Mechanisms of blood pressure variability-induced cardiac hypertrophy and dysfunction in mice with impaired baroreflex

Augmented resting beat-to-beat blood pressure variability in patients with chronic kidney disease

PURPOSE

Our aim was to test the hypothesis that patients with chronic kidney disease (CKD) would exhibit augmented resting beat-to-beat blood pressure variability (BPV) that is associated with poor clinical outcomes independent of mean blood pressure (BP). In addition, since the arterial baroreflex plays a critical role in beat-to-beat BP regulation, we further hypothesized that an impaired baroreflex control would be associated with an augmented resting beat-to-beat BPV.

METHODS

In 25 sedentary patients with CKD stages III-IV (62 ± 9 years) and 20 controls (57 ± 10 years), resting beat-to-beat BP (finger photoplethysmography) and heart rate (electrocardiography) were continuously measured for 10 min. We calculated the standard deviation (SD), average real variability (ARV) and other indices of BPV. The sequence technique was used to estimate spontaneous cardiac baroreflex sensitivity.

RESULTS

Compared with controls (CON), the CKD group had significantly increased resting BPV. The ARV (2.2 ± 0.6 versus 1.6 ± 0.5 mmHg, P < 0.001; 1.6 ± 0.7 versus 1.3 ± 0.3 mmHg, P = 0.039; 1.4 ± 0.5 versus 1.0 ± 0.2 mmHg, P < 0.001) of systolic, diastolic and mean BP, respectively, was increased in CKD versus controls. Other traditional measures of variability showed similar results. The cardiac baroreflex sensitivity was lower in CKD compared with controls (CKD: 8.4 ± 4.5 ms/mmHg versus CON: 14.0 ± 8.2 ms/mmHg, P = 0.008). In addition, cardiac baroreflex sensitivity was negatively associated with BPV [systolic blood pressure (SBP) ARV; r = -0.44, P = 0.003].

CONCLUSION

In summary, our data demonstrate that patients with CKD have augmented beat-to-beat BPV and lower cardiac baroreflex sensitivity. BPV and cardiac baroreflex sensitivity were negatively correlated in this cohort. These findings may further our understanding about cardiovascular dysregulation observed in patients with CKD.

Systolic Blood Pressure Time in Target Range and Incident Atrial Fibrillation in Patients With Hypertension: Insights From the SPRINT Trial

BACKGROUND

Systolic blood pressure (SBP) time in target range (TTR) indicates the mean value, exposure time, and variability in blood pressure over time. The prognostic value of SBP TTR for incident atrial fibrillation (AF) in patients with hypertension is unclear.

METHODS

We performed a post hoc analysis of SPRINT (Systolic Blood Pressure Intervention Trial), a randomized controlled trial comparing intensive (<120 mm Hg) and standard (<140 mm Hg) SBP interventions in participants with hypertension. SBP target ranges for intensive and standard arms were defined as 110 to 130 and 120 to 140 mm Hg, respectively. TTR was calculated by linear interpolation method using SBP from months 0 to 3. We used Cox proportional regression models to assess the association of SBP TTR with incident AF.

RESULTS

Among 7939 participants included in this analysis, 187 incident AF cases occurred during follow-up. After multivariable adjustment, a 10% increase in SBP TTR was independently associated with a 7% lower risk of incident AF (hazard ratio, 0.93 [95% CI, 0.88-0.97]; P=0.003). The restricted spline curve depicted a linear and inverse relationship between SBP TTR and incident AF. Sensitivity analyses generated consistent results when calculating TTR over a longer period or setting target range as 110 to 140 mm Hg for the whole population.

CONCLUSIONS

Higher SBP TTR independently predicts a lower risk of incident AF. Efforts to attain SBP within 110 to 140 mm Hg over time may be an effective strategy to prevent AF.

REGISTRATION

URL: https://www.

CLINICALTRIALS

gov; Unique identifier: NCT01206062.

Baroreflex sensitivity in facioscapulohumeral muscular dystrophy

Facioscapulohumeral muscular dystrophy (FSHD), a common form of muscular dystrophy, is caused by a genetic mutation that alters DUX4 gene expression. This mutation contributes to significant skeletal muscle loss. Although it is suggested that cardiac muscle may be spared, people with FSHD have demonstrated autonomic dysregulation. It is unknown if baroreflex function, an important regulator of blood pressure (BP), is impaired in people with FSHD. We examined if baroreflex sensitivity (BRS) is blunted in patients with FSHD. Thirty minutes of resting BP, heart rate, and cardiovagal BRS were measured in 13 patients with FSHD (age: 50 ± 13 years, avg ± SD) and 17 sex- and age-matched controls (age: 47 ± 14 years, p > 0.05). People with FSHD were less active (Activity Metabolic Index, AMI) (FSHD: 24 ± 30; controls: 222 ± 175 kcal/day; p < 0.001) but had a similar body mass index compared with controls (FSHD: 27 ± 4; controls: 27 ± 4 kg/m2 ; p > 0.05). BRSup (hypertensive response), BRSdown (hypotensive response), and total BRS were similar between groups (BRSup: FSHD: 12 ± 8; controls: 12 ± 5 ms/mmHg; BRSdown: FSHD: 10 ± 4; controls: 13 ± 6 ms/mmHg; BRS: FSHD: 14 ± 9; controls: 13 ± 6 ms/mmHg; p > 0.05). Mean arterial pressure was similar between groups (FSHD: 96 ± 7; controls: 91 ± 6mmHg). Individuals with FSHD had an elevated heart rate compared with controls (FSHD: 65 ± 8; controls: 59 ± 8 BPM; p = 0.03), but when co-varied for AMI, this relationship disappeared (p = 0.39). These findings suggest that BRS is not attenuated in people with FSHD, but an elevated heart rate may be due to low physical activity levels, a potential consequence of limited mobility.

Tentonin 3/TMEM150C senses blood pressure changes in the aortic arch

The baroreceptor reflex is a powerful neural feedback that regulates arterial pressure (AP). Mechanosensitive channels transduce pulsatile AP to electrical signals in baroreceptors. Here we show that tentonin 3 (TTN3/TMEM150C), a cation channel activated by mechanical strokes, is essential for detecting AP changes in the aortic arch. TTN3 was expressed in nerve terminals in the aortic arch and nodose ganglion (NG) neurons. Genetic ablation of Ttn3 induced ambient hypertension, tachycardia, AP fluctuations, and impaired baroreflex sensitivity. Chemogenetic silencing or activation of Ttn3+ neurons in the NG resulted in an increase in AP and heart rate, or vice versa. More important, overexpression of Ttn3 in the NG of Ttn3-/- mice reversed the cardiovascular changes observed in Ttn3-/- mice. We conclude that TTN3 is a molecular component contributing to the sensing of dynamic AP changes in baroreceptors.

Smart Baroreceptor Activation Therapy Strikingly Attenuates Blood Pressure Variability in Hypertensive Rats With Impaired Baroreceptor

Increased blood pressure (BP) variability (BPV) is an independent risk factor of cardiovascular events among hypertensive patients. The arterial baroreceptor reflex is a powerful regulator of BP and attenuates BPV via a sympathetic negative feedback control. Conventional baroreceptor activation therapy (cBAT) electrically stimulates the carotid baroreceptors with constant stimulation parameters. While cBAT lowers BP, it does not mount a pressure feedback mechanism. We hypothesized that baroreceptor activation therapy with a pressure feedback system (smart BAT [sBAT]) is able to reduce BPV as well as lower BP. We developed sBAT that electrically stimulated baroreceptors at a frequency proportional to the difference between instantaneous BP and a preset reference pressure, and compared its performance with cBAT. In 14-week-old spontaneously hypertensive rats (n=6), we implanted BP telemeter and created impaired arterial baroreceptors by modified sino-aortic denervation. One week after surgical preparation, we administered sBAT, cBAT or no stimulation (sham) for 15 minutes and compared BP and BPV under freely moving condition. Both cBAT and sBAT significantly lowered mean BP (sham, 141.3±12.8; cBAT, 114.3±11.4; and sBAT, 112.0±7.3 mm Hg). Conventional BAT did not affect BPV at all, while sBAT significantly reduced BPV (sham, 15.4±2.6; cBAT, 16.0±5.2; and sBAT, 9.7±3.3 mm Hg). sBAT also prevented transient excessive BP rise and fall. In conclusion, sBAT was capable of reducing BP and attenuating BPV in hypertensive rats with impaired baroreceptor. sBAT is a novel treatment option for hypertensive patients with increased BPV.

PIEZOs mediate neuronal sensing of blood pressure and the baroreceptor reflex

Activation of stretch-sensitive baroreceptor neurons exerts acute control over heart rate and blood pressure. Although this homeostatic baroreflex has been described for more than 80 years, the molecular identity of baroreceptor mechanosensitivity remains unknown. We discovered that mechanically activated ion channels PIEZO1 and PIEZO2 are together required for baroreception. Genetic ablation of both Piezo1 and Piezo2 in the nodose and petrosal sensory ganglia of mice abolished drug-induced baroreflex and aortic depressor nerve activity. Awake, behaving animals that lack Piezos had labile hypertension and increased blood pressure variability, consistent with phenotypes in baroreceptor-denervated animals and humans with baroreflex failure. Optogenetic activation of Piezo2-positive sensory afferents was sufficient to initiate baroreflex in mice. These findings suggest that PIEZO1 and PIEZO2 are the long-sought baroreceptor mechanosensors critical for acute blood pressure control.

Greater nighttime blood pressure variability is associated with left atrial enlargement in atrial fibrillation patients with preserved ejection fraction

Left atrial enlargement is an independent risk factor for ischemic stroke in patients with atrial fibrillation. Little is known regarding the association between nighttime blood pressure variability and left atrial enlargement in patients with atrial fibrillation and preserved ejection fraction. The study population consisted of 140 consecutive patients with atrial fibrillation (mean age 64 ± 10 years) with preserved ejection fraction (≥50%). Nighttime blood pressure was measured at hourly intervals, using a home blood pressure monitoring device. Nighttime blood pressure variability was expressed as the standard deviation of all readings. Left atrial volume index was measured using the modified Simpson's biplane method with transthoracic echocardiography. Multiple regression analysis indicated that nighttime mean systolic/diastolic blood pressure and its variability remained independently associated with left atrial enlargement after adjustment for age, sex, anti-hypertensive medication class, and left ventricular mass index (P < 0.01). When patients were divided into four groups according to nighttime blood pressure and its variability, the group with higher nighttime blood pressure and its variability had significantly larger left atrial volume than the group with lower nighttime blood pressure and its variability (46.6 ml/m² vs. 35.0 ml/m², P < 0.0001). Higher nighttime blood pressure and its variability are associated with left atrial enlargement. The combination of nighttime blood pressure and its variability has additional predictive value for left atrial enlargement. Intensive intervention for these high-risk patients may avoid or delay progression of left atrial enlargement and reduce the risk of stroke.

A Newly Isolated Carboxymethyl-Glucan (CM-G) Restores Depressed Baroreflex Sensitivity in Renovascular Hypertensive Rats

This study was designed to investigate the effects of a newly synthesized carboxymethyl-glucan (CM-G) on blood pressure (BP), baroreflex sensitivity (BRS) and sympathetic vascular modulation in renovascular hypertensive rats. Male Wistar rats were divided into four groups: Sham (n = 10); 2K1C (subjected to renal artery clipping to induce renovascular hypertension, n = 10); Sham + CM-G (treated with CM-G, n = 7) and 2K1C + CM-G (treated with CM-G, n = 7). The daily treatment with CM-G (40 mg/kg) was performed for 2 weeks. Blood pressure, heart rate (HR), systolic BP variability, baroreflex sensitivity (BRS) and sympathetic vascular tone were evaluated. After six weeks of renal artery clipping, 2K1C rats exhibited arterial hypertension (171 ± 11 vs. 118 ± 4 mmHg, p < 0.05), impaired BRS (-1.30 ± 0.10 vs. -2.59 ± 0.17 bpm.mmHg-1, p < 0.05) and enhanced sympathetic activity as shown by the hexamethonium test (-60 ± 5 vs. -33 ± 2 ΔmmHg, p < 0.05) when compared to sham rats. Oral administration of CM-G in renovascular hypertensive rats reduced hypertension (126 ± 4 vs. 171 ± 11 mmHg, p < 0.05) and improved the BRS (-2.03 ± 0.16 vs. -1.30 ± 0.10 bpm.mmHg^-1, p < 0.05) in 2K1C rats when compared to placebo. Those effects seem to be caused by a reduction in sympathetic activity. The present study revealed for the first time that CM-G treatment reduces arterial hypertension and restores arterial baroreflex sensitivity via a reduction in the sympathetic tone in conscious renovascular hypertensive rats.

Beneficial Effects of Paeoniflorin Enriched Extract on Blood Pressure Variability and Target Organ Damage in Spontaneously Hypertensive Rats

Blood pressure variability (BPV) is associated with the development and progression of severe target organ damage (TOD). This study aims to evaluate the protective effect of paeoniflorin enriched extract from Radix Paeoniae Alba (PG) on BPV and TOD in spontaneously hypertensive rats (SHR). All SHR were orally treated with distilled water, metoprolol (MP, 20 mg/kg), and PG (PG-H, 90 mg/kg or PG-L, 30 mg/kg) for a single time or daily for 7 weeks. The 24-hour dynamic blood pressure was monitored and then calculated BPV including long- and short-term systolic blood pressure variability (SBPV), diastolic blood pressure variability (DBPV), mean blood pressure variability (MBPV), and heart rate variability (HRV) as well as the 24-hour-SBP, 24-hour-DBP, and 24-hour-MBP. The protective effects of PG on TOD were observed by histopathologic and biochemical detection. The results indicated that long- and short-term SBPV, DBPV, MBPV, and HRV as well as 24-hour-SBP, 24-hour-DBP, and 24-hour-MBP showed no significant changes after single-dose administration of PG and significantly decreased after administration with PG for 7 weeks. PG could also markedly improve the damage of aorta, heart, kidney, and brain. This study suggested that PG could notably reduce BPV, stabilize blood pressure, and mitigate TOD in SHR.

Exercise prevents development of autonomic dysregulation and hyperalgesia in a mouse model of chronic muscle pain

Chronic musculoskeletal pain (CMP) conditions, like fibromyalgia, are associated with widespread pain and alterations in autonomic functions. Regular physical activity prevents the development of CMP and can reduce autonomic dysfunction. We tested if there were alterations in autonomic function of sedentary mice with CMP, and whether exercise reduced the autonomic dysfunction and pain induced by CMP. Chronic musculoskeletal pain was induced by 2 intramuscular injections of pH 5.0 in combination with a single fatiguing exercise task. A running wheel was placed into cages so that the mouse had free access to it for either 5 days or 8 weeks (exercise groups) and these animals were compared to sedentary mice without running wheels. Autonomic function and nociceptive withdrawal thresholds of the paw and muscle were assessed before and after induction of CMP in exercised and sedentary mice. In sedentary mice, we show decreased baroreflex sensitivity, increased blood pressure variability, decreased heart rate variability, and decreased withdrawal thresholds of the paw and muscle 24 hours after induction of CMP. There were no sex differences after induction of the CMP in any outcome measure. We further show that both 5 days and 8 weeks of physical activity prevent the development of autonomic dysfunction and decreases in withdrawal threshold induced by CMP. Thus, this study uniquely shows the development of autonomic dysfunction in animals with chronic muscle hyperalgesia, which can be prevented with as little as 5 days of physical activity, and suggest that physical activity may prevent the development of pain and autonomic dysfunction in people with CMP.

Blood Pressure Variability and Prediction of Target Organ Damage in Patients With Uncomplicated Hypertension

BACKGROUND

The average of multiple blood pressure (BP) readings (mean BP) independently predicts target organ damage (TOD). Observational studies have also shown an independent relationship between BP variability (BPV) and TOD, but there is limited longitudinal data. This study aimed to determine the effects of changes in mean BP levels compared with BPV on left ventricular mass index (LVMI) and aortic pulse wave velocity (aPWV).

METHODS

Mean BP levels (research-protocol clinic BP (clinic BP), 24-hour ambulatory BP, and 7-day home BP) and BPV were assessed in 286 patients with uncomplicated hypertension (mean age 64±8 SD years, 53% women) over 12 months. Reading-to-reading BPV (from 24-hour ambulatory BP) and day-to-day BPV (from 7-day home BP) were assessed at baseline and 12 months, and visit-to-visit BPV (clinic BP) was assessed from 5 visits over 12 months. LVMI was measured by 3D echocardiography and aPWV with applanation tonometry.

RESULTS

The strongest predictors of the changes in LVMI (ΔLVMI) were the changes in mean 24-hour systolic BPs (SBPs) (P < 0.02). Similarly, the strongest predictors of the changes in aPWV (ΔaPWV) were the changes in mean 24-hour ambulatory SBPs (P < 0.01) and the changes in mean clinic SBP (P < 0.001). However, none of the changes in BPV were independently associated with ΔLVMI or ΔaPWV (P > 0.05 for all).

CONCLUSIONS

Changes in mean BP levels, but not BPV, were most relevant to changes in TOD in patients with uncomplicated hypertension. Thus, from this point of view, BPV appears to have limited clinical utility in this patient population.

Hypertension, Blood Pressure Variability, and Target Organ Lesion

Hypertensive patients have a higher risk of developing health complications, particularly cardiovascular (CV) events, than individuals with normal blood pressure (BP). Severity of complications depends on the magnitude of BP elevation and other CV risk factors associated with the target organ damage. Therefore, BP control and management of organ damage may contribute to reduce this risk. BP variability (BPV) has been considered a physiological marker of autonomic nervous system control and may be implicated in increased CV risk in hypertension. This review will present some evidence relating BPV and target organ damage in hypertension in clinical and experimental settings.

Molecular Mechanisms Linking Autonomic Dysfunction and Impaired Cardiac Contractility in Critical Illness

OBJECTIVES

Molecular mechanisms linking autonomic dysfunction with poorer clinical outcomes in critical illness remain unclear. We hypothesized that baroreflex dysfunction alone is sufficient to cause cardiac impairment through neurohormonal activation of (nicotinamide adenine dinucleotide phosphate oxidase dependent) oxidative stress resulting in increased expression of G-protein-coupled receptor kinase 2, a key negative regulator of cardiac function.

DESIGN

Laboratory/clinical investigations.

SETTING

University laboratory/medical centers.

SUBJECTS

Adult rats; wild-type/nicotinamide adenine dinucleotide phosphate oxidase subunit-2-deficient mice; elective surgical patients.

INTERVENTIONS

Cardiac performance was assessed by transthoracic echocardiography following experimental baroreflex dysfunction (sino-aortic denervation) in rats and mice. Immunoblots assessed G-protein-coupled receptor recycling proteins expression in rodent cardiomyocytes and patient mononuclear leukocytes. In surgical patients, heart rate recovery after cardiopulmonary exercise testing, time/frequency measures of parasympathetic variables were related to the presence/absence of baroreflex dysfunction (defined by spontaneous baroreflex sensitivity of <6 ms mm Hg). The associations of baroreflex dysfunction with intraoperative cardiac function and outcomes were assessed.

MEASUREMENTS AND MAIN RESULTS

Experimental baroreflex dysfunction in rats and mice resulted in impaired cardiac contractility and upregulation of G-protein-coupled receptor kinase 2 expression. In mice, genetic deficiency of gp91 nicotinamide adenine dinucleotide phosphate oxidase subunit-2 prevented upregulation of G-protein-coupled receptor kinase 2 expression in conditions of baroreflex dysfunction and preserved cardiac function. Baroreflex dysfunction was present in 81 of 249 patients (32.5%) and was characterized by lower parasympathetic tone and increased G-protein-coupled receptor kinase 2 expression in mononuclear leukocytes. Baroreflex dysfunction in patients was also associated with impaired intraoperative cardiac contractility. Critical illness and mortality were more frequent in surgical patients with baroreflex dysfunction (relative risk, 1.66 [95% CI, 1.16-2.39]; p = 0.006).

CONCLUSIONS

Reduced baroreflex sensitivity is associated with nicotinamide adenine dinucleotide phosphate oxidase subunit-2-mediated upregulation of G-protein-coupled receptor kinase 2 expression in cardiomyocytes and impaired cardiac contractility. Autonomic dysfunction predisposes patients to the development of critical illness and increases mortality.

Cardiac Organ Damage and Arterial Stiffness in Autonomic Failure: Comparison With Essential Hypertension

Autonomic failure (AF) is characterized by orthostatic hypotension, supine hypertension, and increased blood pressure (BP) variability. AF patients develop cardiac organ damage, similarly to essential hypertension (EH), and have higher arterial stiffness than healthy controls. Determinants of cardiovascular organ damage in AF are not well known: both BP variability and mean BP values may be involved. The aim of the study was to evaluate cardiac organ damage, arterial stiffness, and central hemodynamics in AF, compared with EH subjects with similar 24-hour BP and a group of healthy controls, and to evaluate determinants of target organ damage in patients with AF. Twenty-seven patients with primary AF were studied (mean age, 65.7±11.2 years) using transthoracic echocardiography, carotid-femoral pulse wave velocity, central hemodynamics, and 24-hour ambulatory BP monitoring. They were compared with 27 EH subjects matched for age, sex, and 24-hour mean BP and with 27 healthy controls. AF and EH had similar left ventricular mass (101.6±33.3 versus 97.7±28.1 g/m(2), P=0.59) and carotid-femoral pulse wave velocity (9.3±1.8 versus 9.2±3.0 m/s, P=0.93); both parameters were significantly lower in healthy controls (P<0.01). Compared with EH, AF patients had higher augmentation index (31.0±7.6% versus 26.1±9.2%, P=0.04) and central BP values. Nighttime systolic BP and 24-hour systolic BP predicted organ damage, independent of BP variability. AF patients develop hypertensive heart disease and increased arterial stiffness, similar to EH with comparable mean BP values. Twenty-four-hour and nighttime systolic BP were determinants of cardiovascular damage, independent of BP variability.

QT variability improves risk stratification in patients with dilated cardiomyopathy

Recently it could be demonstrated that systolic and diastolic blood pressure variability (BPV) as well as segmented Poincare plot analysis (SPPA) contribute to risk stratification in patients suffering from dilated cardiomyopathy (DCM). The aim of this study was to improve the risk stratification applying a multivariate technique including QT variability (QTV). We enrolled and significantly separated 56 low risk and 13 high risk DCM patients by nearly all applied BPV and QTV methods, but not with traditional heart rate variability analysis. The optimum set of two indices calculating the multivariate discriminate analysis (DA) included one BPV index calculated by symbolic dynamics method (DBP(Shannon)) and one index calculated from QTV (QTV(log)) achieving an area under the receiver operating characteristics curve (AUC) of 92%, sensitivity of 92.3% and specificity of 89.3%. Performing only electrocardiogram analysis, the optimum multivariate approach including indices from segmented Poincaré plot analysis and QTV still achieved a remarkable AUC of 88.3%. Increasing the number of indices for multivariate DA up to three, we achieved an AUC of 95.7%, sensitivity of 100% and specificity of 85.7% including one clinical, one BPV and one QTV index. Summarizing, we identified DCM patients with an increased risk of sudden cardiac death applying QTV analysis in a multivariate approach.

Toll-like receptor 9 plays a key role in the autonomic cardiac and baroreflex control of arterial pressure

The crosstalk between the immune and the autonomic nervous system may impact the cardiovascular function. Toll-like receptors are components of the innate immune system and play developmental and physiological roles. Toll-like receptor 9 (TLR9) is involved in the pathogenesis of cardiovascular diseases, such as hypertension and heart failure. Since such diseases are commonly accompanied by autonomic imbalance and lower baroreflex sensitivity, we hypothesized that TLR9 modulates cardiac autonomic and baroreflex control of arterial pressure (AP). Toll-like receptor 9 knockout (TLR9 KO) and wild-type (WT) mice were implanted with catheters into carotid artery and jugular vein and allowed to recover for 3 days. After basal recording of AP, mice received methyl-atropine or propranolol. AP and pulse interval (PI) variability were evaluated in the time and frequency domain (spectral analysis), as well as by multiscale entropy. Spontaneous baroreflex was studied by sequence technique. Behavioral and cardiovascular responses to fear-conditioning stress were also evaluated. AP was similar between groups, but TLR9 KO mice exhibited lower basal heart rate (HR). AP variability was not different, but PI variability was increased in TLR9 KO mice. The total entropy was higher in TLR9 KO mice. Moreover, baroreflex function was found higher in TLR9 KO mice. Atropine-induced tachycardia was increased in TLR9 KO mice, whereas the propranolol-induced bradycardia was similar to WT mice. TLR9 KO mice exhibit increased behavioral and decreased tachycardia responses to fear-conditioning stress. In conclusion, our findings suggest that TLR9 may negatively modulate cardiac vagal tone and baroreflex in mice.

Blood pressure variability and left ventricular mass index in children

Clinical implications of blood pressure variability (BPV) on subclinical organ damage in children are unknown. The authors sought to explore the potential utility of two newly derived BPV indices: weighted standard deviation (wBPSD) and real average variability (ARV), as well as two standard ambulatory blood pressure indices: average 24-hour systolic blood pressure (SBP) and 24-hour SBP load, to identify children at high risk for left ventricular (LV) hypertrophy (LVH). The study group consisted of 67 consecutive children who were referred to our institution for evaluation of suspected hypertension. LV mass was estimated by M-mode echocardiography using Devereux's formula according to the Penn convention and indexed for height(2.7) . We found a statistically significant, positive correlation between 24-hour wBPSD and LV mass index (LVMI) (ρ=0.389; P=.002) and no correlation between 24-hour ARV and LVMI (P>.05). However, partial correlation analysis of 24-hour wBPSD adjusted for body mass index (BMI) and LVMI showed only a weak correlation (ρ=0.3; P=.022). By using multiple linear regression analysis in a model with LVMI as a dependent variable and 24-hour wBPSD, 24-hour ARV, and BMI as independent variables, only BMI showed statistically significant independent positive associations with LVMI (P=.028). Results of our study showed that currently used BPV indices (24-hour wBPSD and 24-hour ARV) are not clinically reliable parameters to identify children at risk for LVH. Apparent contribution of the 24-hour wBPSD parameter to LVMI is negligible and is secondary to its close correlation with BMI (ρ=0.335 P=.009).

Modulated pressure waves in large elastic tubes

Modulational instability is the direct way for the emergence of wave patterns and localized structures in nonlinear systems. We show in this work that it can be explored in the framework of blood flow models. The whole modified Navier-Stokes equations are reduced to a difference-differential amplitude equation. The modulational instability criterion is therefore derived from the latter, and unstable patterns occurrence is discussed on the basis of the nonlinear parameter model of the vessel. It is found that the critical amplitude is an increasing function of α, whereas the region of instability expands. The subsequent modulated pressure waves are obtained through numerical simulations, in agreement with our analytical expectations. Different classes of modulated pressure waves are obtained, and their close relationship with Mayer waves is discussed.

Neural control of arterial pressure variability in the neuromuscularly blocked rat

The baroreflexes stabilize moment-to-moment arterial pressure. Sinoaortic denervation (SAD) of the baroreflexes results in a large increase in arterial pressure variability (APV) across various species. Due to an incomplete understanding of the nonlinear interactions between central and peripheral systems, the major source of APV remains controversial. While some studies suggested that the variability is endogenous to the central nervous system (CNS), others argued that peripheral influences may be the main source. For decades, abnormal cardiovascular variability has been associated with a number of cardiovascular diseases including hypertension, heart failure, and stroke. Delineating mechanisms of the APV is critical for the improvement of current strategies that use APV as a clinical tool for the diagnosis and prognosis of cardiovascular diseases. In this study, with a unique chronic neuromuscularly blocked (NMB) rat preparation that largely constrains peripheral influences, we determined the CNS contribution to the post-SAD APV. First, we confirmed that SAD significantly increased APV in the NMB rat, then demonstrated that post-SAD ganglionic blockade substantially reduced APV, and subsequent intravenous infusions of phenylephrine and epinephrine (in presence of ganglionic blockade) only slightly increased APV. These data suggest that the CNS is an important source, and skeletal activity, thermal challenges or other forms of peripherally generated cardiovascular stress are not required for the post-SAD APV. In addition, we showed that bilateral aortic denervation produced a larger increase in APV than bilateral carotid sinus denervation, suggesting that the aortic baroreflex plays a more dominant role in the control of APV than the carotid sinus.

Effect of baroreceptor denervation on the autonomic control of arterial pressure in conscious mice

This study evaluated the role of arterial baroreceptors in arterial pressure (AP) and pulse interval (PI) regulation in conscious C57BL mice. Male animals, implanted with catheters in a femoral artery and a jugular vein, were submitted to sino-aortic (SAD), aortic (Ao-X) or carotid sinus denervation (Ca-X), 5 days prior to the experiments. After basal recording of AP, the lack of reflex bradycardia elicited by administration of phenylephrine was used to confirm the efficacy of SAD, and cardiac autonomic blockade with methylatropine and propranolol was performed. The AP and PI variability were calculated in the time and frequency domains (spectral analysis/fast Fourier transform) with the spectra quantified in low- (LF; 0.25-1 Hz) and high-frequency bands (HF; 1-5 Hz). Basal AP and AP variability were higher after SAD, Ao-X or Ca-X than in intact mice. Pulse interval was similar among the groups, whereas PI variability was lower after SAD. Atropine elicited a slight tachycardia in control mice but did not change PI after total or partial denervation. The bradycardia caused by propranolol was higher after SAD, Ao-X or Ca-X compared with intact mice. The increase in the variability of AP was accompanied by a marked increase in the LF and HF power of the AP spectra after baroreceptor denervation. The LF and HF power of the PI were reduced by SAD and by Ao-X or Ca-X. Therefore, both sino-aortic and partial baroreceptor denervation in mice elicits hypertension and a remarkable increase in AP variability and cardiac sympathetic tonus. Spectral analysis showed an important contribution of the baroreflex in the power of LF oscillations of the PI spectra. Both sets of baroreceptors seem to be equally important in the autonomic regulation of the cardiovascular system in mice.

Scavenging superoxide selectively in mouse forebrain is associated with improved cardiac function and survival following myocardial infarction

Dysregulation in central nervous system (CNS) signaling that results in chronic sympathetic hyperactivity is now recognized to play a critical role in the pathogenesis of heart failure (HF) following myocardial infarction (MI). We recently demonstrated that adenovirus-mediated gene transfer of cytoplasmic superoxide dismutase (Ad-Cu/ZnSOD) to forebrain circumventricular organs, unique sensory structures that lack a blood-brain barrier and link peripheral blood-borne signals to central nervous system cardiovascular circuits, inhibits both the MI-induced activation of these central signaling pathways and the accompanying sympathoexcitation. Here, we tested the hypothesis that this forebrain-targeted reduction in oxidative stress translates into amelioration of the post-MI decline in myocardial function and increase in mortality. Adult C57BL/6 mice underwent left coronary artery ligation or sham surgery along with forebrain-targeted gene transfer of Ad-Cu/ZnSOD or a control vector. The results demonstrate marked MI-induced increases in superoxide radical formation in one of these forebrain regions, the subfornical organ (SFO). Ad-Cu/ZnSOD targeted to this region abolished the increased superoxide levels and led to significantly improved myocardial function compared with control vector-treated mice. This was accompanied by diminished levels of cardiomyocyte apoptosis in the Ad-Cu/ZnSOD but not the control vector-treated group. These effects of superoxide scavenging with Ad-Cu/ZnSOD in the forebrain paralleled increased post-MI survival rates compared with controls. This suggests that oxidative stress in the SFO plays a critical role in the deterioration of cardiac function following MI and underscores the promise of CNS-targeted antioxidant therapy for the treatment of MI-induced HF.

Changes in hemodynamic and neurohumoral control cause cardiac damage in one-kidney, one-clip hypertensive mice

Sympathovagal balance and baroreflex control of heart rate (HR) were evaluated during the development (1 and 4 wk) of one-kidney, one-clip (1K1C) hypertension in conscious mice. The development of cardiac hypertrophy and fibrosis was also examined. Overall variability of systolic arterial pressure (AP) and HR in the time domain and baroreflex sensitivity were calculated from basal recordings. Methyl atropine and propranolol allowed the evaluation of the sympathovagal balance to the heart and the intrinsic HR. Staining of renal ANG II in the kidney and plasma renin activity (PRA) were also evaluated. One and four weeks after clipping, the mice were hypertensive and tachycardic, and they exhibited elevated sympathetic and reduced vagal tone. The intrinsic HR was elevated only 1 wk after clipping. Systolic AP variability was elevated, while HR variability and baroreflex sensitivity were reduced 1 and 4 wk after clipping. Renal ANG II staining and PRA were elevated only 1 wk after clipping. Concentric cardiac hypertrophy was observed at 1 and 4 wk, while cardiac fibrosis was observed only at 4 wk after clipping. In conclusion, these data further support previous findings in the literature and provide new features of neurohumoral changes during the development of 1K1C hypertension in mice. In addition, the 1K1C hypertensive model in mice can be an important tool for studies evaluating the role of specific genes relating to dependent and nondependent ANG II hypertension in transgenic mice.

Autonomic neuropathy precedes cardiovascular dysfunction in rats with diabetes

BACKGROUND

Our team previously demonstrated arterial stiffening and cardiac hypertrophy in type 2 diabetic rats at 8 but not 4 weeks after being administered streptozotocin (STZ) and nicotinamide (NA). The present study focused on investigating the effects of type 2 diabetes on cardiac autonomic nerve function in the STZ- and NA-treated animals, using modern spectral estimation technique.

DESIGN

An autoregressive process was performed to each detrended signal of heart rate and systolic blood pressure measured in the 4- and 8-week STZ-NA rats with anaesthesia. The power of low-frequency and high-frequency oscillations was automatically quantified with each spectral peak by computing the residuals. The closed-loop baroreflex gain was estimated using the square root of the ratio between heart rate and systolic blood pressure powers in the low-frequency band.

RESULTS

Compared with the age-matched controls, both the 4- and 8-week STZ-NA diabetic rats had significantly decreased low-frequency oscillations of heart rate but not systolic blood pressure variability, showing a decline in baroreflex gain (0.451 +/- 0.060 and 0.484 +/- 0.056 vs. 1.196 +/- 0.064 ms mmHg(-1), P < 0.05). On the other hand, the low frequency-high frequency power ratio of the heart period was also diminished in the two diabetic groups, indicating a shift in sympatho-vagal balance of the heart control (0.472 +/- 0.109 and 0.504 +/- 0.090 vs. 1.857 +/- 0.336, P < 0.05).

CONCLUSIONS

The cardiac autonomic dysfunction in the absence of any significant changes in vascular dynamics, 4 but not 8 weeks after induction of type 2 diabetes, suggests that the diabetic autonomic neuropathy may precede arterial stiffening and cardiac hypertrophy in the STZ- and NA-treated rats.

Cardiovascular responses to peripheral chemoreflex activation and comparison of different methods to evaluate baroreflex gain in conscious mice using telemetry

Peripheral chemoreceptors located in the carotid bodies are the primary sensors of systemic hypoxia. Although the pattern of responses elicited by peripheral chemoreceptor activation is well established in rats, lambs, and rabbits, the cardiovascular responses to peripheral chemoreflex activation in conscious mice have not been delineated. Here we report that stimulation of peripheral chemoreceptors by potassium cyanide (KCN) in conscious mice elicits a unique biphasic response in blood pressure that is characterized by an initial and robust rise followed by a decrease in blood pressure, which is accompanied by a marked reduction in heart rate. The depressor and bradycardic responses to KCN were abolished by muscarinic receptor blockade with atropine, and the pressor response was abolished by alpha-adrenergic receptor blockade with prazosin, suggesting that vagal and sympathetic drive to the heart and sympathetic drive to the vasculature mediate these cardiovascular responses. These studies characterized the chemoreflex in conscious mice and established the reliability of using them for studying hypoxia-related diseases such as obstructive sleep apnea. In another series of experiments, two methods for analyzing baroreflex sensitivity were compared: the classical pharmacological approach using phenylephrine and sodium nitroprusside (i.e., the Oxford technique) or the sequence method for analyzing spontaneous baroreflex activity. Our findings indicate that both methods are reliable, and the sequence method certainly has its benefits as a predictive tool in the context of long-term noninvasive studies using telemetry. However, for absolute determination of baroreflex function, analysis of spontaneous baroreflex activity should be complemented by the classical pharmacological method.

Evaluation of baroreflex control of heart rate in renovascular hypertensive mice

The objective of the present study was to evaluate the baroreflex and the autonomic control of heart rate (HR) in renovascular hypertensive mice. Experiments were carried out in conscious C57BL/6 (n = 16) mice 28 days after a 2-kidney 1-clip procedure (2K1C mice) or a sham operation (sham mice). Baroreflex sensitivity was evaluated by measuring changes in heart rate (HR) in response to increases or decreases in mean arterial pressure (MAP) induced by phenylephrine or sodium nitroprusside. Cardiac autonomic tone was determined by use of atropine and atenolol. Basal HR and MAP were significantly higher in 2K1C mice than in sham mice. The reflex tachycardia induced by decreases in MAP was greatly attenuated in 2K1C mice compared with sham mice. Consequently, the baroreflex sensitivity was greatly decreased (2.2 +/- 0.4 vs. 4.4 +/- 0.3 beats x min(-1) x mmHg(-1)) in hypertensive mice compared with sham mice. The reflex bradycardia induced by increases in MAP and the baroreflex sensitivity were similar in both groups. Evaluation of autonomic control of HR showed an increased sympathetic tone and a tendency to a decreased vagal tone in 2K1C mice compared with that in sham mice. 2K1C hypertension in mice is accompanied by resting tachycardia, increased predominance of the cardiac sympathetic tone over the cardiac vagal tone, and impairment of baroreflex sensitivity.

Applicability of recent methods used to estimate spontaneous baroreflex sensitivity to resting mice

Short-term blood pressure (BP) variability is limited by the arterial baroreflex. Methods for measuring the spontaneous baroreflex sensitivity (BRS) aim to quantify the gain of the transfer function between BP and pulse interval (PI) or the slope of the linear relationship between parallel BP and PI changes. These frequency-domain (spectral) and time-domain (sequence) techniques were tested in conscious mice equipped with telemetric devices. The autonomic relevance of these indexes was evaluated using pharmacological blockades. The significant changes of the spectral bandwidths resulting from the autonomic blockades were used to identify the low-frequency (LF) and high-frequency (HF) zones of interest. The LF gain was 1.45 +/- 0.14 ms/mmHg, with a PI delay of 0.5 s. For the HF gain, the average values were 2.0 +/- 0.19 ms/mmHg, with a null phase. LF and HF bands were markedly affected by atropine. On the same 51.2-s segments used for cross-spectral analysis, an average number of 26.4 +/- 2.2 slopes were detected, and the average slope in resting mice was 4.4 +/- 0.5 ms/mmHg. Atropine significantly reduced the slopes of the sequence method. BRS measurements obtained using the sequence technique were highly correlated to the spectral estimates. This study demonstrates the applicability of the recent methods used to estimate spontaneous BRS in mice. There was a vagal predominance in the baroreflex control of heart rate in conscious mice in the present conditions.

Altered melusin expression in the hearts of aortic stenosis patients

BACKGROUND

The role of melusin, a necessary component in pressure-induced left-ventricular hypertrophy (LVH) in mice, has not yet been determined in human cardiac hypertrophy. We analyzed for the first time the expression and regional distribution of melusin in human LVH due to aortic stenosis (AS) and determined AKT phosphorylation as a potential downstream effector of melusin signalling.

METHODS

Regional distribution of melusin was evaluated in four normal hearts. Melusin staining, gene expression and protein content were assessed in biopsies from normal and diseased hearts and melusin gene expression was correlated with LV functional changes. The pAKT/AKT ratio was determined in parallel and correlated with melusin protein content.

RESULTS

In normal hearts, melusin was found in the myocytes with a uniform regional distribution. Melusin staining, mRNA and protein were significantly decreased in human AS hearts. The reduction in melusin mRNA was significantly correlated with LVEF, LVEDD and LVESD. pAKT/AKT ratio was significantly decreased in human AS and was correlated with melusin content.

CONCLUSION

Reduction in melusin expression parallels the functional cardiac impairment in human AS. The simultaneous decrease of melusin and AKT phosphorylation suggests a connection between the loss of melusin and the decrease in systolic function.

Elastin-insufficient mice show normal cardiovascular remodeling in 2K1C hypertension despite higher baseline pressure and unique cardiovascular architecture

Mice heterozygous for the elastin gene (ELN(+/-)) show unique cardiovascular properties, including increased blood pressure and smaller, thinner arteries with an increased number of lamellar units. Some of these properties are also observed in humans with supravalvular aortic stenosis, a disease caused by functional heterozygosity of the elastin gene. The arterial geometry in ELN(+/-) mice is contrary to the increased thickness that would be expected in an animal demonstrating hypertensive remodeling. To determine whether this is due to a decreased capability for cardiovascular remodeling or to a novel adaptation of the ELN(+/-) cardiovascular system, we increased blood pressure in adult ELN(+/+) and ELN(+/-) mice using the two-kidney, one-clip Goldblatt model of hypertension. Successfully clipped mice have a systolic pressure increase of at least 15 mmHg over sham-operated animals. ELN(+/+) and ELN(+/-)-clipped mice show significant increases over sham-operated mice in cardiac weight, arterial thickness, and arterial cross-sectional area with no changes in lamellar number. There are no significant differences in most mechanical properties with clipping in either genotype. These results indicate that ELN(+/+) and ELN(+/-) hearts and arteries remodel similarly in response to adult induced hypertension. Therefore, the cardiovascular properties of ELN(+/-) mice are likely due to developmental remodeling in response to altered hemodynamics and reduced elastin levels.

Very low-frequency blood pressure variability depends on voltage-gated L-type Ca2+ channels in conscious rats

The mechanisms generating high- frequency (HF) and low-frequency (LF) blood pressure variability (BPV) are reasonably well understood. However, little is known about the origin of very low-frequency (VLF) BPV. We tested the hypothesis that VLF BPV is generated by L-type Ca2+ channel-dependent mechanisms. In conscious rats, arterial blood pressure was recorded during control conditions ( n = 8) and ganglionic blockade ( n = 7) while increasing doses (0.01–5.0 mg·100 μl−1·h−1) of the L-type Ca2+ channel blocker nifedipine were infused intravenously. VLF (0.02–0.2 Hz), LF (0.2–0.6 Hz), and HF (0.6–3.0 Hz) BPV were assessed by spectral analysis of systolic blood pressure. During control conditions, nifedipine caused dose-dependent declines in VLF and LF BPV, whereas HF BPV was not affected. At the highest dose of nifedipine, VLF BPV was reduced by 86% compared with baseline, indicating that VLF BPV is largely mediated by L-type Ca2+ channel-dependent mechanisms. VLF BPV appeared to be relatively more dependent on L-type Ca2+ channels than LF BPV because lower doses of nifedipine were required to significantly reduce VLF BPV than to reduce LF BPV. Ganglionic blockade markedly reduced VLF and LF BPV and abolished the nifedipine-induced dose-dependent declines in VLF and LF BPV, suggesting that VLF and LF BPV require sympathetic activity to be evident. In conclusion, VLF BPV is largely mediated by L-type Ca2+ channel-dependent mechanisms. We speculate that VLF BPV is generated by myogenic vascular responses to spontaneously occurring perturbations of blood pressure. Other factors, such as sympathetic nervous system activity, may elicit a permissive effect on VLF BPV by increasing vascular myogenic responsiveness.

Treatment of hypertension based on measurement of blood pressure variability: lessons from animal studies

PURPOSE OF REVIEW

Blood pressure variability, a quantitative index for the spontaneous variation in blood pressure, has been proposed as a risk factor for end-organ damage and to determine the efficacy of hypertension treatment.

RECENT FINDINGS

Animal studies indicate that blood pressure variability is as important as blood pressure level in determining end-organ damage, and that high blood pressure variability is associated with end-organ damage, including myocardial lesions, aortic hypertrophy, vascular remodeling and renal damage. Although the organ damage induced by high blood pressure variability was similar to that induced by hypertension, comparative studies in sinoaortic-denervated and spontaneously hypertensive rats revealed that aortic hypertrophy is a sensitive index of high blood pressure variability, and left ventricular hypertrophy is a sensitive index of high blood pressure level. The possible mechanisms for high blood pressure variability-induced end-organ damage include: direct endothelial lesions, renin-angiotensin system activation, inflammation initiation and cardiomyocyte apoptosis augmentation. Blood pressure variability reduction contributes importantly to the organ-protective effect of some antihypertensive drugs.

SUMMARY

Although animal studies suggest some advantages in blood pressure variability measurements, clinical trials are necessary before the widespread use of blood pressure variability as a predictor of hypertensive organ damage and a new strategy for the treatment of hypertension.