Mechanisms of resetting of arterial baroreceptors: an overview

Impaired Cardiovagal Activity as a Link Between Hyperglycemia and Arterial Stiffness in Adults With Type 2 Diabetes Mellitus Among an Eastern Indian Population: A Cross-sectional Study

OBJECTIVES

Cardiac autonomic neuropathy (CAN) is one of the most common yet overlooked complications of type 2 diabetes mellitus (T2DM). Individuals with T2DM with CAN have a 5-fold higher rate of cardiovascular morbidity and mortality. The presence of CAN in T2DM could potentially lead to arterial stiffness. However, only sparse data are available suggesting any association between autonomic dysfunction and arterial stiffness in T2DM.

METHODS

We recruited 80 people with T2DM and 74 healthy controls for our study. Heart rate variability (HRV) testing was performed to assess autonomic function. Assessment of arterial stiffness was done by measuring the brachial pulse wave velocity (baPWV) and augmentation index (AI).

RESULTS

The time-domain parameters were significantly decreased (p<0.001) and frequency-domain parameters, such as total power and high-frequency band expressed as a normalized unit, were found to be significantly reduced in people with T2DM (p<0.001). Both baPWV and AI were significantly higher in people with T2DM compared with healthy controls (p<0.001). We observed a moderate correlation between standard deviation of normal to normal interval (SDNN) and baPWV (r=-0.437, p=0.002) and AI (r=-0.403, p=0.002). A multiple linear regression model showed an association between SDNN and arterial stiffness parameters, such as baPWV and AI, which were statistically significant (p<0.05) in a fully adjusted model that included the conventional risk factors for atherosclerosis.

CONCLUSIONS

Impaired cardiovagal activity is an independent risk factor for the development of arterial stiffness. Incorporation of HRV testing into the diabetes management protocol would have potential benefits for identifying individuals at high risk of developing cardiovascular events. Hence, preventive measures can be taken as early as possible to improve patient outcomes.

Key challenges in exploring the rat as a preclinical neurostimulation model for aortic baroreflex modulation in hypertension

Electrode-based electrophysiological interfaces with peripheral nerves have come a long way since the 1960s, with several neurostimulation applications witnessing widespread clinical implementation since then. In resistant hypertension, previous clinical trials have shown that "carotid" baroreflex stimulation using device-based baroreflex activation therapy (BAT) can effectively lower blood pressure (BP). However, device-based "aortic" baroreflex stimulation remains untouched for clinical translation. The rat is a remarkable animal model that facilitates exploration of mechanisms pertaining to the baroreceptor reflex and preclinical development of novel therapeutic strategies for BP modulation and hypertension treatment. Specifically, the aortic depressor nerve (ADN) in rats carries a relatively pure population of barosensitive afferent neurons, which enable selective investigation of the aortic baroreflex function. In a rat model of essential hypertension, the spontaneously hypertensive rat (SHR), we have recently investigated the aortic baroreceptor afferents as an alternate target for BP modulation, and showed that "low intensity" stimulation is able to evoke clinically meaningful reductions in BP. Deriving high quality short-term and long-term data on aortic baroreflex modulation in rats is currently hampered by a number of unresolved experimental challenges, including anatomical variations across rats which complicates identification of the ADN, the use of unrefined neurostimulation tools or paradigms, and issues arising from anesthetized and conscious surgical preparations. With the goal of refining existing experimental protocols designed for preclinical investigation of the baroreflex, this review seeks to outline current challenges hindering further progress in aortic baroreflex modulation studies in rats and present some practical considerations and recently emerging ideas to overcome them. Aortic baroreflex modulation.

Autonomic Regulation of Inflammation in Conscious Animals

Bioelectronic medicine is a novel field in modern medicine based on the specific neuronal stimulation to control organ function, cardiovascular, and immune homeostasis. However, most studies addressing neuromodulation of the immune system have been conducted on anesthetized animals, which can affect the nervous system and neuromodulation. Here, we review recent studies involving conscious experimental rodents (rats and mice) to better understand the functional organization of neural control of immune homeostasis. We highlight typical experimental models of cardiovascular regulation, such as electrical activation of the aortic depressor nerve or the carotid sinus nerve, bilateral carotid occlusion, the Bezold-Jarisch reflex, and intravenous administration of the bacterial endotoxin lipopolysaccharide. These models have been used to investigate the relationship between neuromodulation of the cardiovascular and immune systems in conscious rodents (rats and mice). These studies provide critical information about the neuromodulation of the immune system, particularly the role of the autonomic nervous system, i.e., the sympathetic and parasympathetic branches acting both centrally (hypothalamus, nucleus ambiguus, nucleus tractus solitarius, caudal ventrolateral medulla, and rostral ventrolateral medulla), and peripherally (particularly spleen and adrenal medulla). Overall, the studies in conscious experimental models have certainly highlighted to the reader how the methodological approaches used to investigate cardiovascular reflexes in conscious rodents (rats and mice) can also be valuable for investigating the neural mechanisms involved in inflammatory responses. The reviewed studies have clinical implications for future therapeutic approaches of bioelectronic modulation of the nervous system to control organ function and physiological homeostasis in conscious physiology.

Low intensity stimulation of aortic baroreceptor afferent fibers as a potential therapeutic alternative for hypertension treatment

Carotid baroreceptor stimulation has been clinically explored for antihypertensive benefits, but neuromodulation of aortic baroreceptor afferents remains unexplored for potential translation into the clinic. Published studies have used supramaximal stimulations, which are unphysiological and energy inefficient. The objective of the present study was to identify optimal low-charge nerve stimulation parameters that would provide a clinically-relevant (20–30 mmHg) decrease in mean arterial pressure (MAP) in anesthetized spontaneously hypertensive rats. Stimulations of 20 s were delivered to the left aortic depressor nerve (ADN) of these rats using low ranges of pulse amplitudes (≤ 0.6 mA), widths (≤ 0.5 ms) and frequencies (≤ 5 Hz). We also assessed the effects of continuous (20 s) versus intermittent (5 s ON/3 s OFF and 5 s ON/3 s OFF for 20 s) stimulation on MAP, heart rate (HR), mesenteric (MVR) and femoral (FVR) vascular resistance using low (5 Hz) and high (15 Hz) frequencies. Lower pulse amplitudes (0.2 mA) produced 9 ± 2 to 18 ± 2 mmHg decreases in MAP. Higher pulse amplitudes (0.4 mA) produced a median MAP reduction of 28 ± 4 mmHg at 0.2 ms and 5 Hz, with no added benefit seen above 0.4 mA. Continuous and intermittent low frequency stimulation at 0.4 mA and 0.2 ms produced similar sustained decreases in MAP, HR, MVR and FVR. Continuous high frequency stimulation at 0.4 mA and 0.2 ms produced larger reductions in MAP, HR, MVR and FVR compared with all low frequency and/or intermittent high frequency stimulations. We conclude from these findings that “low intensity intermittent” electrical stimulation is an effective alternate way for neuromodulation of the aortic baroreceptor afferents and to evoke a required restoration of MAP levels in spontaneously hypertensive rats. This approach enables low energy consumption and markedly lowers the excessive decreases in MAP and hemodynamic disturbances elicited by continuous high-charge injection protocols.

The Bezold-Jarisch Reflex and The Inflammatory Response Modulation in Unanesthetized Endotoxemic Rats

Evidence indicates that the activation of the parasympathetic branch of the autonomic nervous system may be effective in treating inflammatory diseases. Previously, we have described that baroreflex activation displays anti-inflammatory properties. Analogous to the baroreflex, the Bezold-Jarisch reflex also promotes parasympathetic activation with simultaneous inhibition of the sympathetic system. Thus, the present study aimed to evaluate whether the activation of the Bezold-Jarisch reflex would also have the ability to reduce inflammation in unanesthetized rats. We used lipopolysaccharide (LPS) injection (5mg/kg, i.p.) to induce systemic inflammation in male Wistar Hannover rats and phenylbiguanide (PBG) administration (5μg/kg, i.v.) to activate the Bezold-Jarisch reflex. Spleen, heart, hypothalamus, and blood samples were collected to determine the levels of cytokines. Compared to baseline, PBG reduced the arterial pressure (115±2 vs. 88±5mmHg) and heart rate (380±7 vs. 114±26bpm), immediately after its administration, confirming the activation of the parasympathetic system and inhibition of the sympathetic system. From the immunological point of view, the activation of the Bezold-Jarisch reflex decreased the plasma levels of TNF (LPS: 775±209 vs. PBG + LPS: 248±30pg/ml) and IL-6 levels in the spleen (LPS: 39±6 vs. PBG + LPS: 24±4pg/mg of tissue). However, it did not change the other cytokines in the plasma or the other tissues evaluated. These findings confirm that the activation of the Bezold-Jarisch reflex can modulate inflammation and support the understanding that the cardiovascular reflexes regulate the immune system.

Physiological Sympathetic Activation Reduces Systemic Inflammation: Role of Baroreflex and Chemoreflex

Baroreflex and chemoreflex act through the autonomic nervous system, which is involved with the neural regulation of inflammation. The present study reports the effects of reflex physiological sympathetic activation in endotoxemic rats using bilateral carotid occlusion (BCO), a physiological approach involving the baroreflex and chemoreflex mechanisms and the influence of the baroreceptors and peripheral chemoreceptors in the cardiovascular and systemic inflammatory responses. After lipopolysaccharide (LPS) administration, the arterial pressure was recorded during 360 min in unanesthetized rats, and serial blood samples were collected to analyze the plasma cytokine levels. BCO elicited the reflex activation of the sympathetic nervous system, providing the following outcomes: (I) increased the power of the low-frequency band in the spectrum of the systolic arterial pressure during the BCO period; (II) reduced the levels of pro-inflammatory cytokines in plasma, including the tumor necrosis factor (TNF) and the interleukin (IL)-1β; (III) increased the plasma levels of anti-inflammatory cytokine IL-10, 90 min after LPS administration. Moreover, selective baroreceptor or chemoreceptor denervation deactivated mechanosensitive and chemical sensors, respectively, and decreased the release of the LPS-induced cytokine but did not alter the BCO modulatory effects. These results show, for the first time, that physiological reflex activation of the sympathetic circuit decreases the inflammatory response in endotoxemic rats and suggest a novel function for the baroreceptors as immunosensors during the systemic inflammation.

Carotid sinus nerve stimulation attenuates alveolar bone loss and inflammation in experimental periodontitis

Baroreceptor and chemoreceptor reflexes modulate inflammatory responses. However, whether these reflexes attenuate periodontal diseases has been poorly examined. Thus, the present study determined the effects of electrical activation of the carotid sinus nerve (CSN) in rats with periodontitis. We hypothesized that activation of the baro and chemoreflexes attenuates alveolar bone loss and the associated inflammatory processes. Electrodes were implanted around the CSN, and bilateral ligation of the first mandibular molar was performed to, respectively, stimulate the CNS and induce periodontitis. The CSN was stimulated daily for 10 min, during nine days, in unanesthetized animals. On the eighth day, a catheter was inserted into the left femoral artery and, in the next day, the arterial pressure was recorded. Effectiveness of the CNS electrical stimulation was confirmed by hypotensive responses, which was followed by the collection of a blood sample, gingival tissue, and jaw. Long-term (9 days) electrical stimulation of the CSN attenuated bone loss and the histological damage around the first molar. In addition, the CSN stimulation also reduced the gingival and plasma pro-inflammatory cytokines induced by periodontitis. Thus, CSN stimulation has a protective effect on the development of periodontal disease mitigating alveolar bone loss and inflammatory processes.

Laterality Influences Central Integration of Baroreceptor Afferent Input in Male and Female Sprague Dawley Rats

We explored the effects of baroreceptor afferents laterality and sexual dimorphism on the expression of cardiovascular reflex responses to baroreflex activation in Sprague Dawley (SD) rats. Under urethane anesthesia, rats of either sex (total n = 18) were instrumented for left, right and bilateral aortic depressor nerve (ADN) stimulation (1–40 Hz, 0.2 ms, 0.4 mA for 20 s) and measurement of mean arterial pressure (MAP), heart rate (HR) and mesenteric (MVR) and femoral (FVR) vascular resistance. Female rats were matched for the diestrus phase of the estrus cycle. Left, right and bilateral ADN stimulation evoked frequency-dependent drops in MAP, HR, and MVR, and increases in FVR. Irrespective of sex, left and bilateral ADN stimulation as compared to right-sided stimulation mediated greater reflex reductions in MAP, HR, and MVR but not in FVR. In males, reflex bradycardic responses were greater in response to bilateral stimulation relative to both left- and right-sided stimulation. In females, left ADN stimulation evoked the largest increase in FVR. Left and bilateral ADN stimulations evoked greater reductions in MAP and MVR while left-sided stimulation produced larger increases in FVR in females compared with males. All other reflex responses to ADN stimulation were relatively comparable between males and females. These results show a differential baroreflex processing of afferent neurotransmission promoted by left versus right baroreceptor afferent inputs and sexual dimorphism in the expression of baroreflex responses in rats of either sex. Collectively, these data add to our understanding of physiological mechanisms pertaining to baroreflex control in both males and females.

Sleep-disordered breathing does not impact maternal outcomes in women with hypertensive disorders of pregnancy

Objective

Sleep-disordered breathing (SDB) is characterised by intermittent hypoxemia, sympathetic activation and widespread endothelial dysfunction, sharing pathophysiologic features with the hypertensive disorders of pregnancy. We sought to determine whether coexisting SDB would adversely impact the outcomes of women with gestational hypertension (GH) and preeclampsia (PE), and healthy matched controls.

Study design

Women diagnosed with GH or PE along with BMI- and gestation-matched normotensive controls underwent polysomnography in late pregnancy to establish the presence or absence of SDB (RDI ≥ 5). Clinical outcomes of hypertensive disease severity were compared between groups, and venous blood samples were taken in the third trimester and at delivery to examine for any impact of SDB on the anti-angiogenic markers of PE.

Results

Data was available for 17 women with PE, 24 women with GH and 44 controls. SDB was diagnosed in 41% of the PE group, 63% of the GH group and 39% of the control group. Women with PE and co-existing SDB did not have worse outcomes in terms of gestation at diagnosis of PE (SDB = 29.1 (25.9, 32.1) weeks vs. no SDB = 32.0 (29.0, 33.9), p = n.s.) and days between diagnosis of PE and delivery (SDB = 20.0 (4.0, 35.0) days vs. no SDB = 10.5 (9.0, 14.0), p = n.s.). There were also no differences in severity of hypertension, antihypertensive treatment and biochemical, haematological and anti-angiogenic markers of PE between SDB and no SDB groups. Similar results were observed among women with GH. Healthy control women with SDB were no more likely to develop a hypertensive disorder of pregnancy in the later stages of pregnancy (SDB = 5.9% vs. no SDB = 7.4%, p = n.s.). Increasing the threshold for diagnosis of SDB to RDI ≥ 15 did not unmask a worse prognosis.

Conclusion

The presence of SDB during pregnancy did not worsen the disease course of GH or PE, and was not associated with high blood pressure or anti-angiogenic markers of hypertensive disease amongst healthy pregnant women. Given the numerous reports of the relationship between SDB and diagnosis of hypertensive disorders of pregnancy, it appears more work is required to distinguish causal, versus confounding, pathways.

Selective denervation of the aortic and carotid baroreceptors in rats

NEW FINDINGS

What is the central question of this study? The traditional surgical approach for sino-aortic denervation in rats leads to simultaneous carotid baroreceptor and chemoreceptor deactivation, which does not permit their individual study in different situations. What is the main finding and its importance? We have described a new surgical approach capable of selective denervation of the arterial (aortic and carotid) baroreceptors, keeping the carotid bodies (chemoreceptors) intact. It is understood that this technique might be a useful tool for investigating the relative role of the baro- and chemoreceptors in several physiological and pathophysiological conditions.

ABSTRACT

Studies have demonstrated that the traditional surgical approach for sino-aortic denervation in rats leads to simultaneous carotid baroreceptor and chemoreceptor deactivation. The present study reports a new surgical approach to denervate the aortic and the carotid baroreceptors selectively, keeping the carotid bodies (peripheral chemoreceptors) intact. Wistar rats were subjected to specific aortic and carotid baroreceptor denervation (BAROS-X) or sham surgery (SHAM). Baroreflex activation was achieved by i.v. administration of phenylephrine, whereas peripheral chemoreflex activation was produced by i.v. administration of potassium cyanide. The SHAM and BAROS-X rats displayed significant hypertensive responses to phenylephrine administration. However, the reflex bradycardia following the hypertensive response caused by phenylephrine was remarkable in SHAM, but not significant in the BAROS-X animals, confirming the efficacy of the surgical procedure to abolish the baroreflex. In addition, the baroreflex activation elicited by phenylephrine increased carotid sinus nerve activity only in SHAM, but not in the BAROS-X animals, providing support to the notion that the baroreceptor afferents were absent. Instead, the classical peripheral chemoreflex hypertensive and bradycardic responses to potassium cyanide were similar in both groups, suggesting that the carotid body chemoreceptors were preserved after BAROS-X. In summary, we describe a new surgical approach in which only the baroreceptors are eliminated, while the carotid chemoreceptors are preserved. Therefore, it is understood that this procedure is potentially a useful tool for examining the relative roles of the arterial baroreceptors versus the chemoreceptors in several pathophysiological conditions, for instance, arterial hypertension and heart failure.

Even weak vasoconstriction from rilmenidine can be unmasked in vivo by opening the baroreflex feedback loop

AIMS

Rilmenidine and moxonidine are centrally acting antihypertensive agents that are more selective for I₁-imidazoline receptors than for α₂-adrenergic receptors. Moxonidine previously showed a peripheral vasoconstrictive effect stronger than generally recognized, which counteracted an arterial pressure (AP) lowering effect resulting from central sympathoinhibition. We tested whether rilmenidine also showed a significant vasoconstrictive effect that could attenuate its AP lowering effect.

MAIN METHODS

Efferent sympathetic nerve activity (SNA) and AP responses to changes in carotid sinus pressure were compared in nine anesthetized Wistar-Kyoto rats before and after low, medium, and high doses (40, 100, and 250 μg/kg, respectively) of intravenous rilmenidine.

KEY FINDINGS

High-dose rilmenidine narrowed the range of the SNA response (from 89.6 ± 2.9% to 50.4 ± 7.9%, P < 0.001) and reduced the lower asymptote of SNA (from 13.5 ± 3.0% to 2.7 ± 1.5%, P < 0.001). High-dose rilmenidine significantly increased the intercept (from 57.1 ± 3.8 to 78.2 ± 2.7 mm Hg, P < 0.001) but reduced the slope (from 0.82 ± 0.08 to 0.51 ± 0.07 mm Hg/%, P < 0.001) of the SNA-AP relationship. The reduction in the operating-point AP induced by high-dose rilmenidine did not significantly differ based on whether the peripheral effect was considered (-19.8 ± 2.2 vs. -26.4 ± 5.3 mm Hg, not significant).

SIGNIFICANCE

Rilmenidine increased AP in the absence of SNA, which suggests a peripheral vasoconstrictive effect; however, the vasoconstrictive effect was weak and did not significantly counteract the AP-lowering effect through central sympathoinhibition.

Baroreflex stimulation attenuates central but not peripheral inflammation in conscious endotoxemic rats

We previously reported that activation of the baroreflex, a critical physiological mechanism controlling cardiovascular homeostasis, through electrical stimulation of the aortic depressor nerve attenuates joint inflammation in experimental arthritis. However, it is unknown whether baroreflex activation can control systemic inflammation. Here, we investigate whether baroreflex activation controls systemic inflammation in conscious endotoxemic rats. Animals underwent sham or electrical aortic depressor nerve stimulation initiated 10 min prior to a lipopolysaccharide (LPS) challenge, while inflammatory cytokine levels were measured in the blood, spleen, heart and hypothalamus 90 min after LPS treatment. Baroreflex activation did not affect LPS-induced levels of pro-inflammatory (tumor necrosis factor, interleukin 1β and interleukin 6) or anti-inflammatory (interleukin 10) cytokines in the periphery (heart, spleen and blood). However, baroreflex stimulation attenuated LPS-induced levels of all these cytokines in the hypothalamus. Notably, these results indicate that the central anti-inflammatory mechanism induced by baroreflex stimulation is independent of cardiovascular alterations, since aortic depressor nerve stimulation that failed to induce hemodynamic changes was also efficient at inhibiting inflammatory cytokines in the hypothalamus. Thus, aortic depressor nerve stimulation might represent a novel therapeutic strategy for neuroprotection, modulating inflammation in the central nervous system.

Carotid sinus nerve electrical stimulation in conscious rats attenuates systemic inflammation via chemoreceptor activation

Recent studies demonstrated a critical functional connection between the autonomic (sympathetic and parasympathetic) nervous and the immune systems. The carotid sinus nerve (CSN) conveys electrical signals from the chemoreceptors of the carotid bifurcation to the central nervous system where the stimuli are processed to activate sympathetic and parasympathetic efferent signals. Here, we reported that chemoreflex activation via electrical CSN stimulation, in conscious rats, controls the innate immune response to lipopolysaccharide attenuating the plasma levels of inflammatory cytokines such as tumor necrosis factor (TNF), interleukin 1β (IL-1β) and interleukin 6 (IL-6). By contrast, the chemoreflex stimulation increases the plasma levels of anti-inflammatory cytokine interleukin 10 (IL-10). This chemoreflex anti-inflammatory network was abrogated by carotid chemoreceptor denervation and by pharmacological blockade of either sympathetic - propranolol - or parasympathetic - methylatropine – signals. The chemoreflex stimulation as well as the surgical and pharmacological procedures were confirmed by real-time recording of hemodynamic parameters [pulsatile arterial pressure (PAP) and heart rate (HR)]. These results reveal, in conscious animals, a novel mechanism of neuromodulation mediated by the carotid chemoreceptors and involving both the sympathetic and parasympathetic systems.

Consequences of Obstructive Sleep Apnea: Cardiovascular Risk of Obstructive Sleep Apnea and Whether Continuous Positive Airway Pressure Reduces that Risk

Obstructive sleep apnea (OSA) is present in up to 25% of otherwise healthy individuals. OSA is associated with intermittent hypoxia, oxidative stress, sympathetic activation, and an inflammatory response. These perturbations mediate the role of OSA as an independent and modifiable risk factor for cardiovascular disease (CVD). OSA can induce CVD or accelerate the progression of CVD into an end-stage disorder, including heart failure and stroke. Current clinical recommendations are based on existing clinical trial data and the clinical experience of our program; current and future clinical trials will help to optimize management of OSA in the setting of CVD.

Renal Denervation

Sympathetic nervous system over-activity is closely linked with elevation of systemic blood pressure. Both animal and human studies suggest renal sympathetic nerves play an important role in this respect. Historically, modulation of sympathetic activity has been used to treat hypertension. More recently, catheter based renal sympathetic denervation was introduced for the management of treatment resistant hypertension. Sound physiological principles and surgical precedent underpin renal denervation as a therapy for treatment of resistant hypertension. Encouraging results of early studies led to a widespread adoption of the procedure for management of this condition. Subsequently a sham controlled randomised controlled study failed to confirm the benefit of renal denervation leading to a halt in its use in most countries in the world. However, critical analysis of the sham-controlled study indicates a number of flaws. A number of lessons have been learnt from this and other studies which need to be applied in future trials to ascertain the actual role of renal denervation in the management of treatment resistant hypertension before further implementation. This chapter deals with all these issues in detail.

Cardiovascular responses elicited by continuous versus intermittent electrical stimulation of the aortic depressor nerve in conscious rats

AIMS

Short-term (seconds or minutes) continuous electrical activation of the aortic depressor nerve (ADN) in conscious rats has been successfully used to investigate baroafferent function in experimental hypertension, heart failure, and peripheral inflammation. The aim of this study was to characterize the hemodynamic responses elicited by longer periods (60min) of continuous or intermittent electrical baroreflex activation.

MAIN METHODS

Wistar rats were implanted with an electrode around the left ADN and a catheter into a femoral artery. The systolic, diastolic and mean arterial pressure and heart rate were recorded in subjects randomly assigned to continuous or intermittent electrical stimulation. The time-course of cardiovascular responses in conscious rats was examined during longer-term (60min) continuous (n=6) or intermittent (5s ON/3s OFF; n=10) electrical stimulation (0.5mA; 0.25ms; 30Hz) of the ADN.

KEY FINDINGS

The prompt (20s) hypotensive response was greater under continuous stimulation, but no difference was detected in the bradycardic response. The hypotensive response was sustained only by continuous stimulation while no sustained bradycardia was observed in either protocol.

SIGNIFICANCE

These findings indicate that continuous stimulation of the ADN is more effective in reducing arterial pressure over a longer period (60min) of stimulation. Nevertheless, both protocols - continuous or intermittent - were unable to elicit a sustained bradycardia.

Baroreflex activation in conscious rats modulates the joint inflammatory response via sympathetic function

The baroreflex is a critical physiological mechanism controlling cardiovascular function by modulating both the sympathetic and parasympathetic activities. Here, we report that electrical activation of the baroreflex attenuates joint inflammation in experimental arthritis induced by the administration of zymosan into the femorotibial cavity. Baroreflex activation combined with lumbar sympathectomy, adrenalectomy, celiac subdiaphragmatic vagotomy or splenectomy dissected the mechanisms involved in the inflammatory modulation, highlighting the role played by sympathetic inhibition in the attenuation of joint inflammation. From the immunological standpoint, baroreflex activation attenuates neutrophil migration and the synovial levels of inflammatory cytokines including TNF, IL-1β and IL-6, but does not affect the levels of the anti-inflammatory cytokine IL-10. The anti-inflammatory effects of the baroreflex system are not mediated by IL-10, the vagus nerve, adrenal glands or the spleen, but by the inhibition of the sympathetic drive to the knee. These results reveal a novel physiological neuronal network controlling peripheral local inflammation.

The role of the kidney and the sympathetic nervous system in hypertension

Nearly one-third of the world's population has hypertension. The human and societal impact of hypertension is enormous. Primary hypertension accounts for 95 % of cases of hypertension in adults. The pathogenesis of primary hypertension is complex. The kidney and the sympathetic nervous system play important roles in the development and maintenance of hypertension. This review discusses their respective roles, the interaction between the two, implications of sympathetic overactivity in kidney disease and therapeutic interventions that have been developed on the basis of this knowledge, especially modulation of the sympathetic nervous system.

Cardiac autonomic function and insulin resistance for the development of hypertension: a six-year epidemiological follow-up study

BACKGROUND AND AIMS

To explore the impact of cardiac autonomic function (CAF) and insulin resistance (IR) on incident hypertension.

METHODS AND RESULTS

In 1996, 1638 subjects finished baseline examination, which included anthropometry, blood pressures, CAF, blood biochemistry, plasma insulin, urine examination and electrocardiogram. CAF included standard deviation of normal-to-normal intervals or RR intervals (SDNN), low- and high-frequency power spectrum (LF and HF), and LF/HF ratio at supine for 5 min, the RR interval changes during lying-to-standing maneuver, and the ratio between the longest RR interval during expiration and the shortest RR interval during inspiration (E/I ratio). We used homeostasis model assessment to define beta cell function (HOMA-B) and insulin resistance (HOMA-IR). In total, 992 non-hypertensive participants completed the follow-up assessment in 2003 and 959 participants were included for the final analysis. Incident hypertension was determined by blood pressure status at follow-up. In unadjusted model, both square root of HOMA-IR (OR:3.37, 95%CI: 2.10-6.64) and HOMA-B (OR:0.996, 95%CI: 0.992-0.999) were related to incident hypertension. In multivariate model, square root of HOMA-IR (OR:1.97, 95%CI: 1.05-3.70), but not HOMA-B, was associated with incident hypertension. After further adjustment for baseline CAF, the positive relationship between the square root of HOMA-IR and incident hypertension disappeared. In contrast, LF/HF ratio (OR:1.18, 95%CI: 1.01-1.37), HF power (OR:0.98, 95%CI: 0.96-0.999), and E/I ratio (OR:0.71, 95%CI: 0.54-0.95) were each independently associated with incident hypertension after further adjustment for HOMA measures.

CONCLUSION

Sympathovagal imbalance with an apparently decreased parasympathetic tone is an important predictor of incident hypertension independent of IR.

Hemodynamic and ventilatory response to different levels of hypoxia and hypercapnia in carotid body-denervated rats

OBJECTIVE

Chemoreceptors play an important role in the autonomic modulation of circulatory and ventilatory responses to changes in arterial O(2) and/or CO(2). However, studies evaluating hemodynamic responses to hypoxia and hypercapnia in rats have shown inconsistent results. Our aim was to evaluate hemodynamic and respiratory responses to different levels of hypoxia and hypercapnia in conscious intact or carotid body-denervated rats.

METHODS

Male Wistar rats were submitted to bilateral ligature of carotid body arteries (or sham-operation) and received catheters into the left femoral artery and vein. After two days, each animal was placed into a plethysmographic chamber and, after baseline measurements of respiratory parameters and arterial pressure, each animal was subjected to three levels of hypoxia (15, 10 and 6% O(2)) and hypercapnia (10% CO(2)).

RESULTS

The results indicated that 15% O(2) decreased the mean arterial pressure and increased the heart rate (HR) in both intact (n = 8) and carotid body-denervated (n = 7) rats. In contrast, 10% O(2) did not change the mean arterial pressure but still increased the HR in intact rats, and it decreased the mean arterial pressure and increased the heart rate in carotid body-denervated rats. Furthermore, 6% O(2) increased the mean arterial pressure and decreased the HR in intact rats, but it decreased the mean arterial pressure and did not change the HR in carotid body-denervated rats. The 3 levels of hypoxia increased pulmonary ventilation in both groups, with attenuated responses in carotid body-denervated rats. Hypercapnia with 10% CO(2) increased the mean arterial pressure and decreased HR similarly in both groups. Hypercapnia also increased pulmonary ventilation in both groups to the same extent.

CONCLUSION

This study demonstrates that the hemodynamic and ventilatory responses varied according to the level of hypoxia. Nevertheless, the hemodynamic and ventilatory responses to hypercapnia did not depend on the activation of the peripheral carotid chemoreceptors.

Baroreflex activation therapy for the treatment of drug-resistant hypertension: new developments

In the past few years, novel accomplishments have been obtained in carotid baroreflex activation therapy (BAT) for the treatment of resistant hypertension. In addition, this field is still evolving with promising results in the reduction of blood pressure and heart rate. This overview addresses the latest developments in BAT for the treatment of drug-resistant hypertension. Although not totally understood considering the working mechanisms of BAT, it appeared to be possible to achieve at least as much efficacy of single-sided as bilateral stimulation. Therefore unlike the first-generation Rheos system, the second-generation Barostim neo operates by unilateral baroreflex activation, using a completely different carotid electrode. Also significant improvements in several cardiac parameters have been shown by BAT in hypertensive patients, which set the basis for further research to evaluate BAT as a therapy for systolic heart failure. Yet important uncertainties need to be clarified to guarantee beneficial effects; hence not all participants seem to respond to BAT.

Fluid Structure Interaction With Contact Surface Methodology for Evaluation of Endovascular Carotid Implants for Drug-Resistant Hypertension Treatment

Drug-resistant hypertensive patients may be treated by mechanical stimulation of stretch-sensitive baroreceptors located in the sinus of carotid arteries. To evaluate the efficacy of endovascular devices to stretch the carotid sinus such that the induced strain might trigger baroreceptors to increase action potential firing rate and thereby reduce systemic blood pressure, numerical simulations were conducted of devices deployed in subject-specific carotid models. Two models were chosen—a typical physiologic carotid and a diminutive atypical physiologic model representing a clinically worst case scenario—to evaluate the effects of device deployment in normal and extreme cases, respectively. Based on the anatomical dimensions of the carotids, two different device sizes were chosen out of five total device sizes available. A fluid structure interaction (FSI) simulation methodology with contact surface between the device and the arterial wall was implemented for resolving the stresses and strains induced by device deployment. Results indicate that device deployment in the carotid sinus of the physiologic model induces an increase of 2.5% and 7.5% in circumferential and longitudinal wall stretch, respectively, and a maximum of 54% increase in von Mises arterial stress at the sinus wall baroreceptor region. The second device, deployed in the diminutive carotid model, induces an increase of 6% in both circumferential and longitudinal stretch and a 50% maximum increase in von Mises stress at the sinus wall baroreceptor region. Device deployment has a minimal effect on blood-flow patterns, indicating that it does not adversely affect carotid bifurcation hemodynamics in the physiologic model. In the smaller carotid model, deployment of the device lowers wall shear stress at sinus by 16% while accelerating flow entering the external carotid artery branch. Our FSI simulations of carotid arteries with deployed device show that the device induces localized increase in wall stretch at the sinus, suggesting that this will activate baroreceptors and subsequently may control hypertension in drug-resistant hypertensive patients, with no consequential deleterious effects on the carotid sinus hemodynamics.

Expression of P2X(2) and P2X (3) receptors in the rat carotid sinus, aortic arch, vena cava, and heart, as well as petrosal and nodose ganglia

With single- and double-labeling immunofluorescence techniques, the distribution patterns and morphological characteristics of P2X(2)- and P2X(3)-immunoreactive nerve fiber terminals and neuronal bodies have been studied in the main circulatory system baroreceptors and the nodose and petrosal ganglia of rats. A high density of P2X(2)- and P2X(3)-immunoreactive nerve fiber terminals was detected in the carotid sinus. P2X(2)- and P2X(3)-immunoreactive nerve fiber terminals were also distributed widely in the aortic arch, atrium, vena cava, and ventricles. Almost all the P2X(2)-immunoreactive nerve fiber terminals were immunoreactive for P2X(3) receptors. P2X(2)- and P2X(3)-immunoreactive neuronal bodies were also detected in the nodose and petrosal ganglia, which are the sources of the P2X(2)- and P2X(3)-immunoreactive nerve terminals. P2X(2) and P2X(3) receptors were expressed in the same ganglionic neurons. These data indicate that extracellular ATP, via the homomeric P2X(2) and P2X(3) receptors, and heteromeric P2X(2/3) receptor in the sensory receptors of carotid sinus, aortic arch, atrium, and vena cava, may be involved in the regulation of systematic circulation blood pressure.

Carotid baroreceptor stimulation for the treatment of resistant hypertension

Interventional activation of the carotid baroreflex has been an appealing idea for the management of resistant hypertension for several decades, yet its clinical application remained elusive and a goal for the future. It is only recently that the profound understanding of the complex anatomy and pathophysiology of the circuit, combined with the accumulation of relevant experimental and clinical data both in animals and in humans, has allowed the development of a more effective and well-promising approach. Indeed, current data support a sustained over a transient reduction of blood pressure through the resetting of baroreceptors, and technical deficits have been minimized with a subsequent recession of adverse events. In addition, clinical outcomes from the application of a new implantable device (Rheos) that induces carotid baroreceptor stimulation point towards a safe and effective blood pressure reduction, but longer experience is needed before its integration in the everyday clinical practice. While accumulating evidence indicates that carotid baroreceptor stimulation exerts its benefits beyond blood pressure reduction, further research is necessary to assess the spectrum of beneficial effects and evaluate potential hazards, before the extraction of secure conclusions.

Sympathetic nervous system overactivity and its role in the development of cardiovascular disease

This review examines how the sympathetic nervous system plays a major role in the regulation of cardiovascular function over multiple time scales. This is achieved through differential regulation of sympathetic outflow to a variety of organs. This differential control is a product of the topographical organization of the central nervous system and a myriad of afferent inputs. Together this organization produces sympathetic responses tailored to match stimuli. The long-term control of sympathetic nerve activity (SNA) is an area of considerable interest and involves a variety of mediators acting in a quite distinct fashion. These mediators include arterial baroreflexes, angiotensin II, blood volume and osmolarity, and a host of humoral factors. A key feature of many cardiovascular diseases is increased SNA. However, rather than there being a generalized increase in SNA, it is organ specific, in particular to the heart and kidneys. These increases in regional SNA are associated with increased mortality. Understanding the regulation of organ-specific SNA is likely to offer new targets for drug therapy. There is a need for the research community to develop better animal models and technologies that reflect the disease progression seen in humans. A particular focus is required on models in which SNA is chronically elevated.

Baroreflex Control of Sympathetic Nerve Activity in Hypertensive Pregnant Rats with Reduced Uterine Perfusion

OBJECTIVE

Baroreflex sensitivity is reduced in women with preeclampsia. The aim of this study was to determine whether baroreflex control of renal sympathetic nerve activity (RSNA) is altered in pregnant rats with reduced uterine perfusion (a model of human preeclampsia).

METHODS

Uterine perfusion was reduced in the third trimester by clipping the distal aorta and uterine branches of the ovarian arteries. RSNA baroreflex parameters were compared at term gestation in rats with reduced uterine perfusion (n = 12), in normal pregnant rats (n = 14) and in nonpregnant rats (n = 14).

RESULTS

Reduced perfusion rats were hypertensive (123.6 +/- 2.3 mm Hg), and normal pregnant rats were hypotensive (97.7 +/- 2.2 mm Hg), compared with controls. In rats with reduced perfusion, the baroreflex was shifted to a higher set-point, and maximum and minimum RSNA were increased compared with normal pregnant rats.

CONCLUSION

The blunted baroreflex gain of normal pregnancy is maintained in rats with reduced uterine perfusion, but a hypertensive shift in baroreflex function exists in this rat model that is associated with a reversal of the reflex maximum and minimum RSNA observed in normal pregnancy.

Carotid baroreceptor stimulation as a therapeutic target in hypertension and other cardiovascular conditions

BACKGROUND

The role of the carotid baroreflex in blood pressure regulation has been known for a long time but its effects were thought to be short lived. Recent data indicate that stimulation of carotid baroreceptors may lower blood pressure not only for short periods of time, but also in the long run.

OBJECTIVE/METHODS

Recent advances in technology permitted the development of a new device (Rheos) that addresses problems with older devices. Several questions remain to be addressed before Rheos can be used widely, and several potential clinical applications remain to be clarified. This review examines these issues and comprehensively describes this therapeutic approach.

RESULTS/CONCLUSIONS

The carotid baroreceptor reflex is probably not completely in control of blood pressure. Baroreflexes are one of many control systems acting in concert.

Role of cardiopulmonary and carotid sinus baroreceptors in regulating renal sympathetic nerve activity during water immersion in conscious dogs

The aim of the present study was to investigate how loading of cardiopulmonary baroreceptors induced by water immersion (WI) modifies baroreflex control of renal sympathetic nerve activity (RSNA) in conscious dogs. Nine dogs were chronically instrumented for measuring carotid sinus nerve activity (CSNA), RSNA, carotid arterial (Pca), and central venous (Pcv) pressures. The stimulus-response relationships of Pca-CSNA and Pca-RSNA were determined simultaneously in the same dog by changing Pca using rapid intravenous infusions vasoactive drugs during pre-WI and WI. WI increased central venous pressure significantly (P < 0.05) by 10.4 mmHg. WI shifted the Pca-RSNA curve acutely leftward compared with the pre-WI period, which was associated with significant (P < 0.05) decreases in the saturation pressure by 39.0 mmHg and operating range by 43.1 mmHg. WI relocated the operating pressure to near the saturation pressure, where the gain was low. The Pca-CSNA curve obtained during WI was identical to that obtained during pre-WI period. These results suggest that the shift in baroreflex control of RSNA could be attributed to the inhibitory influence of the cardiopulmonary mechanoreceptor loading and not by the resetting of carotid baroreceptors per se.

Obstructive sleep apnea: the new cardiovascular disease. Part I: Obstructive sleep apnea and the pathogenesis of vascular disease

Obstructive sleep apnea (OSA) is increasingly recognized as a novel cardiovascular risk factor. OSA is implicated in the pathogenesis of hypertension, left ventricular dysfunction, coronary artery disease and stroke. OSA exerts its negative cardiovascular consequences through its unique pattern of intermittent hypoxia. Endothelial dysfunction, oxidative stress, and inflammation are all consequences of OSA directly linked to intermittent hypoxia and critical pathways in the pathogenesis of cardiovascular disease in patients with OSA. This review will discuss the known mechanisms of vascular dysfunction in patients with OSA and their implications for cardiovascular disease.

Electrophysiological and neuroanatomical evidence of sexual dimorphism in aortic baroreceptor and vagal afferents in rat

Evidence for sexual dimorphism in autonomic control of cardiovascular function is both compelling and confounding. Across healthy and disease populations sex-associated differences in neurocirculatory hemodynamics are far too complex to be entirely related to sex hormones. As an initial step toward identifying additional physiological mechanisms, we investigated whether there is a sex bias in the relative expression of low-threshold-myelinated and high-threshold-unmyelinated aortic baroreceptor afferents in rats. These two types of afferent fibers have markedly different reflexogenic effects upon heart rate and blood pressure and thus the potential impact upon baroreflex dynamics could be substantial. Our results, using a combination of a patch-clamp study of fluorescently identified aortic baroreceptor neurons (ABN) and morphometric analysis of aortic baroreceptor nerve fibers, demonstrate that females exhibit a greater percentage of myelinated baroreceptor fibers (24.8% vs. 18.7% of total baroreceptor fiber population, P < 0.01) and express a functional subtype of myelinated ABN rarely found in age-matched males (11% vs. 2.3%, n = 107, P < 0.01). Interestingly, this neuronal phenotype is more prevalent in the general population of female vagal afferent neurons (17.7% vs. 3.8%, n = 169, P < 0.01), and ovariectomy does not alter its expression but does lessen neuronal excitability. These data suggest there are fundamental neuroanatomical and electrophysiological differences between aortic baroreceptor afferents of female and male rats. Possible explanations are presented as to how such a greater prevalence of low-threshold myelinated afferents could be a contributing factor to the altered baroreflex sensitivity and vagal tone of females compared with males.

Effects of birth weight on spontaneous baroreflex sensitivity in adult life

BACKGROUND AND AIM

Several epidemiological studies have suggested a link between low birth weight and coronary heart disease; this may be partly due to the association between low birth weight and conventional risk factors. Among the factors involved in the regulation of cardiovascular homeostasis, baroreflexes play a crucial role. The objective of the present study was to investigate if baroreflex sensitivity (BRS) in adulthood is associated with birth weight.

METHODS AND RESULTS

Two hundred and eleven adults from Turin, Italy, aged 22-24 years, were examined in a cross sectional survey. Birth weight, blood pressure, pulse rate, family history of hypertension, anthropometric and environmental parameters and spontaneous baroreflex sensitivity were evaluated. In this study we observed a significant increase in baroreflex sensitivity across the tertiles of birth weight, even after correction for gender, blood pressure and heart rate; in a regression model, birth weight was positively and independently associated with BRS; moreover, BRS showed a significant negative correlation with adult pulse rate.

CONCLUSION

This finding may be helpful in understanding the association between low birth weight and cardiovascular disease outcome in later life, since baroreflex failure is associated with an increased cardiovascular morbidity and mortality.

Hierarchical recruitment of the sympathetic and parasympathetic limbs of the baroreflex in normotensive and spontaneously hypertensive rats

The arterial baroreflex acts to buffer acute changes in blood pressure by reciprocal modulation of sympathetic and parasympathetic activity that controls the heart and vasculature. We have examined the baroreflex pressure-function curves for changes in heart rate and non-cardiac sympathetic nerve activity (SNA, thoracic chain T8-12) in artificially perfused in situ rat preparations. We found that the non-cardiac SNA baroreflex is active over a lower range of pressures than the cardiac baroreflex (threshold 66 +/- 1 mmHg versus 82 +/- 5 mmHg and mid-point 77 +/- 3 versus 87 +/- 4 mmHg, respectively, P < 0.05, n = 6). This can manifest as a complete dissociation of the baroreflex limbs at low pressures. This difference between the cardiac and non-cardiac SNA baroreflex is also seen in end-organ sympathetic outflows (adrenal and renal nerves). Recordings of the cardiac vagal (parasympathetic) and the inferior cardiac (sympathetic) nerves identify the cardiac parasympathetic baroreflex component as being active over a higher range of pressures. This difference in the operating range of the baroreflex-function curves is exaggerated in the spontaneously hypertensive rat where the cardiac component has selectively reset by 20-25 mmHg to a higher pressure range (threshold of 104 +/- 4 mmHg and mid-point 113 +/- 4, n = 6). The difference in the pressure-function curves for the cardiac versus the vascular baroreflex indicates that there is a hierarchical recruitment of the output limbs of the baroreflex with a sympathetic predominance at lower arterial pressures.

Baroreflex responses to electrical stimulation of aortic depressor nerve in conscious SHR

Baroreflex responses to changes in arterial pressure are impaired in spontaneously hypertensive rats (SHR). Mean arterial pressure (MAP), heart rate (HR), and regional vascular resistances were measured before and during electrical stimulation (5-90 Hz) of the left aortic depressor nerve (ADN) in conscious SHR and normotensive control rats (NCR). The protocol was repeated after beta-adrenergic-receptor blockade with atenolol. SHR exhibited higher basal MAP (150 +/- 5 vs. 103 +/- 2 mmHg) and HR (393 +/- 9 vs. 360 +/- 5 beats/min). The frequency-dependent hypotensive response to ADN stimulation was preserved or enhanced in SHR. The greater absolute fall in MAP at higher frequencies (-68 +/- 5 vs. -38 +/- 3 mmHg at 90-Hz stimulation) in SHR was associated with a preferential decrease in hindquarter (-43 +/- 5%) vs. mesenteric (-27 +/- 3%) resistance. In contrast, ADN stimulation decreased hindquarter and mesenteric resistances equivalently in NCR (-33 +/- 7% and -30 +/- 7%). Reflex bradycardia was also preserved in SHR, although its mechanism differed. Atenolol attenuated the bradycardia in SHR (-88 +/- 14 vs. -129 +/- 18 beats/min at 90-Hz stimulation) but did not alter the bradycardia in NCR (-116 +/- 16 vs. -133 +/- 13 beats/min). The residual bradycardia under atenolol (parasympathetic component) was reduced in SHR. MAP and HR responses to ADN stimulation were also preserved or enhanced in SHR vs. NCR after deafferentation of carotid sinuses and contralateral right ADN. The results demonstrate distinct differences in central baroreflex control in conscious SHR vs. NCR. Inhibition of cardiac sympathetic tone maintains reflex bradycardia during ADN stimulation in SHR despite impaired parasympathetic activation, and depressor responses to ADN stimulation are equivalent or even greater in SHR due to augmented hindquarter vasodilation.

Inhibitory neurotransmission in the nucleus tractus solitarii: implications for baroreflex resetting during exercise

Inhibitory neurotransmission plays a crucial role in the processing of sensory afferent signals in the nucleus of the solitary tract (NTS). The aim of this review is to provide a critical overview of inhibitory mechanisms that may be responsible for altering arterial baroreflex function during physical activity or exercise. Over a decade ago, the view of reflex control of cardiovascular function during exercise was revised because of the finding that the arterial baroreflex is reset in humans, enabling continuous beat-to-beat reflex regulation of blood pressure and heart rate. During the ensuing decade, many investigators proposed that resetting was mediated by central neural mechanisms that were intrinsic to the brain. Recent experimental data suggest that rapid and reversible changes in gamma-aminobutyric acid (GABA) inhibitory neurotransmission within the NTS play a fundamental role in this process. The hypothesis will be presented that baroreflex resetting by somatosensory input is mediated by: (1) selective inhibition of barosensitive NTS neurones; and (2) excitation of sympathoexcitatory neurones in the rostral ventrolateral medulla. Current research findings will be discussed that support an interaction between GABA and substance P (SP) signalling mechanisms in the NTS. An understanding of these mechanisms may prove to be essential for future detailed analysis of the cellular and molecular mechanisms underlying sensory integration in the NTS.

Problems, possibilities, and pitfalls in studying the arterial baroreflexes’ influence over long-term control of blood pressure

While there is no disputing the critical role of baroreflexes in buffering rapid changes in arterial pressure, their role in long-term pressure control has become an area of controversy. Recent experiments using novel techniques have challenged the traditional view that arterial baroreflexes are not involved in setting chronic arterial pressure levels. Resetting of the arterial baroreflex, often used as an argument against the arterial baroreflex playing a role in long-term pressure control is rarely complete. The arterial baroreflex is just one of the many neural, hormonal, and intrinsic mechanisms involved in arterial pressure control and while the removal of the arterial baroreflex alone has little effect on mean arterial pressure it is too simplistic to suggest that the baroreflex has no role in long-term pressure control. Renal sympathetic nerve activity appears to be particularly resistant to resetting in response to ANG II-induced hypertension. Given the important role of the kidneys in long-term pressure control, we suggest there is a clear need to develop experimental techniques whereby sympathetic nerve activity to the kidneys and other organs can be monitored over periods of weeks to months.

Maternal nutrient restriction in sheep: hypertension and decreased nephron number in offspring at 9 months of age

Pregnant ewes were fed either a 50% nutrient-restricted (NR; n= 8) or a control 100% (C; n= 8) diet from day 28 to day 78 of gestation (dGA; term = 150 dGA). Lambs were born naturally, and fed to appetite throughout the study period. At 245 +/- 1 days postnatal age (DPNA), offspring were instrumented for blood pressure measurements, with tissue collection at 270 DPNA. Protein expression was assessed using Western blot, glomerulus number determined via acid maceration and hormone changes by radioimmunoassay (RIA) or enzyme-linked immunosorbent assay (ELISA). NR lambs had higher mean arterial pressure (MAP; 89.0 +/- 6.6 versus 73.4 +/- 1.6 mmHg; P < 0.05), fewer renal glomeruli (57.8 +/- 23.8 versus 64.6 +/- 19.3 x 10(4); P < 0.05), increased expression of angiotensin converting enzyme (ACE) in the renal cortex (942 +/- 130 versus 464 +/- 60 arbitrary pixel units (apu); P < 0.03), and increased angiotensin II receptor AT2 expression in the renal medulla (63.3 +/- 12.1 versus 19.5 +/- 44.2 x 10(4) apu; P < 0.03). All data are presented as mean +/-S.E.M. The present data indicate that global maternal nutrient restriction (50%) during early to mid-gestation impairs renal nephrogenesis, increases MAP, and alters expression of AT2 and ACE without an associated change in birth weight. These data demonstrate the existence of a critical window of fetal susceptibility during early to mid-gestation that alters kidney development and blood pressure regulation in later life.

Role of brain and peripheral angiotensin II in hypertension and altered arterial baroreflex programmed during fetal life in rat

Intrauterine programming of hypertension is associated with evidence of increased renin-angiotensin system (RAS) activity. The current study was undertaken to investigate whether arterial baroreflex and blood pressure variability are altered in a model of in utero programming of hypertension secondary to isocaloric protein deprivation and whether activation of the RAS plays a role in this alteration. Pregnant Wistar rats were fed a normal-protein (18%) or low-protein (9%) diet during gestation, which had no effect on litter size, birth weight, or pup survival. Mean arterial blood pressure (MABP; 126 +/- 3 mm Hg 9% versus 108 +/- 4 mm Hg 18%; p < 0.05) and blood pressure variability were significantly greater in the adult offspring of the 9% protein-fed mothers. Arterial baroreflex control of heart rate, generated by graded i.v. infusion of phenylephrine and nitroprusside, was significantly shifted toward higher pressure; i.v. angiotensin-converting enzyme inhibitor normalized MABP and shifted the arterial baroreflex curve of the 9% offspring toward lower pressure without affecting the 18% offspring. For examining whether brain RAS is also involved in programming of hypertension, angiotensin-converting enzyme inhibitor and losartan (specific AT(1) receptor antagonist) were administered intracerebroventricularly; both significantly reduced MABP of the 9% but not the 18% offspring. Autoradiographic receptor binding studies demonstrated an increase in brain AT(1) expression in the subfornical organ and the vascular organ of the lamina terminalis in the 9% offspring. These data demonstrate a major tonic role of brain and peripheral RAS on hypertension associated with antenatal nutrient deprivation.

Ontogeny of baroreflex control in the American alligator Alligator mississippiensis

Baroreflex regulation appears in different species at different points in embryonic development. This study was designed to understand the development of the baroreflex in embryos of the American alligator at four different points of embryonic development (60%, 70%, 80% and 90% of a total incubation period of 72 days) and in 1-week-old hatchlings. Data from a separate study on 1-year-old alligators were included for comparison. The gain of the cardiac limb of the baroreflex was calculated from heart rate changes triggered by pharmacological manipulation of arterial pressure with sodium nitroprusside and phenylephrine. The results demonstrated that a vagally mediated hypertensive baroreflex was present during the final 30% of alligator development. A hypotensive baroreflex was not present in embryos but appeared in hatchlings, mediated by a combined effect of vagal and sympathetic efferents. Absolute baroreflex gain was maximal at 80% of incubation (41.22 beats kPa(-1) min(-1)) and dropped thereafter, reaching a minimum in 1-year-old alligators (9.69 beats kPa(-1) min(-1)). When the baroreflex gain was normalized to resting arterial pressure and heart rate, the maximum gain was observed in 1-year-old alligators (normalized index of 2.12 versus 0.75 in hatchlings and 0.69 as the highest gain in embryos). In conclusion, baroreflex regulation appeared during embryonic development with a substantial gain. These findings indicate that embryonic development is a period of preparation for cardiovascular regulatory mechanisms that will be necessary in adult life and that the baroreflex control mechanism is required for cardiovascular control during ontogeny.

Mental stress response, arterial stiffness, and baroreflex sensitivity in healthy aging

This study examined the relationship of pressor responses during mental stress to arterial stiffness and baroreflex sensitivity. Hemodynamic responses of 24 healthy individuals (51-86 years old) to two mental stress tasks (math and speech) were compared with common carotid artery mechanical stiffness and autonomic nervous system regulation of blood pressure as measured by using the modified Oxford technique. At the ages studied, no effect of age on stress task responsiveness, carotid stiffness, or baroreflex sensitivity was observed. Carotid stiffness and baroreflex sensitivity demonstrated a strong inverse relation. Change in heart rate during the speech task was correlated with arterial stiffness, and the increase in mean arterial pressure was associated with carotid stiffness and was inversely correlated to baroreflex sensitivity. These associations suggest that acute hemodynamic reactions to mental stress among healthy adults are determined, in part, by structural properties of arterial vessels and sensitivity of arterial baroreflex. These observations may provide a mechanistic link between the physiology of cardiovascular reactivity to stress and risk of cardiovascular events in middle-aged and older individuals.

Blood pressure in the very low birth weight neonate

Few aspects of management of very low birth weight (VLBW; <1500 g) neonates have generated as much controversy as the assessment of blood pressure (BP) and need for treatment of perceived abnormalities of this physiologic variable. The approach to this problem may differ greatly among various institutions and even among clinicians within a given center. The purpose of this manuscript is to review available information regarding physiologic determinants and measurement of BP in VLBW neonates, normative data for BP, clinical factors that may affect BP in these at-risk neonates and studies in which presumed abnormalities of BP resulted in adverse clinical outcomes. Options for treatment of low BP in VLBW neonates also will be discussed.