Cardiac excitation-contraction coupling

Abnormal spectra alteration observed in Triton calibration method for measuring [Ca2+]i with fluorescence indicator, fura-2

We compared two commonly used calibration methods for measuring the concentration of intracellular free calcium ([Ca2+]i) by ratiometric fluorescence dye, fura-2 in mouse neuroblastoma-rat glioma hybrid cells (NG108-15). One calibration method, the Triton method, employs detergent Triton X-100, while the other, the Ionomycin method, uses a calcium-specific ionophore, Ionomycin. In the Triton method, we observed that at excitation 380 nm, the fura-2 fluorescence intensity of steady-state cells abnormally situated beyond the limiting intensity for calibration. By excitation scan, we demonstrated that this abnormality was caused by the change of fura-2 isosbestic points, which in turn was due to cell lysis after the addition of Triton X-100. This problem was resolved in the Ionomycin method by avoidance of cell lysis. Our results showed the correlation between inconsistent isosbestic points and cell lysis. As the basis for [Ca2+]i calibration, the proportionality between the fluorescence intensity and the concentration of dye species was impaired because of inconsistent isosbestic points. This inconsistency can be eliminated by a preliminary experiment of excitation scan to test the feasibility of different calibration methods.

In situcardiac performance of Pacific bluefin tuna hearts in response to acute temperature change

This study reports the cardiovascular physiology of the Pacific bluefin tuna (Thunnus orientalis) in an in situ heart preparation. The performance of the Pacific bluefin tuna heart was examined at temperatures from 30°C down to 2°C. Heart rates ranged from 156 beats min–1 at 30°C to 13 beats min–1 at 2°C. Maximal stroke volumes were 1.1 ml kg–1 at 25°C and 1.3 ml kg–1 at 2°C. Maximal cardiac outputs were 18.1 ml kg–1 min–1 at 2°C and 106 ml kg–1 min–1 at 25°C. These data indicate that cardiovascular function in the Pacific bluefin tuna exhibits a strong temperature dependence, but cardiac function is retained at temperatures colder than those tolerated by tropical tunas. The Pacific bluefin tuna's cardiac performance in the cold may be a key adaptation supporting the broad thermal niche of the bluefin tuna group in the wild. In situ data from Pacific bluefin are compared to in situ measurements of cardiac performance in yellowfin tuna and preliminary results from albacore tuna.

Insights into cardioprotection obtained from study of cellular Ca2+ handling in myocardium of true hibernating mammals

Mammalian hibernators exhibit remarkable resistance to low body temperature, whereas non-hibernating (NHB) mammals develop ventricular dysfunction and arrhythmias. To investigate this adaptive change, we compared contractile and electrophysiological properties of left ventricular myocytes isolated from hibernating (HB) woodchucks (Marmota monax) and control NHB woodchucks. The major findings of this study were the following: 1) the action potential duration in HB myocytes was significantly shorter than in NHB myocytes, but the amplitude of peak contraction was unchanged; 2) HB myocytes had a 33% decreased L-type Ca2+ current (I(Ca)) density and twofold faster I(Ca) inactivation but no change in the current-voltage relationship; 3) there were no changes in the density of inward rectifier K+ current, transient outward K+ current, or Na+/Ca2+ exchange current, but HB myocytes had increased sarcoplasmic reticulum Ca2+ content as estimated from caffeine-induced Na+/Ca2+ exchange current values; 4) expression of the L-type Ca2+ channel alpha(1C)-subunit was decreased by 30% in HB hearts; and 5) mRNA and protein levels of sarco(endo)plasmic reticulum Ca2+-ATPase 2a (SERCA2a), phospholamban, and the Na+/Ca2+ exchanger showed a pattern that is consistent with functional measurements: SERCA2a was increased and phospholamban was decreased in HB relative to NHB hearts with no change in the Na+/Ca2+ exchanger. Thus reduced Ca2+ channel density and faster I(Ca) inactivation coupled to enhanced sarcoplasmic reticulum Ca2+ release may underlie shorter action potentials with sustained contractility in HB hearts. These changes may account for natural resistance to Ca2+ overload-related ventricular dysfunction and point to an important cardioprotective mechanism during true hibernation.

Ca2+ syntillas, miniature Ca2+ release events in terminals of hypothalamic neurons, are increased in frequency by depolarization in the absence of Ca2+ influx

Localized, brief Ca2+ transients (Ca2+ syntillas) caused by release from intracellular stores were found in isolated nerve terminals from magnocellular hypothalamic neurons and examined quantitatively using a signal mass approach to Ca2+ imaging. Ca2+ syntillas (scintilla, L., spark, from a synaptic structure, a nerve terminal) are caused by release of approximately 250,000 Ca ions on average by a Ca2+ flux lasting on the order of tens of milliseconds and occur spontaneously at a membrane potential of -80 mV. Syntillas are unaffected by removal of extracellular Ca2+, are mediated by ryanodine receptors (RyRs) and are increased in frequency, in the absence of extracellular Ca2+, by physiological levels of depolarization. This represents the first direct demonstration of mobilization of Ca2+ from intracellular stores in neurons by depolarization without Ca2+ influx. The regulation of syntillas by depolarization provides a new link between neuronal activity and cytosolic [Ca2+] in nerve terminals.

Direct measurement of Ca2+ concentration in the SR of living cardiac myocytes

Although abnormal sarcoplasmic reticulum (SR) Ca(2+) handling may cause heart failure, there has been no method to directly measure Ca(2+) concentration in SR ([Ca(2+)](SR)) of living cardiomyocytes. We have measured [Ca(2+)](SR) by expressing novel fluorescent Ca(2+) indicators yellow cameleon (YC) 2.1, YC3er, and YC4er in cultured neonatal rat cardiomyocytes. The distribution of YC2.1 was uniform in the cytoplasm, while that of YC3er/YC4er, containing the signal sequence which recruits them to SR, showed reticular pattern and was co-localized with SERCA2a. The treatment with caffeine reversibly decreased the emission ratio (R) in YC3er/YC4er-expressing myocytes, and the treatment with ryanodine and thapsigargin decreased R irreversibly. During the contraction-relaxation cycle, R was changed periodically in the YC2.1- and YC3er-expressing myocytes, but its direction of the change was opposite. These results suggest that YC3er/YC4er were specifically localized and functioned in SR as a [Ca(2+)](SR) indicator. This technique would be useful to understand the function of SR in failing myocardium.

Temperature dependence of the Ca2+-ATPase (SERCA2) in the ventricles of tuna and mackerel

Recent physiological studies on the cardiovascular performance of tunas suggest that the elevated heart rates of these fish may rely on increased use of intracellular sarcoplasmic reticulum (SR) Ca2+stores. In this study, we compare the cellular cardiac performance in endothermic tunas (bluefin, albacore, yellowfin) and their ectothermic sister taxa (mackerel) in response to acute temperature change. The cardiac sarco/endoplasmic reticulum Ca2+-ATPase (SERCA2) plays a major role during cardiac excitation-contraction (E-C) coupling, transporting Ca2+from the cytosol into the lumen of the SR and thus promoting the relaxation of the muscle. Measurements of oxalate-supported Ca2+uptake in SR-enriched ventricular vesicles indicated that tunas were capable of sustaining a rate of Ca2+uptake that was significantly higher than the mackerel. Among tunas, the cold-tolerant bluefin had the highest rates of SR Ca2+uptake and ATPase activity. The differences among Ca2+uptake and ATP hydrolysis rates do not seem to result from intrinsic differences between the SERCA2 present in the different tunas, as shown by their similar temperature sensitivities and similar values for activation energy. Western blots reveal that increased SERCA2 protein content is associated with the higher Ca2+uptake and ATPase activities seen in bluefin ventricles compared with albacore, yellowfin, and mackerel. We hypothesize that a key step in the evolution of high heart rate and high metabolic rate in tunas is increased activity of the SERCA2 enzyme. We also suggest that high levels of SERCA2 in bluefin tuna hearts may be important for retaining cardiac function at cold temperatures.

Phosphoinositide 3-kinase regulates excitation-contraction coupling in neonatal cardiomyocytes

The phosphoinositide 3-kinase (PI3K) inhibitor LY-294002 decreased steady-state contraction in neonatal rat ventricular myocytes (NRVM). To determine whether the effect on steady-state contraction could be due to decreased intracellular Ca2+content, Ca2+content was assessed with fluorescent plate reader analysis by using the caffeine-releasable Ca2+stores as an index of sarcoplasmic reticulum (SR) Ca2+content. Caffeine-releasable Ca2+content was diminished in a dose-dependent manner with LY-294002, suggesting that the decrease in steady-state contraction was due to diminished intracellular Ca2+content. Activation of the L-type Ca2+channel by BAY K 8644 was attenuated by LY-294002, suggesting the effect of LY-294002 is to reduce Ca2+influx at this channel. To investigate whether additional proteins involved in excitation-contraction (EC) coupling are likewise regulated by PI3K activity, the effects of compounds acting at sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA2a), the ryanodine receptor, and the Na/Ca exchanger (NCX) were compared with LY-294002. Inhibition of SERCA2a by thapsigargin increased basal Ca2+levels in contrast to LY-294002, indicating that SERCA2a activity is sustained in the presence of LY-294002. Ryanodine decreased SR Ca2+content. The additive effect with coadministration of LY-294002 could be attributed to a decrease in Ca2+influx at the L-type Ca2+channel. The NCX inhibitor Ni2+was used to investigate whether the decrease in intracellular Ca2+content with LY-294002 could be due to inhibition of the NCX reverse-mode activity. The minimal effect of LY-294002 with Ni2+suggests that the primary effect of LY-294002 on EC coupling occurs through inhibition of PI3K-mediated L-type Ca2+channel activity.

Homer Protein Increases Activation of Ca2+ Sparks in Permeabilized Skeletal Muscle

Members of the Homer family of proteins are known to form multimeric complexes capable of cross-linking plasma membrane channels (e.g. metabotropic glutamate receptor) and intracellular Ca2+ release channels (e.g. inositol trisphosphate receptor) in neurons, which potentiates Ca2+ release. Recent work has demonstrated direct interaction of Homer proteins with type 1 and type 2 ryanodine receptor (RyR) isoforms. Moreover, Homer proteins have been shown to modulate RyR-dependent Ca2+ release in isolated channels as well as in whole cell preparations. We now show that long and short forms of Homer H1 (H1c and H1-EVH1) are potent activators of Ca2+ release via RyR in skeletal muscle fibers (e.g. Ca2+ sparks) and potent modulators of ryanodine binding to membranes enriched with RyR, with H1c being significantly more potent than H1-EVH1. Homer did not significantly alter the spatio-temporal properties of the sparks, demonstrating that Homer increases the rate of opening of RyRs, with no change in the overall RyR channel open time and amount of Ca2+ released during a spark. No changes in Ca2+ spark frequency or properties were observed using a full-length H1c with mutation in the EVH1 binding domain (H1c-G89N). One novel finding with each Homer agonist (H1c and H1-EVH1) was that in combination their actions on [3H]ryanodine binding was additive, an effect also observed for these Homer agonists in the Ca2+ spark studies. Finally, in Ca2+ spark studies, excess H1c-G89N prevented the effects of H1c in a dominant negative manner. Taken together our results suggest that the EVH1 domain is critical for the agonist behavior on Ca2+ sparks and ryanodine binding, and that the coiled-coil domain, present in long but not short form Homer, confers an increase in agonist potential apparently through the multimeric association of Homer ligand.

Modulation of in vivo cardiac function by myocyte-specific nitric oxide synthase-3

Nitric oxide (NO) functions principally as a diffusible paracrine effector. The exception is in cardiomyocytes where both NO synthases (NOS) and target proteins coexist, allowing NO to work in an autocrine/intracrine fashion. However, the most abundant myocyte isoform (NOS3) is far more expressed in vascular endothelium; thus, the in vivo contribution of myocyte-NOS3 remains less clear. The present study tested this role by transfecting whole hearts of NOS3-null (NOS3(-/-)) mice with adenovirus-expressing NOS3 coupled to a alpha-MHC promoter (AdV(NOS3)), comparing results to hearts transfected with marker-gene beta-galactosidase (AdVbeta(gal)). Total myocardial NOS3 protein and activity were restored to near wild-type (WT) levels in NOS3(-/-)+AdV(NOS3) hearts, and NOS3 relocalized normally with caveolin-3. Ejection function by pressure-volume analysis was enhanced in NOS3(-/-)+AdVbeta(gal) over WT or NOS3(-/-)+AdV(NOS3). More prominently, isoproterenol (ISO)-stimulated systolic and diastolic function in WT was amplified in NOS3(-/-)+AdVbeta(gal), whereas NOS3(-/-)+AdV(NOS3) returned the response to control. ISO-activated systolic function was inhibited 85% by concomitant muscarinic stimulation (carbachol) in NOS3(-/-)+AdV(NOS3) but not NOS3(-/-)+AdVbeta(gal) hearts. Lastly, NOS3(-/-)+AdVbeta(gal) mice displayed enhanced inotropy and lusitropy over WT at slower heart rates but a blunted rate augmentation versus controls. A more positive rate response was restored in NOS3(-/-)+AdV(NOS3) (P<0.001). Thus, myocyte autocrine/intracrine NOS3 regulation in vivo can underlie key roles in beta-adrenergic, muscarinic, and frequency-dependent cardiac regulation.

Calcium influx through hyperpolarization-activated cation channels (I(h) channels) contributes to activity-evoked neuronal secretion

The hyperpolarization-activated cation channels (I(h)) play a distinct role in rhythmic activities in a variety of tissues, including neurons and cardiac cells. In the present study, we investigated whether Ca(2+) can permeate through the hyperpolarization-activated pacemaker channels (HCN) expressed in HEK293 cells and I(h) channels in dorsal root ganglion (DRG) neurons. Using combined measurements of whole-cell currents and fura-2 Ca(2+) imaging, we found that there is a Ca(2+) influx in proportion to I(h) induced by hyperpolarization in HEK293 cells. The I(h) channel blockers Cs(+) and ZD7288 inhibit both HCN current and Ca(2+) influx. Measurements of the fractional Ca(2+) current showed that it constitutes 0.60 +/- 0.02% of the net inward current through HCN4 at -120 mV. This fractional current is similar to that of the low Ca(2+)-permeable AMPA-R (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor) channels in Purkinje neurons. In DRG neurons, activation of I(h) for 30 s also resulted in a Ca(2+) influx and an elevated action potential-induced secretion, as assayed by the increase in membrane capacitance. These results suggest a functional significance for I(h) channels in modulating neuronal secretion by permitting Ca(2+) influx at negative membrane potentials.

Protein kinase A phosphorylation at serine-2808 of the cardiac Ca2+-release channel (ryanodine receptor) does not dissociate 12.6-kDa FK506-binding protein (FKBP12.6)

Dissociation of FKBP12.6 from the cardiac Ca2+-release channel (RyR2) as a consequence of protein kinase A (PKA) hyperphosphorylation of RyR2 at a single amino acid residue, serine-2808, has been proposed as an important mechanism underlying cardiac dysfunction in heart failure. However, the issue of whether PKA phosphorylation of RyR2 can dissociate FKBP12.6 from RyR2 is controversial. To additionally address this issue, we investigated the effect of PKA phosphorylation and mutations at serine-2808 of RyR2 on recombinant or native FKBP12.6-RyR2 interaction. Site-specific antibodies, which recognize the serine-2808 phosphorylated or nonphosphorylated form of RyR2, were used to unambiguously correlate the phosphorylation state of RyR2 at serine-2808 with its ability to bind FKBP12.6. We found that FKBP12.6 can bind to both the serine-2808 phosphorylated and nonphosphorylated forms of RyR2. The S2808D mutant thought to mimic constitutive phosphorylation also retained the ability to bind FKBP12.6. Complete phosphorylation at serine-2808 by exogenous PKA disrupted neither the recombinant nor native FKBP12.6-RyR2 complex. Furthermore, binding of site-specific antibodies to the serine-2808 phosphorylation site did not dissociate FKBP12.6 from or prevent FKBP12.6 from binding to RyR2. Taken together, our results do not support the notion that PKA phosphorylation at serine-2808 dissociates FKBP12.6 from RyR2.

Molecular determinants of altered contractility in heart failure

Heart failure remains a leading cause of mortality in the Western world. An important hallmark of heart failure is reduced myocardial contractility. Alterations in intracellular Ca2+ handling play a major role in the pathophysiology of these contractile abnormalities. Several defects in the excitation-contraction (EC) coupling system have been identified in patients with heart failure. Alterations in the density and function of proteins relevant for EC coupling have been reported. Chronic stimulation of the beta-adrenergic signaling pathway leads to protein kinase A (PKA) hyperphosphorylation of the cardiac ryanodine receptor (RyR2), which dissociates FKBP12.6 from RyR2, thereby altering channel gating and promoting diastolic sarcoplasmic reticulum (SR) Ca2+ release. This may deplete the SR Ca2+ stores, which may reduce myocardial contractility. Clinical studies have demonstrated that beta-adrenergic receptor blockers reduce morbidity and mortality in all grades of congestive heart failure. Our experimental data indicate that beta-blockers reverse RyR2 hyperphosphorylation and normalize channel gating, which is associated with increased contractility in heart failure. In conclusion, chronic hyperactivity of the beta-adrenergic signaling pathway impairs intracellular Ca2+ handling, which leads to reduced contractility in patients with heart failure.

Left Ventricular Mechanoenergetics in Small Animals

Studies on left ventricular mechanical work and energetics in rat and mouse hearts are reviewed. First, left ventricular linear end-systolic pressure-volume relation (ESPVR) and curved end-diastolic pressure-volume relation (EDPVR) in canine hearts and left ventricular curved ESPVR and curved EDPVR in rat hearts are reviewed. Second, as an index for total mechanical energy per beat in rat hearts as in canine hearts, a systolic pressure-volume area (PVA) is proposed. By the use of our original system for measuring continuous oxygen consumption for rat left ventricular mechanical work, the linear left ventricular myocardial oxygen consumption per beat (VO2)-PVA relation is obtained as in canine hearts. The slope of VO2-PVA relation (oxygen cost of PVA) indicates a ratio of chemomechanical energy transduction. VO2 intercept (PVA-independent VO2) indicates the summation of oxygen consumption for Ca2+ handling in excitation-contraction coupling and for basal metabolism. An equivalent maximal elastance (eEmax) is proposed as a new left ventricular contractility index based on PVA at the midrange left ventricular volume. The slope of the linear relation between PVA-independent VO2 and eEmax (oxygen cost of eEmax) indicates changes in oxygen consumption for Ca2+ handling in excitation-contraction coupling per unit changes in left ventricular contractility. The key framework of VO2-PVA-eEmax can give us a better understanding for the biology and mechanisms of physiological and various failing rat heart models in terms of mechanical work and energetics.

Gene therapy for heart failure

Despite our continued advances in the management of coronary artery disease, there have been no significant reductions in the morbidity or mortality related to end-stage heart failure. The syndrome of heart failure represents a common endpoint for several disease processes, however, at the molecular level there are certain biochemical similarities common to all failing myocardium. Targeting these derangements with gene therapy represents a promising option in the treatment of heart failure. In this review, we will discuss the common biochemical changes that occur in the failing heart, novel therapeutic targets, including the beta-adrenergic receptor system and intracellular calcium regulation, and the vectors and transfer methodology responsible for delivering these transgenes to the myocardium.

The Predicted TM10 Transmembrane Sequence of the Cardiac Ca2+ Release Channel (Ryanodine Receptor) Is Crucial for Channel Activation and Gating

The predicted TM10 transmembrane sequence, (4844)IIFDITFFFFVIVILLAIIQGLII(4867), has been proposed to be the pore inner helix of the ryanodine receptor (RyR) and to play a crucial role in channel activation and gating, as with the inner helix of bacterial potassium channels. However, experimental evidence for the involvement of the TM10 sequence in RyR channel activation and gating is lacking. In the present study, we have systematically investigated the effects of mutations of each residue within the 24-amino acid TM10 sequence of the mouse cardiac ryanodine receptor (RyR2) on channel activation by caffeine and Ca(2+). Intracellular Ca(2+) release measurements in human embryonic kidney 293 cells expressing the RyR2 wild type and TM10 mutants revealed that several mutations in the TM10 sequence either abolished caffeine response or markedly reduced the sensitivity of the RyR2 channel to activation by caffeine. By assessing the Ca(2+) dependence of [(3)H]ryanodine binding to RyR2 wild type and TM10 mutants we also found that mutations in the TM10 sequence altered the sensitivity of the channel to activation by Ca(2+) and enhanced the basal activity of [(3)H]ryanodine binding. Furthermore, single I4862A mutant channels exhibited considerable channel openings and altered gating at very low concentrations of Ca(2+). Our data indicate that the TM10 sequence constitutes an essential determinant for channel activation and gating, in keeping with the proposed role of TM10 as an inner helix of RyR. Our results also shed insight into the orientation of the TM10 helix within the RyR channel pore.

p38 MAP kinase-mediated negative inotropic effect of HIV gp120 on cardiac myocytes

Myocardial dysfunction leading to dilated cardiomyopathy has been documented with surprisingly high frequency in human immunodeficiency virus (HIV)-infected individuals. p38 MAP kinase has been implicated as a mediator of myocardial dysfunction. We previously reported p38 MAP kinase activation by the HIV coat protein gp120 in neonatal rat cardiac myocytes. We now report the direct inotropic effects of HIV gp120 on adult rat ventricular myocytes (ARVM). ARVM were continuously superfused with gp120, and percent fractional shortening (FS) was determined by automated border detection and simultaneous intracellular ionized free Ca2+concentration ([Ca2+]i) measured by fura 2-AM fluorescence: gp120 alone increased FS and increased [Ca2+]iwithin 5 min and then depressed FS without a decrease in [Ca2+]iby 20–60 min, which persisted for at least 2 h. Exposure of ARVM to gp120 also resulted in the phosphorylation of the upstream regulator of p38 MAP kinase MKK3/6, p38 MAP kinase itself, and its downstream effector, ATF-2, over a similar time course. ERK (p44/42) and JNK stress signaling pathways were not similarly activated. The effects of the p38 MAP kinase inhibitor were concentration dependent. SB-203580 (10 μM) blocked both p38 MAP kinase phosphorylation and the delayed negative inotropic effect of gp120. SB-203580 (5 μM) selectively blocked phosphorylation of ATF-2 without blocking the phosphorylation of MKK3/6 or p38 MAP kinase itself. SB-203580 (5 μM) administered before, with, or after gp120 blocked the negative inotropic effect of gp120 in ARVM. p38 MAP kinase activation may be a common stress-response mechanism contributing to myocardial dysfunction in HIV and other nonischemic as well as ischemic cardiomyopathies.

Cytoskeletal modulation of electrical and mechanical activity in cardiac myocytes

The cardiac myocyte has an intracellular scaffold, the cytoskeleton, which has been implicated in several cardiac pathologies including hypertrophy and failure. In this review we describe the role that the cytoskeleton plays in modulating both the electrical activity (through ion channels and exchangers) and mechanical (or contractile) activity of the adult heart. We focus on the 3 components of the cytoskeleton, actin microfilaments, microtubules, and desmin filaments. The limited visual data available suggest that the subsarcolemmal actin cytoskeleton is sparse in the adult myocyte. Selective disruption of cytoskeletal actin by pharmacological tools has yet to be verified in the adult cell, yet evidence exists for modulation of several ionic currents, including I(CaL), I(Na), I(KATP), I(SAC) by actin microfilaments. Microtubules exist as a dense network throughout the adult cardiac cell, and their structure, architecture, kinetics and pharmacological manipulation are well described. Both polymerised and free tubulin are functionally significant. Microtubule proliferation reduces contraction by impeding sarcomeric motion; modulation of sarcoplasmic reticulum Ca(2+) release may also be involved in this effect. The lack of effect of microtubule disruption on cardiac contractility in adult myocytes, and the concentration-dependent modulation of the rate of contraction by the disruptor nocodazole in neonatal myocytes, support the existence of functionally distinct microtubule populations. We address the controversy regarding the stimulation of the beta-adrenergic signalling pathway by free tubulin. Work with mice lacking desmin has demonstrated the importance of intermediate filaments to normal cardiac function, but the precise role that desmin plays in the electrical and mechanical activity of cardiac muscle has yet to be determined.

Forefront of Na+/Ca2+ Exchanger Studies: Molecular Pharmacology of Na+/Ca2+ Exchange Inhibitors

The Na+/Ca2+ exchanger (NCX) is an ion transporter that exchanges Na+ and Ca2+ in either Ca2+ efflux or Ca2+ influx mode, depending on membrane potential and transmembrane ion gradients. In myocytes, neurons, and nephron cells, NCX is thought to play an important role in the regulation of intracellular Ca2+ concentration. Recently, the benzyloxyphenyl derivatives KB-R7943, SEA0400, and SN-6 have been developed as selective NCX inhibitors. Currently, SEA0400 is the most potent and selective inhibitor. These inhibitors possess different isoform-selectivities, although they have similar properties, such as Ca2+ influx mode-selectivity and I1 inactivation-dependence. Recent site-directed mutagenesis has revealed that these inhibitors possess some molecular determinants (Phe-213, Val-227, Tyr-228, Gly-833, and Asn-839) for interaction with NCX1. These benzyloxyphenyl derivatives are expected to be useful tools to study the physiological roles of NCX. Moreover, such inhibitors may have therapeutic potential as a new remedy for ischemic disease, arrhythmias, heart failure, and hypertension.

NADH oxidase activity of rat cardiac sarcoplasmic reticulum regulates calcium-induced calcium release

NADH and Ca2+ have important regulatory functions in cardiomyocytes related to excitation-contraction coupling and ATP production. To elucidate elements of these functions, we examined the effect of NADH on sarcoplasmic reticulum (SR) Ca2+ release and the mechanisms of this regulation. Physiological concentrations of cytosolic NADH inhibited ryanodine receptor type 2 (RyR2)-mediated Ca2+-induced Ca2+ release (CICR) from SR membranes (IC50=120 micromol/L) and significantly lowered single channel open probability. In permeabilized single ventricular cardiomyocytes, NADH significantly inhibited the amplitude and frequency of spontaneous Ca2+ release. Blockers of electron transport prevented the inhibitory effect of NADH on CICR in isolated membranes and permeabilized cells, as well as on the activity of RyR2 channels reconstituted in lipid bilayer. An endogenous NADH oxidase activity from rat heart copurified with SR enriched with RyR2. A significant contribution by mitochondria was excluded as NADH oxidation by SR exhibited >9-fold higher catalytic activity (8.8 micromol/mg protein per minute) in the absence of exogenous mitochondrial complex I (ubiquinone) or complex III (cytochrome c) electron acceptors, but was inhibited by rotenone and pyridaben (IC50=2 to 3 nmol/L), antimycin A (IC50=13 nmol/L), and diphenyleneiodonium (IC50=28 micromol/L). Cardiac junctional SR treated with [3H](trifluoromethyl)diazirinyl-pyridaben specifically labeled a single 23-kDa PSST-like protein. These data indicate that NADH oxidation is tightly linked to, and essential for, negative regulation of the RyR2 complex and is a likely component of an important physiological negative-feedback mechanism coupling SR Ca2+ fluxes and mitochondrial energy production.

Molecular determinants of Na+/Ca2+ exchange (NCX1) inhibition by SEA0400

SEA0400 is a potent and selective Na(+)/Ca(2+) exchanger (NCX) inhibitor. We evaluated the inhibitory effects of SEA0400 on Na(+)(i)-dependent (45)Ca(2+) uptake and whole-cell Na(+)/Ca(2+) exchange currents in NCX-transfected fibroblasts. SEA0400 preferentially inhibited (45)Ca(2+) uptake by NCX1 compared with inhibitions by NCX2, NCX3, and NCKX2. SEA0400 also selectively blocked outward exchange currents from NCX1 transfectants. We searched for regions that may form the SEA0400 receptor in the NCX1 molecule by NCX1/NCX3 chimeric analysis. The results suggest that the first intracellular loop and the fifth transmembrane segment are mostly responsible for the differential drug responses between NCX1 and NCX3. Further site-directed mutagenesis revealed that multiple mutations at Phe-213 markedly reduced sensitivity to SEA0400 without affecting that to KB-R7943. We also found that Gly-833-to-Cys mutation (within the alpha-2 repeat) greatly reduced the inhibition by SEA0400, but unexpectedly the NCX1 chimera with an alpha-2 repeat from NCKX2 possessed normal drug sensitivity. In addition, exchangers with mutated exchanger inhibitory peptide regions, which display either undetectable or accelerated Na(+)-dependent inactivation, had a markedly reduced sensitivity or hypersensitivity to SEA0400, respectively. To verify the efficacy of the NCX inhibitor, we examined the renoprotective effect of SEA0400 in a hypoxic injury model using porcine renal tubular cells. SEA0400 protected against hypoxia/reoxygenation-induced cell damage in tubular cells expressing wild-type NCX1 but not in cells expressing SEA0400-insensitive mutants. These results suggest that Phe-213, Gly-833, and residues that eliminate Na(+)-dependent inactivation are critical determinants for the inhibition by SEA0400, and their mutants are very useful for checking the pharmacological importance of NCX inhibition by SEA0400.

What is the role of beta-adrenergic signaling in heart failure?

This review addresses open questions about the role of beta-adrenergic receptors in cardiac function and failure. Cardiomyocytes express all three beta-adrenergic receptor subtypes-beta1, beta2, and, at least in some species, beta3. The beta1 subtype is the most prominent one and is mainly responsible for positive chronotropic and inotropic effects of catecholamines. The beta2 subtype also increases cardiac function, but its ability to activate nonclassical signaling pathways suggests a function distinct from the beta1 subtype. In heart failure, the sympathetic system is activated, cardiac beta-receptor number and function are decreased, and downstream mechanisms are altered. However, in spite of a wealth of data, we still do not know whether and to what extent these alterations are adaptive/protective or detrimental, or both. Clinically, beta-adrenergic antagonists represent the most important advance in heart failure therapy, but it is still debated whether they act by blocking or by resensitizing the beta-adrenergic receptor system. Newer experimental therapeutic strategies aim at the receptor desensitization machinery and at downstream signaling steps.

Regulation of cardiac excitation-contraction coupling: a cellular update

The primary purpose of this paper is to present a basic overview of some "relatively" new ideas related to the regulation of cardiac performance and underlying excitation-contraction (EC) coupling that have yet to be incorporated to textbooks currently used for introductory graduate-level physiology courses. Within the context of cardiac EC coupling, this review incorporates information on microdomains and local control theory, with particular emphasis on the role of Ca(2+) sparks as a key regulatory component of ventricular myocyte contraction dynamics. Recent information pertaining to Ca(2+) release mechanisms specific to the sarcoplasmic reticulum is also presented, as well as the idea of the ryanodine receptor as a macromolecular signaling complex. Because of the potential relationship to maladaptive functional responses under conditions of cardiovascular pathology, the regulatory role of cardiac adrenergic and additional G protein-coupled receptors known to regulate cardiac function is included, and fundamental concepts related to intracellular signaling are discussed. Finally, information on the roles of vascular and cardiac nitric oxide as an important regulator of cardiac performance is included to allow students to begin to think about the ubiquitous role of nitric oxide in the regulation of the cardiovascular system. An important point of emphasis is that whole organ cardiac dynamics can be traced back to the cellular events regulating intracellular Ca(2+) homeostasis and as such provides an important conceptual framework from which the students can begin to think about whole organ physiology in health and disease.

Mechanism of enhanced cardiac function in mice with hypertrophy induced by overexpressed Akt

Transgenic mice with cardiac-specific overexpression of active Akt (TG) not only exhibit hypertrophy but also show enhanced left ventricular (LV) function. In 3-4-month-old TG, heart/body weight was increased by 60% and LV ejection fraction was elevated (84 +/- 2%, p < 0.01) compared with nontransgenic littermates (wild type (WT)) (73 +/- 1%). An increase in isolated ventricular myocyte contractile function (% contraction) in TG compared with WT (6.1 +/- 0.2 versus 3.5 +/- 0.2%, p < 0.01) was associated with increased Fura-2 Ca2+ transients (396 +/- 50 versus 250 +/- 24 nmol/liter, p < 0.05). The rate of relaxation (+dL/dt) was also enhanced in TG (214 +/- 15 versus 98 +/- 18 microm/s, p < 0.01). L-type Ca2+ current (ICa) density was increased in TG compared with WT (-9.0 +/- 0.3 versus 7.2 +/- 0.3 pA/pF, p < 0.01). Sarcoplasmic reticulum Ca2+ ATPase 2a (SERCA2a) protein levels were increased (p < 0.05) by 6.6-fold in TG, which could be recapitulated in vitro by adenovirus-mediated overexpression of Akt in cultured adult ventricular myocytes. Conversely, inhibiting SERCA with either ryanodine or thapsigargin affected myocyte contraction and relaxation and Ca2+ channel kinetics more in TG than in WT. Thus, myocytes from mice with overexpressed Akt demonstrated enhanced contractility and relaxation, Fura-2 Ca2+ transients, and Ca2+ channel currents. Furthermore, increased protein expression of SERCA2a plays an important role in mediating enhanced LV function by Akt. Up-regulation of SERCA2a expression and enhanced LV myocyte contraction and relaxation in Akt-induced hypertrophy is opposite to the down-regulation of SERCA2a and reduced contractile function observed in many other forms of LV hypertrophy.

Modeling beta-adrenergic control of cardiac myocyte contractility in silico

The beta-adrenergic signaling pathway regulates cardiac myocyte contractility through a combination of feedforward and feedback mechanisms. We used systems analysis to investigate how the components and topology of this signaling network permit neurohormonal control of excitation-contraction coupling in the rat ventricular myocyte. A kinetic model integrating beta-adrenergic signaling with excitation-contraction coupling was formulated, and each subsystem was validated with independent biochemical and physiological measurements. Model analysis was used to investigate quantitatively the effects of specific molecular perturbations. 3-Fold overexpression of adenylyl cyclase in the model allowed an 85% higher rate of cyclic AMP synthesis than an equivalent overexpression of beta 1-adrenergic receptor, and manipulating the affinity of Gs alpha for adenylyl cyclase was a more potent regulator of cyclic AMP production. The model predicted that less than 40% of adenylyl cyclase molecules may be stimulated under maximal receptor activation, and an experimental protocol is suggested for validating this prediction. The model also predicted that the endogenous heat-stable protein kinase inhibitor may enhance basal cyclic AMP buffering by 68% and increasing the apparent Hill coefficient of protein kinase A activation from 1.0 to 2.0. Finally, phosphorylation of the L-type calcium channel and phospholamban were found sufficient to predict the dominant changes in myocyte contractility, including a 2.6x increase in systolic calcium (inotropy) and a 28% decrease in calcium half-relaxation time (lusitropy). By performing systems analysis, the consequences of molecular perturbations in the beta-adrenergic signaling network may be understood within the context of integrative cellular physiology.

Spatial and dynamic interactions between phospholamban and the canine cardiac Ca2+ pump revealed with use of heterobifunctional cross-linking agents

Heterobifunctional thiol to amine cross-linking agents were used to gain new insights on the dynamics and conformational factors governing the interaction between the cardiac Ca2+ pump (SERCA2a) and phospholamban (PLB). PLB is a small protein inhibitor of SERCA2a that reduces enzyme affinity for Ca2+ and thereby regulates cardiac contractility. We found that the PLB monomer with Asn27 or Asn30 changed to Cys (N27C-PLB or N30C-PLB) cross-linked to lysine of SERCA2a within seconds with > or =80% efficiency. Optimal cross-linking occurred at spacer chain lengths of 10 and 15 A for N27C and N30C, respectively. The rapid time course of cross-linking indicated that neither dissociation of PLB pentamers nor binding of PLB monomers to SERCA2a was rate-limiting. Cross-linking occurred only to the E2 (Ca2+-free) conformation of SERCA2a, was strongly favored by nucleotide binding to this state, and was completely inhibited by thapsigargin. Protein sequencing in combination with mutagenesis identified of Lys328 of SERCA2a as the target of cross-linking. A three-dimensional map of interacting residues indicated that the cross-linking distances were entirely compatible with the 10-A distance recently determined between N30C of PLB and Cys318 of SERCA2a. In contrast, Lys3 of PLB did not cross-link to any Lys (or Cys) of SERCA2a, suggesting that previous three-dimensional models that constrain Lys3 near residues 397-400 of thapsigargin-inhibited SERCA2a should be viewed with caution. Furthermore, although earlier models of PLB.SERCA2a are based on thapsigargin-bound SERCA, our results suggest that the nucleotide-bound, E2 conformation is substantially different and represents the key conformational state for interacting with PLB.

Relationships among Doppler-derived umbilical artery absolute velocities, cardiac function, and placental volume blood flow and resistance in fetal sheep

We hypothesized that umbilical artery (UA) absolute blood flow velocities measured by Doppler ultrasonography reflect placental volume blood flow (Q(UA)) and placental vascular resistance (R(UA)) in a late gestation fetal sheep model. In addition, we examined the relationships between umbilical artery absolute blood flow velocities and parameters of fetal cardiac function. Twenty-six sheep fetuses were instrumented at 112-132 days of gestation. After a 5-day recovery period, experiments were performed under general anesthesia in 16 normal fetuses, in 5 fetuses after maternal administration of phenylephrine, and in 5 fetuses after placental embolization. The Q(UA) and arterial blood pressures were measured using a transit-time ultrasonic flow probe and a catheter placed into the descending aorta, respectively. UA peak systolic velocity (PSV), end-diastolic velocity (EDV), time-averaged maximum velocity (TAMXV), pulsatility index (PI), mean velocity (V(mean)), fetal cardiac output, ventricular ejection forces, and the proportion of isovolumetric relaxation time (IRT%) in the cardiac cycle were measured with the use of Doppler ultrasonography. Significant positive linear correlations were found between UA EDV, TAMXV, and V(mean) versus Q(UA), whereas UA PI had a significant negative correlation with Q(UA). Significant negative correlations were shown between UA EDV, TAMXV, and V(mean) versus R(UA). A significant positive correlation was present between UA PI and R(UA). Doppler-derived UA parameters did not correlate with fetal arterial blood pressures, cardiac output, ventricular ejection forces or IRT%. In fetal sheep, Doppler-derived UA PI and absolute velocities, except PSV, are closely related to directly measured Q(UA) and R(UA), validating the use of noninvasive Doppler velocimetry in the assessment of placental circulation.

A model of graded calcium release and L-type Ca2+ channel inactivation in cardiac muscle

We have developed a model of Ca(2+) handling in ferret ventricular myocytes. This model includes a novel L-type Ca(2+) channel, detailed intracellular Ca(2+) movements, and graded Ca(2+)-induced Ca(2+) release (CICR). The model successfully reproduces data from voltage-clamp experiments, including voltage- and time-dependent changes in intracellular Ca(2+) concentration ([Ca(2+)](i)), L-type Ca(2+) channel current (I(CaL)) inactivation and recovery kinetics, and Ca(2+) sparks. The development of graded CICR is critically dependent on spatial heterogeneity and the physical arrangement of calcium channels in opposition to ryanodine-sensitive release channels. The model contains spatially distinct subsystems representing the subsarcolemmal regions where the junctional sarcoplasmic reticulum (SR) abuts the T-tubular membrane and where the L-type Ca(2+) channels and SR ryanodine receptors (RyRs) are localized. There are eight different types of subsystems in our model, with between one and eight L-type Ca(2+) channels distributed binomially. This model exhibits graded CICR and provides a quantitative description of Ca(2+) dynamics not requiring Monte-Carlo simulations. Activation of RyRs and release of Ca(2+) from the SR depend critically on Ca(2+) entry through L-type Ca(2+) channels. In turn, Ca(2+) channel inactivation is critically dependent on the release of stored intracellular Ca(2+). Inactivation of I(CaL) depends on both transmembrane voltage and local [Ca(2+)](i) near the channel, which results in distinctive inactivation properties. The molecular mechanisms underlying many I(CaL) gating properties are unclear, but [Ca(2+)](i) dynamics clearly play a fundamental role.

Electrophysiological effects of 5-hydroxytryptamine on isolated human atrial myocytes, and the influence of chronic beta-adrenoceptor blockade

5-Hydroxytryptamine (5-HT) has been postulated to play a proarrhythmic role in the human atria via stimulation of 5-HT4 receptors. The aims of this study were to examine the effects of 5-HT on the L-type Ca2+ current (ICaL) action potential duration (APD), the effective refractory period (ERP) and arrhythmic activity in human atrial cells, and to assess the effects of prior treatment with beta-adrenoceptor antagonists. Isolated myocytes, from the right atrial appendage of 27 consenting patients undergoing cardiac surgery who were in sinus rhythm, were studied using the whole-cell perforated patch-clamp technique at 37 degrees C. 5-HT (1 nm-10 microm) caused a concentration-dependent increase in ICaL, which was potentiated in cells from beta-blocked (maximum response to 5-HT, Emax=299+/-12% increase above control) compared to non-beta-blocked patients (Emax=220+/-6%, P<0.05), but with no change in either the potency (log EC50: -7.09+/-0.07 vs -7.26+/-0.06) or Hill coefficient (nH: 1.5+/-0.6 vs 1.5+/-0.3) of the 5-HT concentration-response curve. 5-HT (10 microm) produced a greater increase in the APD at 50% repolarisation (APD50) in cells from beta-blocked patients (of 37+/-10 ms, i.e. 589+/-197%) vs non-beta-blocked patients (of 10+/-4 ms, i.e. 157+/-54%; P<0.05). Both the APD90 and the ERP were unaffected by 5-HT. Arrhythmic activity was observed in response to 5-HT in five of 17 cells (29%) studied from beta-blocked, compared to zero of 16 cells from the non-beta-blocked patients (P<0.05). In summary, the 5-HT-induced increase in calcium current was associated with a prolonged early plateau phase of repolarisation, but not late repolarisation or refractoriness, and the enhancement of these effects by chronic beta-adrenoceptor blockade was associated with arrhythmic potential.

Influence of age and run training on cardiac Na+/Ca2+ exchange

Effects of age and training on myocardial Na+/Ca2+ exchange were examined in young sedentary (YS; 14-15 mo), aged sedentary (AS; 27-31 mo), and aged trained (AT; 8- to 11-wk treadmill run training) male Fischer Brown Norway rats. Whole heart performance and isolated cardiocyte Na+/Ca2+ exchange characteristics were measured. At the whole heart level, a small but significant slowing of late isovolumic left ventricular (LV) relaxation, which may be indicative of altered Na+/Ca2+ exchange activity, was seen in hearts from AS rats. This subtle impairment in relaxation was not observed in hearts from AT rats. At the single-cardiocyte level, late action potential duration was prolonged, resting membrane potential was more positive, and overshoot potential was greater in cardiocytes from AS rats than from YS rats (P < 0.05). Training did not influence any of these age-related action potential characteristics. In electrically paced cardiocytes, neither shortening nor intracellular Ca2+ concentration ([Ca2+]i) dynamics was influenced by age or training. Similarly, neither age nor training influenced the rate of [Ca2+]i clearance via forward (Nain+ /Caout2+) Na+/Ca2+ exchange after caffeine-induced Ca2+ release from the sarcoplasmic reticulum or cardiac Na+/Ca2+ exchanger protein (NCX1) expression. However, when whole cell patch-clamp techniques combined with fluorescence microscopy were used to evaluate the ability of Na+/Ca2+ exchange to alter cytosolic [Ca2+] ([Ca2+]c) under conditions where membrane potential (Vm) and internal and external [Na+] and [Ca2+] could be controlled, we observed age-associated increases in forward Na+/Ca2+ exchange-mediated [Ca2+]c clearance (P < 0.05) that were not influenced by training. The age-related increase in forward Na+/Ca2+ exchange activity provides a hypothetical explanation for the late action potential prolongation observed in this study.

Mesenteric Lymph From Rats With Thermal Injury Prolongs the Action Potential and Increases Ca2+ Transient in Rat Ventricular Myocytes

Although gut-derived mesenteric lymph from animals with thermal injury appears to lead to myocardial contractile dysfunction, the cellular mechanisms remain unclear. We examined the direct effects of intestinal lymph on excitation-contraction coupling in rat ventricular myocytes. Lymph from rats receiving burn injury (burn lymph), but not from sham-burned rats, rapidly enhanced myocyte contraction and the amplitude of Ca2+ transient; the average percentage of shortening was increased from 5.5 +/- 0.3% to 10.5 +/- 0.9%. 90% and the Ca2+ transients increased by 80% +/- 20%. Burn lymph had no effect on the amplitude of L-type Ca2+ current (ICa) or the inward rectifier K+ current, but the transient outward K+ currents (Ito) were reduced significantly by burn lymph. Inhibition of Ito was not altered by an alpha1-adrenergic receptor (AR) antagonist, prazosin, indicating that the block was not mediated via alpha1-AR signaling pathway. Action potential (AP) duration, measured at 50% and 90% repolarization, was prolonged by burn lymph. Stimulation of myocytes with AP voltage-clamp waveforms derived from prolonged AP induced by burn lymph revealed a 1.7-fold increase in Ca2+ influx via ICa compared with the Ca2+ influx induced by control AP. Blocking of Ito by 4-aminopyridine prolonged AP duration and increased Ca2+ transients, mimicking the effects of burn lymph. Burn lymph did not affect Na+/Ca2+ exchange currents or caffeine-induced SR Ca2+ release. Thus, acute exposure of normal cardiac myocytes to burn lymph increases Ca2+ transients by a prolongation of AP as a result of a reduction of Ito with no intrinsic change in ICa or exchanger. The electrophysiological changes are similar to those that occur during compensated cardiac hypertrophy, suggesting a common mechanistic link between burn lymph- and hypertrophy-induced cardiac dysfunction.

Quantitative Reconstruction of Cardiac Electromechanics in Human Myocardium:

INTRODUCTION

Myocytes from normal and failing myocardium show significant differences in electromechanical behavior. Mathematical modeling of the behavior provides insights into the underlying physiologic and pathophysiologic mechanisms. Electromechanical models of cardiomyocytes exist for various species, but models of human myocytes are lacking.

METHODS AND RESULTS

A mathematical model of electromechanics in normal and failing cardiac myocytes in humans was created by assembly and adaptation of parameters of an electrophysiologic model at the level of single cells and a force development model at the level of the sarcomere. The adaptation was performed using data from recent studies of ventricular myocytes and myocardium. The model was applied to quantitatively reconstruct measurement data from different experimental studies of normal and failing myocardium. Several simulations were performed to quantify the transmembrane voltage Vm, intracellular concentration of calcium[Ca2+]i, the [Ca2+]i-force relationship, and force transients. Furthermore, frequency dependencies and restitution of action voltage duration to 90% recovery APD90, peak [Ca2+]i, duration to 50% force recovery FD50, and peak force were determined.

CONCLUSION

The presented mathematical model was capable of quantitatively reconstructing data obtained from different studies of electrophysiology and force development in normal and failing myocardium of humans. In future work, the model can serve as a component for studying macroscopic mechanisms of excitation propagation, metabolism, and electromechanics in human myocardium.

The Electrical Restitution Curve Revisited:

The electrical restitution curve (ERC) traditionally describes the recovery of action potential duration (APD) as a function of the interbeat interval or, more correctly, the diastolic interval (DI). Often overlooked in modeling studies, the normal ventricular ERC is triphasic, starting with a steep initial recovery at the shortest DIs, a transient decline, and a final asymptotic rise to a plateau phase reached at long DIs. Recent studies have proposed that it would be advantageous to lower the slope of the ERC by drug intervention, as this might reduce the potential for electrical alternans and ventricular fibrillation. This review discusses the pros and cons of a flat versus steep slope of the ERC and draws attention to mechanisms thatjustify the (physiologically) steep slope, rather than a flat slope, as a better design against arrhythmias. Five potential mechanisms are discussed, which allows for a different interpretation of the effect of the slope on arrhythmogenicity. The most important appears to be the physiologic rate adaptive shortening of APD that, by reciprocal lengthening of the DI, allows the subsequent APD to move more quickly from the steep initial ERC phase onto the flat phase. A less steep initial ERC phase would protract the transition toward more fully recovered APD and, in fact, may perpetuate electrical alternans. The triphasic ERC time course in normal myocardium cannot be explained by or fitted to single exponentials or single ion channel recovery kinetics. A simple tri-ionic model is suggested that may help explain the shape of the ERC at various repolarization levels and place APD recovery into perspective with intracellular calcium recycling and recovery of contractile force.

Beta-adrenergic regulation requires direct anchoring of PKA to cardiac CaV1.2 channels via a leucine zipper interaction with A kinase-anchoring protein 15

Activation of beta-adrenergic receptors and consequent phosphorylation by cAMP-dependent protein kinase A (PKA) greatly increases the L-type Ca2+ current through CaV1.2 channels in isolated cardiac myocytes. A kinase-anchoring protein 15 (AKAP15) coimmunoprecipitates with CaV1.2 channels isolated from rat heart membrane extracts and transfected cells, and it colocalizes with CaV1.2 channels and PKA in the transverse tubules of isolated ventricular myocytes. Site-directed mutagenesis studies reveal that AKAP15 directly interacts with the distal C terminus of the cardiac CaV1.2 channel via a leucine zipper-like motif. Disruption of PKA anchoring to CaV1.2 channels via AKAP15 using competing peptides markedly inhibits the beta-adrenergic regulation of CaV1.2 channels via the PKA pathway in ventricular myocytes. These results identify a conserved leucine zipper motif in the C terminus of the CaV1 family of Ca2+ channels that directly anchors an AKAP15-PKA signaling complex to ensure rapid and efficient regulation of L-type Ca2+ currents in response to beta-adrenergic stimulation and local increases in cAMP.

G-proteins as transducers in transmembrane signalling

The G-protein-mediated signalling system has evolved as one of the most widely used transmembrane signalling mechanisms in mammalian organisms. All mammalian cells express G-protein-coupled receptors as well as several types of heterotrimeric G-proteins and effectors. G-protein-mediated signalling is involved in many physiological and pathological processes. This review summarizes some general aspects of G-protein-mediated signalling and focusses on recent data especially from studies in mutant mice which have elucidated some of the cellular and biological functions of heterotrimeric G-prtoteins.

Protein phosphatases decrease sarcoplasmic reticulum calcium content by stimulating calcium release in cardiac myocytes

Phosphorylation/dephosphorylation of Ca2+ transport proteins by cellular kinases and phosphatases plays an important role in regulation of cardiac excitation-contraction coupling; furthermore abnormal protein kinase and phosphatase activities have been implicated in heart failure. However, the precise mechanisms of action of these enzymes on intracellular Ca2+ handling in normal and diseased hearts remains poorly understood. We have investigated the effects of protein phosphatases PP1 and PP2A on spontaneous Ca2+ sparks and SR Ca2+ load in myocytes permeabilized with saponin. Exposure of myocytes to PP1 or PP2A caused a dramatic increase in frequency of Ca2+ sparks followed by a nearly complete disappearance of events. These effects were accompanied by depletion of the SR Ca2+ stores, as determined by application of caffeine. These changes in Ca2+ release and SR Ca2+ load could be prevented by the inhibitors of PP1 and PP2A phosphatase activities okadaic acid and calyculin A. At the single channel level, PP1 increased the open probability of RyRs incorporated into lipid bilayers. PP1-mediated RyR dephosphorylation in our permeabilized myocytes preparations was confirmed biochemically by quantitative immunoblotting using a phosphospecific anti-RyR antibody. Our results suggest that increased intracellular phosphatase activity stimulates RyR-mediated SR Ca2+ release leading to depleted SR Ca2+ stores in cardiac myocytes.

Calsequestrin determines the functional size and stability of cardiac intracellular calcium stores: Mechanism for hereditary arrhythmia

Calsequestrin is a high-capacity Ca-binding protein expressed inside the sarcoplasmic reticulum (SR), an intracellular Ca release and storage organelle in muscle. Mutations in the cardiac calsequestrin gene (CSQ2) have been linked to arrhythmias and sudden death. We have used Ca-imaging and patch-clamp methods in combination with adenoviral gene transfer strategies to explore the function of CSQ2 in adult rat heart cells. By increasing or decreasing CSQ2 levels, we showed that CSQ2 not only determines the Ca storage capacity of the SR but also positively controls the amount of Ca released from this organelle during excitation-contraction coupling. CSQ2 controls Ca release by prolonging the duration of Ca fluxes through the SR Ca-release sites. In addition, the dynamics of functional restitution of Ca-release sites after Ca discharge were prolonged when CSQ2 levels were elevated and accelerated in the presence of lowered CSQ2 protein levels. Furthermore, profound disturbances in rhythmic Ca transients in myocytes undergoing periodic electrical stimulation were observed when CSQ2 levels were reduced. We conclude that CSQ2 is a key determinant of the functional size and stability of SR Ca stores in cardiac muscle. CSQ2 appears to exert its effects by influencing the local luminal Ca concentration-dependent gating of the Ca-release channels and by acting as both a reservoir and a sink for Ca in SR. The abnormal restitution of Ca-release channels in the presence of reduced CSQ2 levels provides a plausible explanation for ventricular arrhythmia associated with mutations of CSQ2.

Ryanodine receptor mutations associated with stress-induced ventricular tachycardia mediate increased calcium release in stimulated cardiomyocytes

Ca2+ release from the sarcoplasmic reticulum mediated by the cardiac ryanodine receptor (RyR2) is a fundamental event in cardiac muscle contraction. RyR2 mutations suggested to cause defective Ca2+ channel function have recently been identified in catecholaminergic polymorphic ventricular tachycardia (CPVT) and arrhythmogenic right ventricular dysplasia (ARVD) affected individuals. We report expression of three CPVT-linked human RyR2 (hRyR2) mutations (S2246L, N4104K, and R4497C) in HL-1 cardiomyocytes displaying correct targeting to the endoplasmic reticulum. N4104K also localized to the Golgi apparatus. Phenotypic characteristics including intracellular Ca2+ handling, proliferation, viability, RyR2:FKBP12.6 interaction, and beat rate in resting HL-1 cells expressing mutant hRyR2 were indistinguishable from wild-type (WT) hRyR2. However, Ca2+ release was augmented in cells expressing mutant hRyR2 after RyR activation (caffeine and 4-chloro-m-cresol) or beta-adrenergic stimulation (isoproterenol). RyR2:FKBP12.6 interaction remained intact after caffeine or 4-CMC activation, but was dramatically disrupted by isoproterenol or forskolin, an activator of adenylate cyclase. Isoproterenol and forskolin elevated cyclic-AMP to similar magnitudes in all cells and were associated with equivalent hyperphosphorylation of mutant and WT hRyR2. CPVT-linked mutations in hRyR2 did not alter resting cardiomyocyte phenotype but mediated augmented Ca2+ release on RyR-agonist or beta-AR stimulation. Furthermore, equivalent interaction between mutant and WT hRyR2 and FKBP12.6 was demonstrated.

Gonadectomy of adult male rats reduces contractility of isolated cardiac myocytes

Sex-related differences in cardiac function have been well documented. The extent to which sex hormones are responsible for these differences is unclear. The current study was designed to determine whether castration and androgen replacement resulted in changes in functional expression of genes encoding the L-type calcium channel and Na/Ca exchanger in isolated rat ventricular myocytes. Sixteen weeks of castration produced a 50% decline in dihydropyridine receptor expression levels and a 16% (P < 0.05) increase in time to peak shortening. Furthermore, cardiac myocytes isolated from castrated animals also displayed an 18% (P < 0.001) increase in time to relengthening and an 80% decrease in Na/Ca exchanger gene expression when compared with intact controls. Testosterone treatment of castrated animals completely reversed these effects. These results provide the first evidence that androgens regulate functional expression of the L-type calcium channel and the Na/Ca exchanger in isolated rat ventricular myocytes and thus may play a role in modulating cardiac performance in males and thereby contribute to the observed gender differences in cardiac function.

The endogenous inhibitor of NCX1 does not resemble the properties of digitalis compound

In our previous study, we ware successful in isolation and purification of an endogenous inhibitor of the Na/Ca exchanger (NCX1) from the calf ventricle extracts. The purified factor has characterized to have strong positive inotropic effect on isometric contractions of isolated ventricle strips of guinea pig. A possibility is that besides the NCX1 the endogenous factor may also interact with other ion-transport systems (e.g., Na,K-ATPase) involved in modulation of muscle contractility-relaxation. Therefore, a primary goal of the present study was to detect a possible effect of newly found NCX1 inhibitor on Na,K-ATPase and Ca-ATPase activities. The preparations of isolated sarcolemma vesicles were used for this goal. Although the crude extracts of calf ventricles can inhibit both the Na/Ca exchange and Na,K-ATPase, these two inhibitory activities can be separated on the Sephadex G-10 column, meaning that different molecular entities might be responsible for inhibition of Na/Ca exchange and Na,K-ATPase. Addition of 100 U of purified endogenous factor to the assay medium results in nearly complete inhibition of forward (Na(i)-dependent Ca-uptake) and reverse (Na(o)-dependent Ca-efflux) modes of Na/Ca exchange. On the other hand, no effect was detected on activities of Na,K-ATPase and Ca-ATPase even in the presence of 500 U of purified factor in the assay medium. In light of the present data, it is concluded that the endogenous inhibitor of NCX1 does not resemble the targeting properties of digitalis like compound. Obviously, more systematic studies are required in the future for resolving a possible interaction of the endogenous inhibitor of NCX1 with other ion-transport systems involved in calcium homeostasis and action potential.

Increase of intracellular Ca(2+) during ischemia/reperfusion injury of heart is mediated by cyclic ADP-ribose

While the molecular mechanisms by which oxidants cause cytotoxicity are still poorly understood, disruption of Ca(2+) homeostasis appears to be one of the critical alterations during the oxidant-induced cytotoxic process. Here, we examined the possibility that oxidative stress may alter the metabolism of cyclic ADP-ribose (cADPR), a potent Ca(2+)-mobilizing second messenger in the heart. Isolated heart perfused by Langendorff technique was subjected to ischemia/reperfusion injury and endogenous cADPR level was determined using a specific radioimmunoassay. Following ischemia/reperfusion injury, a significant increase in intracellular cADPR level was observed. The elevation of cADPR content was closely correlated with the increase in ADP-ribosyl cyclase activity. Inclusion of oxygen free radical scavengers, 2,2,6,6-tetramethyl-1-piperidinyloxy and mannitol, in the reperfusate prevented the ischemia/reperfusion-induced increases in cADPR level and the ADP-ribosyl cyclase activity. Exposure of isolated cardiomyocytes to t-butyl hydroperoxide increased the ADP-ribosyl cyclase activity, cADPR level, and intracellular Ca(2+) concentration ([Ca(2+)](i)) and consequently resulting in cell lethal damage. The oxidant-induced elevation of [Ca(2+)](i) as well as cell lethal damage was blocked by a cADPR antagonist, 8-bromo-cADPR. These results provide evidence for involvement of cADPR and its producing enzyme in alteration of Ca(2+) homeostasis during the ischemia/reperfusion injury of the heart.

Benefit and risk of exercise on myocardial function in diabetes

Regular physical activity promotes cardiorespiratory fitness and has been considered a cornerstone for non-pharmacological treatment of more than 17 million Americans with diabetes mellitus. Physical exercise has been shown to positively affect certain cardiovascular risk factors such as insulin resistance, glucose metabolism, blood pressure and body fat composition, which are closely associated with diabetes and heart disease. With the increasingly sedentary life style in our society, routine daily exercise of moderate intensity is highly recommended to reduce cardiovascular risk, the leading cause of death in diabetic patients. Exercise produces many beneficial effects to the heart function such as reduced incidence of coronary heart disease, attenuated severity of diabetic cardiomyopathy, improved cardiac performance, cardiac reserve and autonomic regulation. Nevertheless, many diabetic patients do not appear to gain much benefit from exercise or may even be at risk of performing physical exercise. This review summarizes the benefit and risk of exercise on diabetic heart function, with a special emphasis on myocardial and autonomic function.

Inotropic effects on mammalian skeletal muscle change with contraction frequency

Over the last decade, we have attempted to determine if mammalian skeletal muscle's steady-level force development as established by mechanical and stimulation parameters can be increased or decreased by physiological signals. In these experiments, nitric oxide (NO), endothelin-1 (ET-1), adenosine (Ado), and beta-adrenergic agonists (beta) modified force production in the soleus and (or) the extensor digitorum longus (EDL) of the mouse. NO and beta increased the force produced by 0.5-s tetanic contractions at 0.6 contractions/min in both muscles. While EDL did not respond to either Ado or ET-1, the developed force of the soleus was amplified by Ado but attenuated by ET-1. Increased cAMP analogue concentrations amplified developed force in both muscles, but a cGMP analogue had no effect on either muscle. Following an increase in the contraction frequency of the soleus, the increased force in response to NO disappeared, as did the decreased force to ET-1. The increase in force due to a cAMP analogue disappeared during fatigue but reappeared quickly during recovery. Thus, steady-level developed force can be modified by a number of substances that can be released from locations in the body or muscle. The response to a given compound is determined by a complex interaction of metabolic and intracellular signals on the force-generating cascade.

Sorcin regulates excitation-contraction coupling in the heart

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

Effects of adenovirus-mediated sorcin overexpression on excitation-contraction coupling in isolated rabbit cardiomyocytes

To evaluate the effect of sorcin on cardiac excitation-contraction coupling, adult rabbit ventricular myocytes were transfected with a recombinant adenovirus coding for human sorcin (Ad-sorcin). A beta-galactosidase adenovirus (Ad-LacZ) was used as a control. Fractional shortening in response to 1-Hz field stimulation (at 37 degrees C) was significantly reduced in Ad-sorcin-transfected myocytes compared with control myocytes (2.10+/-0.05% [n=311] versus 2.42+/-0.06% [n=312], respectively; P<0.001). Action potential duration (at 20 degrees C) was significantly less in the Ad-sorcin group (458+/-22 ms, n=11) compared with the control group (520+/-19 ms, n=10; P<0.05). In voltage-clamped, fura 2-loaded myocytes (20 degrees C), a reduced peak-systolic and end-diastolic [Ca2+]i was observed after Ad-sorcin transfection. L-type Ca2+ current amplitude and time course were unaffected. Caffeine-induced Ca2+ release from the sarcoplasmic reticulum (SR) and the accompanying inward Na+-Ca2+ exchanger (NCX) current revealed a significantly lower SR Ca2+ content and faster Ca2+-extrusion kinetics in Ad-sorcin-transfected cells. Higher NCX activity after Ad-sorcin transfection was confirmed by measuring the NCX current-voltage relationship. beta-Escin-permeabilized rabbit cardiomyocytes were used to study the effects of sorcin overexpression on Ca2+ sparks imaged with fluo 3 at 145 to 160 nmol/L [Ca2+] using a confocal microscope. Under these conditions, caffeine-mediated SR Ca2+ release was not different between the two groups. Spontaneous spark frequency, duration, width, and amplitude were lower in sorcin-overexpressing myocytes. In summary, sorcin overexpression in rabbit cardiomyocytes decreased Ca2+-transient amplitude predominantly by lowering SR Ca2+ content via increased NCX activity. The effect of sorcin overexpression on Ca2+ sparks indicates an effect on the ryanodine receptor that may also influence excitation-contraction coupling.

Ca2+ scraps: local depletions of free [Ca2+] in cardiac sarcoplasmic reticulum during contractions leave substantial Ca2+ reserve

Free [Ca2+] inside the sarcoplasmic reticulum ([Ca2+]SR) is difficult to measure yet critically important in controlling many cellular systems. In cardiac myocytes, [Ca2+]SR regulates cardiac contractility. We directly measure [Ca2+]SR in intact cardiac myocytes dynamically and quantitatively during beats, with high spatial resolution. Diastolic [Ca2+]SR (1 to 1.5 mmol/L) is only partially depleted (24% to 63%) during contraction. There is little temporal delay in the decline in [Ca2+]SR at release junctions and between junctions, indicating rapid internal diffusion. The incomplete local Ca2+ release shows that the inherently positive feedback of Ca2+-induced Ca2+ release terminates, despite a large residual driving force. These findings place stringent novel constraints on how excitation-contraction coupling works in heart and also reveal a Ca2+ store reserve that could in principle be a therapeutic target to enhance cardiac function in heart failure.

Endothelial NO formation does not control myocardial O2 consumption in mouse heart

To test whether endothelium-derived nitric oxide (NO) regulates mitochondrial respiration, NO was pharmacologically modulated in isolated mouse hearts, which were perfused at constant flow to sensitively detect small changes in myocardial O2 consumption (MVO2). Stimulation of NO formation by 10 microM bradykinin (BK) increased coronary venous nitrite release fivefold to 58 +/- 33 nM (n = 17). Vasodilatation by BK, adenosine (1 microM), or papaverine (10 microM) decreased perfusion pressure, left ventricular developed pressure (LVDP), and MVO2. In the presence of adenosine-induced vasodilatation, stimulation of endothelial NO synthesis by BK had no effect on LVDP and MVO2. Also, inhibition of NO formation by NG-monomethyl-l-arginine (l-NMMA, 100 microM) did not significantly alter LVDP and MVO2. Similarly, intracoronary infusion of authentic NO <or=2 microM did not influence LVDP or MVO2 (-1 +/- 1%). Only when NO was >2 microM were contractile dysfunction and MVO2 reduction observed. Because BK-induced stimulation of endothelial NO formation and basal NO are not sufficient to impair MVO2 in the saline-perfused mouse heart, a tonic control of the respiratory chain by endothelial NO is difficult to conceive.