Chemical Sympathectomy Restores Baroreceptor-Heart Rate Reflex and Heart Rate Variability in Rats With Chronic Nitric Oxide Deficiency

Comprehensive viewpoints on heart rate variability at high altitude

OBJECTIVES

Hypoxia is a physiological state characterized by reduced oxygen levels in organs and tissues. It is a common clinicopathological process and a major cause of health problems in highland areas.  Heart rate variability (HRV) is a measure of the balance in autonomic innervation to the heart. It provides valuable information on the regulation of the cardiovascular system by neurohumoral factors, and changes in HRV reflect the complex interactions between multiple systems. In this review, we provide a comprehensive overview of the relationship between high-altitude hypoxia and HRV. We summarize the different mechanisms of diseases caused by hypoxia and explore the changes in HRV across various systems. Additionally, we discuss relevant pharmaceutical interventions. Overall, this review aims to provide research ideas and assistance for in-depth studies on HRV. By understanding the intricate relationship between high-altitude hypoxia and HRV, we can gain insights into the underlying mechanisms and potential therapeutic approaches to mitigate the effects of hypoxia on cardiovascular and other systems.

METHODS

The relevant literature was collected systematically from scientific database, including PubMed, Web of Science, China National Knowledge Infrastructure (CNKI), Baidu Scholar, as well as other literature sources, such as classic books of hypoxia.

RESULTS

There is a close relationship between heart rate variability and high-altitude hypoxia. Heart rate variability is an indicator that evaluates the impact of hypoxia on the cardiovascular system and other related systems. By improving the observation of HRV, we can estimate the progress of cardiovascular diseases and predict the impact on other systems related to cardiovascular health. At the same time, changes in heart rate variability can be used to observe the efficacy of preventive drugs for altitude related diseases.

CONCLUSIONS

HRV can be used to assess autonomic nervous function under various systemic conditions, and can be used to predict and monitor diseases caused by hypoxia at high altitude. Investigating the correlation between high altitude hypoxia and heart rate variability can help make HRV more rapid, accurate, and effective for the diagnosis of plateau-related diseases.

General anesthesia and electrocardiographic parameters in in vivo experiments involving rats

In in vivo cardiovascular or toxicological studies involving rat models, changes in selected electrocardiographic (ECG) parameters are monitored after various interventions to assess the origin and development of heart rhythm disorders. Each ECG parameter has diagnostic significance; as such, commonly evaluated ECG parameters, including heart rate, PR interval, P wave duration, P wave amplitude, QRS complex, QT and QTc interval duration, R wave and T wave amplitude, of rats under various types of general anesthesia were the focus of this study. Studies that performed in vivo cardiovascular or toxicological experiments in rats were retrieved from a search of the Web of Science database for articles published mainly between 2000 and 2021. In total, the search retrieved 123 articles. ECG parameters that were reported as baseline or control values were summarized and averages with ranges were calculated. It is important to be cautious when interpreting results and, in discussions addressing the mechanisms underlying a given type of arrhythmia, acknowledge that initial ECG parameters may already be affected to some extent by the general anesthesia as well as by sex and the time of day the experiments were performed.

Effect of Intensity on Changes in Cardiac Autonomic Control of Heart Rate and Arterial Stiffness After Equated Continuous Running Training Programs

Background: It is well known that exercise training has positive effects on both cardiac autonomic function and arterial stiffness (AS). However, it is not clear that which exercise training variables, intensity or volume, or both, play a crucial role in this regard. This study investigates the chronic effects of high-volume moderate-intensity training (HVMIT) and low-volume high-intensity training (LVHIT) on heart rate variability (HRV) and AS in sedentary adult men.

Materials and Methods: Notably, 45 males (age: 42 ± 5.7 years) were randomly assigned to a control (n = 15), HVMIT (n = 15), or LVHIT (n = 15). The HVMIT group ran three times per week on a treadmill at 50–60% of VO₂max for 45–60 min, while the LVHIT trained at 70–85% of VO₂max for 25–40 min. Both training protocols were equated by caloric expenditure. HRV, pulse wave velocity (PWV), hemodynamic variables, and body composition were measured before and after 12 weeks.

Results: Both protocols (i.e., HVMIT and LVHIT) significantly increased the SD of normal sinus beat intervals (SDNN) and high-frequency (HF) bands (p < 0.05) after 12 weeks. Whereas the low-frequency (LF)-HF ratio decreased significantly in both training protocols (p < 0.05); however, these changes were significantly greater in the LVHIT protocol (p < 0.05). Furthermore, the root mean square of successive RR interval differences (RMSSD) significantly increased only in the LVHIT (p < 0.05). Moreover, a significant decrease in LF and PWV was only observed following the LVHIT protocol (p < 0.05). Some measures of HRV and PWV were significantly correlated (r = 0.275–0.559; p < 0.05).

Conclusion: These results show that the LVHIT protocol was more efficient for improving HRV variables and PWV than the HVMIT protocol after 12 weeks of continuous running training. Interestingly, changes in some HRV parameters were related to changes in PWV. Further studies should elaborate on the link between central and peripheral cardiovascular adaptations after continuous and intermittent training regimens differing in intensity.

Beetroot juice intake with different amounts of nitrate does not change aerobic exercise-mediated responses in heart rate variability in hypertensive postmenopausal women: A randomized, crossover and double-blind study

Objectives

To compare the acute effects of a single dose of beetroot juice (BJ) with different concentrations of nitrate (NO₃⁻) on heart rate variability (HRV) in postmenopausal hypertensive women.

Methods

Thirteen hypertensive postmenopausal women (58.1 ± 4.6 years of age and 27 ± 4 kg/m² of BMI) completed the protocol that consisted of three visits with different beverage intakes in a randomized and crossover design. The three beverages were BJ with a high content of nitrate (high-NO₃^-), BJ with a low content of nitrate (low-NO₃^-), and an orange flavored non-caloric drink (OFD). Heart rate (HR) were evaluated during 20 min after sitting rest at 7:20 a.m. (baseline), after they drank one of the drinks, and remained at sitting rest for 120 min and then performed 40 min of aerobic exercise at 65–70% of the HR reserve on a treadmill. HR was recorded for 90 min after exercise for time, frequency, and non-linear domains of HRV index analysis.

Results

Two-way ANOVA showed that there were no interaction effects (time∗sessions) in any of the HRV indexes after exercise in all three sessions. HRV indexes increased after exercise (p = <0.05) similarly in all three sessions when compared with the baseline time point.

Conclusion

Therefore, a single dose of BJ, independent of NO₃⁻ content, does not change aerobic exercise-mediated responses in HRV indexes in hypertensive postmenopausal women.

Chronic L-Name-Treatment Produces Hypertension by Different Mechanisms in Peripheral Tissues and Brain: Role of Central eNOS

The goal of our study was to analyze the time course of the effect of N^G-nitro-L-arginine methyl ester (L-NAME) on nitric oxide synthase (NOS) isoforms and nuclear factor–κB (NF-κB) protein expression, total NOS activity, and blood pressure (BP) in rats. Adult 12-week-old male Wistar rats were subjected to treatment with L-NAME (40 mg/kg/day) for four and seven weeks. BP was increased after 4- and 7-week L-NAME treatments. NOS activity decreased after 4-week-L-NAME treatment; however, the 7-week treatment increased NOS activity in the aorta, heart, and kidney, while it markedly decreased NOS activity in the brainstem, cerebellum, and brain cortex. The 4-week-L-NAME treatment increased eNOS expression in the aorta, heart, and kidney and this increase was amplified after 7 weeks of treatment. In the brain regions, eNOS expression remained unchanged after 4-week L-NAME treatment and prolonged treatment led to a significant decrease of eNOS expression in these tissues. NF-κB expression increased in both peripheral and brain tissues after 4 weeks of treatment and prolongation of treatment decreased the expression in the aorta, heart, and kidney. In conclusion, decreased expression of eNOS in the brain regions after 7-week L-NAME treatment may be responsible for a remarkable decrease of NOS activity in these regions. Since the BP increase persisted after 7 weeks of L-NAME treatment, we hypothesize that central regulation of BP may contribute significantly to L-NAME-induced hypertension.

Dexamethasone Causes Hypertension in Rats Even Under Chemical Blockade of Peripheral Sympathetic Nerves

Synthetic glucocorticoids (GCs) are widely used to treat inflammatory conditions. However, chronic use of GCs can lead to hypertension. The cause of this undesired side effect remains unclear. Previously, we developed an in vivo rat model to study the mechanisms underlying hypertension induced by the chronic administration of the potent synthetic GC, dexamethasone (DEX) and found that the catecholamine biosynthetic pathway plays an important role. In the current study, we used this model to investigate the role of the adrenal medulla, renal nerves, and other peripheral sympathetic nerves in DEX-induced hypertension. After 5 days of baseline telemetric recording of mean arterial pressure (MAP) and heart rate (HR), rats were subjected to one of the following treatments: renal denervation (RDNX), adrenal medullectomy (ADMX), 6-hydroxydopamine (6-OHDA, 20 mg/kg, i.p.) to induce chemical sympathectomy, or a combination of ADMX and 6-OHDA. On day 11, the animals received vehicle (VEH) or DEX in drinking water for 7 days, with the latter causing an increase in MAP in control animals. ADMX and RDNX by themselves exacerbated the pressor effect of DEX. In the chemical sympathectomy group, DEX still caused a rise in MAP but the response was lower (ΔMAP of 6-OHDA/DEX < VEH/DEX, p = 0.039). However, when ΔMAP was normalized to day 10, 6-OHDA + DEX did not show any difference from VEH + DEX, certainly not an increase as observed in DEX + ADMX or RDNX groups. This indicates that sympathetic nerves do not modulate the pressor effect of DEX. TH mRNA levels increased in the adrenal medulla in both VEH/DEX (p = 0.009) and 6-OHDA/DEX (p = 0.031) groups. In the 6-OHDA group, DEX also increased plasma levels of norepinephrine (NE) (p = 0.016). Our results suggest that the activation of catecholamine synthetic pathway could be involved in the pressor response to DEX in animals even under chemical sympathectomy with 6-OHDA.

Blood pressure and hemodynamics: Mayer waves in different phases of ovarian and menstrual cycle in women

The goal of the research is to investigate the special effect of ovarian-menstrual cycle phases on the level of women's blood pressure and characteristics of Mayer waves. 77 women aged 18-19 were tested under condition close to the state of basal metabolism in follicular phase (I), ovulation (II) and luteal phase (III) of ovarian-menstrual cycle. In phases II and III, the increase of mean and diastolic blood pressure level, in comparison with phase I in the prone position at rest and with psycho-emotional loading, were observed. The distinctions between variation parameters of R-R interval duration, stroke volume and its synchronization in phases II and III, in comparison with phase I, were observed in the prone position at rest, during tilt-test and with psycho-emotional loading. The substantial level of relationship between the power of Mayer waves and mean and diastolic blood pressure, mainly in phase I under conditions of all types, is observed. The maximum peak amplitude of stroke volume spectrogram is associated with pressure levels in the range of 0.04-0.15 Hz (rho from -0.33 to -0.64). The obtained results indicate the possible participation of spontaneous baroreflex sensitivity characteristics in keeping blood pressure level in women.