Cardiac enzyme activities in fetal and adult pregnant and nonpregnant sheep exposed to high-altitude hypoxemia

Genetic signatures of high-altitude adaptation and geographic distribution in Tibetan sheep

Most sheep breeding programs designed for the tropics and sub-tropics have to take into account the impacts of environmental adaptive traits. However, the genetic mechanism regulating the multiple biological processes driving adaptive responses remains unclear. In this study, we applied a selective sweep analysis by combing 1% top values of F_st and ZHp on both altitude and geographic subpopulations (APS) in 636 indigenous Tibetan sheep breeds. Results show that 37 genes were identified within overlapped genomic regions regarding F_st significantly associated with APS. Out of the 37 genes, we found that 8, 3 and 6 genes at chromosomes (chr.) 13, 23 and 27, respectively, were identified in the genomic regions with 1% top values of ZHp. We further analyzed the INDEL variation of 6 genes at chr.27 (X chromosome) in APS together with corresponding orthologs of 6 genes in Capra, Pantholops, and Bos Taurus. We found that an INDEL was located within 5′UTR region of HAG1 gene. This INDEL of HAG1 was strongly associated with the variation of APS, which was further confirmed by qPCR. Sheep breeds carrying “C-INDEL” of HAG1 have significantly greater body weight, shear amount, corpuscular hemoglobin and globulin levels, but lower body height, than those carrying “CA-INDEL” of HAG1. We concluded that “C-INDEL” variation of HAG1 gene confers better hypoxia tolerance in the highlands of Tibetan and explains well geographic distributions in this population. These results contribute to our understanding of adaptive responses to altitude and geographic adaptation in Tibetan sheep populations and will help to guide future conservation programs for Tibetan sheep native to Qinghai-Tibetan Plateau.

Sex-dependent effect of perinatal hypoxia on cardiac tolerance to oxygen deprivation in adults

Epidemiological studies have demonstrated a relationship between the adverse influence of perinatal development and increased risk of ischemic heart disease in adults. From negative factors to which the fetus is subjected, the most important is hypoxia. The fetus may experience hypoxic stress under different conditions, including pregnancy at high altitude, pregnancy with anemia, placental insufficiency, and heart, lung, and kidney disease. One of the most common insults during the early stages of postnatal development is hypoxemia due to congenital cyanotic heart defects. Experimental studies have demonstrated a link between early hypoxia and increased risk of ischemia/reperfusion injury (I/R) in adults. Furthermore, it has been observed that late myocardial effects of chronic hypoxia, experienced in early life, may be sex-dependent. Unlike in males, perinatal hypoxia significantly increased cardiac tolerance to acute I/R injury in adult females, expressed as decreased infarct size and lower incidence of ischemic arrhythmias. It was suggested that early hypoxia may result in sex-dependent programming of specific genes in the offspring with the consequence of increased cardiac susceptibility to I/R injury in adult males. 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.

Cardiorespiratory consequences of intrauterine growth restriction: Influence of timing, severity and duration of hypoxaemia

At birth, weight of the neonate is used as a marker of the 9-month journey as a fetus. Those neonates born less than the 10th centile for their gestational age are at risk of being intrauterine growth restricted. However, this depends on their genetic potential for growth and the intrauterine environment in which they grew. Alterations in the supply of oxygen and nutrients to the fetus will decrease fetal growth, but these alterations occur due to a range of causes that are maternal, placental or fetal in nature. Consequently, IUGR neonates are a heterogeneous population. For this reason, it is likely that these neonates will respond differently to interventions compared not only to normally grown fetuses, but also to other neonates that are IUGR but have travelled a different path to get there. Thus, a range of models of IUGR should be studied to determine the effects of IUGR on the development and function of the heart and lung and subsequently the impact of interventions to improve development of these organs. Here we focus on a range of models of IUGR caused by manipulation of the maternal, placental or fetal environment on cardiorespiratory outcomes.

Age Related Bioenergetics Profiles in Isolated Rat Cardiomyocytes Using Extracellular Flux Analyses

Mitochondrial dysfunction is increasingly recognized and studied as a mediator of heart disease. Extracellular flux analysis (XF) has emerged as a powerful tool to investigate cellular bioenergetics in the context of cardiac health and disease, however its use and interpretation requires improved understanding of the normal metabolic differences in cardiomyocytes (CM) at various stages of maturation. This study standardized XF analyses methods (mitochondrial stress test, glycolytic stress test and palmitate oxidation test) and established age related differences in bioenergetics profiles of healthy CMs at newborn (NB1), weaning (3WK), adult (10WK) and aged (12–18MO) time points. Findings show that immature CMs demonstrate a more robust and sustained glycolytic capacity and a relative inability to oxidize fatty acids when compared to older CMs. The study also highlights the need to recognize the contribution of CO₂ from the Krebs cycle as well as lactate from anaerobic glycolysis to the proton production rate before interpreting glycolytic capacity in CMs. Overall, this study demonstrates that caution should be taken to assure that translatable developmental time points are used to investigate mitochondrial dysfunction as a cause of cardiac disease. Specifically, XF analysis of newborn CMs should be reserved to study fetal/neonatal disease and older CMs (≥10 weeks) should be used to investigate adult disease pathogenesis. Knowledge gained will aid in improved investigation of developmentally programmed heart disease and stress the importance of discerning maturational differences in bioenergetics when developing mitochondrial targeted preventative and therapeutic strategies for cardiac disease.

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.

Homogenous protein programming in the mammalian left and right ventricle free walls

Despite identical cardiac outputs, the right (RV) and left ventricle (LV) have very different embryological origins and resting workload. These differences suggest that the ventricles have different protein programming with regard to energy metabolism and contractile elements. The objective of this study was to determine the relative RV and LV protein expression levels, with an emphasis on energy metabolism. The RV and LV protein contents of the rabbit and porcine heart were determined with quantitative gel electrophoresis (2D-DIGE), mass spectrometry, and optical spectroscopy techniques. Surprisingly, the expression levels for more than 600 RV and LV proteins detected were similar. This included proteins many different compartments and metabolic pathways. In addition, no isoelectric shifts were detected in 2D-DIGE consistent with no differential posttranslational modifications in these proteins. Analysis of the RV and LV metabolic response to work revealed that the metabolic rate increases much faster with workload in the RV compared with LV. This implies that the generally lower metabolic stress of the RV actually approaches LV metabolic stress at maximum workloads. Thus, identical levels of energy conversion and mechanical elements in the RV and LV may be driven by the performance requirements at maximum workloads. In summary, the ventricles of the heart manage the differences in overall workload by modifying the amounts of cytosol, not its composition. The constant myocyte composition in the LV and RV implies that the ratio of energy metabolism and contractile elements may be optimal for the sustained cardiac contractile activity in the mammalian heart.

Development of aerobic and anaerobic metabolism in cardiac and skeletal muscles from harp and hooded seals

In diving animals, skeletal muscle adaptations to extend underwater time despite selective vasoconstriction include elevated myoglobin (Mb) concentrations, high acid buffering ability (beta) and high aerobic and anaerobic enzyme activities. However, because cardiac muscle is perfused during dives, it may rely less heavily on Mb, beta and anaerobic pathways to support contractile activity. In addition, because cardiac tissue must sustain contractile activity even before birth, it may be more physiologically mature at birth and/or develop faster than skeletal muscles. To test these hypotheses, we measured Mb levels, beta and the activities of citrate synthase (CS), beta-hydroxyacyl-CoA dehydrogenase (HOAD) and lactate dehydrogenase (LDH) in cardiac and skeletal muscle samples from 72 harp and hooded seals, ranging in age from fetuses to adults. Results indicate that in adults cardiac muscle had lower Mb levels (14.7%), beta (55.5%) and LDH activity (36.2%) but higher CS (459.6%) and HOAD (371.3%) activities (all P<0.05) than skeletal muscle. In addition, while the cardiac muscle of young seals had significantly lower [Mb] (44.7%) beta (80.7%) and LDH activity (89.5%) than adults (all P<0.05), it was relatively more mature at birth and weaning than skeletal muscle. These patterns are similar to those in terrestrial species, suggesting that seal hearts do not exhibit unique adaptations to the challenges of an aquatic existence.

Regulation of cardiac energy metabolism in newborn

Energy in the form of ATP is supplied from the oxidation of fatty acids and glucose in the adult heart in most species. In the fetal heart, carbohydrates, primarily glucose and lactate, are the preferred sources for ATP production. As the newborn matures the contribution of fatty acid oxidation to overall energy production increases and becomes the dominant substrate for the adult heart. The mechanisms responsible for this switch in energy substrate preference in the heart are complicated to identify due to slight differences between species and differences in techniques that are utilized. Nevertheless, our current knowledge suggests that the switch in energy substrate preference occurs due to a combination of events. During pregnancy, the fetus receives a constant supply of nutrients that is rich carbohydrates and poor in fatty acids in many species. Immediately after birth, the newborn is fed with milk that is high in fat and low in carbohydrates. The hormonal environment is also different between the fetal and the newborn. Moreover, direct subcellular changes occur in the newborn period that play a major role in the adaptation of the newborn heart to extrauterin life. The newborn period is unique and provides a very useful model to examine not only the metabolic changes, but also the effects of hormonal changes on the heart. A better understanding of developmental physiology and metabolism is also very important to approach certain disorders in energy substrate metabolism.

Prenatal hypoxia and cardiac programming

Epidemiologic studies have shown a clear association of adverse intrauterine environment and an increased risk of hypertension and coronary heart disease in the adult. Many studies have been focused on the effects of maternal undernutrition and fetal glucocorticoid exposure on fetal programming and later adult disease. Although it is relatively less clear, there is evidence that fetal exposure to hypoxia, alcohol, tobacco smoking, and cocaine may also cause in utero programming leading to an increased risk of adult disease. Chronic hypoxia during the course of pregnancy is thought to result in fetal intrauterine growth retardation. Among other effects, chronic hypoxia suppresses fetal cardiac function, alters cardiac gene expression, increases myocyte apoptosis, and results in a premature exit of the cell cycle of cardiomyocytes and myocyte hypertrophy. This review discusses recent evidence of an association of prenatal hypoxic exposure with an increased vulnerability of adult heart disease, and the possible mechanisms involved.

Adaptations to diving hypoxia in the heart, kidneys and splanchnic organs of harbor seals (Phoca vitulina)

Pinnipeds (seals and sea lions) have an elevated mitochondrial volume density [VV(mt)] and elevated citrate synthase (CS) and beta-hydroxyacyl-CoA dehydrogenase (HOAD) activities in their swimming muscles to maintain an aerobic, fat-based metabolism during diving. The goal of this study was to determine whether the heart, kidneys and splanchnic organs have an elevated VV(mt) and CS and HOAD activities as parallel adaptations for sustaining aerobic metabolism and normal function during hypoxia in harbor seals (Phoca vitulina). Samples of heart, liver, kidney, stomach and small intestine were taken from 10 freshly killed harbor seals and fixed in glutaraldehyde for transmission electron microscopy or frozen in liquid nitrogen for enzymatic analysis. Samples from dogs and rats were used for comparison. Within the harbor seal, the liver and stomach had the highest VV(mt). The liver also had the highest CS activity. The kidneys and heart had the highest HOAD activities, and the liver and heart had the highest lactate dehydrogenase (LDH) activities. Mitochondrial volume densities scaled to tissue-specific resting metabolic rate [VV(mt)/RMR] in the heart, liver, kidneys, stomach and small intestine of harbor seals were elevated (range 1.2-6.6x) when compared with those in the dog and/or rat. In addition, HOAD activity scaled to tissue-specific RMR in the heart and liver of harbor seals was elevated compared with that in the dog and rat (3.2x and 6.2x in the heart and 8.5x and 5.5x in the liver, respectively). These data suggest that organs such as the liver, kidneys and stomach possess a heightened ability for aerobic, fat-based metabolism during hypoxia associated with routine diving. However, a heightened LDH activity in the heart and liver indicates an adaptation for the anaerobic production of ATP on dives that exceed the animal's aerobic dive limit. Hence, the heart, liver, kidneys and gastrointestinal organs of harbor seals exhibit adaptations that promote an aerobic, fat-based metabolism under hypoxic conditions but can provide ATP anaerobically if required.

Fetal cardiac and cerebrovascular acclimatization responses to high altitude, long-term hypoxia

In response to high altitude long-term hypoxemia, the heart of fetal sheep shows a decrease in cardiac output that is secondary to a decrease in myocardial cell contractile function. The intracellular mechanisms responsible for these reductions might include reduced myofibrillar Mg(2+)-activated ATPase. There is also a decrease in beta(1)-adrenergic receptor stimulated augmentation of myocardial contraction. An overproduction of cAMP by beta(1)-adrenergic receptor stimulation, resulting in overphosphorylation of troponin I, may reduce calcium binding by troponin C. Fetal coronary arteries have a reduced contractile response to K(+) depolarization and a reduced sensitivity to a thromboxane A(2) receptor agonist-stimulated contraction. Cerebral arteries of adult sheep (but not the fetus) show decreased responses to both K(+)-depolarization and norepinephrine-induced contraction. Nonetheless, cerebral arteries in the long-term hypoxic fetus demonstrated a number of significant changes from control. For the cerebral arteries in general, high altitude hypoxia is associated with augmented or upregulation of presynaptic functions. In contrast, postsynaptic functions tend to be significantly depressed or downregulated. The results emphasize the role of high altitude, long-term hypoxemia in modulating adrenergic- and serotonergic-mediated signal transduction in the cerebral vasculature. They specifically highlight the significant differences in acclimatization responses between the fetus and adult.

Effects of chronic hypoxia on fetal coronary responses

This review examines the effect of high altitude and/or chronic hypoxia on cardiac mechanisms that influence perfusion of the fetal heart (e.g., tissue metabolism, coronary vessel growth, and coronary blood flow and vessel responsiveness). In response to intrauterine hypoxia, the fetal heart may either reduce its energy demand or increase its substrate and oxygen delivery as a means of sustaining cardiac function. Cardiac glycolysis predominates as a metabolic pathway of ATP synthesis in the fetal heart under both normoxic and hypoxic conditions. During prolonged oxygen insufficiency, normal cardiac function is sustained by anaerobic glycolysis relying primarily on high levels of stored glycogen in the heart. Chronic hypoxia increases coronary vessel growth and myocardial vascularization in fetal hearts, although the response may depend on the presence of ventricular hypertrophy. Recent studies demonstrate that high altitude hypoxia increases both resting fetal coronary flow and coronary flow reserve as an adaptive response toward increasing oxygen delivery. Hypoxia may also directly effect local vascular smooth muscle mechanisms, resulting in altered coronary artery reactivity to circulating vasoactive substances and contributing to enhanced perfusion. Further study is needed to understand the relative importance of each of these cardiac adaptations in contributing to fetal survival. It is likely that differences in fetal coronary responses to intrauterine hypoxia are highly dependent on the gestational age and relative maturity of the animal species.

Effect of maternal chronic hypoxic exposure during gestation on apoptosis in fetal rat heart

Chronic hypoxia during pregnancy is one of the most common insults to fetal development. We tested the hypothesis that maternal hypoxia induced apoptosis in the hearts of near-term fetal rats. Pregnant rats were divided into two groups, normoxic control and continuous hypoxic exposure (10.5% O2) from day 15 to 21 of gestation. Hearts were isolated from fetal rats of 21-day gestational age. Maternal hypoxia increased hypoxia-inducible factor-1alpha protein in fetal hearts. Chronic hypoxia significantly increased the percentage and size of binucleated myocytes and increased apoptotic cells from 1.4 +/- 0.14% to 2.7 +/- 0.3% in the fetal heart. In addition, the active cleaved form of caspase 3 was significantly increased in the hypoxic heart, which was associated with an increase in caspase 3 activity. There was a significant increase in Fas protein levels in the hypoxic heart. Chronic hypoxia did not change Bax protein levels but significantly decreased Bcl-2 proteins. In addition, chronic hypoxia significantly suppressed expression of heat shock protein 70. However, chronic hypoxia significantly increased expression of the anti-apoptotic protein 14-3-3, among other 14-3-3 isoforms. Chronic hypoxia differentially regulated beta-adrenoreceptor (beta-AR) subtypes with an increase in beta1-AR levels but no changes in beta2-AR. The results demonstrate that maternal hypoxia increases apoptosis in fetal rat heart, which may be mediated by an increase in Fas and a decrease in Bcl-2 proteins. Chronic hypoxia-mediated increase in beta1-AR and decrease in heat shock proteins may also play an important role in apoptosis in the fetal heart.

Quantitative electron microscopic study of the hypoxic fetal sheep heart

In order to determine the effects of chronic, high-altitude hypoxia on the ovine fetal heart, we exposed pregnant ewes to 3,820 m beginning at 30 days gestation. We previously showed that following approximately 110 days of hypoxia the fetal heart showed significant reduction in cardiac output (76% of control) and contractility, and elevated levels of citrate synthase and lactate dehydrogenase. To investigate ultrastructural influences on these observed physiologic changes at altitude, we hypothesized that the volume densities of myofibrils and mitochondria, and glycogen content would be reduced in the ovine fetal heart and that this may contribute to contraction and cardiac output deficits in hypoxia. Mitochondria and myofibril volume density were determined by standard point-counting techniques and glycogen content was determined by biochemical analysis. The glycogen content from the hypoxic right ventricle (4.8 +/- 0.3%) was significantly lower than in control right ventricle (6.8 +/- 0.5%) and both left ventricles (hypoxia, 7.2 +/- 0.5; control, 7.8 +/- 0. 4%). Total mitochondrial volume density was also significantly reduced following hypoxia (15.5 +/- 0.7%) compared to controls (16.9 +/- 0.4%). As is common in the ovine fetal heart, the myofibril volume density of the right ventricle from both groups was significantly higher than the left ventricle (RV, 58.6 +/- 1.6; LV 54.3 +/- 0.9%). However, it was not different between control and high altitude. In support of our hypothesis, we may speculate that deficits in the quantity of myocyte glycogen and mitochondria contribute to the observed reduction in cardiac output and contractility, despite the upregulation of citrate synthase and lactate dehydrogenase. In contrast, myofibril volume density was unchanged.

Effects of long-term, high-altitude hypoxia on the capillarity of the ovine fetal heart

To determine the effect of chronic hypoxia on myocardial capillarity, we exposed pregnant ewes to an altitude of 3,820 m from day 30 to day 139 of gestation and compared the fetus to low-altitude (approximately 300 m) controls. We hypothesized that capillarity would increase in the hypoxic myocardium to optimize oxygen and metabolite flux to hypoxic tissues. Fetal hearts were fixed by retrograde aortic perfusion and processed for microscopy and stereological evaluation. Fiber cross-sectional area and capillary density were measured and standardized to sarcomere length. Capillary volume density and capillary diameter were measured, capillary-to-fiber ratio and capillary length density were calculated, and the capillary anisotropy coefficient was obtained from a table of known values. Capillary-to-fiber ratio, capillary volume density, and the capillary anisotropy coefficient were not different between hypoxia and control groups. Capillary diameter was significantly larger in the right compared with the left ventricle of hypoxic but not control hearts; fiber cross-sectional area tended to be larger in the right ventricle of both groups, but this was not significant. As a result of larger fiber size, capillary density and capillary length density were significantly smaller in the right ventricle of hypoxic but not control fetal hearts. Contrary to our hypothesis, the ovine fetus does not show morphological adaptation in the myocardium after approximately 109 days of high-altitude hypoxic stress.

Myocardial blood flow and coronary reserve in chronically anemic fetal lambs

Chronic fetal anemia produces large compensatory increases in coronary blood flow in the near-term fetal lamb. To determine if increased coronary flow in anemic fetuses is associated with decreased coronary flow reserve or, alternatively, an increase in coronary conductance, we measured maximal coronary artery conductance during adenosine infusion before and during anemia. Isovolemic hemorrhage over 7 days reduced hematocrit from 30.6 +/- 2. 7 to 15.8 +/- 2.4% (P < 0.02) and the oxygen content from 7.3 +/- 1. 4 to 2.6 +/- 0.4 ml/dl (P < 0.001). Coronary blood flow increased from control (202 +/- 60) to 664 +/- 208 ml. min(-1). 100 g(-1) with adenosine to 726 +/- 169 ml. min(-1). 100 g(-1) during anemia and to 1,162 +/- 250 ml. min(-1). 100 g(-1) (left ventricle) during anemia with adenosine infusion (all P < 0.001). Coronary conductance, determined during maximal vasodilation, was 18.2 +/- 7.7 before and 32.8 +/- 11.9 ml. min(-1). 100 g(-1). mmHg(-1) during anemia (P < 0. 001). Coronary reserve, the difference between resting and maximal myocardial blood flow interpolated at 40 mmHg, was unchanged in control and anemic fetuses (368 +/- 142 and 372 +/- 201 ml/min). Because hematocrit affects viscosity, anemic fetuses were transfused with blood to acutely increase the hematocrit back to control, and conductance was remeasured. Coronary blood flow decreased 57.3 +/- 18.9% but was still 42.6 +/- 18.9% greater than control. We conclude that in chronically anemic fetal sheep coronary conductance is increased and coronary reserve is maintained, and this is attributed in part to angiogenesis as well as changes in viscosity.

Cardiac hypertrophy in chronically anemic fetal sheep: Increased vascularization is associated with increased myocardial expression of vascular endothelial growth factor and hypoxia-inducible factor 1

OBJECTIVE

Our purpose was to determine whether the increase in fetal cardiac mass and cardiac output in chronic anemia is accompanied by changes in capillary density or size or changes in levels of vascular endothelial growth factor and hypoxia-inducible factor 1, a basic helix-loop-helix transcription factor that has previously been shown to activate vascular endothelial growth factor gene transcription when cultured cells are subjected to hypoxia.

STUDY DESIGN

Anemia was induced in near-term ovine fetuses by daily isovolemic hemorrhage. In five fetuses the heart was arrested in diastole, isolated, and fixed at physiologic pressures with adenosine-paraformaldehyde, and morphometric measurements of capillaries were made. In six fetuses cardiac expression of vascular endothelial growth factor and hypoxia-inducible factor 1 protein was detected by Western analysis and vascular endothelial growth factor messenger ribonucleic acid by Northern blot analysis. Eleven age-matched fetuses served as controls.

RESULTS

The anemic fetuses compared with controls had a lower hematocrit (14.8% +/- 0.7% vs 35.3% +/- 1.5%) and a greater heart-to-body weight ratio (10.5 +/- 1.1 vs 7.7 +/- 0.5 gm/kg). The minimal capillary diameter was increased and the intercapillary distance was decreased in both right and left ventricles of anemic fetuses compared with controls. Vascular endothelial growth factor protein was increased 4.5-fold, vascular endothelial growth factor messenger ribonucleic acid 3.2-fold, and hypoxia-inducible factor 1alpha protein 3.8-fold in ventricular tissue from anemic fetuses.

CONCLUSIONS

In chronic fetal anemia cardiac hypertrophy is accompanied by anatomic changes in myocardial capillary morphometry along with induction of hypoxia-inducible factor 1 and vascular endothelial growth factor. These results provide evidence for a pathway by which anemia-hypoxia may stimulate myocardial vascularization.