactivates. In this study, we used membrane potential (E(m)) and I(NCX) to control Ca entry into and Ca efflux from intact cardiac myocytes to investigate whether this allosteric regulation (Ca activation) occurs with [Ca](i) in the physiological range. In the absence of Ca activation, the electrochemical effect of increasing [Ca](i) would be to increase inward I(NCX) (Ca efflux) and to decrease outward I(NCX). On the other hand, Ca activation would increase I(NCX) in both directions. Thus, we attributed [Ca](i)-dependent increases in outward I(NCX) to allosteric regulation. Ca activation of I(NCX) was observed in ferret myocytes but not in wild-type mouse myocytes, suggesting that Ca regulation of NCX may be species dependent. We also studied transgenic mouse myocytes overexpressing either normal canine NCX or this same canine NCX lacking Ca regulation (Delta680-685). Animals with the normal canine NCX transgene showed Ca activation, whereas animals with the mutant transgene did not, confirming the role of this region in the process. In native ferret cells and in mice with expressed canine NCX, allosteric regulation by Ca occurs under physiological conditions (K(mCaAct) = 125 +/- 16 nM SEM approximately resting [Ca](i)). This, along with the observation that no delay was observed between measured [Ca](i) and activation of I(NCX) under our conditions, suggests that beat to beat changes in NCX function can occur in vivo. These changes in the I(NCX) activation state may influence SR Ca load and resting [Ca](i), helping to fine tune Ca influx and efflux from cells under both normal and pathophysiological conditions. Our failure to observe Ca activation in mouse myocytes may be due to either the extent of Ca regulation or to a difference in K(mCaAct) from other species. Model predictions for Ca activation, on which our estimates of K(mCaAct) are based, confirm that Ca activation strongly influences outward I(NCX), explaining why it increases rather than declines with increasing [Ca](i).

Sarcoplasmic reticulum Ca(2+) release causes myocyte depolarization. Underlying mechanism and threshold for triggered action potentials

Spontaneous sarcoplasmic reticulum (SR) Ca(2+) release causes delayed afterdepolarizations (DADs) via Ca(2+)-induced transient inward currents (I:(ti)). However, no quantitative data exists regarding (1) Ca(2+) dependence of DADs, (2) Ca(2+) required to depolarize the cell to threshold and trigger an action potential (AP), or (3) relative contributions of Ca(2+)-activated currents to DADs. To address these points, we evoked SR Ca(2+) release by rapid application of caffeine in indo 1-AM-loaded rabbit ventricular myocytes and measured caffeine-induced DADs (cDADs) with whole-cell current clamp. The SR Ca(2+) load of the myocyte was varied by different AP frequencies. The cDAD amplitude doubled for every 88+/-8 nmol/L of Delta[Ca(2+)](i) (simple exponential), and the Delta[Ca(2+)](i) threshold of 424+/-58 nmol/L was sufficient to trigger an AP. Blocking Na(+)-Ca(2+) exchange current (I(Na/Ca)) by removal of [Na](o) and [Ca(2+)](o) (or with 5 mmol/L Ni(2+)) reduced cDADs by >90%, for the same Delta[Ca(2+)](i). In contrast, blockade of Ca(2+)-activated Cl(-) current (I(Cl(Ca))) with 50 micromol/L niflumate did not significantly alter cDADs. We conclude that DADs are almost entirely due to I(Na/Ca), not I(Cl(Ca)) or Ca(2+)-activated nonselective cation current. To trigger an AP requires 30 to 40 micromol/L cytosolic Ca(2+) or a [Ca(2+)](i) transient of 424 nmol/L. Current injection, simulating I(ti)s with different time courses, revealed that faster I:(ti)s require less charge for AP triggering. Given that spontaneous SR Ca(2+) release occurs in waves, which are slower than cDADs or fast I(ti)s, the true Delta[Ca(2+)](i) threshold for AP activation may be approximately 3-fold higher in normal myocytes. This provides a safety margin against arrhythmia in normal ventricular myocytes.

A mathematical model of cardiocyte Ca(2+) dynamics with a novel representation of sarcoplasmic reticular Ca(2+) control

Cardiac contraction and relaxation dynamics result from a set of simultaneously interacting Ca(2+) regulatory mechanisms. In this study, cardiocyte Ca(2+) dynamics were modeled using a set of six differential equations that were based on theories, equations, and parameters described in previous studies. Among the unique features of the model was the inclusion of bidirectional modulatory interplay between the sarcoplasmic reticular Ca(2+) release channel (SRRC) and calsequestrin (CSQ) in the SR lumen, where CSQ acted as a dynamic rather than simple Ca(2+) buffer, and acted as a Ca(2+) sensor in the SR lumen as well. The inclusion of this control mechanism was central in overcoming a number of assumptions that would otherwise have to be made about SRRC kinetics, SR Ca(2+) release rates, and SR Ca(2+) release termination when the SR lumen is assumed to act as a simple, buffered Ca(2+) sink. The model was sufficient to reproduce a graded Ca(2+)-induced Ca(2+) release (CICR) response, CICR with high gain, and a system with reasonable stability. As constructed, the model successfully replicated the results of several previously published experiments that dealt with the Ca(2+) dependence of the SRRC (, J. Gen. Physiol. 85:247-289), the refractoriness of the SRRC (, Am. J. Physiol. 270:C148-C159), the SR Ca(2+) load dependence of SR Ca(2+) release (, Am. J. Physiol. 268:C1313-C1329;, J. Biol. Chem. 267:20850-20856), SR Ca(2+) leak (, J. Physiol. (Lond.). 474:463-471;, Biophys. J. 68:2015-2022), SR Ca(2+) load regulation by leak and uptake (, J. Gen. Physiol. 111:491-504), the effect of Ca(2+) trigger duration on SR Ca(2+) release (, Am. J. Physiol. 258:C944-C954), the apparent relationship that exists between sarcoplasmic and sarcoplasmic reticular calcium concentrations (, Biophys. J. 73:1524-1531), and a variety of contraction frequency-dependent alterations in sarcoplasmic [Ca(2+)] dynamics that are normally observed in the laboratory, including rest potentiation, a negative frequency-[Ca(2+)] relationship, and extrasystolic potentiation. Furthermore, under the condition of a simulated Ca(2+) overload, an alternans-like state was produced. In summary, the current model of cardiocyte Ca(2+) dynamics provides an integrated theoretical framework of fundamental cellular Ca(2+) regulatory processes that is sufficient to predict a broad array of observable experimental outcomes.

Loading of calcium and strontium into the sarcoplasmic reticulum in rat ventricular muscle

Previous work suggests that strontium ions (Sr(2+)) are less effective than calcium ions (Ca(2+)) at supporting excitation-contraction (EC) coupling in cardiac muscle. We therefore tested whether this was due to differences in the uptake and release of Ca(2+)and Sr(2+)by the sarcoplasmic reticulum (SR) of rat ventricular trabeculae and myocytes at 22-24 degrees C. In permeabilized trabeculae, isometric contractions activated by exposure to Ca(2+)- and Sr(2+)-containing solutions produced similar maximal force, but were four times more sensitive to Ca(2+)than to Sr(2+). The rate of loading and maximal SR capacity for caffeine-releasable Ca(2+)and Sr(2+)were similar. In isolated, voltage-clamped ventricular myocytes, the SR content was measured as Na(+)-Ca(2+)exchange current during caffeine-induced SR cation releases. The SR Ca(2+)load reached a steady maximum during a train of voltage clamp depolarizations. A similar maximal Sr(2+)load was not observed, suggesting that the SR capacity for Sr(2+)exceeds that for Ca(2+). Therefore, the relative inability of Sr(2+)to support cardiac EC coupling appears not to be due to failure of the SR to sequester Sr(2+). Instead, increases in cytosolic [Sr(2+)] seem to poorly activate Sr(2+)release from the SR.

Potentiation of fractional sarcoplasmic reticulum calcium release by total and free intra-sarcoplasmic reticulum calcium concentration

Our aim was to measure the influence of sarcoplasmic reticulum (SR) calcium content ([Ca](SRT)) and free SR [Ca] ([Ca](SR)) on the fraction of SR calcium released during voltage clamp steps in isolated rabbit ventricular myocytes. [Ca](SRT), as measured by caffeine application, was progressively increased by conditioning pulses. Sodium was absent in both the intracellular and in the extracellular solutions to block sodium/calcium exchange. Total cytosolic calcium flux during the transient was inferred from I(Ca), [Ca](SRT), [Ca](i), and cellular buffering characteristics. Fluxes via the calcium current (I(Ca)), the SR calcium pump, and passive leak from the SR were evaluated to determine SR calcium release flux (J(rel)). Excitation-contraction (EC) coupling was characterized with respect to both gain (integral J(rel)/integral I(Ca)) and fractional SR calcium release. Both parameters were virtually zero for a small, but measurable [Ca](SRT). Gain and fractional SR calcium release increased steeply and nonlinearly with both [Ca](SRT) and [Ca](SR). We conclude that potentiation of EC coupling can be correlated with both [Ca](SRT) and [Ca](SR). While fractional SR calcium release was not linearly dependent upon [Ca](SR), intra-SR calcium may play a crucial role in regulating the SR calcium release process.

Reverse mode of the sarcoplasmic reticulum calcium pump and load-dependent cytosolic calcium decline in voltage-clamped cardiac ventricular myocytes

We have characterized [Ca](i) decline in voltage-clamped rabbit ventricular myocytes with progressive increases in sarcoplasmic reticulum (SR) calcium load. "Backflux" through the SR calcium pump is a critical feature which allows realistically small values for SR calcium leak fluxes to be used. Total cytosolic calcium was calculated from the latter part of [Ca](i) decline using rate constants for cellular calcium buffers. Intra-SR calcium buffering characteristics were also deduced. We found that the net SR calcium pump flux and rate of [Ca](i) decline decreased as the SR free [Ca] rose, with pump parameters held constant. We have therefore characterized for the first time in intact myocytes both forward and reverse SR calcium pump kinetics as well as intra-SR calcium buffering and SR calcium leak. We conclude that the reverse flux through the SR calcium pump is an important factor in comprehensive understanding of dynamic SR calcium fluxes.

Subcellular [Ca2+]i gradients during excitation-contraction coupling in newborn rabbit ventricular myocytes

The central role of T-tubule and sarcoplasmic reticulum (SR) diadic junctions in excitation-contraction (EC) coupling in adult (AD) ventricular myocytes suggests that their absence in newborn (NB) cells may manifest as an altered EC coupling phenotype. We used confocal microscopy to compare fluo-3 [Ca2+]i transients in the subsarcolemmal space and cell center of field-stimulated NB and AD rabbit ventricular myocytes. Peak systolic [Ca2+]i occurred sooner and was higher in the subsarcolemmal space compared with the cell center in NB myocytes. In AD myocytes, [Ca2+]i rose and declined with similar profiles at the cell center and subsarcolemmal space. Disabling the SR (10 micromol/L thapsigargin) slowed the rate of rise and decline of Ca2+ in AD myocytes but did not alter Ca2+ transient kinetics in NB myocytes. In contrast to adults, localized SR Ca2+ release events ("Ca2+ sparks") occurred predominantly at the cell periphery of NB myocytes. Immunolabeling experiments demonstrated overlapping distributions of the Na(+)-Ca2+ exchanger and ryanodine receptors (RyR2) in AD myocytes. In contrast, RyR2s were spatially separated from the sarcolemma in NB myocytes. Confocal sarcolemmal imaging of di-8-ANEPPS-treated myocytes confirmed an extensive T-tubule network in AD cells, and that T-tubules are absent in NB myocytes. A mathematical model of subcellular Ca2+ dynamics predicts that Ca2+ flux via the Na(+)-Ca2+ exchanger during an action potential can account for the subsarcolemmal Ca2+ gradients in NB myocytes. Spatial separation of sarcolemmal Ca2+ entry from SR Ca2+ release channels may minimize the role of SR Ca2+ release during normal EC coupling in NB ventricular myocytes.

Cardiac ionic currents and acute ischemia: from channels to arrhythmias

The aim of this review is to provide basic information on the electrophysiological changes during acute ischemia and reperfusion from the level of ion channels up to the level of multicellular preparations. After an introduction, section II provides a general description of the ion channels and electrogenic transporters present in the heart, more specifically in the plasma membrane, in intracellular organelles of the sarcoplasmic reticulum and mitochondria, and in the gap junctions. The description is restricted to activation and permeation characterisitics, while modulation is incorporated in section III. This section (ischemic syndromes) describes the biochemical (lipids, radicals, hormones, neurotransmitters, metabolites) and ion concentration changes, the mechanisms involved, and the effect on channels and cells. Section IV (electrical changes and arrhythmias) is subdivided in two parts, with first a description of the electrical changes at the cellular and multicellular level, followed by an analysis of arrhythmias during ischemia and reperfusion. The last short section suggests possible developments in the study of ischemia-related phenomena.

Ca2+ waves during triggered propagated contractions in intact trabeculae. Determinants of the velocity of propagation

During triggered propagated contractions, Ca2+ waves travel along cardiac trabeculae with a constant velocity (Vprop) ranging from 0. 34 to 5.47 mm/s. To explore the determinants of Vprop, we studied (1) the relationship between [Ca2+]i and Vprop and (2) the effect of low concentrations of caffeine on Vprop. Trabeculae were dissected from the right ventricle of rat hearts. [Ca2+]i was measured using electrophoretically injected fura-2 and an image-intensified CCD camera. Force was measured using a silicon strain gauge, and sarcomere length was measured using laser diffraction techniques. After induction of reproducible Ca2+ waves by trains of electrical stimuli (2.5 Hz) at 21.9+/-0.2 degrees C, the number of stimuli or [Ca2+]o was varied in 9 trabeculae. In 5 trabeculae, the effects of caffeine (0.1 to 1.0 mmol/L) at [Ca2+]o of 2.2+/-0.3 mmol/L were determined. All images were recorded under stable conditions of wave propagation. The increment in [Ca2+]i during the last electrically stimulated transient (DeltaCaT) and [Ca2+]i just before onset of the Ca2+ waves (CaD) were used to estimate the Ca2+ loading of the sarcoplasmic reticulum (SR) and the myoplasm, respectively. The ratio (DeltaCaW/DeltaCaT) of the [Ca2+]i increment during the waves (DeltaCaW) to DeltaCaT was used to estimate the probability of opening of the SR-Ca2+ release channel during wave propagation. As a result of an increase of the number of stimuli or [Ca2+]o, Vprop increased in proportion to (1) DeltaCaT (r=0.82); (2) CaD (r=0.88); (3) DeltaCaW (r=0.85); and (4) DeltaCaW/DeltaCaT (r=0.74). The addition of caffeine (</=0.3 mmol/L) increased Vprop for any DeltaCaT and any DeltaCaW, revealing an increased sensitivity of Vprop to DeltaCaT and DeltaCaW. In contrast, caffeine had little effect on the relationship between Vprop and CaD and no effect on that between Vprop and DeltaCaW/DeltaCaT. These results suggest that both the cellular Ca2+ loading and open probability of the SR-Ca2+ release channels determine the velocity of propagation of Ca2+ waves.