Adult cardiac stem cells are multipotent and support myocardial regeneration

The notion of the adult heart as terminally differentiated organ without self-renewal potential has been undermined by the existence of a subpopulation of replicating myocytes in normal and pathological states. The origin and significance of these cells has remained obscure for lack of a proper biological context. We report the existence of Lin(-) c-kit(POS) cells with the properties of cardiac stem cells. They are self-renewing, clonogenic, and multipotent, giving rise to myocytes, smooth muscle, and endothelial cells. When injected into an ischemic heart, these cells or their clonal progeny reconstitute well-differentiated myocardium, formed by blood-carrying new vessels and myocytes with the characteristics of young cells, encompassing approximately 70% of the ventricle. Thus, the adult heart, like the brain, is mainly composed of terminally differentiated cells, but is not a terminally differentiated organ because it contains stem cells supporting its regeneration. The existence of these cells opens new opportunities for myocardial repair.

Cardiac stem cells possess growth factor-receptor systems that after activation regenerate the infarcted myocardium, improving ventricular function and long-term survival

Cardiac stem cells and early committed cells (CSCs-ECCs) express c-Met and insulin-like growth factor-1 (IGF-1) receptors and synthesize and secrete the corresponding ligands, hepatocyte growth factor (HGF) and IGF-1. HGF mobilizes CSCs-ECCs and IGF-1 promotes their survival and proliferation. Therefore, HGF and IGF-1 were injected in the hearts of infarcted mice to favor, respectively, the translocation of CSCs-ECCs from the surrounding myocardium to the dead tissue and the viability and growth of these cells within the damaged area. To facilitate migration and homing of CSCs-ECCs to the infarct, a growth factor gradient was introduced between the site of storage of primitive cells in the atria and the region bordering the infarct. The newly-formed myocardium contained arterioles, capillaries, and functionally competent myocytes that with time increased in size, improving ventricular performance at healing and long thereafter. The volume of regenerated myocytes was 2200 microm3 at 16 days after treatment and reached 5100 microm3 at 4 months. In this interval, nearly 20% of myocytes reached the adult phenotype, varying in size from 10,000 to 20,000 microm3. Moreover, there were 43+/-13 arterioles and 155+/-48 capillaries/mm2 myocardium at 16 days, and 31+/-6 arterioles and 390+/-56 capillaries at 4 months. Myocardial regeneration induced increased survival and rescued animals with infarcts that were up to 86% of the ventricle, which are commonly fatal. In conclusion, the heart has an endogenous reserve of CSCs-ECCs that can be activated to reconstitute dead myocardium and recover cardiac function.

Cardiac stem cell and myocyte aging, heart failure, and insulin-like growth factor-1 overexpression

To determine whether cellular aging leads to a cardiomyopathy and heart failure, markers of cellular senescence, cell death, telomerase activity, telomere integrity, and cell regeneration were measured in myocytes of aging wild-type mice (WT). These parameters were similarly studied in insulin-like growth factor-1 (IGF-1) transgenic mice (TG) because IGF-1 promotes cell growth and survival and may delay cellular aging. Importantly, the consequences of aging on cardiac stem cell (CSC) growth and senescence were evaluated. Gene products implicated in growth arrest and senescence, such as p27Kip1, p53, p16INK4a, and p19ARF, were detected in myocytes of young WT mice, and their expression increased with age. IGF-1 attenuated the levels of these proteins at all ages. Telomerase activity decreased in aging WT myocytes but increased in TG, paralleling the changes in Akt phosphorylation. Reduction in nuclear phospho-Akt and telomerase resulted in telomere shortening and uncapping in WT myocytes. Senescence and death of CSCs increased with age in WT impairing the growth and turnover of cells in the heart. DNA damage and myocyte death exceeded cell formation in old WT, leading to a decreased number of myocytes and heart failure. This did not occur in TG in which CSC-mediated myocyte regeneration compensated for the extent of cell death preventing ventricular dysfunction. IGF-1 enhanced nuclear phospho-Akt and telomerase delaying cellular aging and death. The differential response of TG mice to chronological age may result from preservation of functional CSCs undergoing myocyte commitment. In conclusion, senescence of CSCs and myocytes conditions the development of an aging myopathy.

Social stress, autonomic neural activation, and cardiac activity in rats

Animal models of social stress represent a useful experimental tool to investigate the relationship between psychological stress, autonomic neural activity and cardiovascular disease. This paper summarizes the results obtained in a series of experiments performed on rats and aimed at verifying whether social challenges produce specific modifications in the autonomic neural control of heart rate and whether these changes can be detrimental for cardiac electrical stability. Short-term electrocardiographic recordings were performed via radiotelemetry and the autonomic input to the heart evaluated by means of time-domain heart rate variability measures. Compared to other stress contexts, a social defeat experience produces a strong shift of autonomic balance toward sympathetic dominance, poorly antagonized by vagal rebound, and associated with the occurrence of cardiac tachyarrhythmias. These effects were particularly severe when a wild-type strain of rats was studied. The data also suggest that the cardiac autonomic responses produced by different types of social contexts (dominant-subordinate interaction, dominant-dominant confrontation, social defeat) are related to different degrees of emotional activation, which in turn are likely modulated by the social rank of the experimental animal and the opponent, the prior experience with the stressor, and the level of controllability over the stimulus.

Electrode positioning for reliable telemetry ECG recordings during social stress in unrestrained rats

We describe a surgical procedure for optimizing the location of telemetry ECG leads in rats. The new location was aimed at obtaining an accurate representation of ECG features throughout the cardiac cycle by limiting the voltage instability usually observed during intense somatomotor activity and improving the signal-to-noise ratio. The two electrodes (wire loops) were fixed on the dorsal surface of the xiphoid process and in the anterior mediastinum close to the right atrium. The implantation procedure was fast, little invasive, and allowed animals to completely recover from intervention. The performance of the "improved" location (IL, n = 10) with respect to two subcutaneous (SC) positionings ("conventional positioning" CSP, n = 5; "updated location," USL, n = 5) was evaluated by comparing ECGs obtained in baseline, stress and recovery conditions and during different behavioral activities (immobility and grooming). The resident-intruder test (emotional/physical challenge) was chosen as experimental stress paradigm. The noise level of ECGs obtained from IL rats was lower than in CSP and USL animals, in all recording conditions. Percentages of correctly recognized beats (CRBs) over the total number of beats (TBs) were significantly higher in IL rats than in CSP and USL animals, both in baseline conditions (99% vs. 11% and 40%) and situations involving high somatomotor activity (stress: 97%, 5% and 16% recovery; 97%, 7% and 15%) (p < 0.01). The performance of IL as compared to CSP and USL was also better when percentages during grooming and immobility were considered (grooming: 93% vs. 4% and 23%: immobility: 97%, 6%, and 33%; p < 0.01).

Nuclear targeting of Akt enhances ventricular function and myocyte contractility

Cytoplasmic overexpression of Akt in the heart results in a myopathy characterized by organ and myocyte hypertrophy. Conversely, nuclear-targeted Akt does not lead to cardiac hypertrophy, but the cellular basis of this distinct heart phenotype remains to be determined. Similarly, whether nuclear-targeted Akt affects ventricular performance and mechanics, calcium metabolism, and electrical properties of myocytes is unknown. Moreover, whether the expression and state of phosphorylation of regulatory proteins implicated in calcium cycling and myocyte contractility are altered in nuclear-targeted Akt has not been established. We report that nuclear overexpression of Akt does not modify cardiac size and shape but results in an increased number of cardiomyocytes, which are smaller in volume. Additionally, the heart possesses enhanced systolic and diastolic function, which is paralleled by increased myocyte performance. Myocyte shortening and velocity of shortening and relengthening are increased in transgenic mice and are coupled with a more efficient reuptake of calcium by the sarcoplasmic reticulum (SR). This process increases calcium loading of the SR during relengthening. The enhanced SR function appears to be mediated by an increase in SR Ca2+-ATPase2a activity sustained by a higher degree of phosphorylation of phospholamban. This posttranslational modification was associated with an increase in phospho-protein kinase A and a decrease in protein phosphatase-1. Together, these observations provide a plausible biochemical mechanism for the potentiation of myocyte and ventricular function in Akt transgenic mice. Therefore, nuclear-targeted Akt in myocytes may have important implications for the diseased heart.

Human cardiac and bone marrow stromal cells exhibit distinctive properties related to their origin

AIMS

Bone marrow mesenchymal stromal cell (BMStC) transplantation into the infarcted heart improves left ventricular function and cardiac remodelling. However, it has been suggested that tissue-specific cells may be better for cardiac repair than cells from other sources. The objective of the present work has been the comparison of in vitro and in vivo properties of adult human cardiac stromal cells (CStC) to those of syngeneic BMStC.

METHODS AND RESULTS

Although CStC and BMStC exhibited a similar immunophenotype, their gene, microRNA, and protein expression profiles were remarkably different. Biologically, CStC, compared with BMStC, were less competent in acquiring the adipogenic and osteogenic phenotype but more efficiently expressed cardiovascular markers. When injected into the heart, in rat a model of chronic myocardial infarction, CStC persisted longer within the tissue, migrated into the scar, and differentiated into adult cardiomyocytes better than BMStC.

CONCLUSION

Our findings demonstrate that although CStC and BMStC share a common stromal phenotype, CStC present cardiovascular-associated features and may represent an important cell source for more efficient cardiac repair.

Different sympathovagal modulation of heart rate during social and nonsocial stress episodes in wild-type rats

The acute consequences of a social aversive stimulus (defeat) on the autonomic control upon the electrical activity of the heart were measured and compared to those observed in three nonsocial stress paradigms, namely restraint, shock-probe test, and swimming. Electrocardiograms were recorded from rats via radiotelemetry, and the autonomic neural control of the heart was evaluated via measures of heart rate and heart rate variability, such as the average R-R interval (RR), the standard deviation of RR (SD), the coefficient of variance (SD/RR), and the root-mean-square of successive R-R interval differences (r-MSSD). Although all stressors induced significant reductions of average R-R interval, the effect of defeat was significantly larger (p < 0.05). The social stimulus also determined a significant decrease in the variability indexes (p < 0.01 for all), whereas in the other stress conditions they were either unchanged or increased (SD/RR during restraint, p < 0.05; SD and SD/RR during swimming, p < 0.05 and p < 0.01). Cardiac arrhythmias (mostly ventricular premature beats, VPBs) were far more frequent during defeat than during the other challenging situations (p < 0.01), with an average of 33.5 +/- 6.5 VPBs per 15-min test recording. These data suggest that during defeat autonomic control was shifted toward a sympathetic dominance, whereas in rats exposed to nonsocial stressors, although significant heart rate accelerations were also found, sympathovagal balance was substantially maintained. These differences in autonomic stress responsivity explain the different susceptibility to ventricular arrhythmias and indicate that a social challenge can be far more detrimental for cardiac electrical stability than other nonsocial aversive stimuli.

Cardiac autonomic reactivity and salivary cortisol in men and women exposed to social stressors: relationship with individual ethological profile

The degree of cardiovascular stress responsivity and its possible implications for the onset and progression of cardiovascular pathologies seem to be linked to the individual strategy of behavioral coping with stressors. This study was designed to investigate the relationship among cardiac autonomic, endocrine and behavioral responses to real-life stress episodes. Thirty university students were exposed to two brief social challenges (stress interviews), during which the state of sympathovagal balance (time-domain indexes of heart rate variability) and a number of non-verbal behaviors were quantified. Psychometric measurements were also obtained via SPRAS questionnaire, administered just after each stress interview. Samples of saliva were collected for cortisol determination immediately prior and after the experimental session. Subjects showing higher levels of sympathetic dominance were characterized by higher scores of submissive behavior, larger cortisol increments, and higher perception of psychophysiological arousal. A clear consistency in the individual response to the two stress interviews was found, at the behavioral, physiological and psychophysiological level. Finally, the gender of the subjects did not clearly influence their stress responsivity. These results support the hypothesis of a close relationship between the degree of physiological arousal and the style of behavioral adaptation to social stressors.

The young mouse heart is composed of myocytes heterogeneous in age and function

The recognition that the adult heart continuously renews its myocyte compartment raises the possibility that the age and lifespan of myocytes does not coincide with the age and lifespan of the organ and organism. If this were the case, myocyte turnover would result at any age in a myocardium composed by a heterogeneous population of parenchymal cells which are structurally integrated but may contribute differently to myocardial performance. To test this hypothesis, left ventricular myocytes were isolated from mice at 3 months of age and the contractile, electrical, and calcium cycling characteristics of these cells were determined together with the expression of the senescence-associated protein p16(INK4a) and telomere length. The heart was characterized by the coexistence of young, aged, and senescent myocytes. Old nonreplicating, p16(INK4a)-positive, hypertrophied myocytes with severe telomeric shortening were present together with young, dividing, p16(INK4a)-negative, small myocytes with long telomeres. A class of myocytes with intermediate properties was also found. Physiologically, evidence was obtained in favor of the critical role that action potential (AP) duration and I(CaL) play in potentiating Ca(2+) cycling and the mechanical behavior of young myocytes or in decreasing Ca(2+) transients and the performance of senescent hypertrophied cells. The characteristics of the AP appeared to be modulated by the transient outward K(+) current I(to) which was influenced by the different expression of the K(+) channels subunits. Collectively, these observations at the physiological and structural cellular level document that by necessity the heart has to constantly repopulate its myocyte compartment to replace senescent poorly contracting myocytes with younger more efficient cells. Thus, cardiac homeostasis and myocyte turnover regulate cardiac function.

Intermittent exposure to social defeat and open-field test in rats: acute and long-term effects on ECG, body temperature and physical activity

This study investigated the effects of exposure to an intermittent homotypic stressor on: (i) habituation of acute autonomic responsivity (i.e. cardiac sympathovagal balance and susceptibility to arrhythmias), and (ii) circadian rhythmicity of heart rate, body temperature, and physical activity. After implantation of a transmitter for the radiotelemetric recording of electrocardiogram (ECG), body temperature and physical activity, adult male rats (Rattus norvegicus, Wild Type Groningen strain) were repeatedly exposed (10 consecutive times, on alternate days) to either a social stressor (defeat by a con-specific, n = 15) or an open-field, control challenge (transfer to a new cage; n = 8). ECGs, body temperature and physical activity were continuously recorded in baseline, test and recovery periods (each lasting 15 min), at the 1st and 10th episodes of both defeat and open-field challenge. The circadian rhythms of heart rate, body temperature and physical activity were monitored before (5 days), during (16 days) and after (21 days) the intermittent stress protocol. This study indicates that there is no clear habituation of either acute cardiac autonomic responsivity (as estimated by means of time-domain indexes of heart rate variability) or arrhythmia occurrence to a brief, intermittent, homotypic challenge, regardless of the nature of the stressor (social or non-social). On the other hand, rats exposed to social challenge also failed to show adaptation of acute temperature and activity stress responsiveness, whereas rats facing open-field challenge developed habituation of activity and sensitization of temperature responses. Repeated social challenge produced remarkable reductions of the heart rate circadian rhythm amplitude (this effect being significantly greater than that produced by intermittent open-field), but only minor changes in the daily rhythms of body temperature and physical activity.

Individual differences in cardiovascular response to social challenge

An important determinant of cardiovascular stress reactivity and morbidity is the individual behavioral strategy of coping with social challenge. This review summarizes the results of a number of studies that we performed in rats, aimed at investigating the relationship between aggression and cardiovascular responsivity under social stress conditions. We show that rats belonging to the 'aggressive tail' of a population are characterized by a higher sympathetic-adrenomedullary activation during social and non-social stress episodes. Wild-type rats are characterized by a larger sympathetic dominance and a higher susceptibility to cardiac arrhythmias during defeat as compared to Wistars. Cardiovascular habituation takes place when social challenge is an intermittent victory experience, whereas no habituation is observed across repeated defeat episodes. Dominant rats whose social dominance is challenged by the aggression of another subject display long-term alterations of heart rate circadian rhythmicity. Such changes are linked to individual proness to defend social dominance: the more the animal counterattacks the aggressor, the smaller the subsequent rhythm disturbance. These data underline how important it is to carefully consider individual differences in aggression and the context in which aggression is expressed, when studying cardiovascular effects of social interactions.

Cardiac autonomic responses to intermittent social conflict in rats

Intermittent exposure to the same stressor can lead to a gradual decline in physiological, neuroendocrine and behavioral stress responses (habituation). We investigated possible habituation of cardiac autonomic responsiveness and susceptibility to cardiac arrhythmias in male rats exposed to either intermittent social victory (VIC) or defeat (DEF) stress (10 exposures in each case). Electrocardiograms were recorded via radiotelemetry and the sympathovagal balance at the level of the heart was evaluated via time-domain measurements of heart rate variability, namely average R--R interval (average time interval between two consecutive heart beats, RR), the standard deviation of RR (SD(RR)) and the root-mean-square of successive R--R interval differences (r-MSSD). Values of these parameters were significantly lower in DEF as compared to VIC rats in the second part of the test period (from Minute 6 to Minute 15), suggesting a more pronounced sympathetic dominance in the former group of animals. Accordingly, the occurrence of the most frequent cardiac arrhythmias (ventricular and supraventricular premature beats) was higher in DEF rats. Habituation of cardiac autonomic responsivity was observed across repeated exposure to victory, both in terms of sympathovagal balance and susceptibility to cardiac tachyarrhythmias, whereas no habituation was found in repeatedly defeated animals. A possible explanation to this discrepancy could be the different degree of controllability characterizing the two social challenging situations.

Ventricular activation is impaired in aged rat hearts

Ventricular arrhythmias are frequently observed in the elderly population secondary to alterations of electrophysiological properties that occur with the normal aging process of the heart. However, the underlying mechanisms remain poorly understood. The aim of the present study was to determine specific age-related changes in electrophysiological properties and myocardial structure in the ventricles that can be related to a structural-functional arrhythmogenic substrate. Multiple unipolar electrograms were recorded in vivo on the anterior ventricular surface of four control and seven aged rats during normal sinus rhythm and ventricular pacing. Electrical data were related to morphometric and immunohistochemical parameters of the underlying ventricular myocardium. In aged hearts total ventricular activation time was significantly delayed (QRS duration: +69%), while ventricular conduction velocity did not change significantly compared with control hearts. Moreover, ventricular activation patterns displayed variable numbers of epicardial breakthrough points whose appearance could change with time. Morphological analysis in aged rats revealed that heart weight and myocyte transverse diameter increased significantly, scattered microfoci of interstitial fibrosis were mostly present in the ventricular subendocardium, and gap junction connexin expression decreased significantly in ventricular myocardium compared with control rats. Our results show that in aged hearts delayed total ventricular activation time and abnormal activation patterns are not due to delayed myocardial conduction and suggest the occurrence of impaired impulse propagation through the conduction system leading to uncoordinated myocardial excitation. Impaired interaction between the conduction system and ventricular myocardium might create a potential reentry substrate, contributing to a higher incidence of ventricular arrhythmias in the elderly population.

Correlation of alpha-skeletal actin expression, ventricular fibrosis and heart function with the degree of pressure overload cardiac hypertrophy in rats

We have analysed alterations of alpha-skeletal actin expression and volume fraction of fibrosis in the ventricular myocardium and their functional counterpart in terms of arrhythmogenesis and haemodynamic variables, in rats with different degrees of compensated cardiac hypertrophy induced by infra-renal abdominal aortic coarctation. The following coarctation calibres were used: 1.3 (AC1.3 group), 0.7 (AC0.7) and 0.4 mm (AC0.4); age-matched rats were used as controls (C group). One month after surgery, spontaneous and sympathetic-induced ventricular arrhythmias were telemetrically recorded from conscious freely moving animals, and invasive haemodynamic measurements were performed in anaesthetized animals. After killing, subgroups of AC and C rats were used to evaluate in the left ventricle the expression and spatial distribution of alpha-skeletal actin and the amount of perivascular and interstitial fibrosis. As compared with C, all AC groups exhibited higher values of systolic pressure, ventricular weight and ventricular wall thickness. AC0.7 and AC0.4 rats also showed a larger amount of fibrosis and upregulation of alpha-skeletal actin expression associated with a higher vulnerability to ventricular arrhythmias (AC0.7 and AC0.4) and enhanced myocardial contractility (AC0.4). Our results illustrate the progressive changes in the extracellular matrix features accompanying early ventricular remodelling in response to different degrees of pressure overload that may be involved in the development of cardiac electrical instability. We also demonstrate for the first time a linear correlation between an increase in alpha-skeletal actin expression and the degree of compensated cardiac hypertrophy, possibly acting as an early compensatory mechanism to maintain normal mechanical performance.

Myocardial remodeling and arrhythmogenesis in moderate cardiac hypertrophy in rats

In 47 male adult Wistar rats with 4-wk aortic coarctation (AC) and 39 age-matched sham-operated rats (SO) chronically instrumented for telemetry electrocardiogram recording, we investigated the mechanisms of arrhythmogenesis in moderate cardiac hypertrophy, with an approach from "in vivo" toward the cellular level, analyzing 1) stress-induced cardiac arrhythmias in all rats and 2) myocardial fibrosis in 35 animals and action potential duration and density of hyperpolarization-activated current in 19 others at the ventricular level. Aortic banding increased arterial blood pressure, cardiac weight, and ventricular myocyte volume by 11, 25, and 14%, respectively (P < 0.001-0.05). Ventricular arrhythmias occurred at similar rates in AC and SO rats throughout the stress procedure. Action potential duration and hyperpolarization-activated current were about twice as great and myocardial fibrosis about four times greater in AC animals (P < 0.005-0.05). Electrocardiogram data also revealed more supraventricular arrhythmias in AC rats during the baseline period and after stress and fewer atrioventricular block episodes after stress (P < 0.05). Thus stress-induced supraventricular and atrioventricular nodal, but not ventricular, arrhythmias were affected in moderate cardiac hypertrophy when ventricular morphofunctional alterations were evident.

Body surface maps in left bundle branch block uncomplicated or complicated by myocardial infarction, left ventricular hypertrophy or myocardial ischemia

We provided a topographic and quantitative description of body surface maps (BSM) during the entire QRST interval in seven uncomplicated LBBBs and 31 LBBBs complicated by: myocardial infarction (MI, seven cases), left ventricular hypertrophy (LVH, eight cases), myocardial ischemia (IS, seven cases), MI + LVH (six cases) and LVH + IS (three cases). In all patients we observed abnormal map configurations attributable to the LBBB. We were unable to identify consistent effects of the complicating heart condition(s) on the general pattern of chest potentials. Conversely, the surface voltages were generally decreased by MI and IS and increased by LVH. By considering the 38 patients as a preliminary learning set we applied a stepwise discriminant analysis to 77 voltage-related variables derived from BSM to produce a model for discriminating between LBBBs with and without MI. We properly allocated more than 90% of the patients. We also attempted to classify the patients into four groups: pure LBBB, LBBB + MI, LBBB + LVH and LBBB + IS, with a percentage of correct classification of about 80%. The two classifying procedures were applied to ten new LBBB patients with results similar to those obtained in the 38 of the study group.

Resident cardiac stem cells

The introduction of stem cells in cardiology provides new tools in understanding the regenerative processes of the normal and pathologic heart and opens new options for the treatment of cardiovascular diseases. The feasibility of adult bone marrow autologous and allogenic cell therapy of ischemic cardiomyopathies has been demonstrated in humans. However, many unresolved questions remain to link experimental with clinical observations. The demonstration that the heart is a self-renewing organ and that its cell turnover is regulated by myocardial progenitor cells offers novel pathogenetic mechanisms underlying cardiac diseases and raises the possibility to regenerate the damaged heart. Indeed, cardiac stem progenitor cells (CSPCs) have recently been isolated from the human heart by several laboratories although differences in methodology and phenotypic profile have been described. The present review points to the potential role of CSPCs in the onset and development of congestive heart failure and its reversal by regenerative approaches aimed at the preservation and expansion of the resident pool of progenitors.

Growth factor-induced mobilization of cardiac progenitor cells reduces the risk of arrhythmias, in a rat model of chronic myocardial infarction

Heart repair by stem cell treatment may involve life-threatening arrhythmias. Cardiac progenitor cells (CPCs) appear best suited for reconstituting lost myocardium without posing arrhythmic risks, being commissioned towards cardiac phenotype. In this study we tested the hypothesis that mobilization of CPCs through locally delivered Hepatocyte Growth Factor and Insulin-Like Growth Factor-1 to heal chronic myocardial infarction (MI), lowers the proneness to arrhythmias. We used 133 adult male Wistar rats either with one-month old MI and treated with growth factors (GFs, n = 60) or vehicle (V, n = 55), or sham operated (n = 18). In selected groups of animals, prior to and two weeks after GF/V delivery, we evaluated stress-induced ventricular arrhythmias by telemetry-ECG, cardiac mechanics by echocardiography, and ventricular excitability, conduction velocity and refractoriness by epicardial multiple-lead recording. Invasive hemodynamic measurements were performed before sacrifice and eventually the hearts were subjected to anatomical, morphometric, immunohistochemical, and molecular biology analyses. When compared with untreated MI, GFs decreased stress-induced arrhythmias and concurrently prolonged the effective refractory period (ERP) without affecting neither the duration of ventricular repolarization, as suggested by measurements of QTc interval and mRNA levels for K-channel α-subunits Kv4.2 and Kv4.3, nor the dispersion of refractoriness. Further, markers of cardiomyocyte reactive hypertrophy, including mRNA levels for K-channel α-subunit Kv1.4 and β-subunit KChIP2, interstitial fibrosis and negative structural remodeling were significantly reduced in peri-infarcted/remote ventricular myocardium. Finally, analyses of BrdU incorporation and distribution of connexin43 and N-cadherin indicated that cytokines generated new vessels and electromechanically-connected myocytes and abolished the correlation of infarct size with deterioration of mechanical function. In conclusion, local injection of GFs ameliorates electromechanical competence in chronic MI. Reduced arrhythmogenesis is attributable to prolongation of ERP resulting from improved intercellular coupling via increased expression of connexin43, and attenuation of unfavorable remodeling.

Behavioral and electrocardiographic responses to social stress in male rats

Telemetry ECGs were recorded from Wistar male rats during social stress induced by exposure to aggressive lactating female rats. Behavioral response to maternal attack was evaluated in terms of relative duration of passive submissive (p/s) and active/nonsubmissive (a/ns) patterns. A decrease of R-R interval (R-R) compared to baseline conditions was found, significantly more pronounced than that observed in control animals exposed just to novel environment. R-R variability during social stress was positively correlated with the amount of p/s behavior. R-R fluctuations, episodes of II degree A-V block, and ventricular arrhythmias were also observed. Most R-R fluctuations and II degree A-V blocks were temporally associated with phases of p/s behavior and periods of high R-R variability. Ventricular arrhythmias generally appeared during a/ns behavior and were temporally linked with periods of low R-R variability. Ventricular arrhythmias, low R-R variability, and concomitant a/ns behavior might be related to an increased sympathetic activity. R-R fluctuations and II degree A-V blocks, associated with high R-R variability and p/s behavior, might be related to a predominant inhibitory effect of vagal activation (accentuated antagonism).

Modulation of actin isoform expression before the transition from experimental compensated pressure-overload cardiac hypertrophy to decompensation

In a rat model of long-lasting pressure-overload hypertrophy, we investigated whether changes in the relative expression of myocardial actin isoforms are among the early signs of ventricular mechanical dysfunction before the transition toward decompensation. Forty-four rats with infrarenal aortic banding (AC rats) were studied. Hemodynamic parameters were measured 1 mo (AC(1) group; n = 20) or 2 mo (AC(2); n = 24) after aortic ligature. Then subgroups of AC(1) and AC(2) left ventricles (LV) were used to evaluate 1) LV anatomy and fibrosis (morphometry), 2) expression levels (immunoblotting) and spatial distribution (immunohistochemistry) of alpha-skeletal actin (alpha-SKA), alpha-cardiac actin (alpha-CA), and alpha-smooth muscle actin (alpha-SMA), and 3) cell mechanics and calcium transients in enzimatically isolated myocytes. Although the two AC groups exhibited a comparable degree of hypertrophy (+30% in LV mass; +20% in myocyte surface) and a similar increase in the amount of fibrosis compared with control animals (C group; n = 22), a worsening of LV mechanical performance was observed only in AC(2) rats at both organ and cellular levels. Conversely, AC(1) rats exhibited enhanced LV contractility and preserved cellular contractile behavior associated with increased calcium transients. Alpha-SKA expression was upregulated (+60%) in AC(1). In AC(2) ventricles, prolonged hypertension also induced a significant increase in alpha-SMA expression, mainly at the level of arterial vessels. No significant differences among groups were observed in alpha-CA expression. Our findings suggest that alpha-SKA expression regulation and wall remodeling of coronary arterioles participate in the development of impaired kinetics of contraction and relaxation in prolonged hypertension before the occurrence of marked histopathologic changes.

Preservation of ventricular performance at early stages of diabetic cardiomyopathy involves changes in myocyte size, number and intercellular coupling

In a rat model of diabetic cardiomyopathy, we tested whether specific changes in myocyte turnover and intercellular coupling contribute to preserving ventricular performance after a short period of hyperglycemia. In 41 rats with streptozotocin-induced diabetes and 24 control animals, cardiac electromechanical properties were assessed by telemetry ECG, epicardial potential mapping, and hemodynamic measurements to document normal ventricular function. Myocardial remodeling, expression of gap-junction proteins and myocyte regeneration were evaluated by tissue morphometry, immunohistochemistry and immunoblotting. Ventricular myocyte number and volume were also determined. In diabetic hearts, after 3 weeks of hyperglycemia, left ventricular mass was lowered by 23%, while left ventricular wall thickness and chamber volume were maintained, in the absence of fibrosis and myocyte hypertrophy. In the presence of a marked DNA oxidative damage, an increased rate of DNA replication and mitotic divisions associated with generation of new myocytes were detected. The number of cells expressing the receptor for Stem Cell Factor (c-kit) and their rate of proliferation were preserved in the left ventricle while the atrial storage of these primitive cells was severely reduced by diabetes-induced oxidative stress. Despite a down-regulation of Connexin43 and over-expression of both Connexin40 and Connexin45, the junctional proteins were normally distributed in diabetic ventricular myocardium,justifying the preserved tissue excitability and conduction velocity. In conclusion, before the appearance of the diabetic cardiomyopathic phenotype,myocardial cell proliferation associated with gap junction protein remodeling may contribute to prevent marked alterations of cardiac structure and electrophysiological properties, preserving ventricular performance.