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1Cardiovascular Research Center, Massachusetts General Hospital, Harvard Medical School, Charlestown, Massachusetts; 2Department of Physiology II, Nara Medical University School of Medicine, Kashihara, Nara, Japan; and 3Department of Life Science, Gwangju Institute of Science and Technology, Gwangju, Korea
Submitted 29 November 2006 ; accepted in final form 9 January 2007
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ABSTRACT |
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 TOP ABSTRACT MATERIALS AND METHODSRESULTSDISCUSSIONGRANTSREFERENCES |
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-galactosidase (-Gal) gene or saline by a catheter-based technique. LV mechanical and energetic function was measured in cross-circulated heart preparations 2–3 days after the infection. The end-systolic pressure-volume relation was shifted upward, end-systolic pressure at 0.1 ml of intraballoon water volume was higher, and equivalent maximal elastance, i.e., enhanced LV contractility, was higher in the SERCA group than in the normal, -Gal, and saline groups. Moreover, the LV relaxation rate was faster in the SERCA group. There was no significant difference in myocardial O2 consumption per beat-systolic pressure-volume area relation among the groups. Finally, O2 cost of LV contractility was decreased to subnormal levels in the SERCA group but remained unchanged in the -Gal and saline groups. This lowered O2 cost of LV contractility in SERCA hearts indicates energy saving in Ca2+ handling during excitation-contraction coupling. Thus overexpression of SERCA2a transformed the normal energy utilization to a more efficient state in Ca2+ handling and superinduced the supranormal contraction/relaxation due to enhanced Ca2+ handling.
contractile function; energetics; oxygen consumption; SERCA2a
Thus restoration of SERCA2a activity may be valuable therapeutically in failing hearts. We previously showed that adenoviral gene transfer of SERCA2a can modify intracellular Ca2+ handling and normalize contractile function in isolated cardiomyocytes from neonatal rats and failing human hearts and in senescent and aortic-banded failing rat whole hearts (6, 12, 18, 24). Moreover, global cardiac gene transfer of SERCA2a improved survival and energetic state [as measured by phosphocreatine (PCr)-to-ATP ratio] in aortic-banded rats (8) and reduced ventricular arrhythmias in a rat model of ischemia (7). In addition, SERCA2a gene transfer improved left ventricular (LV) mechanical and energetic functions in terms of O2 cost of LV contractility in diabetes-induced heart failure in rats (21).
In addition to such studies using adenovirus-mediated overexpression of SERCA2a, transgenic mouse and rat models overexpressing SERCA2a in the heart have been generated to examine the effect of chronic SERCA2a overexpression on mechanical performance, as well as Ca2+ transients, in normal hearts (2, 16, 19). Transgenic hearts overexpressing SERCA2a showed enhanced contractility, with a concomitant boost in Ca2+ transient amplitude, compared with wild-type hearts. However, during development, in the transgenic animals the expression of other genes associated with Ca2+ handling may be altered (16), and this may influence the effects of SERCA2a overexpression. The advantage of catheter-based adenoviral gene transfer is direct overexpression of a specific gene without the deceptive effects of developmental adaptations that may be present in transgenic animals. Therefore, a short-term transgene expression system of adenoviral vectors would be useful for analysis of the direct effects of overexpression of a specific gene.
In our previous study, we found that global overexpression of SERCA2a in nonfailing sham-operated hearts enhances contractility and accelerates relaxation, as evidenced by increases of pressure development in contraction/relaxation (±dP/dt) in the rats (18). However, energetic function has not been analyzed in normal hearts subjected to adenoviral gene transfer of SERCA2a. The aim of the present study was to examine how adenoviral gene transfer of SERCA2a to normal rat hearts can influence LV mechanical and energetic function, especially in terms of O2 cost of LV contractility, in cross-circulated excised heart preparations. Thus this study may provide a novel insight into the specific role of SERCA2a in LV mechanical and energetic function in normal nonfailing hearts without complex changes in cardiac function or expression of other Ca2+-handling proteins and also may have significant implications for the efficacy and safety of SERCA2a gene therapy.
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MATERIALS AND METHODS |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONGRANTSREFERENCES |
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All animal experiments were performed with the approval of the Animal Care Committee of Massachusetts General Hospital and in accordance with the National Institutes of Health Guide for the Care and Use of Laboratory Animals. Male Wistar rats (4–6 mo old; Charles River, MA) were randomized into four groups: 1) normal rats (n = 6), 2) rats subjected to adenoviral -galactosidase (Ad.gal) transfer (-Gal rats, n = 7), 3) saline-injected rats (n = 6), and 4) rats subjected to adenoviral SERCA2a (Ad.SERCA) transfer (SERCA rats, n = 6).
Recombinant Adenoviral Vectors
Recombinant adenoviral vectors with cytomegalovirus-driven expression cassettes for SERCA2a or -Gal were used; a second cassette in each adenovirus containing green fluorescent protein was substituted for E1 by homologous recombination. Concentrations of Ad.SERCA and Ad.gal were 6.2 x 1010 and 4.8 x 1010 plaque-forming units/ml, respectively, with a 40:1 particle-to-plaque-forming unit ratio. Wild-type adenovirus contamination was excluded by the absence of PCR-detectable E1 sequences.
Adenoviral Gene Delivery Protocol
The adenoviral gene delivery system has been described previously by our group (11). Briefly, after the rats were anesthetized with pentobarbital sodium (50 mg/kg ip) and subjected to thoracotomy, a 22-gauge catheter containing 200 µl of adenovirus and 50 µl of adenosine (3 mg/ml) was advanced from the apex of the LV to the aortic root. The aorta and main pulmonary artery were clamped distal to the site of the catheter for 30 s, and the solution was injected. The chest was closed, and the animals were extubated and returned to their cages. Saline, instead of adenovirus, was administered to the saline group.
LV Mechanical and Energetic Studies
Surgical preparations.
LV mechanical and energetic studies were performed on the excised cross-circulated rat heart preparations 2–3 days after injection of adenovirus or saline. The surgical preparations have been described previously in detail (15, 20). Briefly, for each experiment, two male 500- to 650-g Wistar rats (blood supplier and metabolic supporter) and one heart donor rat were anesthetized with pentobarbital sodium (50 mg/kg ip), intubated, and heparinized (1,000 U iv). The common carotid arteries and the right external jugular vein of the supporter rat were cannulated and connected to the arterial and venous cross-circulation tubing, respectively. The arterial and venous cross-circulation tubing from the supporter rat were connected by a cannula to the brachiocephalic artery and the right ventricle (RV) via the superior vena cava, respectively, of the heart donor rat. In the excised beating heart maintained at 37°C, a thin latex balloon (balloon material volume 0.08 ml), connected to a pressure transducer for measuring LV pressure (LVP), was inserted into the LV and primed with water. Systolic unstressed volume (V0 = 0.08 ml) could be determined as the volume at which peak isovolumic pressure was zero. Heart rate was maintained constant at 300 beats/min by electrical pacing of the right atrium. Total coronary blood flow was continuously measured with an ultrasonic flowmeter (model T206, Transonic System, Ithaca, NY), in which the inline flow probe was placed in the middle of the coronary venous drainage tubing from the RV. LV thebesian flow was negligible. Systemic arterial blood pressure of the supporter rat served as coronary perfusion pressure, which was almost constant (mean 
90 mmHg) throughout the experiment. Arterial blood pH, PO2, PCO2, O2 saturation, and O2 content of the supporter rat and perfused blood were monitored with a blood gas analyzer (model GEM 3000, Instrumentation Laboratory) and an oximeter (model IL682 CO-Oxymeter, Instrumentation Laboratory) and maintained within their physiological ranges by an increase of the O2 supplementation in inspiration of the supporter rat and by addition of 3–4 ml of 8.4% sodium bicarbonate solution to the perfused blood throughout the experiment.
Calculation of O2 consumption.
Myocardial O2 consumption (
O2) was obtained as the product of coronary blood flow and arteriovenous blood O2 content difference. The RV component of total O2, which is considered constant irrespective of LV volume, was calculated as follows: biventricular O2 under LV volume unloading (i.e., free of intraballoon water) x RV weight/(RV weight + LV weight). LV O2 was calculated as follows: total O2 – RV O2. LV VO2 per beat, obtained by dividing LVO2 by the heart rate (300 beats/min), is expressed as VO2, unless otherwise specified below.
Experimental protocol.
LVP, LV O2, and systolic pressure-volume area (PVA) data were obtained at five different LV volumes from 0.08 to 0.18 ml in 0.025-ml increments (i.e., from 0 to 0.1 ml of intraballoon water), without inotropic interventions (control volume-loading run). After the control volume-loading run, the Ca2+ inotropism run was performed at a midrange LV volume (mLVV) (i.e., 0.05 ml of intraballoon water) by intracoronary infusion of 1% CaCl2 solution. The infusion rate of CaCl2 solution was increased stepwise from 2 to 6 ml/h. In 10 heart preparations (n = 1 normal, 3 -Gal, 2 saline, and 4 SERCA), a dobutamine (78 µM) inotropism run was also performed at an infusion rate of 2–4 ml/h 1 h after the Ca2+ inotropism run. Steady-state O2 was reached 2–3 min after change of LV volume and 4 min after change of infusion rate. Finally, cardiac arrest was induced by intracoronary infusion of 1 M KCl (12 ml/h) to obtain O2 for basal metabolism. At each steady state, data were sampled at 500 Hz for 2 s simultaneously, and the sampling was usually repeated three times at 0.5- to 1-min intervals.
Data Analysis
Calculation of PVA. The best-fit end-systolic pressure (ESP)/end-diastolic pressure (EDP)-volume relations (ESPVR/EDPVR) were obtained by fitting the data with the exponential functions (23). PVA was defined as the pressure-volume area circumscribed by the curvilinear ESPVR, the EDPVR, and the systolic portion of the ventricular pressure-volume trajectory. The areas under the ESPVR and EDPVR were obtained by integration of the best-fit exponential functions.
VO2 for Ca2+ handling during the inotropic run.
In previous mechanoenergetic studies in rats (15, 20, 23), a linear VO2-PVA relation obtained during Ca2+ infusion (Ca2+ VO2-PVA relation) was shifted upward in parallel with the control VO2-PVA relation before Ca2+ infusion. To confirm such parallelism in adenovirus-infected rat hearts, in the preliminary experiments we compared the slopes of the control and Ca2+ VO2-PVA relation at a Ca2+ infusion rate of 6 ml/h in two -Gal and two SERCA hearts. Both slopes were similar (control vs. Ca2+: 1.65 vs. 1.62 and 1.36 vs. 1.43 in -Gal; 1.91 vs. 1.94 and 1.47 vs. 1.53 in SERCA). The VO2-intercept (PVA-independent VO2) corresponds primarily to the VO2 for Ca2+ handling in E-C coupling and the VO2 for basal metabolism (27). The increased VO2 intercept of the Ca2+ VO2-PVA relation is attributable to the increased VO2 for enhanced Ca2+ handling due to the unchanged VO2 for basal metabolism during Ca2+ infusion (27). During the Ca2+/dobutamine inotropic run at mLVV, ESPVR values at different infusion rates were obtained as a best-fit exponential function curve. We calculated PVA at mLVV (PVAmLVV) by integrating each ESPVR from V0 to mLVV. We then obtained the two composite VO2-PVAmLVV data points. After we drew the lines, including each VO2-PVAmLVV data point, in parallel to the control VO2-PVA relation, we obtained the VO2 intercept at each infusion rate. We subtracted basal metabolic VO2 per beat, measured in KCl-arrested hearts, from PVA-independent VO2 to obtain VO2 for Ca2+ handling in E-C coupling.
LV contractility. To obtain eEmax, an index of LV contractility, we calculated the ESP-to-volume ratio of the specific virtual triangle, which is energetically equivalent to the real PVAmLVV.
O2 cost of LV contractility. The O2 cost of LV contractility was obtained as the slope of the linear relation between VO2 for Ca2+ handling in E-C coupling and eEmax during the Ca2+ or dobutamine inotropism run. This slope is considered an index quantifying the VO2 for Ca2+ handling per unit change in LV contractility.
Logistic time constant. To evaluate the LV relaxation rate, we used the logistic time constant (TL) derived from a logistic model to analyze LV isovolumic relaxation pressure-time curves at mLVV.
Western blot for SERCA2a protein. Lysates from the hearts, obtained after the cross-circulation studies, were matched for protein concentration, separated by SDS-PAGE, and transferred to nitrocellulose membranes. For immunoreaction, the blots were incubated with SERCA2a antibodies and then subjected to enhanced chemiluminescence for detection.
Statistical analysis. Values are means ± SD. Multiple comparisons were performed by ANOVA followed by a Student-Newman-Keuls post hoc test with STATVIEW (Abacus Concepts, Berkeley, CA). Statistical significance was accepted at P < 0.05.
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RESULTS |
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There was no statistical difference in body weight, LV weight, RV weight, LV weight-to-body weight ratio, and RV weight-to-body weight ratio among the four groups (Table 1).
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Figure 1A shows representative control ESPVR and EDPVR values without inotropic interventions in saline, -Gal, and SERCA hearts. Curvilinear ESPVR and EDPVR of normal hearts were similar to saline and -Gal hearts (data not shown). ESPVR of SERCA hearts was shifted upward compared with saline and -Gal hearts. A summary of LV mechanics is shown in Table 1. There were no significant differences in all the best-fitting parameters of ESPVR and EDPVR equations. In the SERCA group, ESP at 0.1 ml of intraballoon water volume (ESP0.1) was increased over 200 mmHg and was significantly higher than in the normal group, although ESP0.1 in the -Gal and saline groups was not significantly different from the normal group. On the other hand, there was no significant difference in EDP at 0.1 ml of intraballoon water volume among the four groups. Moreover, TL values obtained from the LV isovolumic relaxation pressure-time curves of all groups at mLVV were analyzed (Fig. 1B). TL was significantly shorter in the SERCA than in the normal group, although TL in the -Gal and saline groups remained as long as in the normal group.
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Representative control VO2-PVA relations without inotropic interventions in saline, -Gal, and SERCA hearts are shown in Fig. 1C. Similar linear VO2-PVA relations were obtained in normal hearts (data not shown). Data for LV energetics are summarized in Table 2. There was no significant difference in the slope and VO2 intercept of the VO2-PVA relation among all four groups. In adenovirus- or saline-injected groups, moreover, the minute VO2 for basal metabolism and for Ca2+ handling in E-C coupling, of which the beat values are components of the VO2 intercept of the VO2-PVA relation, were not significantly different from the normal group.
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Changes in ESPmLVV in response to Ca2+ infusion are shown in Fig. 2. Mean ESPmLVV before Ca2+ infusion was higher, although not significantly, in the SERCA group than in the three other groups. ESPmLVV was gradually increased as the infusion rate of Ca2+ solution was increased stepwise from 2 to 6 ml/h. The maximal increase in ESPmLVV in response to Ca2+ infusion (6 ml/h) in the SERCA group was not significantly different from that in the three other groups (Fig. 2). In 10 heart preparations, we infused the 1-adrenergic receptor agonist dobutamine into the coronary perfusion tubing after the Ca2+ infusion. Changes in ESPmLVV in response to dobutamine were similar to those in response to Ca2+ in all groups (data not shown).
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eEmax at mLVV, an index of LV contractility, was obtained before Ca2+ infusion and compared among all groups (Fig. 3). The mean value of eEmax at mLVV was significantly higher in the SERCA group than in the saline group.
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Representative relations between VO2 for Ca2+ handling in E-C coupling and eEmax at mLVV during Ca2+ inotropism in saline, -Gal, and SERCA hearts are shown in Fig. 4A. The slope was gentler (i.e., lower O2 cost of LV contractility) in SERCA hearts than in saline and -Gal hearts. After Ca2+ infusion, dobutamine was infused into the same 10 heart preparations. There was a fairly good correlation between the O2 cost of LV contractility in response to Ca2+ and in response to dobutamine (r = 0.93; data not shown). The O2 cost of LV contractility was significantly lower in the SERCA group than in the normal group, but it remained as high in the -Gal and saline groups as in the normal group (Fig. 4B).
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Finally, we examined SERCA2a protein expression in the hearts used for the analysis of mechanical and energetic function (Fig. 5). SERCA2a expression was increased 2.5- and 4.3-fold in SERCA hearts compared with normal and -Gal hearts, respectively.
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DISCUSSION |
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LV Mechanical Function and SERCA2a Overexpression
In this study, adenoviral gene transfer of SERCA2a results in an upward shift of ESPVR, an increase in ESP0.1, and an increase in eEmax at mLVV compared with normal control data. In the SERCA2a-transferred rats, moreover, LV relaxation rate was accelerated, as shown by the shortened TL. Thus, in normal hearts, overexpression of SERCA2a superinduced enhancement of LV systolic and diastolic function. The present results correlate well with our previous observation that adenoviral gene transfer of SERCA2a increases maximal ±dP/dt in nonfailing hearts (18). In addition, these results also agree well with those from transgenic mice and rats overexpressing SERCA2a, where higher LV contractile function, as evidenced by increased LV systolic pressure, increased ±dP/dt, and shortened time constant of isovolumic relaxation, was demonstrated in isolated work-performing heart preparations (2, 19). The molecular and cellular mechanism for enhanced contractile function by SERCA2a gene transfer in the present in vivo study is provided by the following results from an in vitro study: in neonatal rat cardiomyocytes overexpressing SERCA2a, increased SERCA2a activity produced a shorter Ca2+ transient and lower resting intracellular Ca2+ levels, reflecting an enhanced Ca2+ uptake, as well as an increase in peak Ca2+ levels, reflecting more Ca2+ available for release (12). An increase in Ca2+ release would result in more Ca2+ available for myofibrillar activation. Therefore, this enhanced Ca2+ handling resulted in enhanced shortening and faster relaxation in isolated cardiomyocytes. Thus the high LV contractility in our SERCA group seems to be due mainly to the SERCA2a overexpression-induced enhancement of Ca2+ handling. Furthermore, in our SERCA hearts, which show higher levels of ESPmLVV, the maximal increase in ESPmLVV in response to Ca2+ infusion was not significantly different from that of the three other control groups. Thus the gene transfer of SERCA2a was capable of increasing contractility and preserving the inotropic response to Ca2+ (i.e., contractile reserve) in normal hearts, suggesting that the overexpressed SERCA2a functions well and enhances Ca2+ handling. These results together provide convincing evidence that SERCA2a is a primary determinant of myocardial contractility and that overexpression of SERCA2a is capable of superinducing the supranormal contractility.
LV Energetic Function and SERCA2a Overexpression
The VO2 intercept (i.e., PVA-independent VO2) and the slope of the VO2-PVA relation in our SERCA hearts did not differ from the three other groups, as previously reported for SERCA2a-overexpressing diabetic rat hearts (21). The PVA-independent VO2 reflects VO2 for nonmechanical work consisting of Ca2+ handling during E-C coupling and basal metabolism (27). We found no difference in minute O2 for Ca2+ handling and basal metabolism among the groups. This finding shows that adenoviral gene transfer never affects O2 for Ca2+ handling and basal metabolism at steady state in mechanically unloaded and Ca2+-unloaded normal hearts. This unchanged VO2 for Ca2+ handling in SERCA hearts may be attributable to a low level of diastolic intracellular free Ca2+ in the physiologically unloaded condition. It seems likely that overexpression of SERCA2a does not work well because of the limited amount of intracellular Ca2+ available for Ca2+ sequestration into the SR. On the other hand, the unchanged slope of the VO2-PVA relation in all the groups shows that adenoviral gene transfer never affects contractile efficiency, which is the reciprocal of the slope of the VO2-PVA relation, in normal hearts. Contractile efficiency, which reflects the chemomechanical energy transduction efficiency of the contractile machinery, is the product of the efficiency from VO2 to ATP (mitochondrial oxidative phosphorylation) and the efficiency from ATP to PVA (cross-bridge cycling) (27). Therefore, SERCA2a overexpression appears to have no influence on the efficiency of mitochondrial oxidative phosphorylation and cross-bridge cycling in normal hearts.
The most important finding of this study is that the O2 cost of LV contractility, defined as the slope of the relation of VO2 for Ca2+ handling to eEmax, was decreased to subnormal levels in SERCA2a-expressing normal hearts. The O2 cost of contractility, which reflects the energy cost of nonmechanical activities from VO2 to eEmax, is the product of the energy cost from VO2 to ATP (mitochondrial oxidative phosphorylation) and the energy cost from ATP to eEmax (E-C coupling), which consists of the cost of Ca2+ handling and the Ca2+ responsiveness of myofilaments (27). Therefore, it appears that the decreased O2 cost of contractility in our SERCA hearts can be ascribed to the decreased energy cost of Ca2+ handling, rather than the decreased energy cost of mitochondrial oxidative phosphorylation, which seems unlikely, as suggested by the unchanged slope of the VO2-PVA relation, or the increased Ca2+ responsiveness of myofilaments. One possible explanation for this decreased energy cost of Ca2+ handling, i.e., the energy saving in Ca2+ handling during E-C coupling, is as follows. SERCA2a removes cytosolic Ca2+ on the basis of the stoichiometry of 2 Ca2+:1 ATP. On the other hand, NCX removes cytosolic Ca2+ in exchange with Na+ influx on the basis of the stoichiometry of 3 Na+:1 Ca2+ without ATP consumption, and the influx of Na+ pumps out by Na+-K+-ATPase with a stoichiometry of 3 Na+:2 K+:1 ATP, resulting in the net stoichiometry of 1 Ca2+:1 ATP (3). Therefore, the Ca2+ uptake by SERCA2a into the SR leads to half the energy expenditure of Ca2+ extrusion via NCX if the same amount of Ca2+ is handled by SERCA2a and NCX. Although the contribution of SERCA2a to reduction of cytosolic Ca2+ is high (92%) and Ca2+ extrusion via NCX is low (7%) in normal rat hearts (4), SERCA2a overexpression may induce the further increase in Ca2+ uptake into the SR and, thereby, the further decrease in Ca2+ extrusion via NCX, resulting in the decreased energy cost of Ca2+ handling. Thus the decreased O2 cost of LV contractility in the Ca2+-loaded SERCA2a hearts may be caused mainly by the altered distribution of Ca2+ removal from the cytoplasm during relaxation.
The heart requires a continuous supply of energy in the form of ATP, which is mostly produced by oxidative phosphorylation in mitochondria, with the major energy reserve molecule represented by PCr. In normal hearts, the majority of energy consumption is due to cross-bridge cycling, and 15% of the energy expenditure is used to remove Ca2+ from the cytoplasm during relaxation. In aortic banding-induced heart failure in rats (22), SERCA2a overexpression improved the high O2 cost of PVA (total mechanical energy) and LV contractility, i.e., energy wasting in chemomechanical energy transduction and in Ca2+ handling during E-C coupling, and, consequently, restored and normalized the reduced PCr-to-ATP ratio, i.e., less energy reserve (8). In SERCA2a-overexpressing sham-operated hearts, however, the PCr-to-ATP ratio was significantly decreased (8). We have speculated on one possible mechanism for this finding: superinduction of supranormal mechanical contraction by SERCA2a overexpression in normal rat hearts. This increase in contractility would increase the amount and rate of ATP hydrolysis in cross-bridge cycling and, thereby, drive PCr down, although there is the more efficient, but minor, energy utilization in Ca2+ handling.
Therapeutic Implications
In patients with congestive heart failure (CHF), inotropic agents can improve contractility and hemodynamics in the short term but cause an energetic imbalance due to the increased O2 in the long term. Therefore, long-term inotropic interventions resulted in the increased mortality in patients with CHF (26). In cross-circulated normal heart preparations, the O2 cost of LV contractility for many inotopic agents, including catecholamine (20), phosphodiesterase inhibitor (9), and Ca2+ sensitizer (14), was as high as for Ca2+, supporting the clinical observation. In this study, however, gene transfer of SERCA2a decreased the O2 cost of LV contractility for Ca2+ or dobutamine, i.e., improved the energy utilization in Ca2+ handling during E-C coupling, even in normal hearts, as well as in diabetes-induced (21) and aortic banding-induced (22) failing rat hearts. Similarly, such efficient energy utilization may be observed in the transgenic animals expressing SERCA2a in the heart. Thus gene transfer of SERCA2a may provide the great advantage of efficient energy utilization over many inotropic agents.
Limitations of the Study and Future Directions
To clarify how gene transfer of SERCA2a alters the distribution of Ca2+ removal from cytoplasm and contributes to the energy saving in Ca2+ handling, quantitative electrophysiological analyses of L-type Ca2+ currents, SR Ca2+ contents, NCX function (Na+/Ca2+ exchange current), intracellular Ca2+ transients, action potential, and contraction are required in ventricular myocytes isolated from SERCA2a-overexpressing normal rat hearts. In rodents, 92% of Ca2+ is removed by SERCA2a, as mentioned above; in humans, however, 75% is removed by SERCA2a (4). Thus the contribution of SERCA2a to reduction of cytosolic Ca2+ levels varies between rodents and large mammals. In this study, SERCA2a overexpression enhanced LV mechanical performance and decreased O2 cost of contractility, even in normal rat hearts with high SERCA2a function, as well as in diabetes-induced (21) and aortic banding-induced (22) heart failure in rats with downregulated expression of SERCA2a. In addition, SERCA2a overexpression improved contractile function in failing human cardiomyocytes (6). Therefore, SERCA2a gene transfer will be expected to be a potential therapy for correction of LV mechanical and energetic dysfunction in failing human hearts with deteriorated SERCA2a function, regardless of etiology and the degree of cardiac dysfunction. However, excessive overexpression of SERCA2a may induce hypertension by the supranormal mechanical performance of LV. To reach the goal of gene therapy for CHF, large animal studies are required in a long-term transgene expression system of adeno-associated virus-based vectors.
In conclusion, in normal rats subjected to adenoviral gene transfer of SERCA2a, cardiac mechanical performance was enhanced and O2 cost of LV contractility was decreased compared with normal, -Gal, or saline rats. Thus SERCA2a overexpression was capable of transforming the normal energy utilization to a more efficient state in Ca2+ handling during E-C coupling and superinducing the supranormal contraction/relaxation due to enhanced Ca2+ handling.
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FOOTNOTES |
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The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
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), 
), and SERCA (
) hearts.
P > 0.05 vs. normal, 
3). *P < 0.05 vs.