Ivana Ostadalova

Gender Differences in Cardiac Ischemic Injury and Protection—Experimental Aspects

This review summarizes some available information on gender differences of myocardial injury with particular attention to experimental approach. It has been observed that significant gender differences exist already in normal heart. They involve among others cardiac growth, contractile function, calcium metabolism and function of mitochondria. Differences, characteristic of the normal myocardium, generate the logical presumption of the different reaction of the male and female heart to various pathogenic factors. Most of the experimental studies confirm the clinical observations: increased resistance of the female heart to ischemia/reperfusion injury was shown in dogs, rats, mice and rabbits. Furthermore, gender differences in the ischemic tolerance of the adult myocardium can be influenced by interventions (e.g. hypoxia) imposed during the early phases of ontogenetic development. The already high tolerance of the adult female heart can be increased by adaptation to chronic hypoxia and ischemic preconditioning. It seems that the protective effect depends on age: it was absent in young, highly tolerant heart but it appeared with the decrease of natural resistance during aging. Both experimental and clinical studies have indicated that female gender influences favorably also the remodeling and the adaptive response to myocardial infarction. It follows from the data available that male and female heart differs significantly in many parameters under both physiological and pathological conditions. Detailed molecular and cellular mechanisms of these differences are still unknown; they involve genomic and non-genomic effects of sex steroid hormones, particularly the most frequently studied estrogens. The cardiovascular system is, however, influenced not only by estrogens but also by other sex hormones, e.g. androgens. Moreover, steroid hormone receptors do not act alone but interact with a broad array of co-regulatory proteins to alter transcription. The differences are so important that they deserve serious consideration in clinical practice in search for proper diagnostic and therapeutic procedures.

Reciprocal changes in the postnatal expression of the sarcolemmal Na+-Ca2+-exchanger and SERCA2 in rat heart

The aim of this study was to examine the relationship between sarcolemmal Na(+)-Ca2+ exchangers and sarcoplasmic reticulum (SR) Ca(2+) -ATPase (SERCA2) expression and the developmental differences in cardiac Ca2+ handling. Postnatal steady-state mRNA and protein levels were analysed in rat ventricular myocardium by Northern and immunoblot analysis, respectively. This was compared to Na+ gradient-induced and SR oxalate-supported Ca2 transport in isolated membranes. Na(+)-Ca2+ exchanger mRNA declined by 75% between day 1 and 30, whereas SR Ca2+ ATPase mRNA levels increased by 97% during this period. The Na(+)-Ca2+ exchanger mRNA/Ca(2+)-ATPase mRNA ratio was found to be inversely related to post-natal age. The changes in mRNA levels were associated with corresponding developmental differences in the Ca2+ transport activities of the respective membrane proteins. In crude membranes, the Na(+)-dependent Ca2+ transport activity (at 75 microM Ca2+) declined gradually (P < 0.01; mean +/- S.E.) from 17.7 +/- 2.4 nmoles Ca2+/g wet tissue/2s at day 1-3 (n = 5) to a value of 4.2 +/- 1.1 at day 40 (n =4). Conversely, SR Ca2+ uptake increased (P < 0.01) 2.6-fold during this period. The inversely related changes in the post-natal expression and function of the Na(+)-Ca2+ exchanger and SR Ca(2+)-ATPase suggest a coordinated control at the pretranslational level of the cellular Ca2+ transport processes mediated by the two membrane proteins.

Ischemic preconditioning in chronically hypoxic neonatal rat heart.

Rat hearts isolated on d 1, 4, 7, and 10 of postnatal life were perfused (in Langendorff mode) with Krebs-Henseleit solution at constant pressure, temperature, and stimulation rate. Recovery of the contractile function after global ischemia was measured by an isometric force transducer and analyzed using an online computer. Ischemic preconditioning (IP) was induced by three 3-min periods of global ischemia, each separated by a 5-min period of reperfusion. Prenatal hypoxia was induced by exposure of pregnant mothers to intermittent high altitude (IHA), simulated in a barochamber (8 h/d, 5000 m) from d 15 to 20 of pregnancy. Postnatal hypoxia was simulated by the identical procedure from postnatal d 1 to 6 and 9. Prenatal exposure to IHA failed to improve ischemic tolerance on d 1, but postnatal exposure to IHA improved recovery of the developed force after ischemia on d 7 (33 ± 3%versus 43 ± 4%) and 10 (39 ± 2%versus 54 ± 2%). Combination of IHA and IP induced higher protective effects in all age groups, including postnatal d 1 (48 ± 2%versus 56 ± 3%), whereas IHA and IP applied separately failed to improve ischemic tolerance. Neither the mitochondrial KATP channel blocker 5-hydroxydecanoate nor the nitric oxide synthase inhibitor Nω-nitro-l-arginine methyl ester abolished protection by IP in normoxic animals, but they decreased significantly protection by IHA hypoxia. The final recovery was even lower than the corresponding normoxic values. It seems likely that mitochondrial KATP channels and nitric oxide may be involved in the protective mechanisms of adaptation to chronic hypoxia but not to that of IP, at least in neonates.

Neonatal cardiac mitochondria and ischemia/reperfusion injury

Postnatal maturation of the heart is characterized by decreasing tolerance to ischemia/reperfusion (I/R) injury associated with significant changes in mitochondrial function. The aim of this study is to test the hypothesis that the role of the mitochondrial membrane permeability transition pore (MPTP) in the I/R injury differs in the neonatal and in the adult heart. For this purpose, the effect of blockade of MPTP on the degree of I/R injury and the sensitivity of MPTP to swelling-inducing agents was compared in hearts from neonatal (7 days old) and adult (90 days old) Wistar rats. It was found that the release of NAD+ from the perfused heart induced by I/R can be prevented by sanglifehrin A (SfA) only in the adult myocardium; SfA had no protective effect in the neonatal heart. Furthermore, the extent of Ca-induced swelling of mitochondria from neonatal rats was significantly lower than that from the adult animals; mitochondria from neonatal rats were more resistant at higher concentrations of calcium. In addition, not only the extent but also the rate of calcium-induced swelling was about twice higher in adult than in neonatal mitochondria. The results support the idea that lower sensitivity of the neonatal MPTP to opening may be involved in the mechanism of the higher tolerance of the neonatal heart to I/R injury.

Effect of perinatal hypoxia on cardiac tolerance to acute ischaemia in adult male and female rats

1 The number of adult patients undergoing surgery for congenital cyanotic defects in childhood has increased significantly. Therefore, the aim of the present study was to examine the effect of perinatal hypoxia on the tolerance of the adult myocardium to acute ischaemia–reperfusion injury. 2 Pregnant Wistar rats were exposed to intermittent hypobaric hypoxia 7 days before delivery; pups were born under normoxic conditions and exposed to hypoxia again for 10 postnatal days. After the last hypoxic exposure, all animals were kept for an additional 3 months under normoxic conditions. All experiments were performed on 90‐day‐old rats. 3 Ventricular arrhythmias were assessed on isolated perfused hearts during 30 min occlusion of the left anterior descending coronary artery. Infarct size was measured on isolated hearts (40 min regional ischaemia and 120 min reperfusion) and on open‐chest animals (20 min regional ischaemia and 3 h reperfusion). 4 Perinatal exposure to hypoxia significantly increased cardiac tolerance to ischaemic injury in adult females, as evidenced by the lower incidence and severity of ischaemic ventricular arrhythmias, compared with the normoxic group. The effect of perinatal hypoxia on ischaemic arrhythmias in males was quite the opposite. Myocardial infarct size measured in open‐chest animals only was significantly smaller in normoxic females compared with normoxic males. Perinatal exposure to hypoxia had no effect on infarct size in either setting or sex. 5 The results of the present study support the hypothesis that perinatal hypoxia is a primary programming stimulus in the heart that may lead to sex‐dependent changes in cardiac tolerance to acute ischaemia in later adult life. This would have important implications for patients who have experienced prolonged hypoxaemia in early life.

Cardiac Adaptation to Chronic Hypoxia

Publisher Summary This chapter discusses cardiac adaptation to chronic hypoxia. The degree of hypoxic injury depends on the intensity and duration of the hypoxic stimulus, and also on the level of cardiac tolerance to oxygen deprivation. Chronic myocardial hypoxia, the result of disproportion between oxygen supply and demand at the tissue level, may be induced by several mechanisms. The most common causes are undoubtedly (1) ischemic hypoxia, induced by the reduction or interruption of the coronary blood flow, and (2) systemic hypoxia, characterized by a drop in PO 2 in the arterial blood. One can also add (3) anemic hypoxia, in which the arterial PO 2 is normal, but the oxygen transport capacity of the blood is decreased. On the other hand, the most frequent causes of raised oxygen consumption are increased physical activity, mental stress, or administration of substances with positive inotropic and chronotropic effects. The most frequently used experimental model in research on chronic hypoxia is that of high altitude, either as seen in the mountain environment or as simulated under laboratory conditions in a normobaric or hypobaric chamber.

Developmental determinants of cardiac sensitivity to hypoxia.

Cardiac sensitivity to oxygen deprivation changes significantly during ontogenetic development. However, the mechanisms for the higher tolerance of the immature heart, possibilities of protection, and the potential impact of perinatal hypoxia on cardiac tolerance to oxygen deprivation in adults have not yet been satisfactorily clarified. The hypoxic tolerance of an isolated rat heart showed a triphasic pattern: significant decrease from postnatal day 1 to 7, followed by increase to the weaning period, and final decline to adulthood. We have observed significant ontogenetic changes in mitochondrial oxidative phosphorylation and mitochondrial membrane potential, as well as in the role of the mitochondrial permeability transition pores in myocardial injury. These results support the hypothesis that cardiac mitochondria are deeply involved in the regulation of cardiac tolerance to oxygen deprivation during ontogenetic development. Ischemic preconditioning failed to increase tolerance to oxygen deprivation in the highly tolerant hearts of newborn rats. Chronic hypoxic exposure during early development may cause in-utero or neonatal programming of several genes that can change the susceptibility of the adult heart to ischemia-reperfusion injury; this effect is sex dependent. These results would have important clinical implications, since cardiac sensitivity in adult patients may be significantly affected by perinatal hypoxia in a sex-dependent manner.

Impact of Perinatal Chronic Hypoxia on Cardiac Tolerance to Acute Ischemia

Perinatal period is critical for the normal cardiac development, and different interventions imposed on the heart may significantly influence myocardial structure and function. Perinatal hypoxemia, although transient, may thus have serious early and late consequences on the cardiovascular system. Epidemiological and experimental studies have repeatedly suggested a possible link between perinatal hypoxia and increased sensitivity to ischemia/reperfusion (I/R) injury in adults. The mechanisms of this increased susceptibility are not known at present. It has been found that prenatal chronic hypoxia sensitizes the apoptosis pathway in the adult male heart in response to I/R stimulation. In addition, cardiac heat shock proteins (Hsp) 70 expression was significantly lower in prenatal hypoxic hearts than in controls; this fact may play a role in the increased susceptibility of the adult heart to I/R injury. The decreased eNOS levels in adult prenatal hypoxic hearts may also contribute to their increased sensitivity. These studies suggest that chronic hypoxic exposure during early development may cause in utero or neonatal programming of several genes which can play an important role in the increased susceptibility of the adult male heart to I/R injury. Furthermore, it has been observed in the rat model that late myocardial effects of chronic hypoxia, experienced in early life, may be sex-dependent. Unlike in males, perinatal exposure to chronic hypoxia significantly increased cardiac tolerance to acute I/R injury in adult females, expressed as the lower incidence of ischemic arrhythmias, decreased infarct size, decreased cardiac enzyme release, and increased postischemic recovery of left ventricular function. It was suggested that these sex-dependent changes may be due to differences in fetal programming of PKCe gene expression, which play a pivotal role in cardioprotection; down-regulation of PKCe function was observed in the hearts of adult male offspring only. These results would have important clinical implications, since cardiac sensitivity to oxygen deprivation in adult patients may be significantly influenced by perinatal hypoxia in a sex-dependent manner.

Cardiotoxicity of β-mimetic catecholamines during ontogenetic development - possible risks of antenatal therapy

Catecholamines are involved in the regulation of a wide variety of vital functions. The β-adrenergic receptor (β-AR) - adenylyl cyclase system has been identified early in embryogenesis before the heart has received adrenergic innervation. The structure of β-receptors in the immature myocardium is similar to that in adults; there are, however, significant quantitative developmental changes in the inotropic and chronotropic responsiveness. Information on the toxic effect of the β-AR agonists in the immature heart is surprisingly scarce, even though these agents are used in clinical practice both during pregnancy and in early postnatal development. Large doses of β-AR agonists induce malformations of the cardiovascular system; the type of change depends upon the time at which the β-AR agonist was administered during embryogenesis. During postnatal ontogeny, the cardiotoxicity of β-AR agonists increased from birth to adulthood. It seems likely that despite interspecies differences, developmental changes in the cardiac sensitivity to β-AR agonists may exist in all mammals, depending on the degree of maturation of the system involved in β-adrenergic signaling. All the existing data draw attention to the possible harmful consequences of the clinical use of β-AR agonists during early phases of cardiac development. Late effects of the early disturbances of the cardiac muscle cannot be excluded.

Ontogenetic Aspects of Cardiac Adaptation to Chronic Hypoxia

One of the most common insults during early stages of postnatal ontogenetic development is hypoxemia due to cyanotic congenital heart defects. The question of the presumed cardiac impact will be, therefore, of considerable importance. Experimental results have clearly shown that the immature heart is significantly more tolerant to acute oxygen deficiency than the adult myocardium. However, the mechanisms of this difference have not yet been satisfactorily clarified; they are likely the result of developmental changes in cardiac mitochondrial function and energy metabolism. Adaptation to chronic hypoxia confers long-lasting protection in both adult and immature heart. However, the already high resistance of the newborn heart cannot be further increased; the effects of protective mechanisms appear only when the ischemic tolerance starts to decrease during development. Early chronic hypoxia, although transient, may have serious sex-dependent late consequences on the adult cardiovascular system. These results support the view that precise knowledge of individual developmental periods that are critical for cardiac ontogeny is crucial for better understanding of the mechanism of cardiac adaptation to oxygen deficiency.