Moderate and severe hypoxia elicit divergent effects on cardiovascular function and physiological rhythms

Dose-effect of exercise intervention on heart rate variability of acclimatized young male lowlanders at 3,680 m

This study investigated whether exercise could improve the reduced HRV in an environment of high altitude. A total of 97 young, healthy male lowlanders living at 3,680 m for >1 year were recruited. They were randomized into four groups, of which three performed-low-, moderate-, and high-intensity (LI, MI, HI) aerobic exercise for 4 weeks, respectively. The remaining was the control group (CG) receiving no intervention. For HI, compared to other groups, heart rate (p = 0.002) was significantly decreased, while standard deviation of RR intervals (p < 0.001), SD2 of Poincaré plot (p = 0.046) and the number of successive RR interval pairs that differ by > 50 ms divided by total number of RR (p = 0.032), were significantly increased after intervention. For MI, significantly increase of trigonometric interpolation in NN interval (p = 0.016) was observed after exercise. Further, a decrease in systolic blood pressure (SBP) after high-intensity exercise was found significantly associated with an increase in SD2 (r = - 0.428, p = 0.042). These results indicated that there was a dose effect of different intensities of aerobic exercise on the HRV of acclimatized lowlanders. Moderate and high-intensity aerobic exercise would change the status of the autonomic nervous system (ANS) and decrease the blood pressure of acclimatized lowlanders exposed to high altitude.

Novel roles of cardiac-derived erythropoietin in cardiac development and function

The role of erythropoietin (EPO) has extended beyond hematopoiesis to include cytoprotection, inotropy, and neurogenesis. Extra-renal EPO has been reported for multiple tissue/cell types, but the physiological relevance remains unknown. Although the EPO receptor is expressed by multiple cardiac cell types and human recombinant EPO increases contractility and confers cytoprotection against injury, whether the heart produces physiologically meaningful amounts of EPO in vivo is unclear. We show a distinct circadian rhythm of cardiac EPO mRNA expression in adult mice and increased mRNA expression during embryogenesis, suggesting physiological relevance to cardiac EPO production throughout life. We then generated constitutive, cardiomyocyte-specific EPO knockout mice driven by the Mlc2v promoter (EPOfl/fl:Mlc2v-cre+/-; EPOΔ/Δ-CM). During cardiogenesis, cardiac EPO mRNA expression and cellular proliferation were reduced in EPOΔ/Δ-CM hearts. However, in adult EPOΔ/Δ- CM mice, total heart weight was preserved through increased cardiomyocyte cross-sectional area, indicating the reduced cellular proliferation was compensated for by cellular hypertrophy. Echocardiography revealed no changes in cardiac dimensions, with modest reductions in ejection fraction, stroke volume, and tachycardia, whereas invasive hemodynamics showed increased cardiac contractility and lusitropy. Paradoxically, EPO mRNA expression in the heart was elevated in adult EPOΔ/Δ-CM, along with increased serum EPO protein content and hematocrit. Using RNA fluorescent in situ hybridization, we found that Epo RNA colocalized with endothelial cells in the hearts of adult EPOΔ/Δ-CM mice, identifying the endothelial cells as a cell responsible for the EPO hyper-expression. Collectively, these data identify the first physiological roles for cardiomyocyte-derived EPO. We have established cardiac EPO mRNA expression is a complex interplay of multiple cell types, where loss of embryonic cardiomyocyte EPO production results in hyper-expression from other cells within the adult heart.

Regulation of heart rate in vertebrates during hypoxia: A comparative overview

Acute exposure to low oxygen (hypoxia) places conflicting demands on the heart. Whilst an increase in heart rate (tachycardia) may compensate systemic oxygen delivery as arterial oxygenation falls, the heart itself is an energetically expensive organ that may benefit from slowing (bradycardia) to reduce work when oxygen is limited. Both strategies are apparent in vertebrates, with tetrapods (mammals, birds, reptiles, and amphibians) classically exhibiting hypoxic tachycardia and fishes displaying characteristic hypoxic bradycardia. With a richer understanding of the ontogeny and evolution of the responses, however, we see similarities in the underlying mechanisms between vertebrate groups. For example, in adult mammals, primary bradycardia results from the hypoxic stimulation of carotid body chemoreceptors that are overwhelmed by mechano-sensory feedback from the lung associated with hyperpnoea. Fish-like bradycardia prevails in the mammalian foetus (which, at this stage, is incapable of pulmonary ventilation), and in fish and foetus alike, the bradycardia ensues despite an elevation of circulating catecholamines. In both cases, the reduced heart rate may primarily serve to protect the heart. Thus, the comparative perspective offers fundamental insight into how and why different vertebrates regulate heart rate in different ways during periods of hypoxia.

[Molecular mechanisms for hypoxia development and adaptation to it. Part I]

Hypoxia is a typical pathological process characterized by the occurrence of oxygen deficiency in tissues and cells and accompanied by the development of immediate and delayed compensatory and adaptive reactions. Reprogramming of the mitochondrial electron transport chain (ETC) function is one the most essential regulatory mechanisms that allow for immediate adaptation to hypoxia. Succinic acid, or succinate, is involved in this process not only as one of the intermediates of the tricarboxylic acid (TAC) cycle, but also as a signaling molecule. In this connection, the purpose of this review was to systematize the available data on the molecular mechanisms for the development of hypoxia and its adaptation at the ETC/TAC coupling site, as well as on the role of succinic acid in these processes.

Hypoxia-induced inflammation: Profiling the first 24-hour posthypoxic plasma and central nervous system changes

Central nervous system and visual dysfunction is an unfortunate consequence of systemic hypoxia in the setting of cardiopulmonary disease, including infection with SARS-CoV-2, high-altitude cerebral edema and retinopathy and other conditions. Hypoxia-induced inflammatory signaling may lead to retinal inflammation, gliosis and visual disturbances. We investigated the consequences of systemic hypoxia using serial retinal optical coherence tomography and by assessing the earliest changes within 24h after hypoxia by measuring a proteomics panel of 39 cytokines, chemokines and growth factors in the plasma and retina, as well as using retinal histology. We induced severe systemic hypoxia in adult C57BL/6 mice using a hypoxia chamber (10% O2) for 1 week and rapidly assessed measurements within 1h compared with 18h after hypoxia. Optical coherence tomography revealed retinal tissue edema at 18h after hypoxia. Hierarchical clustering of plasma and retinal immune molecules revealed obvious segregation of the 1h posthypoxia group away from that of controls. One hour after hypoxia, there were 10 significantly increased molecules in plasma and 4 in retina. Interleukin-1β and vascular endothelial growth factor were increased in both tissues. Concomitantly, there was significantly increased aquaporin-4, decreased Kir4.1, and increased gliosis in retinal histology. In summary, the immediate posthypoxic period is characterized by molecular changes consistent with systemic and retinal inflammation and retinal glial changes important in water transport, leading to tissue edema. This posthypoxic inflammation rapidly improves within 24h, consistent with the typically mild and transient visual disturbance in hypoxia, such as in high-altitude retinopathy. Given hypoxia increases risk of vision loss, more studies in at-risk patients, such as plasma immune profiling and in vivo retinal imaging, are needed in order to identify novel diagnostic or prognostic biomarkers of visual impairment in systemic hypoxia.

Changes in the autonomic and respiratory patterns in mice submitted to short-term sustained hypoxia

NEW FINDINGS

What is the central question of this study? Do mice submitted to sustained hypoxia present autonomic and respiratory changes similarly to rats? What is the main finding and its importance? Arterial pressure in the normal range, reduced baseline heart rate and tachypnoea were observed in behaving sustained hypoxia mice. Recordings in the in situ preparation of mice submitted to sustained hypoxia show an increase in cervical vagus nerve activity and a simultaneous reduction in thoracic sympathetic nerve activity correlated with changes in the respiratory cycle. Therefore, mice are an important model for studies on the modulation of sympathetic activity to the cardiovascular system and the vagus innervation of the upper airways due to changes in the respiratory network induced by sustained hypoxia.

ABSTRACT

Short-term sustained hypoxia (SH) in rats induces sympathetic overactivity and hypertension due to changes in sympathetic-respiratory coupling. However, there are no consistent data about the effect of SH on mice due to the different protocols of hypoxia and difficulties associated with the handling of these rodents under different experimental conditions. In situ recordings of autonomic and respiratory nerves in SH mice have not been performed yet. Herein, we evaluated the effects of SH ( F i O 2  = 0.1 for 24 h) on baseline mean arterial pressure (MAP), heart rate (HR), respiratory frequency (f_R ) and responses to chemoreflex activation in behaving SH mice. A characterization of changes in cervical vagus (cVN), thoracic sympathetic (tSN), phrenic (PN) and abdominal (AbN) nerves in SH mice using the in situ working heart-brainstem preparation was also performed. SH mice presented normal MAP, significant reduction in baseline HR, increase in baseline f_R , as well as increase in the magnitude of bradycardic response to chemoreflex activation. In in situ preparations, SH mice presented a reduction in PN discharge frequency, and increases in the time of expiration and incidence of late-expiratory bursts in AbN activity. Nerve recordings also indicated a significant increase in cVN activity and a significant reduction in tSN activity during expiration in SH mice. These findings make SH mice an important experimental model for better understanding how changes in the respiratory network may impact on the modulation of vagal control to the upper airways, as well as in the sympathetic activity to the cardiovascular system.

Disruption of Physiological Rhythms Persist Following Cessation of Cigarette Smoke Exposure in Mice

Background

Physiological rhythms in mammals are essential for maintaining health, whereas disruptions may cause or exacerbate disease pathogenesis. As such, our objective was to characterize how cigarette smoke exposure affects physiological rhythms of otherwise healthy mice using telemetry and cosinor analysis.

Methods

Female BALB/c mice were implanted with telemetry devices to measure body temperature, heart rate, systolic blood pressure (SBP), and activity. Following baseline measurements, mice were exposed to cigarette smoke for approximately 50 min twice daily during weekdays over 24 weeks. Physiological parameters were recorded after 1, 4, 8, and 24 weeks of exposure or after 4 weeks cessation following 4 weeks of cigarette smoke exposure.

Results

Acute cigarette smoke exposure resulted in anapyrexia, and bradycardia, with divergent effects on SBP. Long term, cigarette smoke exposure disrupted physiological rhythms after just 1 week, which persisted across 24 weeks of exposure (as shown by mixed effects on mesor, amplitude, acrophase, and goodness-of-fit using cosinor analysis). Four weeks of cessation was insufficient to allow full recovery of rhythms.

Conclusion

Our characterization of the pathophysiology of cigarette smoke exposure on physiological rhythms of mice suggests that rhythm disruption may precede and contribute to disease pathogenesis. These findings provide a clear rationale and guide for the future use of chronotherapeutics.

Applications of a novel radiotelemetry method for the measurement of intrathoracic pressures and physiological rhythms in freely behaving mice

Techniques to comprehensively evaluate pulmonary function carry a variety of limitations, including the ability to continuously record intrathoracic pressures (ITP), acutely and chronically, in a natural state of freely behaving animals. Measurement of ITP can be used to derive other respiratory parameters, which provide insight to lung health. Our aim was to develop a surgical approach for the placement of a telemetry pressure sensor to measure ITP, providing the ability to chronically measure peak pressure, breath frequency, and timing of the respiratory cycle to facilitate circadian analyses related to breathing patterns. Applications of this technique are shown using a moderate hypoxic challenge. Male C57Bl/6 mice were implanted with radiotelemetry devices to record heart rate, temperature, activity, and ITP during 24-h normoxia, 24-h hypoxia ([Formula: see text] = 0.15), and return to 48-h normoxia. Radiotelemetry of ITP permitted the detection of hypoxia-induced increases in "the ITP equivalent" of ventilation, which were driven by increases in breathing frequency and ITP on a short-term time scale. Respiratory frequency, derived from pressure waveforms, was increased by a decrease in expiratory time without changes in inspiratory time. Chronically, telemetric recording allowed for circadian analyses of respiratory drive, as assessed by inspiratory pressure divided by inspiratory time, which was increased by hypoxia and remained elevated for 48 h of recovery. Furthermore, respiratory frequency demonstrated a circadian rhythm, which was disrupted through the recovery period. In conclusion, radiotelemetry of ITP is a viable, long-term, chronic methodology that extends traditional methods to evaluate respiratory function in mice.NEW & NOTEWORTHY We have demonstrated for the first time in mice that radiotelemetry is an effective tool for the continuous and chronic recording of intrathoracic pressure (ITP) to facilitate circadian rhythm analyses. We show that continuous 24-h hypoxic stress alters the circadian rhythms of heart rate, body temperature, activity, and respiratory parameters, acutely and perpetually, through normoxic recovery. Radiotelemetry of ITP can complement traditional methods for evaluating respiratory function and better our understanding of respiratory pathophysiology.

Hemodynamic assessment of diastolic function for experimental models

Traditionally, the evaluation of cardiac function has focused on systolic function; however, there is a growing appreciation for the contribution of diastolic function to overall cardiac health. Given the emerging interest in evaluating diastolic function in all models of heart failure, there is a need for sensitivity, accuracy, and precision in the hemodynamic assessment of diastolic function. Hemodynamics measure cardiac pressures in vivo, offering a direct assessment of diastolic function. In this review, we summarize the underlying principles of diastolic function, dividing diastole into two phases: 1) relaxation and 2) filling. We identify parameters used to comprehensively evaluate diastolic function by hemodynamics, clarify how each parameter is obtained, and consider the advantages and limitations associated with each measure. We provide a summary of the sensitivity of each diastolic parameter to loading conditions. Furthermore, we discuss differences that can occur in the accuracy of diastolic and systolic indices when generated by automated software compared with custom software analysis and the magnitude each parameter is influenced during inspiration with healthy breathing and a mild breathing load, commonly expected in heart failure. Finally, we identify key variables to control (e.g., body temperature, anesthetic, sampling rate) when collecting hemodynamic data. This review provides fundamental knowledge for users to succeed in troubleshooting and guidelines for evaluating diastolic function by hemodynamics in experimental models of heart failure.

Melatonin alleviates hypoxia-induced cardiac apoptosis through PI3K/Akt pathway

Hypoxia-induced apoptosis is an inevitable problem in cyanotic congenital heart disease. In the present study, we investigated effects of melatonin on hypoxic cardiomyocytes in vitro and in vivo, and explored its underlying mechanism. H9C2 cells were subjected to hypoxia for 48 hours. Mice were subjected to hypoxia treatment (10% O₂) for 4 weeks. Cell viability was detected by the cell counting kit-8 assay. Cellular apoptosis was assessed by Annexin V/7 AAD assay. Western blotting was employed to determine the expression of Bcl-2, Bax, cleaved caspase 3, phosphorylation of PI3K, and AKT. Melatonin increased cell viability and alleviated apoptosis in hypoxic H9C2 cells and cardiomyocytes of hypoxia-treated mice. Melatonin pretreatment increased Bcl-2 and decreased cleaved caspase 3 and Bax levels. Moreover, melatonin activated the PI3K/Akt pathway. The protective effects of melatonin were abolished by a PI3K/Akt-inhibitor, LY294002. Our results demonstrated that melatonin confers cardioprotection by inhibiting apoptosis through the activation of PI3K/Akt signaling pathway in hypoxic cardiomyocytes.