In situ SR function in postinfarction myocytes

Novel BAG3 Variants in African American Patients With Cardiomyopathy: Reduced β-Adrenergic Responsiveness in Excitation-Contraction

BACKGROUND

We reported 3 novel nonsynonymous single nucleotide variants of Bcl2-associated athanogene 3 (BAG3) in African Americans with heart failure (HF) that are associated with a 2-fold increase in cardiac events (HF hospitalization, heart transplantation, or death).

METHODS AND RESULTS

We expressed BAG3 variants (P63A, P380S, and A479V) via adenovirus-mediated gene transfer in adult left ventricular myocytes isolated from either wild-type (WT) or cardiac-specific BAG3 haploinsufficient (cBAG3^+/-) mice: the latter to simulate the clinical situation in which BAG3 variants are only found on 1 allele. Compared with WT myocytes, cBAG3^+/- myocytes expressed approximately 50% of endogenous BAG3 levels and exhibited decreased [Ca²⁺]_i and contraction amplitudes after isoproterenol owing to decreased L-type Ca²⁺ current. BAG3 repletion with WT BAG3 but not P380S, A479V, or P63A/P380S variants restored contraction amplitudes in cBAG3^+/- myocytes to those measured in WT myocytes, suggesting excitation-contraction abnormalities partly account for HF in patients harboring these mutants. Because P63A is near the WW domain (residues 21-55) and A479V is in the BAG domain (residues 420-499), we expressed BAG3 deletion mutants (Δ1-61 and Δ421-575) in WT myocytes and demonstrated that the BAG but not the WW domain was involved in enhancement of excitation-contraction by isoproterenol.

CONCLUSIONS

The BAG3 variants contribute to HF in African American patients partly by decreasing myocyte excitation-contraction under stress, and that both the BAG and PXXP domains are involved in mediating β-adrenergic responsiveness in myocytes.

Digitoxin Attenuates Heart Failure, Reduces Myocardial Hypertrophy, and Preserves the Calcium-Binding Proteins in Infarcted Rats

We previously showed that digitoxin prolongs the survival of rats with heart failure due to myocardial infarction (MI). In this study, we evaluated the effect of digitoxin on myocardial structure, ventricular function, and proteins involved in calcium kinetics. Seventy-two rats with MI >35% of the left ventricle were randomly assigned to 4 treatment groups: sham (n = 15), digitoxin (n = 11), infarction (n = 20), and infarction + digitoxin (n = 26). The rats were assessed 120 days after surgery by echocardiogram, hemodynamics, papillary muscle mechanics, collagen content, cardiomyocyte nuclear volume, and Western blot analysis of proteins involved in calcium kinetics. Digitoxin was administered via the rat chow. Two-way analysis of variance was used for comparisons. Myocardial infarction caused inotropic impairment, pulmonary congestion, increase of nuclear volume, myocardial collagen, and Na⁺/Ca2⁺ exchanger levels, and decreased SERCA2 and phosphorylated phospholamban levels. Treatment with digitoxin showed improvements in cardiac remodeling, inotropism, ventricular performance, pulmonary congestion, collagen accumulation, nuclear volume, and proteins involved in calcium kinetics. In rats with heart failure due to MI, long-term treatment with digitoxin attenuates congestive heart failure, mitigates myocardial remodeling and contractile impairment, and preserves myocardial levels of proteins involved in calcium kinetics.

Methylene blue counteracts cyanide cardiotoxicity: cellular mechanisms

In adult left ventricular mouse myocytes, exposure to sodium cyanide (NaCN) in the presence of glucose dose-dependently reduced contraction amplitude, with ~80% of maximal inhibitory effect attained at 100 µM. NaCN (100 µM) exposure for 10 min significantly decreased contraction and intracellular Ca2+ concentration ([Ca2+]i) transient amplitudes, systolic but not diastolic [Ca2+]i, and maximal L-type Ca2+ current ( ICa) amplitude, indicating acute alteration of [Ca2+]i homeostasis largely accounted for the observed excitation-contraction abnormalities. In addition, NaCN depolarized resting membrane potential ( Em), reduced action potential (AP) amplitude, prolonged AP duration at 50% (APD50) and 90% repolarization (APD90), and suppressed depolarization-activated K+ currents but had no effect on Na+-Ca2+ exchange current ( INaCa). NaCN did not affect cellular adenosine triphosphate levels but depolarized mitochondrial membrane potential (ΔΨm) and increased superoxide (O2·-) levels. Methylene blue (MB; 20 µg/ml) added 3 min after NaCN restored contraction and [Ca2+]i transient amplitudes, systolic [Ca2+]i, Em, AP amplitude, APD50, APD90, ICa, depolarization-activated K+ currents, ΔΨm, and O2·- levels toward normal. We conclude that MB reversed NaCN-induced cardiotoxicity by preserving intracellular Ca2+ homeostasis and excitation-contraction coupling ( ICa), minimizing risks of arrhythmias ( Em, AP configuration, and depolarization-activated K+ currents), and reducing O2·- levels. NEW & NOTEWORTHY Cyanide poisoning due to industrial exposure, smoke inhalation, and bioterrorism manifests as cardiogenic shock and requires rapidly effective antidote. In the early stage of cyanide exposure, adenosine triphosphate levels are normal but myocyte contractility is reduced, largely due to alterations in Ca2+ homeostasis because of changes in oxidation-reduction environment of ion channels. Methylene blue, a drug approved by the U.S. Food and Drug Administration, ameliorates cyanide toxicity by normalizing oxidation-reduction state and Ca2+ channel function.

Methylene blue counteracts H2S toxicity-induced cardiac depression by restoring L-type Ca channel activity

We have previously reported that methylene blue (MB) can counteract hydrogen sulfide (H2S) intoxication-induced circulatory failure. Because of the multifarious effects of high concentrations of H2S on cardiac function, as well as the numerous properties of MB, the nature of this interaction, if any, remains uncertain. The aim of this study was to clarify 1) the effects of MB on H2S-induced cardiac toxicity and 2) whether L-type Ca(2+) channels, one of the targets of H2S, could transduce some of the counteracting effects of MB. In sedated rats, H2S infused at a rate that would be lethal within 5 min (24 μM·kg(-1)·min(-1)), produced a rapid fall in left ventricle ejection fraction, determined by echocardiography, leading to a pulseless electrical activity. Blood concentrations of gaseous H2S reached 7.09 ± 3.53 μM when cardiac contractility started to decrease. Two to three injections of MB (4 mg/kg) transiently restored cardiac contractility, blood pressure, and V̇o2, allowing the animals to stay alive until the end of H2S infusion. MB also delayed PEA by several minutes following H2S-induced coma and shock in unsedated rats. Applying a solution containing lethal levels of H2S (100 μM) on isolated mouse cardiomyocytes significantly reduced cell contractility, intracellular calcium concentration ([Ca(2+)]i) transient amplitudes, and L-type Ca(2+) currents (ICa) within 3 min of exposure. MB (20 mg/l) restored the cardiomyocyte function, ([Ca(2+)]i) transient, and ICa The present results offer a new approach for counteracting H2S toxicity and potentially other conditions associated with acute inhibition of L-type Ca(2+) channels.

BAG3 regulates contractility and Ca(2+) homeostasis in adult mouse ventricular myocytes

Bcl2-associated athanogene 3 (BAG3) is a 575 amino acid anti-apoptotic protein that is constitutively expressed in the heart. BAG3 mutations, including mutations leading to loss of protein, are associated with familial cardiomyopathy. Furthermore, BAG3 levels have been found to be reduced in end-stage non-familial failing myocardium. In contrast to neonatal myocytes in which BAG3 is found in the cytoplasm and involved in protein quality control and apoptosis, in adult mouse left ventricular (LV) myocytes BAG3 co-localized with Na(+)-K(+)-ATPase and L-type Ca(2+) channels in the sarcolemma and t-tubules. BAG3 co-immunoprecipitated with β1-adrenergic receptor, L-type Ca(2+) channels and phospholemman. To simulate decreased BAG3 protein levels observed in human heart failure, we targeted BAG3 by shRNA (shBAG3) in adult LV myocytes. Reducing BAG3 by 55% resulted in reduced contraction and [Ca(2+)]i transient amplitudes in LV myocytes stimulated with isoproterenol. L-type Ca(2+) current (ICa) and sarcoplasmic reticulum (SR) Ca(2+) content but not Na(+)/Ca(2+) exchange current (INaCa) or SR Ca(2+) uptake were reduced in isoproterenol-treated shBAG3 myocytes. Forskolin or dibutyryl cAMP restored ICa amplitude in shBAG3 myocytes to that observed in WT myocytes, consistent with BAG3 having effects upstream and at the level of the receptor. Resting membrane potential and action potential amplitude were unaffected but APD50 and APD90 were prolonged in shBAG3 myocytes. Protein levels of Ca(2+) entry molecules and other important excitation-contraction proteins were unchanged in myocytes with lower BAG3. Our findings that BAG3 is localized at the sarcolemma and t-tubules while modulating myocyte contraction and action potential duration through specific interaction with the β1-adrenergic receptor and L-type Ca(2+) channel provide novel insight into the role of BAG3 in cardiomyopathies and increased arrhythmia risks in heart failure.

Induced overexpression of phospholemman S68E mutant improves cardiac contractility and mortality after ischemia-reperfusion

Phospholemman (PLM), when phosphorylated at Ser(68), inhibits cardiac Na+ / Ca2+ exchanger 1 (NCX1) and relieves its inhibition on Na+ -K+ -ATPase. We have engineered mice in which expression of the phosphomimetic PLM S68E mutant was induced when dietary doxycycline was removed at 5 wk. At 8-10 wk, compared with noninduced or wild-type hearts, S68E expression in induced hearts was ∼35-75% that of endogenous PLM, but protein levels of sarco(endo)plasmic reticulum Ca2+ -ATPase, α1- and α2-subunits of Na+ -K+ -ATPase, α1c-subunit of L-type Ca2+ channel, and phosphorylated ryanodine receptor were unchanged. The NCX1 protein level was increased by ∼47% but the NCX1 current was depressed by ∼34% in induced hearts. Isoproterenol had no effect on NCX1 currents but stimulated Na+ -K+ -ATPase currents equally in induced and noninduced myocytes. At baseline, systolic intracellular Ca2+ concentrations ([Ca2+]i), sarcoplasmic reticulum Ca2+ contents, and [Ca(2+)]i transient and contraction amplitudes were similar between induced and noninduced myocytes. Isoproterenol stimulation resulted in much higher systolic [Ca2+]i, sarcoplasmic reticulum Ca2+ content, and [Ca2+]i transient and contraction amplitudes in induced myocytes. Echocardiography and in vivo close-chest catheterization demonstrated similar baseline myocardial function, but isoproterenol induced a significantly higher +dP/dt in induced compared with noninduced hearts. In contrast to the 50% mortality observed in mice constitutively overexpressing the S68E mutant, induced mice had similar survival as wild-type and noninduced mice. After ischemia-reperfusion, despite similar areas at risk and left ventricular infarct sizes, induced mice had significantly higher +dP/dt and -dP/dt and lower perioperative mortality compared with noninduced mice. We propose that phosphorylated PLM may be a novel therapeutic target in ischemic heart disease.

Induced overexpression of Na(+)/Ca(2+) exchanger does not aggravate myocardial dysfunction induced by transverse aortic constriction

BACKGROUND

Alterations in expression and activity of cardiac Na(+)/Ca(2+) exchanger (NCX1) have been implicated in the pathogenesis of heart failure.

METHODS AND RESULTS

Using transgenic mice in which expression of rat NCX1 was induced at 5 weeks of age, we performed transverse aortic constriction (TAC) at 8 weeks and examined cardiac and myocyte function at 15-18 weeks after TAC (age 23-26 weeks). TAC induced left ventricular (LV) and myocyte hypertrophy and increased myocardial fibrosis in both wild-type (WT) and NCX1-overexpressed mice. NCX1 and phosphorylated ryanodine receptor expression was increased by TAC, whereas sarco(endo)plasmic reticulum Ca(2+)-ATPase levels were decreased by TAC. Action potential duration was prolonged by TAC, but to a greater extent in NCX1 myocytes. Na(+)/Ca(2+) exchange current was similar between WT-TAC and WT-sham myocytes, but was higher in NCX1-TAC myocytes. Both myocyte contraction and [Ca(2+)](i) transient amplitudes were reduced in WT-TAC myocytes, but restored to WT-sham levels in NCX1-TAC myocytes. Despite improvement in single myocyte contractility and Ca(2+) dynamics, induced NCX1 overexpression in TAC animals did not ameliorate LV hypertrophy, increase ejection fraction, or enhance inotropic (maximal first derivative of LV pressure rise, +dP/dt) responses to isoproterenol.

CONCLUSIONS

In pressure-overload hypertrophy, induced overexpression of NCX1 corrected myocyte contractile and [Ca(2+)](i) transient abnormalities but did not aggravate or improve myocardial dysfunction.

Phospholemman deficiency in postinfarct hearts: enhanced contractility but increased mortality

Phospholemman (PLM) regulates [Na(+) ](i), [Ca(2+)](i) and contractility through its interactions with Na(+)-K(+)-ATPase (NKA) and Na(+) /Ca(2+) exchanger (NCX1) in the heart. Both expression and phosphorylation of PLM are altered after myocardial infarction (MI) and heart failure. We tested the hypothesis that absence of PLM regulation of NKA and NCX1 in PLM-knockout (KO) mice is detrimental. Three weeks after MI, wild-type (WT) and PLM-KO hearts were similarly hypertrophied. PLM expression was lower but fractional phosphorylation was higher in WT-MI compared to WT-sham hearts. Left ventricular ejection fraction was severely depressed in WT-MI but significantly less depressed in PLM-KO-MI hearts despite similar infarct sizes. Compared with WT-sham myocytes, the abnormal [Ca(2+) ], transient and contraction amplitudes observed in WT-MI myocytes were ameliorated by genetic absence of PLM. In addition, NCX1 current was depressed in WT-MI but not in PLM-KO-MI myocytes. Despite improved myocardial and myocyte performance, PLM-KO mice demonstrated reduced survival after MI. Our findings indicate that alterations in PLM expression and phosphorylation are important adaptations post-MI, and that complete absence of PLM regulation of NKA and NCX1 is detrimental in post-MI animals.

Constitutive overexpression of phosphomimetic phospholemman S68E mutant results in arrhythmias, early mortality, and heart failure: potential involvement of Na+/Ca2+ exchanger

Expression and activity of cardiac Na(+)/Ca(2+) exchanger (NCX1) are altered in many disease states. We engineered mice in which the phosphomimetic phospholemman S68E mutant (inhibits NCX1 but not Na(+)-K(+)-ATPase) was constitutively overexpressed in a cardiac-specific manner (conS68E). At 4-6 wk, conS68E mice exhibited severe bradycardia, ventricular arrhythmias, increased left ventricular (LV) mass, decreased cardiac output (CO), and ∼50% mortality compared with wild-type (WT) littermates. Protein levels of NCX1, calsequestrin, ryanodine receptor, and α(1)- and α(2)-subunits of Na(+)-K(+)-ATPase were similar, but sarco(endo)plasmic reticulum Ca(2+)-ATPase was lower, whereas L-type Ca(2+) channels were higher in conS68E hearts. Resting membrane potential and action potential amplitude were similar, but action potential duration was dramatically prolonged in conS68E myocytes. Diastolic intracellular Ca(2+) ([Ca(2+)](i)) was higher, [Ca(2+)](i) transient and maximal contraction amplitudes were lower, and half-time of [Ca(2+)](i) transient decline was longer in conS68E myocytes. Intracellular Na(+) reached maximum within 3 min after isoproterenol addition, followed by decline in WT but not in conS68E myocytes. Na(+)/Ca(2+) exchange, L-type Ca(2+), Na(+)-K(+)-ATPase, and depolarization-activated K(+) currents were decreased in conS68E myocytes. At 22 wk, bradycardia and increased LV mass persisted in conS68E survivors. Despite comparable baseline CO, conS68E survivors at 22 wk exhibited decreased chronotropic, inotropic, and lusitropic responses to isoproterenol. We conclude that constitutive overexpression of S68E mutant was detrimental, both in terms of depressed cardiac function and increased arrhythmogenesis.

Overexpression of ornithine decarboxylase decreases ventricular systolic function during induction of cardiac hypertrophy

Ornithine decarboxylase (ODC), the first enzyme of polyamine metabolism, is rapidly upregulated in response to agents that induce a pathological cardiac hypertrophy. Transgenic mice overexpressing ODC in the heart (MHC-ODC mice) experience a much more dramatic left ventricular hypertrophy in response to β-adrenergic stimulation with isoproterenol (ISO) compared to wild-type (WT) controls. ISO also induced arginase activity in transgenic hearts but not in controls. The current work studies the cooperation between the cardiac polyamines and L-arginine (L-Arg) availability in MHC-ODC mice. Although ISO-induced hypertrophy is well-compensated, MHC-ODC mice administered L-Arg along with ISO showed a rapid onset of systolic dysfunction and died within 48 h. Myocytes isolated from MHC-ODC mice administered L-Arg/ISO exhibited reduced contractility and altered calcium transients, suggesting an alteration in [Ca(2+)] homeostasis, and abbreviated action potential duration, which may contribute to arrhythmogenesis. The already elevated levels of spermidine and spermine were not further altered in MHC-ODC hearts by L-Arg/ISO treatment, suggesting alternative L-Arg utilization pathways lead to dysregulation of intracellular calcium. MHC-ODC mice administered an arginase inhibitor (Nor-NOHA) along with ISO died almost as rapidly as L-Arg/ISO-treated mice, while the iNOS inhibitor S-methyl-isothiourea (SMT) was strongly protective against L-Arg/ISO. These results point to the induction of arginase as a protective response to β-adrenergic stimulation in the setting of high polyamines. Further, NO generated by exogenously supplied L-Arg may contribute to the lethal consequences of L-Arg/ISO treatment. Since considerable variations in human cardiac polyamine and L-Arg content are likely, it is possible that alterations in these factors may influence myocyte contractility.

Regulation of in vivo cardiac contractility by phospholemman: role of Na+/Ca2+ exchange

Phospholemman (PLM), when phosphorylated at serine 68, relieves its inhibition on Na(+)-K(+)-ATPase but inhibits Na(+)/Ca(2+) exchanger 1 (NCX1) in cardiac myocytes. Under stress when catecholamine levels are high, enhanced Na(+)-K(+)-ATPase activity by phosphorylated PLM attenuates intracellular Na(+) concentration ([Na(+)](i)) overload. To evaluate the effects of PLM on NCX1 on in vivo cardiac contractility, we injected recombinant adeno-associated virus (serotype 9) expressing either the phosphomimetic PLM S68E mutant or green fluorescent protein (GFP) directly into left ventricles (LVs) of PLM-knockout (KO) mice. Five weeks after virus injection, ∼40% of isolated LV myocytes exhibited GFP fluorescence. Expression of S68E mutant was confirmed with PLM antibody. There were no differences in protein levels of α(1)- and α(2)-subunits of Na(+)-K(+)-ATPase, NCX1, and sarco(endo)plasmic reticulum Ca(2+)-ATPase between KO-GFP and KO-S68E LV homogenates. Compared with KO-GFP myocytes, Na(+)/Ca(2+) exchange current was suppressed, but resting [Na(+)](i), Na(+)-K(+)-ATPase current, and action potential amplitudes were similar in KO-S68E myocytes. Resting membrane potential was slightly lower and action potential duration at 90% repolarization (APD(90)) was shortened in KO-S68E myocytes. Isoproterenol (Iso; 1 μM) increased APD(90) in both groups of myocytes. After Iso, [Na(+)](i) increased monotonically in paced (2 Hz) KO-GFP but reached a plateau in KO-S68E myocytes. Both systolic and diastolic [Ca(2+)](i) were higher in Iso-stimulated KO-S68E myocytes paced at 2 Hz. Echocardiography demonstrated similar resting heart rate, ejection fraction, and LV mass between KO-GFP and KO-S68E mice. In vivo closed-chest catheterization demonstrated enhanced contractility in KO-S68E compared with KO-GFP hearts stimulated with Iso. We conclude that under catecholamine stress when [Na(+)](i) is high, PLM minimizes [Na(+)](i) overload by relieving its inhibition of Na(+)-K(+)-ATPase and preserves inotropy by simultaneously inhibiting Na(+)/Ca(2+) exchanger.

Cardiac-restricted overexpression of the A(2A)-adenosine receptor in FVB mice transiently increases contractile performance and rescues the heart failure phenotype in mice overexpressing the A(1)-adenosine receptor

In the heart, adenosine binds to pharmacologically distinct G-protein-coupled receptors (A(1)-R, A(2A)-R, and A(3)-R). While the role of A(1)- and A(3)-Rs in the heart has been clarified, the effect of genetically manipulating the A(2A)-R has not been defined. Thus, we created mice overexpressing a cardiac-restricted A(2A)-R transgene. Mice with both low (Lo) and high (Hi) levels of A(2A)-R overexpression demonstrated an increase in cardiac contractility at 12 weeks. These changes were associated with a significantly higher systolic but not diastolic [Ca(2+)]i, higher maximal contraction amplitudes, and a significantly enhanced sarcoplasmic reticulum Ca(2+) uptake activity. At 20 weeks, the effects of A(2A)-R overexpression on cardiac contractility diminished. The positive effects elicited by A(2A)-R overexpression differ from the heart failure phenotype we observed with A(1)-R overexpression. Interestingly, coexpression of A(2A)-R TG(Hi), but not A(2A)-R TGLo, enhanced survival, prevented the development of left ventricular dysfunction and heart failure, and improved Ca(2+) handling in mice overexpressing the A(1)-R. These results suggest that adenosine-mediated signaling in the heart requires a balance between A(1)- and A(2A)-Rs--a finding that may have important implications for the ongoing clinical evaluation of adenosine receptor subtype-specific agonists and antagonists for the treatment of cardiovascular diseases.

Induced overexpression of Na+/Ca2+ exchanger transgene: altered myocyte contractility, [Ca2+]i transients, SR Ca2+ contents, and action potential duration

We have produced mice in which expression of the rat cardiac Na(+)/Ca(2+) exchanger (NCX1) transgene was switched on when doxycycline was removed from the feed at 5 wk. At 8 to 10 wk, NCX1 expression in induced (Ind) mouse hearts was 2.5-fold higher but protein levels of sarco(endo)plasmic reticulum Ca(2+)-ATPase, alpha(1)- and alpha(2)-subunits of Na(+)-K(+)-ATPase, phospholamban, ryanodine receptor, calsequestrin, and unphosphorylated and phosphorylated phospholemman were unchanged compared with wild-type (WT) or noninduced (non-Ind) hearts. There was no cellular hypertrophy since WT, non-Ind, and Ind myocytes had similar whole cell membrane capacitance. In Ind myocytes, NCX1 current amplitude was approximately 42% higher, L-type Ca(2+) current amplitude was unchanged, and action potential duration was prolonged compared with WT or non-Ind myocytes. Contraction and intracellular Ca(2+) concentration ([Ca(2+)](i)) transient amplitudes in Ind myocytes were lower at 0.6, not different at 1.8, and higher at 5.0 mM extracellular Ca(2+) concentration ([Ca(2+)](o)) compared with WT or non-Ind myocytes. Despite similar Ca(2+) current amplitude and sarcoplasmic reticulum (SR) Ca(2+) uptake, SR Ca(2+) content at 5.0 mM [Ca(2+)](o) was significantly higher in Ind compared with non-Ind myocytes, indicating that NCX1 directly contributed to SR Ca(2+) loading. Echocardiography demonstrated that heart rate, left ventricular mass, ejection fraction, stroke volume, and cardiac output were similar among the three groups of animals. In vivo close-chest catheterization demonstrated similar contractility and relaxation among the three groups of mice, both at baseline and after stimulation with isoproterenol. We conclude that induced expression of NCX1 transgene resulted in altered [Ca(2+)](i) homeostasis, myocyte contractility, and action potential morphology. In addition, heart failure did not occur 3 to 5 wk after NCX1 transgene was induced to be expressed at levels found in diseased hearts.

Regulation of cardiac myocyte contractility by phospholemman: Na+/Ca2+ exchange versus Na+ -K+ -ATPase

Phospholemman (PLM) regulates cardiac Na(+)/Ca(2+) exchanger (NCX1) and Na(+)-K(+)-ATPase in cardiac myocytes. PLM, when phosphorylated at Ser(68), disinhibits Na(+)-K(+)-ATPase but inhibits NCX1. PLM regulates cardiac contractility by modulating Na(+)-K(+)-ATPase and/or NCX1. In this study, we first demonstrated that adult mouse cardiac myocytes cultured for 48 h had normal surface membrane areas, t-tubules, and NCX1 and sarco(endo)plasmic reticulum Ca(2+)-ATPase levels, and retained near normal contractility, but alpha(1)-subunit of Na(+)-K(+)-ATPase was slightly decreased. Differences in contractility between myocytes isolated from wild-type (WT) and PLM knockout (KO) hearts were preserved after 48 h of culture. Infection with adenovirus expressing green fluorescent protein (GFP) did not affect contractility at 48 h. When WT PLM was overexpressed in PLM KO myocytes, contractility and cytosolic Ca(2+) concentration ([Ca(2+)](i)) transients reverted back to those observed in cultured WT myocytes. Both Na(+)-K(+)-ATPase current (I(pump)) and Na(+)/Ca(2+) exchange current (I(NaCa)) in PLM KO myocytes rescued with WT PLM were depressed compared with PLM KO myocytes. Overexpressing the PLMS68E mutant (phosphomimetic) in PLM KO myocytes resulted in the suppression of I(NaCa) but had no effect on I(pump). Contractility, [Ca(2+)](i) transient amplitudes, and sarcoplasmic reticulum Ca(2+) contents in PLM KO myocytes overexpressing the PLMS68E mutant were depressed compared with PLM KO myocytes overexpressing GFP. Overexpressing the PLMS68A mutant (mimicking unphosphorylated PLM) in PLM KO myocytes had no effect on I(NaCa) but decreased I(pump). Contractility, [Ca(2+)](i) transient amplitudes, and sarcoplasmic reticulum Ca(2+) contents in PLM KO myocytes overexpressing the S68A mutant were similar to PLM KO myocytes overexpressing GFP. We conclude that at the single-myocyte level, PLM affects cardiac contractility and [Ca(2+)](i) homeostasis primarily by its direct inhibitory effects on Na(+)/Ca(2+) exchange.

Remodeling of T-tubules and reduced synchrony of Ca2+ release in myocytes from chronically ischemic myocardium

In ventricular cardiac myocytes, T-tubule density is an important determinant of the synchrony of sarcoplasmic reticulum (SR) Ca2+ release and could be involved in the reduced SR Ca2+ release in ischemic cardiomyopathy. We therefore investigated T-tubule density and properties of SR Ca2+ release in pigs, 6 weeks after inducing severe stenosis of the circumflex coronary artery (91+/-3%, N=13) with myocardial infarction (8.8+/-2.0% of total left ventricular mass). Severe dysfunction in the infarct and adjacent myocardium was documented by magnetic resonance and Doppler myocardial velocity imaging. Myocytes isolated from the adjacent myocardium were compared with myocytes from the same region in weight-matched control pigs. T-tubule density quantified from the di-8-ANEPPS (di-8-butyl-amino-naphthyl-ethylene-pyridinium-propyl-sulfonate) sarcolemmal staining was decreased by 27+/-7% (P<0.05). Synchrony of SR Ca2+ release (confocal line scan images during whole-cell voltage clamp) was reduced in myocardium myocytes. Delayed release (ie, half-maximal [Ca2+]i occurring later than 20 ms) occurred at 35.5+/-6.4% of the scan line in myocardial infarction versus 22.7+/-2.5% in control pigs (P<0.05), prolonging the time to peak of the line-averaged [Ca2+]i transient (121+/-9 versus 102+/-5 ms in control pigs, P<0.05). Delayed release colocalized with regions of T-tubule rarefaction and could not be suppressed by activation of protein kinase A. The whole-cell averaged [Ca2+]i transient amplitude was reduced, whereas L-type Ca2+ current density was unchanged and SR content was increased, indicating a reduction in the gain of Ca2+-induced Ca2+ release. In conclusion, reduced T-tubule density during ischemic remodeling is associated with reduced synchrony of Ca2+ release and reduced efficiency of coupling Ca2+ influx to Ca2+ release.

Insulin improves cardiomyocyte contractile function through enhancement of SERCA2a activity in simulated ischemia/reperfusion

AIM

Insulin exerts anti-apoptotic effects in both cardiomyocytes and coronary endothelial cells following ischemia/reperfusion (I/R) via the Akt-endothelial nitric oxide synthase survival signal pathway. This important insulin signaling might further contribute to the improvement of cardiac function after reperfusion. In this study, we tested the hypothesis that sarcoplasmic reticulum calcium-ATPase (SERCA2a) is involved in the insulin-induced improvement of cardiac contractile function following I/R.

METHODS

Ventricular myocytes were enzymatically isolated from adult SD rats. Simulated I/R was induced by perfusing cells with chemical anoxic solution for 15 min followed by reperfusion with Tyrode's solution with or without insulin for 30 min. Myocyte shortening and intracellular calcium transients were assessed and underlying mechanisms were investigated.

RESULTS

Reperfusion with insulin (10(-7) mol/L) significantly improved the recovery of contractile function (n=15-20 myocytes from 6-8 hearts, P<0.05), and increased calcium transients, as evidenced by the increased calcium [Ca2+] fluorescence ratio, shortened time to peak Ca2+ and time to 50% diastolic Ca2+, compared with those in cells reperfused with vehicle (P<0.05). In addition, Akt phosphorylation and SERCA2a activity were both increased in insulin-treated I/R cardiomyocytes, which were markedly inhibited by pretreatment of cells with a specific Akt inhibitor. Moreover, inhibition of Akt activity abolished insulin-induced positive contractile and calcium transients responses in I/R cardiomyocytes.

CONCLUSION

These data demonstrated for the first time that insulin improves the recovery of contractile function in simulated I/R cardiomyocytes in an Akt-dependent and SERCA2a-mediated fashion.

Altered contractility and [Ca2+]i homeostasis in phospholemman-deficient murine myocytes: role of Na+/Ca2+ exchange

Phospholemman (PLM) regulates contractility and Ca(2+) homeostasis in cardiac myocytes. We characterized excitation-contraction coupling in myocytes isolated from PLM-deficient mice backbred to a pure congenic C57BL/6 background. Cell length, cell width, and whole cell capacitance were not different between wild-type and PLM-null myocytes. Compared with wild-type myocytes, Western blots indicated total absence of PLM but no changes in Na(+)/Ca(2+) exchanger, sarcoplasmic reticulum (SR) Ca(2+)-ATPase, alpha(1)-subunit of Na(+)-K(+)-ATPase, and calsequestrin levels in PLM-null myocytes. At 5 mM extracellular Ca(2+) concentration ([Ca(2+)](o)), contraction and cytosolic [Ca(2+)] ([Ca(2+)](i)) transient amplitudes and SR Ca(2+) contents in PLM-null myocytes were significantly (P < 0.0004) higher than wild-type myocytes, whereas the converse was true at 0.6 mM [Ca(2+)](o). This pattern of contractile and [Ca(2+)](i) transient abnormalities in PLM-null myocytes mimics that observed in adult rat myocytes overexpressing the cardiac Na(+)/Ca(2+) exchanger. Indeed, we have previously reported that Na(+)/Ca(2+) exchange currents were higher in PLM-null myocytes. Activation of protein kinase A resulted in increased inotropy such that there were no longer any contractility differences between the stimulated wild-type and PLM-null myocytes. Protein kinase C stimulation resulted in decreased contractility in both wild-type and PLM-null myocytes. Resting membrane potential and action potential amplitudes were similar, but action potential duration was much prolonged (P < 0.04) in PLM-null myocytes. Whole cell Ca(2+) current densities were similar between wild-type and PLM-null myocytes, as were the fast- and slow-inactivation time constants. We conclude that a major function of PLM is regulation of cardiac contractility and Ca(2+) fluxes, likely by modulating Na(+)/Ca(2+) exchange activity.

Serine 68 phospholemman phosphorylation during forskolin-induced swine carotid artery relaxation

BACKGROUND

Phospholemman (PLM) is an abundant phosphoprotein in the plasma membrane of cardiac, skeletal and smooth muscle. It is a member of the FXYD family of proteins that bind to and regulate the Na,K-ATPase. Protein kinase A (PKA) is known to phosphorylate PLM on serine 68 (S68), although the functional effect of S68 PLM phosphorylation is unclear. We therefore evaluated S68 PLM phosphorylation in swine carotid arteries.

METHODS

Two anti-PLM antibodies, one to S68 phosphorylated PLM and one to unphosphorylated PLM, were made to PLM peptides in rabbits and tested with purified PLM and PKA-treated PLM. Swine carotid arteries were mounted isometrically, contracted, relaxed with forskolin and then homogenized. Proteins were separated on SDS gels and the intensity of immunoreactivity to the two PLM antibodies determined on immunoblots.

RESULTS

The antipeptide antibody 'C2' primarily reacted with unphosphorylated PLM, and the antipeptide antibody 'CP68' detected S68 PLM phosphorylation. Histamine stimulation of intact swine carotid artery induced a contraction, increased the CP68 PLM antibody signal and reduced the C2 PLM antibody signal. High extracellular [K(+)] depolarization induced a contraction without altering the C2 or CP68 PLM signal. Forskolin-induced relaxation of histamine or extracellular [K(+)] contracted arteries correlated with an increased CP68 signal. Nitroglycerin-induced relaxation was not associated with changes in the C2 or CP68 PLM signal.

CONCLUSIONS

These data suggest that a contractile agonist increased S68 PLM phosphorylation. Agents that increase [cAMP], but not agents that increase [cGMP], increased S68 PLM phosphorylation. S68 PLM phosphorylation may be involved in cAMP-dependent regulation of smooth muscle force.

Left ventricular SERCA2a gene down-regulation does not parallel ANP gene up-regulation during post-MI remodelling in rats

BACKGROUND

In most animal models of chronic hemodynamic overload of the left ventricle (LV) as well as in human end stage heart failure, the sarcoplasmic reticulum (SR) Ca2+-ATPase (SERCA2a) mRNA levels are decreased in parallel with increased atrial natriuretic peptide (ANP) mRNA levels. The situation in the remote myocardium following myocardial infarction (MI) is unclear.

AIMS

(1) To examine SERCA2a mRNA levels in the non-infarcted LV myocardium of rats at the chronic stage of experimental MI and (2) To examine whether a negative linear correlation exists between SERCA2a and ANP mRNA levels in this model.

METHODS

Anesthetized adult male Wistar rats underwent left coronary artery ligation or sham operation. Three months later, the rats were divided into three groups: sham-operated rats (sham, n=21), HF-free rats with MI (non-failing (NF)-MI, n=29) and rats with both MI and HF (congestive heart failure (CHF)-MI, n=14). LV remodelling and function were assessed by echocardiography and hemodynamic measurements. SERCA2a and ANP mRNA levels were determined by Northern and dot blot analysis with specific cDNA probes.

RESULTS

LV SERCA2a mRNA levels varied markedly in sham-operated rats (0.9-1.8). Mean ANP mRNA level increased markedly and mean SERCA2a mRNA level decreased moderately in the remote myocardium. In some NF-MI rats, SERCA2a mRNA levels were higher than those in some sham controls. Whereas ANP mRNA levels correlated well with MI severity (r2=0.79, p<0.001), this was not the case for SERCA2a mRNA levels (r2=0.42, p<0.01). We found no negative correlation between ANP and SERCA2a mRNA levels.

CONCLUSION

SERCA2a gene down-regulation in the non-infarcted myocardium of rats with MI does not correlate with ANP gene up-regulation, suggesting that the two genes are not antithetically regulated.

Effects of sarcoplasmic reticulum Ca2+-ATPase overexpression in postinfarction rat myocytes

Previous studies in adult myocytes isolated from rat hearts 3 wk after myocardial infarction (MI) demonstrated abnormal contractility and intracellular Ca(2+) concentration ([Ca(2+)](i)) homeostasis and decreased sarcoplasmic reticulum Ca(2+)-ATPase (SERCA2) expression and activity, but sarcoplasmic reticulum Ca(2+) leak was unchanged. In the present study, we investigated whether SERCA2 overexpression in MI myocytes would restore contraction and [Ca(2+)](i) transients to normal. Compared with sham-operated hearts, 3-wk MI hearts exhibited significantly higher left ventricular end-diastolic and end-systolic volumes but lower fractional shortening and ejection fraction, as measured by M-mode echocardiography. Seventy-two hours after adenovirus-mediated gene transfer, SERCA2 overexpression in 3-wk MI myocytes did not affect Na(+)-Ca(2+) exchanger expression but restored the depressed SERCA2 levels toward those measured in sham myocytes. In addition, the reduced sarcoplasmic reticulum Ca(2+) uptake in MI myocytes was improved to normal levels by SERCA2 overexpression. At extracellular Ca(2+) concentration of 5 mM, the subnormal contraction and [Ca(2+)](i) transient amplitudes in MI myocytes (compared with sham myocytes) were restored to normal by SERCA2 overexpression. However, at 0.6 mM extracellular Ca(2+) concentration, the supernormal contraction and [Ca(2+)](i) transient amplitudes in MI myocytes (compared with sham myocytes) were exacerbated by SERCA2 overexpression. We conclude that SERCA2 overexpression was only partially effective in ameliorating contraction and [Ca(2+)](i) transient abnormalities in our rat model of ischemic cardiomyopathy. We suggest that other Ca(2+) transport pathways, e.g., Na(+)-Ca(2+) exchanger, may also play an important role in contractile and [Ca(2+)](i) homeostatic abnormalities in MI myocytes.

Serine 68 of phospholemman is critical in modulation of contractility, [Ca2+]i transients, and Na+/Ca2+ exchange in adult rat cardiac myocytes

Overexpression of phospholemman (PLM) in normal adult rat cardiac myocytes altered contractile function and cytosolic Ca2+ concentration ([Ca2+]i) homeostasis and inhibited Na+/Ca2+ exchanger (NCX1). In addition, PLM coimmunoprecipitated and colocalized with NCX1 in cardiac myocyte lysates. In this study, we evaluated whether the cytoplasmic domain of PLM is crucial in mediating its effects on contractility, [Ca2+]i transients, and NCX1 activity. Canine PLM or its derived mutants were overexpressed in adult rat myocytes by adenovirus-mediated gene transfer. Confocal immunofluorescence images using canine-specific PLM antibodies demonstrated that the exogenous PLM or its mutants were correctly targeted to sarcolemma, t-tubules, and intercalated discs, with little to none detected in intracellular compartments. Overexpression of canine PLM or its mutants did not affect expression of NCX1, sarco(endo)plasmic reticulum Ca(2+)-ATPase, Na(+)-K(+)-ATPase, and calsequestrin in adult rat myocytes. A COOH-terminal deletion mutant in which all four potential phosphorylation sites (Ser62, Ser63, Ser68, and Thr69) were deleted, a partial COOH-terminal deletion mutant in which Ser68 and Thr69 were deleted, and a mutant in which all four potential phosphorylation sites were changed to alanine all lost wild-type PLM's ability to modulate cardiac myocyte contractility. These observations suggest the importance of Ser68 or Thr69 in mediating PLM's effect on cardiac contractility. Focusing on Ser68, the Ser68 to Glu mutant was fully effective, the Ser63 to Ala (leaving Ser68 intact) mutant was partially effective, and the Ser68 to Ala mutant was completely ineffective in modulating cardiac contractility, [Ca2+]i transients, and NCX1 currents. Both the Ser63 to Ala and Ser68 to Ala mutants, as well as PLM, were able to coimmunoprecipitate NCX1. It is known that Ser68 in PLM is phosphorylated by both protein kinases A and C. We conclude that regulation of cardiac contractility, [Ca2+]i transients, and NCX1 activity by PLM is critically dependent on Ser68. We suggest that PLM phosphorylation at Ser68 may be involved in cAMP- and/or protein kinase C-dependent regulation of cardiac contractility.

Partial prevention of changes in SR gene expression in congestive heart failure due to myocardial infarction by enalapril or losartan

Although activation of the renin-angiotensin system (RAS) is known to produce ventricular remodeling and congestive heart failure (CHF), its role in inducing changes in the sarcoplasmic reticulum (SR) protein and gene expression in CHF is not fully understood. In this study, CHF was induced in rats by ligation of the left coronary artery for 3 weeks and then the animals were treated orally with or without an angiotensin converting enzyme inhibitor, enalapril (10 mg/kg/day) or an angiotensin II receptor antagonist, losartan (20 mg/kg/day) for 4 weeks. Sham-operated animals were used as control. The animals were hemodynamically assessed and protein content as well as gene expression of SR Ca(2+)-release channel (ryanodine receptor, RYR), Ca(2+)-pump ATPase (SERCA2), phospholamban (PLB) and calsequestrin (CQS) were determined in the left ventricle (LV). The infarcted animals showed cardiac hypertrophy, lung congestion, depression in LV +dP/dt and -dP/dt, as well as increase in LV end diastolic pressure. Both protein content and mRNA levels for RYR, SERCA2 and PLB were decreased without any changes in CQS in the failing heart. These alterations in LV function as well as SR protein and gene expression in CHF were partially prevented by treatment with enalapril or losartan. The results suggest that partial improvement in LV function by enalapril and losartan treatments may be due to partial prevention of changes in SR protein and gene expression in CHF and that these effects may be due to blockade of the RAS.

Effects of phospholemman downregulation on contractility and [Ca(2+)]i transients in adult rat cardiac myocytes

Phospholemman (PLM) expression was increased in rat hearts after myocardial infarction (MI). Overexpression of PLM in normal adult rat cardiac myocytes altered contractile function and cytosolic Ca(2+) concentration ([Ca(2+)](i)) homeostasis in a manner similar to that observed in post-MI myocytes. In this study, we tested whether PLM downregulation in normal adult rat myocytes resulted in contractility and [Ca(2+)](i) transient changes opposite to those observed in post-MI myocytes. Compared with control myocytes infected with adenovirus (Adv) expressing green fluorescent protein (GFP) alone, myocytes infected with Adv expressing both GFP and rat antisense PLM (rASPLM) had 23% less PLM protein (P < 0.012) at 3 days, but no differences were found in sarcoplasmic reticulum (SR) Ca(2+)-ATPase, Na(+)/Ca(2+) exchanger (NCX1), Na(+)-K(+)-ATPase, and calsequestrin levels. SR Ca(2+) uptake and whole cell capacitance were not affected by rASPLM treatment. Relaxation from caffeine-induced contracture was faster, and NCX1 current amplitudes were higher in rASPLM myocytes, indicating that PLM downregulation enhanced NCX1 activity. In native rat cardiac myocytes, coimmunoprecipitation experiments indicated an association of PLM with NCX1. At 0.6 mM [Ca(2+)](o), rASPLM myocytes had significantly (P < 0.003) lower contraction and [Ca(2+)](i) transient amplitudes than control GFP myocytes. At 5 mM [Ca(2+)](o), both contraction and [Ca(2+)](i) transient amplitudes were higher in rASPLM myocytes. This pattern of contractile and [Ca(2+)](i) transient behavior in rASPLM myocytes was opposite to that observed in post-MI rat myocytes. We conclude that downregulation of PLM in normal rat cardiac myocytes enhanced NCX1 function and affected [Ca(2+)](i) transient and contraction amplitudes. We suggest that PLM downregulation offers a potential therapeutic strategy for ameliorating contractile abnormalities in MI myocytes.

Rescue of contractile abnormalities by Na+/Ca2+ exchanger overexpression in postinfarction rat myocytes

Previous studies on myocytes isolated from rat hearts 3 wk after myocardial infarction (MI) demonstrated increased cell length, reduced Na(+)/Ca(2+) exchange (NCX1) activity, altered contractility, and intracellular Ca(2+) concentration ([Ca(2+)](i)) transients. In the present study, we investigated whether NCX1 overexpression in MI myocytes would restore contraction and [Ca(2+)](i) transients to normal. When myocytes were placed in culture under continued electrical-field stimulation conditions, differences in contraction amplitudes and cell lengths between sham and MI myocytes were preserved for at least 48 h. Infection of both sham and MI myocytes by adenovirus expressing green fluorescent protein resulted in >95% infection, as evidenced by green fluorescent protein fluorescence, but contraction amplitudes at 6-, 24-, and 48-h postinfection were not affected. NCX1 overexpression in MI myocytes resulted in lower diastolic [Ca(2+)](i) levels at all extracellular Ca(2+) concentrations ([Ca(2+)](o)) examined, suggesting enhanced forward NCX1 activity. At 5 mM [Ca(2+)](o), subnormal contraction and [Ca(2+)](i) transient amplitudes in MI myocytes (compared with sham myocytes) were restored toward normal levels by overexpressing NCX1. At 0.6 mM [Ca(2+)](o), supranormal contraction and [Ca(2+)](i) transient amplitudes in MI myocytes (compared with sham myocytes) were lowered by NCX1 overexpression. We conclude that overexpression of NCX1 in MI myocytes was effective in improving contractile dysfunction, most likely because of enhancement of both Ca(2+) efflux and influx during a cardiac cycle. We suggest that decreased NCX1 activity may play an important role in contractile abnormalities in postinfarction myocytes.

Effects of Na(+)/Ca(2+) exchanger downregulation on contractility and [Ca(2+)](i) transients in adult rat myocytes

Postmyocardial infarction (MI) rat myocytes demonstrated depressed Na(+)/Ca(2+) exchange (NCX1) activity, altered contractility, and intracellular Ca(2+) concentration ([Ca(2+)](i)) transients. We investigated whether NCX1 downregulation in normal myocytes resulted in contractility changes observed in MI myocytes. Myocytes infected with adenovirus expressing antisense (AS) oligonucleotides to NCX1 had 30% less NCX1 at 3 days and 66% less NCX1 at 6 days. The half-time of relaxation from caffeine-induced contracture was twice as long in ASNCX1 myocytes. Sarcoplasmic reticulum (SR) Ca(2+)-ATPase abundance, SR Ca(2+) uptake, resting membrane potential, action potential amplitude and duration, L-type Ca(2+) current density and cell size were not affected by ASNCX1 treatment. At extracellular Ca(2+) concentration ([Ca(2+)](o)) of 5 mM, ASNCX1 myocytes had significantly lower contraction and [Ca(2+)](i) transient amplitudes and SR Ca(2+) contents than control myocytes. At 0.6 mM [Ca(2+)](o), contraction and [Ca(2+)](i) transient amplitudes and SR Ca(2+) contents were significantly higher in ASNCX1 myocytes. At 1.8 mM [Ca(2+)](o), contraction and [Ca(2+)](i) transient amplitudes were not different between control and ASNCX1 myocytes. This pattern of contractile and [Ca(2+)](i) transient abnormalities in ASNCX1 myocytes mimics that observed in rat MI myocytes. We conclude that downregulation of NCX1 in adult rat myocytes resulted in decreases in both Ca(2+) influx and efflux during a twitch. We suggest that depressed NCX1 activity may partly account for the contractile abnormalities after MI.

Effects of sprint training on contractility and [Ca(2+)](i) transients in adult rat myocytes

The effects of 6-8 wk of high-intensity sprint training (HIST) on rat myocyte contractility and intracellular Ca(2+) concentration ([Ca(2+)](i)) transients were investigated. Compared with sedentary (Sed) myocytes, HIST induced a modest (5%) but significant (P < 0.0005) increase in cell length with no changes in cell width. In addition, the percentage of myosin heavy chain alpha-isoenzyme increased significantly (P < 0.02) from 0.566 +/- 0.077% in Sed rats to 0.871 +/- 0.006% in HIST rats. At all three (0.6, 1.8, and 5 mM) extracellular Ca(2+) concentrations ([Ca(2+)](o)) examined, maximal shortening amplitudes and maximal shortening velocities were significantly (P < 0.0001) lower and half-times of relaxation were significantly (P < 0.005) longer in HIST myocytes. HIST myocytes had significantly (P < 0.0001) higher diastolic [Ca(2+)](i) levels. Compared with Sed myocytes, systolic [Ca(2+)](i) levels in HIST myocytes were higher at 0.6 mM [Ca(2+)](o), similar at 1.8 mM [Ca(2+)](o), and lower at 5 mM [Ca(2+)](o). The amplitudes of [Ca(2+)](i) transients were significantly (P < 0.0001) lower in HIST myocytes. Half-times of [Ca(2+)](i) transient decline, an estimate of sarcoplasmic reticulum (SR) Ca(2+) uptake activity, were not different between Sed and HIST myocytes. Compared with Sed hearts, Western blots demonstrated a significant (P < 0.03) threefold decrease in Na(+)/Ca(2+) exchanger, but SR Ca(2+)-ATPase and calsequestrin protein levels were unchanged in HIST hearts. We conclude that HIST effected diminished myocyte contractile function and [Ca(2+)](i) transient amplitudes under the conditions studied. We speculate that downregulation of Na(+)/Ca(2+) exchanger may partly account for the decreased contractility in HIST myocytes.

Normal contractions triggered by I(Ca,L) in ventricular myocytes from rats with postinfarction CHF

Attenuated L-type Ca(2+) current (I(Ca,L)), or current-contraction gain have been proposed to explain impaired cardiac contractility in congestive heart failure (CHF). Six weeks after coronary artery ligation, which induced CHF, left ventricular myocytes from isoflurane-anesthetized rats were current or voltage clamped from -70 mV. In both cases, contraction and contractility were attenuated in CHF cells compared with cells from sham-operated rats when cells were only minimally dialyzed using high-resistance microelectrodes. With patch pipettes, cell dialysis caused attenuation of contractions in sham cells, but not CHF cells. Stepping from -50 mV, the following variables were not different between sham and CHF, respectively: peak I(Ca,L) (4.5 +/- 0.3 vs. 3.8 +/- 0.3 pApF(-1) at 23 degrees C and 9.4 +/- 0.5 vs. 8.4 +/- 0.5 pApF(-1) at 37 degrees C), the bell-shaped voltage-contraction relationship in Cs(+) solutions (fractional shortening, 15.2 +/- 1.0% vs. 14.3 +/- 0.7%, respectively, at 23 degrees C and 7.5 +/- 0.4% vs. 6.7 +/- 0.5% at 37 degrees C) and the sigmoidal voltage-contraction relationship in K(+) solutions. Caffeine-induced Ca(2+) release and sarcoplasmic reticulum Ca(2+)-ATPase-to-phospholamban ratio were not different. Thus CHF contractions triggered by I(Ca,L) were normal, and the contractile deficit was only seen in undialyzed cardiomyocytes stimulated from -70 mV.

Overexpression of phospholemman alters contractility and [Ca(2+)](i) transients in adult rat myocytes

Previous studies showed increased phospholemman (PLM) mRNA after myocardial infarction (MI) in rats (Sehl PD, Tai JTN, Hillan KJ, Brown LA, Goddard A, Yang R, Jin H, and Lowe DG. Circulation 101: 1990-1999, 2000). We tested the hypothesis that, in normal adult rat cardiac myocytes, PLM overexpression alters contractile function and cytosolic Ca(2+) concentration ([Ca(2+)](i)) homeostasis in a manner similar to that observed in post-MI myocytes. Compared with myocytes infected by control adenovirus expressing green fluorescent protein (GFP) alone, Western blots indicated a 41% increase in PLM expression after 72 h (P < 0.001) but no changes in Na(+)/Ca(2+) exchanger, SERCA2, and calsequestrin levels in myocytes infected by adenovirus expressing GFP and PLM. At 5 mM extracellular [Ca(2+)] ([Ca(2+)](o)), maximal contraction amplitudes in PLM-overexpressed myocytes were 24% (P < 0.005) and [Ca(2+)](i) transient amplitudes were 18% (P < 0.05) lower than control myocytes. At 0.6 mM [Ca(2+)](o), however, contraction and [Ca(2+)](i) transient amplitudes were significantly (P < 0.05) higher in PLM-overexpressed than control myocytes (18% and 42%, respectively); at 1.8 mM [Ca(2+)](o), the differences in contraction and [Ca(2+)](i) transient amplitudes were narrowed. This pattern of contractile and [Ca(2+)](i) transient abnormalities in PLM-overexpressed myocytes mimics that observed in post-MI rat myocytes. We suggest that PLM overexpression observed in post-MI myocytes may partly account for contractile abnormalities by perturbing Ca(2+) fluxes during excitation-contraction.

Overexpression of Na+/Ca2+ exchanger alters contractility and SR Ca2+ content in adult rat myocytes

The functional consequences of overexpression of rat heart Na+/Ca2+ exchanger (NCX1) were investigated in adult rat myocytes in primary culture. When maintained under continued electrical field stimulation conditions, cultured adult rat myocytes retained normal contractile function compared with freshly isolated myocytes for at least 48 h. Infection of myocytes by adenovirus expressing green fluorescent protein (GFP) resulted in >95% infection as ascertained by GFP fluorescence, but contraction amplitude at 6-, 24-, and 48-h postinfection was not affected. When they were examined 48 h after infection, myocytes infected by adenovirus expressing both GFP and NCX1 had similar cell sizes but exhibited significantly altered contraction amplitudes and intracellular Ca2+ concentration ([Ca2+]i) transients, and lower resting and diastolic [Ca2+]i when compared with myocytes infected by the adenovirus expressing GFP alone. The effects of NCX1 overexpression on sarcoplasmic reticulum (SR) Ca2+ content depended on extracellular Ca2+ concentration ([Ca2+]o), with a decrease at low [Ca2+]o and an increase at high [Ca2+]o. The half-times for [Ca2+]i transient decline were similar, suggesting little to no changes in SR Ca2+-ATPase activity. Western blots demonstrated a significant (P < or = 0.02) threefold increase in NCX1 but no changes in SR Ca2+-ATPase and calsequestrin abundance in myocytes 48 h after infection by adenovirus expressing both GFP and NCX1 compared with those infected by adenovirus expressing GFP alone. We conclude that overexpression of NCX1 in adult rat myocytes incubated at high [Ca2+]o resulted in enhanced Ca2+ influx via reverse NCX1 function, as evidenced by greater SR Ca2+ content, larger twitch, and [Ca2+]i transient amplitudes. Forward NCX1 function was also increased, as indicated by lower resting and diastolic [Ca2+]i.

Sprint training restores normal contractility in postinfarction rat myocytes

The significance of 6-8 wk of high-intensity sprint training (HIST) on contractile abnormalities of myocytes isolated from rat hearts with prior myocardial infarction (MI) was investigated. Compared with the sedentary (Sed) condition, HIST attenuated myocyte hypertrophy observed post-MI primarily by reducing cell lengths but not cell widths. At high extracellular Ca(2+) concentration (5 mM) and low pacing frequency (0.1 Hz), conditions that preferentially favored Ca(2+) influx over efflux, MI-Sed myocytes shortened less than Sham-Sed myocytes did. HIST significantly improved contraction amplitudes in MI myocytes. Under conditions that favored Ca(2+) efflux, i.e., low extracellular Ca(2+) concentration (0.6 mM) and high pacing frequency (2 Hz), MI-Sed myocytes contracted more than Sham-Sed myocytes. HIST did not appreciably affect contraction amplitudes of MI myocytes under these conditions. Compared with MI-Sed myocytes, HIST myocytes showed significant improvement in time required to reach one-half maximal contraction amplitude shortening, maximal myocyte shortening and relengthening velocities, and half time of relaxation. Our results indicate that HIST instituted shortly after MI improved cellular contraction in surviving myocytes. Because our previous studies demonstrated that, in post-MI myocytes, HIST improved intracellular Ca(2+) dynamics, enhanced sarcoplasmic reticulum Ca(2+) uptake and Ca(2+) content, and restored Na(+)/Ca(2+) exchange current toward normal, we hypothesized that improvement in MI myocyte contractile function by HIST was likely mediated by normalization of cellular Ca(2+) homeostatic mechanisms.

Sprint training normalizes Ca(2+) transients and SR function in postinfarction rat myocytes

Previous studies have shown that myocytes isolated from sedentary (Sed) rat hearts 3 wk after myocardial infarction (MI) undergo hypertrophy, exhibit altered intracellular Ca(2+) concentration ([Ca(2+)](i)) dynamics and abnormal contraction, and impaired sarcoplasmic reticulum (SR) function manifested as prolonged half-time of [Ca(2+)](i) decline. Because exercise training elicits positive adaptations in cardiac contractile function and myocardial Ca(2+) regulation, the present study examined whether 6-8 wk of high-intensity sprint training (HIST) would restore [Ca(2+)](i) dynamics and SR function in MI myocytes toward normal. In MI rats, HIST ameliorated myocyte hypertrophy as indicated by significant (P </= 0.05) decreases in whole cell capacitances [Sham-Sed 179 +/-12 (n = 20); MI-Sed 226 +/- 7 (n = 20); MI-HIST 183 +/- 11 pF (n = 19)]. HIST significantly (P < 0.0001) restored both systolic [Ca(2+)](i) [Sham-Sed 421 +/- 9 (n = 79); MI-Sed 350 +/- 6 (n = 70); MI-HIST 399 +/- 9 nM (n = 70)] and half-time of [Ca(2+)](i) decline (Sham-Sed 0. 197 +/- 0.005; MI-Sed 0.247 +/- 0.006; MI-HIST 0.195 +/- 0.006 s) toward normal. Compared with Sham-Sed myocytes, SR Ca(2+)-ATPase expression significantly (P < 0.001) decreased by 44% in MI-Sed myocytes. Surprisingly, expression of SR Ca(2+)-ATPase was further reduced in MI-HIST myocytes to 26% of that measured in Sham-Sed myocytes. There were no differences in calsequestrin expression among the three groups. Expression of phospholamban was not different between Sham-Sed and MI-Sed myocytes but was significantly (P < 0.01) reduced in MI-HIST myocytes by 25%. Our results indicate that HIST instituted shortly after MI improves [Ca(2+)](i) dynamics in surviving myocytes. Improvement in SR function by HIST is mediated not by increased SR Ca(2+)-ATPase expression, but by modulating phospholamban regulation of SR Ca(2+)-ATPase activity.