Does sex influence the incidence or severity of reperfusion-induced cardiac arrhythmias?

UNLABELLED: Whether sex affects the acute phase of myocardial ischemia in experimental animal models is currently being debated. Our purpose was to determine if sex influences either the incidence or severity of reperfusion-induced arrhythmias resulting from a brief coronary occlusion. Male and female Sprague-Dawley rats were assigned to the study. Anesthetized animals were subjected to a 5-minute coronary artery occlusion followed by 5 minutes of reperfusion. Mortality differed by sex: 10/27 (37%) of males died due to VT/VF while only 1/16 females (6%) died due to VT/VF (p = 0.033). Quantitative analysis of the electrocardiogram was performed on data acquired from 17 male and 15 female survivors. Analysis showed no other significant differences in ventricular arrhythmias between the two groups. CONCLUSION: Lethal reperfusion-induced arrhythmias led to a higher mortality in male rats versus female rats. Among survivors there was no difference in any other arrhythmic parameters measured.

First direct comparison of the late sodium current blocker ranolazine to established antiarrhythmic agents in an ischemia/reperfusion model.

INTRODUCTION: There are few safe antiarrhythmics for ischemic heart disease. Whereas ranolazine is a promising late INa blocker with antiarrhythmic effects, and devoid of pro-arrhythmic properties, there are no direct comparisons between ranolazine and other antiarrhythmic agents in an ischemia/reperfusion setting. HYPOTHESIS AND METHODS: To determine whether ranolazine was as effective as sotalol and lidocaine to reduce ischemia/reperfusion-induced arrhythmias, anesthetized rats were subjected to 5 minutes of proximal left coronary artery occlusion plus 5 minutes of reperfusion, which causes severe ventricular arrhythmias. At 21 minutes prior to coronary occlusion, rats (n = 20 per group) were randomized to receive either sotalol (intravenous [IV] bolus 5 mg/kg, 10 mg/kg per hour infusion), lidocaine (IV bolus 2.5 mg/kg, 2.5 mg/kg/hr infusion), ranolazine (IV bolus 3.3 mg/kg, 3.2 mg/kg per hour infusion), or saline (control). RESULTS: The incidence of ventricular arrhythmias in the sotalol (S), lidocaine (L), ranolazine (R), and control (C) groups was 7/20, 10/20, 9/20, and 16/20, respectively (P = .01 S vs C, P = .10 L vs C, and P = .048 R vs C). Duration of ventricular tachycardia (VT) episodes was reduced from 15.5 seconds (mean) in C to 1.3 seconds in S, 1.4 sec in L and 0.09 sec in R (P < .05 for S vs C and R vs C by Wilcoxon test). The number of rats with any (≥ 10 seconds) sustained VT was 3 in C versus 1, 0, and 0 in the S, L, and R groups, respectively. Two rats in C had reversible ventricular fibrillation versus 0 in the S, L, and R groups. The number of ventricular premature beats (VPBs) per rat was 10.9 in C, 2.3 in S, 4.9 in L, and 5.7 in R (P < .05 for S, L, or R vs C). P = NS for R versus L or S for all analyses. CONCLUSION: In this first head-to-head comparison of R vs other antiarrhythmic agents at therapeutic doses in an ischemia/reperfusion model, ranolazine (which lacks pro-arrhythmic effects) was as effective as either sotalol or lidocaine to reduce reperfusion-induced ventricular arrhythmias.

The antianginal agent ranolazine is a potent antiarrhythmic agent that reduces ventricular arrhythmias: through a mechanism favoring inhibition of late sodium channel.

BACKGROUND: The antianginal agent ranolazine (R) has shown some promise as an antiarrhythmic agent but its mechanism of action is not known. Previously, we have shown that R suppresses ventricular arrhythmias at a concentration >10 µM that may affect multiple ion currents including IKr. PURPOSE: The present study was carried out to determine the effects of low dose (4 µM) of R that primarily inhibits late I(Na) on ischemia/reperfusion induced ventricular arrhythmias. METHODS: We subjected 20 anesthetized rats to 5 min of proximal left coronary artery occlusion followed by 5 min of reperfusion. Rats were randomized to vehicle control (C; n = 10) versus low dose R (n = 10; 3.33 mg/kg i.v. bolus plus 3.2 mg/kg/h R started 20 min prior to occlusion, which yields a concentration of 4 µM, within the known level that blocks late Na channels but well below the level that has effect on IKr or peak I(Na)). Reperfusion-induced arrhythmias were quantitated by electrocardiographic monitoring. RESULTS: In the C group 9/10 rats developed any arrhythmias versus 3/10 in the R group (P = 0.02); 6/10 developed ventricular tachycardia (VT) in the C group versus 0/10 in the R group (P = 0.01). The median number of episodes of VT were 1.5 in the C group versus 0 in the R group (P = 0.005). Sustained VT (>10 sec) occurred in 3/10 C and 0/10 in R (P = 0.21). The median duration of VT was 1.8 seconds in C versus 0 in R (P = 0.005). Ventricular fibrillation occurred in 1/10 in C and 0 in R. Ventricular premature beats (VPBs) occurred in 9/10 C and 3/10 R rats (P = 0.02). The median number of VPBs was 5.5 in the C group versus 0 in R group (P = 0.01). The ischemic risk zones were equivalent in the C and R groups (35 ± 3% and 32 ± 3% of the left ventricle, respectively). CONCLUSIONS: In conclusion, data show that the marked antiarrhythmic effect of R in the setting of acute ischemia/reperfusion occurs at low doses consistent with inhibition of late I(Na) .

Mesenchymal stem cell administration at coronary artery reperfusion in the rat by two delivery routes: a quantitative assessment.

AIMS: Ideally, mesenchymal stem cells (MSC) home to and/or remain at the site of damaged myocardium when administered after myocardial infarction. However, MSC may not remain in the heart, but instead relocate to other areas. We investigated quantitatively the distribution of labeled rat MSC, given by two routes after coronary artery occlusion/reperfusion in rats. MAIN METHODS: Rats were subjected to 45 min of coronary artery occlusion and 7 days of reperfusion. Before reperfusion rats received 2 x 10(6) MSC, labeled with europium, injected directly into the ischemic region of the heart (n = 9) or intravenously (n = 8). After 1 week tissues were analyzed for label content together with a standard curve of known quantities of labeled MSC. KEY FINDINGS: In rats receiving cells injected directly into the myocardium, 15% of labeled cells were retained in the heart. When the cells were administered intravenously, no MSC were detected in the heart. The route of administration did not affect distribution to other organs, as the number of MSC in liver, spleen and lung was similar with both routes of delivery. SIGNIFICANCE: Even with direct intramyocardial injection, only a small proportion of the cells are retained in the heart, instead traveling to other organs. With intravenous injection there was no evidence that cells "homed" to the damaged heart. Although cell delivery to the heart was significantly affected by the route of administration, the distribution of cells to other organs was similar with both routes of administration.

Alterations in myostatin expression are associated with changes in cardiac left ventricular mass but not ejection fraction in the mouse.

Myostatin (Mst) is a negative regulator of skeletal muscle in humans and animals. It is moderately expressed in the heart of sheep and cattle, increasing considerably after infarction. Genetic blockade of Mst expression increases cardiomyocyte growth. We determined whether Mst overexpression in the heart of transgenic mice reduces left ventricular size and function, and inhibits in vitro cardiomyocyte proliferation. Young transgenic mice overexpressing Mst in the heart (Mst transgenic mice (TG) under a muscle creatine kinase (MCK) promoter active in cardiac and skeletal muscle, and Mst knockout (Mst (-/-)) mice were used. Xiscan angiography revealed that the left ventricular ejection fraction did not differ between the Mst TG and the Mst (-/-) mice, when compared with their respective wild-type strains, despite the decrease in whole heart and left ventricular size in Mst TG mice, and their increase in Mst (-/-) animals. The expected changes in cardiac Mst were measured by RT-PCR and western blot. Mst and its receptor (ActRIIb) were detected by RT-PCR in rat H9c2 cardiomyocytes. Transfection of H9c2 with plasmids expressing Mst under muscle-specific creatine kinase promoter, or cytomegalovirus promoter, enhanced p21 and reduced cdk2 expression, when assessed by western blot. A decrease in cell number occurred by incubation with recombinant Mst (formazan assay), without affecting apoptosis or cardiomyocyte size. Anti-Mst antibody increased cardiomyocyte replication, whereas transfection with the Mst-expressing plasmids inhibited it. In conclusion, Mst does not affect cardiac systolic function in mice overexpressing or lacking the active protein, but it reduces cardiac mass and cardiomyocyte proliferation.

Stem cell therapy in the heart and vasculature.

Stem cell therapy is a progressive approach to a pervasive clinical problem; cardiovascular disease is the number 1 killer in the USA and other developed countries, and aspects of it are amenable to stem cell therapy. Many types of stem cells have been used in treating the heart during myocardial infarction, and here, we describe our approach of direct myocardial injection of bone marrow-derived mesenchymal stem cells into the infarct of rats. We will also briefly introduce the methods we have used to inject neonatal cardiomyocytes into the aorta as a first step in attempting to produce an external cardiac pump. Proper surgical technique and postoperative care are as important as adequate injection of the cells and will greatly improve the survival of the animal after surgery. By carefully following the methods presented in this chapter, the reader will be able to perform direct myocardial and vascular injection of stem cells into rats.

In vivo and in vitro models to test the hypothesis of particle-induced effects on cardiac function and arrhythmias.

Exposure to ultrafine particles (UFPs) by inhalation increases the number and severity of cardiac events. The specific mechanism(s) of action are unknown. This study was designed to examine whether UFPs could exert a direct effect on the cardiovascular system without dependence upon lung-mediated responses. The direct effects of UFPs were determined in normal rats (infused intravenously with UFPs), and in the isolated Langendorff perfused rat heart. UFPs from either ambient air (UFAAs) or diesel engine exhaust (UFDGs) were studied. Infusion of UFDGs prepared in our laboratory caused ventricular premature beats (VPBs) in 2 of 3 rats in vivo. Ejection fraction increased slightly (approximately 4.5%) in rats receiving UFPAA and was unchanged in the UFDG and saline groups in vivo. In the isolated rat heart, perfused according to Langendorff, UFDGs caused a marked increase in left-ventricular end-diastolic pressure (LVEDP; from 12.0 +/- 4.6 mmHg to 24.8 +/- 11.2 mmHg, p < 0.05) after 30 min of exposure. UFPs isolated from industrial diesel particulate matter (UFIDs), obtained from the National Institute of Standards and Technology, caused a significant decrease in left-ventricular systolic pressure (LVSP; from 85.7 +/- 4.0 mmHg to 37.9 +/- 20.3 mmHg, p < 0.05) and +/- dP/dt (from 2,365 +/- 158 mmHg/s to 1,188 +/- 858 mmHg/s, p < 0.05) at 30 min after the start of infusion. This effect was absent when the soluble fraction (containing no particles) isolated from the UFIDs was studied. These findings indicate that UFPs can have direct effects on the cardiovascular system that are independent of effects of particles on the lungs.

Thickening of the infarcted wall by collagen injection improves left ventricular function in rats: a novel approach to preserve cardiac function after myocardial infarction.

OBJECTIVES: We determined whether collagen implantation could thicken the infarcted left ventricular (LV) wall and improve LV function. BACKGROUND: We hypothesized that thickening the infarcted wall by using collagen might result in some benefits that are similar to what previously had been reported when the infarcted wall was thickened with cells. METHODS: Fischer rats with one-week-old myocardial infarcts were injected with collagen or saline (100 microl) into the scar (n = 12 each group). Six weeks later, LV angiograms, hemodynamics, and regional myocardial blood flow were assessed. The hearts were processed for measurements of postmortem LV volume and histology. RESULTS: Collagen injection significantly increased scar thickness (719 +/- 26 microm) compared with the saline-treated group (440 +/- 34 microm, p = 2.6 x 10(-6)). By LV angiography, stroke volume was significantly larger in the collagen-treated group (163 +/- 8 microl) than in the saline-treated group (129 +/- 6 microl, p = 0.005), and LV ejection fraction was also greater in the collagen-treated group (48.4 +/- 1.8%) than in the saline-treated group (40.7 +/- 1.0%, p = 0.002). Analysis of regional wall motion demonstrated paradoxical systolic bulging in 5 of 10 saline-treated rats that averaged 20.3 +/- 2.6% of the LV diastolic circumference, but in none of the 11 collagen-treated rats (p = 0.012). The LV end-diastolic and end-systolic volumes were 319 +/- 12 microl and 190 +/- 7 microl in the saline-treated group, respectively. There was a trend for larger LV end-diastolic volumes (343 +/- 23 microl), but smaller end-systolic volumes (180 +/- 16 microl) in the collagen-treated group. CONCLUSIONS: This study shows that collagen injection thickens an infarct scar and improves LV stroke volume and ejection fraction, and prevents paradoxical systolic bulging after myocardial infarction.

Allogeneic mesenchymal stem cell transplantation in postinfarcted rat myocardium: short- and long-term effects.

BACKGROUND: Mesenchymal stem cells (MSCs) have the potential to replace infarct scar, but the long-term effects are unknown. We studied short- and long-term effects of MSC transplantation on left ventricular (LV) function in a rat myocardial infarction model. METHODS AND RESULTS: Saline (n=46) or MSCs labeled with 1,1'-dioctadecyl-3,3,3'3'-testramethylindocarbocyanine perchlorate (DiI; n=49, 2x10(6) cells each) were injected into the scar of a 1-week-old myocardial infarction in Fischer rats. The presence and differentiation of engrafted cells and their effect on LV ejection fraction was assessed. At 4 weeks, LV stroke volume was significantly greater in the MSC-treated group (145+/-9 microL) than in the saline group (122+/-3 microL, P=0.032), and LV ejection fraction was significantly greater in MSC-treated animals (43.8+/-1.0%) than in the saline group (38.8+/-1.1%, P=0.0027). However, at 6 months, these benefits of MSC treatment were lost. DiI-positive cells were observed in the MSC group at 2 weeks and at 3 and 6 months. Expression of the muscle-specific markers alpha-actinin, myosin heavy chain, phospholamban, and tropomyosin was not observed at 2 weeks in DiI-positive cells. At 3 and 6 months, the DiI-positive cells were observed to express the above muscle-specific markers, but they did not fully evolve into an adult cardiac phenotype. Some of the DiI-positive cells expressed von Willebrand factor. CONCLUSIONS: Allogeneic MSCs survive in infarcted myocardium as long as 6 months and express markers that suggest muscle and endothelium phenotypes. MSCs improved global LV function at 4 weeks; however, this benefit was transient, which suggests a possible early paracrine effect.

Stem cell therapy for the heart.

Cellular cardiomyoplasty is an expanding field of research that involves numerous types of immature cells administered via several modes of delivery. The purpose of this review is to investigate the benefits of different types of cells used in stem cell research as well as the most efficient mode of delivery. The authors also present data showing that stem cells isolated from bone marrow are present at both 2 weeks and 3 months after engraftment in a myocardial infarction. These cells express muscle markers at both time points, which suggests that they have begun to differentiate into cardiomyocytes. Several questions must be answered, however, before stem cells can be used routinely in the clinic. Once these questions have been addressed, the use of stem cells in clinical practice can be realized.

No-reflow phenomenon persists long-term after ischemia/reperfusion in the rat and predicts infarct expansion.

BACKGROUND: No-reflow after reperfusion therapy for myocardial infarction is a strong predictor of clinical outcome. But its fate on a long-term basis and potential significance for infarct healing are not yet known. METHODS AND RESULTS: Twenty-nine female Fisher rats were subjected to 60 minutes of coronary occlusion followed by reperfusion. At 4 weeks, 15 survivors were euthanized after measurement of regional myocardial blood flow (radioactive microspheres) and in vivo staining of perfused tissue (0.5 mL 50% Uniperse blue IV). Infarct size (34.3+/-3.4%), scar thickness (1.19+/-0.10 mm), and infarct expansion index (0.51+/-0.04) were assessed from histological sections (2 additional exclusions because of failed occlusion). Regional myocardial blood flow in the reperfused infarct was reduced significantly compared with noninfarcted tissue (1.98+/-0.47 versus 4.55+/-0.86 mL x min(-1) x g(-1), P<0.003, apical slice, and 1.77+/-0.44 versus 5.34+/-0.38 mL x min(-1) x g(-1), P<0.0001, second slice), accompanied by a striking reduction of perfused capillaries within the infarct (n=23+/-4 versus 163+/-8 in the noninfarcted tissue, P<0.0001, microscopically assessed as capillaries containing blue particles per high-power field). Macroscopically, no-reflow areas were visible in 9 of 13 hearts. The number of perfused capillaries within the infarct correlated significantly with infarct expansion index (r=-0.76, P<0.003), infarct thickness (r=0.60, P<0.03), and the ratio of infarct to septum thickness (r=0.74, P<0.004). CONCLUSIONS: The no-reflow phenomenon persists for 1 month after reperfusion and predicts worse scar thinning and infarct expansion. Thus, one might shift the "open-artery" hypothesis downstream to an "open-microvessel" hypothesis, relating infarct healing, infarct expansion, and outcome to the completeness of microvascular reperfusion above and beyond epicardial artery patency.

Transplantation of neonatal cardiomyocytes after permanent coronary artery occlusion increases regional blood flow of infarcted myocardium.

BACKGROUND: Cellular cardiomyoplasty is a promising approach for rebuilding scar tissue after acute myocardial infarction. However, the angiogenic potential of transplanted immature cardiomyocytes and their effect on regional myocardial blood flow (RMBF) after coronary artery occlusion remain to be evaluated. METHODS AND RESULTS: Intramyocardial injection of cultured neonatal cardiomyocytes (4 x 10(6) cells/50-70 microliter) into the scar 1 week after permanent coronary occlusion in rats resulted in improved RMBF in the infarct 4 weeks after transplantation (radioactive microspheres, 0.97 +/- 0.18 ml/min/g) in comparison to medium-injected hearts (0.61 +/- 0.11 ml/min/g, P < 0.047). The macroscopic perfusion defect after in vivo staining with the blue dye 50% Uniperse blue was significantly smaller in the cell transplantation group (1.5 +/- 0.3% of the heart) compared to the medium group (3.0 +/- 0.6%, P < 0.017). Clusters of engrafted cells within the scar demonstrated a high capillary density (1217 +/- 114 perfused (blue) capillaries/mm(2)); however, in the scar tissue itself capillary density in the cell group (156 +/- 62/mm(2)) did not significantly differ from the medium group (125 +/- 10/mm(2)), suggesting that neo-angiogenesis was confined to regions of successful engraftment (non-infarcted tissue: 1924 +/- 114 perfused capillaries/mm(2)). The transplantation group was characterized by smaller diastolic and systolic left ventricular volumes, as assessed by intravenous ventriculography, along with thickened infarcts (0.93 +/- 0.07 vs. 0.75 +/- 0.04 mm, P < 0.020) and lower infarct expansion indices (0.64 +/- 0.07 vs. 0.83 +/- 0.06, P < 0.023), as determined by post-mortem morphometry of histologic slides. CONCLUSIONS: Transplantation of neonatal cardiomyocytes induced neo-angiogenesis in zones of successful cell engraftment within the scar, which effectively enhanced tissue perfusion.

Long-term outcome of fetal cell transplantation on postinfarction ventricular remodeling and function.

OBJECTIVES: The purpose of this study was to determine the long-term outcome of fetal cell transplantation into myocardial infarction on left ventricular (LV) function and remodeling. BACKGROUND: While neonatal cell transplantation improved function for acute myocardial infarction, long-term data on the effects of cell-transplant therapy using a more primitive cell on ventricular remodeling and function are needed.Methods. - Therefore, we injected 4 x 10(6) Fischer 344 fetal cardiac cells or medium into 1-week old infarcts in adult female Fischer rats to assess long-term outcome. RESULTS: Ten months after transplantation histologic analysis showed that cell implants were readily visible within the infarct scar. Infarct wall thickness was greater in cell-treated at 0.69 +/- 0.05 mm (n = 11) vs. medium-treated hearts at 0.33 +/- 0.01 mm (n = 19; P = 0.0001). Postmortem LV volume was 0.41 +/- 0.04 ml in cell-treated vs. 0.51 +/- 0.03 ml in medium-treated hearts (P < 0.04). Ejection fraction assessed by LV angiography was 0.40 +/- 0.02 in cell-treated (n = 16) vs. 0.33 +/- 0.02 in medium-treated hearts (n = 24; P < 0.03) with trends towards smaller in vivo end-diastolic and end-systolic volumes in cell-treated vs. medium-treated hearts. Polymerase chain reaction analysis of the Sry gene of the Y chromosome was positive in four of five cell-treated and zero of five medium-treated hearts confirming viability of male cells in female donors. CONCLUSION: Over the course of 10 months, fetal cardiac cell transplantation into infarcted hearts increased infarct wall thickness, reduced LV dilatation, and improved LV ejection fraction. Thus, fetal cell-transplant therapy mitigated the longer-term adverse effects of LV remodeling following a myocardial infarction.

Development of abnormal tissue architecture in transplanted neonatal rat myocytes.

BACKGROUND: Most myocardial cell transplant studies focus on demonstration of improved function; however, such improvement depends on the development of appropriate tissue structure. Thus, our aim was to assess the architectural changes that occurred after cell transplant into normal and infarcted myocardium. METHODS: Male neonatal cells (1 to 2 days old) were injected into the left ventricular free wall of adult female rats. The tissue was examined 0 to 1 days and 1 to 2, 4 to 6, and 12 weeks later in noninfarcted hearts and 6 months after transplant into infarcts. In histologic sections, we assessed the cells' retardation of polarized light (to measure development of contractile elements), two-dimensional cell orientation, cell nuclear morphology, and collagen content. RESULTS: The transplant cells' retardation of polarized light gradually increased to 81% of that of host cells after 6 months (p < 0.001). The transplant cells were disorganized and although their nuclei increased in size, they always had a rounded appearance. Collagen content in the transplant was 210% to 430% higher than in host tissue (p < 0.01). In addition, scar collagen always separated transplant and host cells. CONCLUSIONS: One architectural feature, the rounded nuclei, provided a distinctive marker to identify transplanted cells. Nevertheless, the transplants' inhibited muscle development together with disorganization, separation from the host muscle, and a substantial increase in collagen resulted in a structure unlikely to play an active role in systolic function.

Rebuilding a damaged heart: long-term survival of transplanted neonatal rat cardiomyocytes after myocardial infarction and effect on cardiac function.

BACKGROUND: The long-term effects of cardiac cell transplantation on cardiac function are unknown. Therefore, we tested the survival and functional impact of rat neonatal cardiac myocytes up to 6 months after transplantation into infarcted hearts. METHODS AND RESULTS: Cardiomyocytes from male neonatal Fischer 344 rats (1 to 2 days, 3 to 5x10(6)) or medium was injected into the infarcts of adult syngeneic female animals 1 week after left coronary artery ligation. Six months later, implanted cardiomyocytes were still present by quantitative TaqMan polymerase chain reaction and histology. In all treated hearts, discrete lumps of cells were present within the infarct scar, which was not observed in media-injected hearts typified by a transmural infarct scar. Infarct thickness was greater in treated animals versus control animals (909+/-97 versus 619+/-43 microm, P<0.02), whereas infarct size and left ventricular volumes were similar. By biplane angiography, left ventricular ejection fractions at 6 months were greater (0.36+/-0.03 versus 0.25+/-0.02, P<0.01) and significantly less infarct zone dyskinesis was seen (0.30+/-0.08 versus 0.55+/-0.07, P=0.035, lateral projection) in treated animals versus control animals. CONCLUSIONS: Grafted neonatal cardiomyocytes were present in infarcts 6 months after transplantation; they thickened the wall of the left ventricle and were associated with enhanced ejection fraction and reduced paradoxical systolic bulging of the infarct. Therefore, neonatal cardiac cell transplants exhibit long-term survival in a myocardial infarct model and contribute to long-term improved cardiac function. These results suggest that a damaged heart can be rebuilt.

Survival and development of neonatal rat cardiomyocytes transplanted into adult myocardium.

Transplantation of neonatal cardiomyocytes is a novel approach for the treatment of heart failure and myocardial infarction, but quantitative information on long-term cell survival and development is limited. Male donor cardiomyocytes were isolated from neonatal Fischer 344 rats (1-2 days), purified, and injected into the left ventricular wall of female syngeneic adult rats. One hour to 12 weeks later, genomic DNA was isolated from recipient hearts. The amount of male DNA per sample was determined by quantitative real-time TaqMan PCR of the male-specific Sry gene. Transplanted cell survival was 57 +/- 9% at 0-1 h, 24 +/- 6% at 24 h, 28 +/- 11% at 7 days, 27 +/- 3% at 14 days, 23 +/- 8% at 4 weeks and 15 +/- 3% at 12 weeks. The caspase inhibitor AcYVADcmk failed to improve transplanted cell survival at 24 h, suggesting that apoptosis did not play a major role in cell loss. Histology revealed that transplanted cells became more elongated over time, developed cross-striations, and that their nuclei increased in size. However, at 12 weeks, transplanted cells and their nuclei were still smaller than those of host myocardium. We established a quantitative survival profile for neonatal cardiomyocytes transplanted into normal adult myocardium. There was significant loss of cells within 24 h, but 15% of transplanted cells survived 12 weeks. Those cells that did survive underwent differentiation and developed visible sarcomeres, suggesting a potential contribution toward ventricular function.