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

Design of a Proteolytically Stable Sodium-Calcium Exchanger 1 Activator Peptide for In Vivo Studies

The cardiac sodium–calcium exchanger (NCX1) is important for normal Na⁺- and Ca²⁺-homeostasis and cardiomyocyte relaxation and contraction. It has been suggested that NCX1 activity is reduced by phosphorylated phospholemman (pSer68-PLM); however its direct interaction with PLM is debated. Disruption of the potentially inhibitory pSer68-PLM-NCX1 interaction might be a therapeutic strategy to increase NCX1 activity in cardiac disease. In the present study, we aimed to analyze the binding affinities and kinetics of the PLM-NCX1 and pSer68-PLM-NCX1 interactions by surface plasmon resonance (SPR) and to develop a proteolytically stable NCX1 activator peptide for future in vivo studies. The cytoplasmic parts of PLM (PLM_cyt) and pSer68-PLM (pSer68-PLM_cyt) were found to bind strongly to the intracellular loop of NCX1 (NCX1_cyt) with similar K_D values of 4.1 ± 1.0 nM and 4.3 ± 1.9 nM, but the PLM_cyt-NCX1_cyt interaction showed higher on/off rates. To develop a proteolytically stable NCX1 activator, we took advantage of a previously designed, high-affinity PLM binding peptide (OPT) that was derived from the PLM binding region in NCX1 and that reverses the inhibitory PLM (S68D)-NCX1 interaction in HEK293. We performed N- and C-terminal truncations of OPT and identified PYKEIEQLIELANYQV as the minimum sequence required for pSer68-PLM binding. To increase peptide stability in human serum, we replaced the proline with an N-methyl-proline (NOPT) after identification of N-terminus as substitution tolerant by two-dimensional peptide array analysis. Mass spectrometry analysis revealed that the half-life of NOPT was increased 17-fold from that of OPT. NOPT pulled down endogenous PLM from rat left ventricle lysate and exhibited direct pSer68-PLM binding in an ELISA-based assay and bound to pSer68-PLM_cyt with a K_D of 129 nM. Excess NOPT also reduced the PLM_cyt-NCX1_cyt interaction in an ELISA-based competition assay, but in line with that NCX1 and PLM form oligomers, NOPT was not able to outcompete the physical interaction between endogenous full length proteins. Importantly, cell-permeable NOPT-TAT increased NCX1 activity in cardiomyocytes isolated from both SHAM-operated and aorta banded heart failure (HF) mice, indicating that NOPT disrupted the inhibitory pSer68-PLM-NCX1 interaction. In conclusion, we have developed a proteolytically stable NCX1-derived PLM binding peptide that upregulates NCX1 activity in SHAM and HF cardiomyocytes.

A new cell-penetrating peptide that blocks the autoinhibitory XIP domain of NCX1 and enhances antiporter activity

The plasma membrane Na(+)/Ca(2+) exchanger (NCX) is a high-capacity ionic transporter that exchanges 3Na(+) ions for 1Ca(2+) ion. The first 20 amino acids of the f-loop, named exchanger inhibitory peptide (XIP(NCX1)), represent an autoinhibitory region involved in the Na(+)-dependent inactivation of the exchanger. Previous research has shown that an exogenous peptide having the same amino acid sequence as the XIP(NCX1) region exerts an inhibitory effect on NCX activity. In this study, we identified another regulatory peptide, named P1, which corresponds to the 562-688aa region of the exchanger. Patch-clamp analysis revealed that P1 increased the activity of the exchanger, whereas the XIP inhibited it. Furthermore, P1 colocalized with NCX1 thus suggesting a direct binding interaction. In addition, site-directed mutagenesis experiments revealed that the binding and the stimulatory effect of P1 requires a functional XIP(NCX1) domain on NCX1 thereby suggesting that P1 increases the exchanger activity by counteracting the action of this autoinhibitory sequence. Taken together, these results open a new strategy for developing peptidomimetic compounds that, by mimicking the functional pharmacophore of P1, might increase NCX1 activity and thus exert a therapeutic action in those diseases in which an increase in NCX1 activity might be helpful.

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.

The effects of ropivacaine at clinically relevant doses on myocardial ischemia in pigs

A major risk associated with bupivacaine during myocardial ischemia is ventricular fibrillation. We investigated the influence of ropivacaine on cardiac contractility and the propensity to ventricular fibrillation before and after myocardial ischemia in a placebo-controlled pig study. Anesthetized domestic pigs were administered 1 mg.kg(-1) of ropivacaine intravenously over 1 min and then 0.03 mg.kg(-1).min(-1) as a 30-min infusion, or saline. The following endpoints were measured before and after ropivacaine administration: (1) the ventricular fibrillation threshold (VFT) before and during myocardial ischemia induced by total transient ligation of the anterior interventricular artery and (2) electrophysiological (sinus heart rate, duration of QRS and QT intervals) and hemodynamic (blood pressure, the time derivative of left ventricular pressure [peak LV dP/dt]) parameters. Ropivacaine induced no changes in sinus heart rate, QRS, and or QT before or during ischemia. In contrast, there was a mild increase in the VFT before ischemia, which was drastically and significantly reduced during ischemia. The reduction of peak LV dP/dt during ischemia was further increased by ropivacaine. We also found that the effect of ropivacaine on the VFT was coronary blood flow-dependent, with a markedly decreased threshold in the presence of ischemia. Similar effects have been observed in humans with several other local anesthetics, as well as with class I antiarrhythmic drugs. The results of this study should be taken into account by anesthesiologists when administering ropivacaine to coronary patients.

Phospholemman overexpression inhibits Na+-K+-ATPase in adult rat cardiac myocytes: relevance to decreased Na+ pump activity in postinfarction myocytes

Messenger RNA levels of phospholemman (PLM), a member of the FXYD family of small single-span membrane proteins with putative ion-transport regulatory properties, were increased in postmyocardial infarction (MI) rat myocytes. We tested the hypothesis that the previously observed reduction in Na+-K+-ATPase activity in MI rat myocytes was due to PLM overexpression. In rat hearts harvested 3 and 7 days post-MI, PLM protein expression was increased by two- and fourfold, respectively. To simulate increased PLM expression post-MI, PLM was overexpressed in normal adult rat myocytes by adenovirus-mediated gene transfer. PLM overexpression did not affect the relative level of phosphorylation on serine68 of PLM. Na+-K+-ATPase activity was measured as ouabain-sensitive Na+-K+ pump current (Ip). Compared with control myocytes overexpressing green fluorescent protein alone, Ip measured in myocytes overexpressing PLM was significantly (P < 0.0001) lower at similar membrane voltages, pipette Na+ ([Na+]pip) and extracellular K+ ([K+]o) concentrations. From -70 to +60 mV, neither [Na+]pip nor [K+]o required to attain half-maximal Ip was significantly different between control and PLM myocytes. This phenotype of decreased V(max) without appreciable changes in K(m) for Na+ and K+ in PLM-overexpressed myocytes was similar to that observed in MI rat myocytes. Inhibition of Ip by PLM overexpression was not due to decreased Na+-K+-ATPase expression because there were no changes in either protein or messenger RNA levels of either alpha1- or alpha2-isoforms of Na+-K+-ATPase. In native rat cardiac myocytes, PLM coimmunoprecipitated with alpha-subunits of Na+-K+-ATPase. Inhibition of Na+-K+-ATPase by PLM overexpression, in addition to previously reported decrease in Na+-K+-ATPase expression, may explain altered V(max) but not K(m) of Na+-K+-ATPase in postinfarction rat myocytes.

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.

Modification of sarcolemmal Na+-K+-ATPase and Na+/Ca2+exchanger expression in heart failure by blockade of renin-angiotensin system

The activities of both sarcolemmal (SL) Na+-K+-ATPase and Na+/Ca2+exchanger, which maintain the intracellular cation homeostasis, have been shown to be depressed in heart failure due to myocardial infarction (MI). Because the renin-angiotensin system (RAS) is activated in heart failure, this study tested the hypothesis that attenuation of cardiac SL changes in congestive heart failure (CHF) by angiotensin-converting enzyme (ACE) inhibitors is associated with prevention of alterations in gene expression for SL Na+-K+-ATPase and Na+/Ca2+exchanger. CHF in rats due to MI was induced by occluding the coronary artery, and 3 wk later the animals were treated with an ACE inhibitor, imidapril (1 mg·kg−1·day−1), for 4 wk. Heart dysfunction and cardiac hypertrophy in the infarcted animals were associated with depressed SL Na+-K+-ATPase and Na+/Ca2+exchange activities. Protein content and mRNA levels for Na+/Ca2+exchanger as well as Na+-K+-ATPase α1-, α2- and β1-isoforms were depressed, whereas those for α3-isoform were increased in the failing heart. These changes in SL activities, protein content, and gene expression were attenuated by treating the infarcted animals with imidapril. The beneficial effects of imidapril treatment on heart function and cardiac hypertrophy as well as SL Na+-K+-ATPase and Na+/Ca2+exchange activities in the infarcted animals were simulated by enalapril, an ACE inhibitor, and losartan, an angiotensin receptor antagonist. These results suggest that blockade of RAS in CHF improves SL Na+-K+-ATPase and Na+/Ca2+exchange activities in the failing heart by preventing changes in gene expression for SL proteins.

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.

Sprint training improves contractility in postinfarction rat myocytes: role of Na+/Ca2+ exchange

Previous studies in adult myocytes isolated from rat hearts 3–9 wk after myocardial infarction (MI) demonstrated abnormal contractility and decreased Na+/Ca2+ exchanger (NCX1) activity. In addition, a program of high-intensity sprint training (HIST) instituted shortly after MI restored both contractility and NCX1 activity toward normal. The present study examined the hypotheses that reduced NCX1 activity caused abnormal contractility in myocytes isolated from sedentary (Sed) rat hearts 9–11 wk after coronary artery ligation and that HIST ameliorated contractile dysfunction in post-MI myocytes by increasing NCX1 activity. The approach was to upregulate NCX1 in MI-sedentary (MISed) myocytes and downregulate NCX1 in MI-exercised (MIHIST) myocytes by adenovirus-mediated gene transfer. Overexpression of NCX1 in MISed myocytes did not affect sarco(endo)plasmic reticulum Ca2+-ATPase and calsequestrin levels but rescued contractile abnormalities observed in MISed myocytes. That is, at 5 mM extracellular Ca2+ concentration, the subnormal contraction amplitude in MISed myocytes (compared with Sham myocytes) was increased toward normal by NCX1 overexpression, whereas at 0.6 mM extracellular Ca2+ concentration the supernormal contraction amplitude in MISed myocytes was lowered. Conversely, NCX1 downregulation by antisense in MIHIST myocytes abolished the beneficial effects of HIST on contraction amplitudes in MI myocytes. We suggest that decreased NCX1 activity may play an important role in contractile abnormalities in post-MI myocytes and that HIST ameliorated contractile dysfunction in post-MI myocytes partly by enhancing NCX1 activity.