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

Effects of Nondipping Blood Pressure Changes: A Nephrologist Prospect

Blood pressure (BP) variations depend on various internal, environmental, and behavioral factors. BP fluctuations occur both in normotensive and hypertensive people. Although it fluctuates over the 24-hr day and night, the morning BP increases after waking up and declines throughout sleep. It is typical for BP to decrease by 10% to 20%, while sleeping, known as dipping BP. However, if there is no decrease in nighttime mean systolic BP or a drop of less than 10 mmHg, it is called nondipping BP. Conversely, reverse dipping BP means an increase in mean systolic BP instead of a drop during the night. Reverse dipping is observed in hypertension (HTN), diabetes mellitus (DM), chronic kidney disease (CKD), and obstructive sleep apnea (OSA) syndrome. The introduction of ambulatory BP monitoring (ABPM) led to the emergence of identifying normal and elevated BP patterns. Non-dipping BP increases the risk of cardiovascular system (CVS) complications such as left ventricular hypertrophy, proteinuria, glomerular filtration rate (GFR) reduction, and CKD progression. A loss or blunting of the normal BP profile is recognized as a deleterious variant, and restoring abnormal BP patterns has been reported to significantly impact end-organ damage, morbidity, and mortality. In this non-systematic clinically-oriented, comprehensive review, we aim to update the BP variables and the pathophysiology of nondipping BP and point out the areas which need more investigation from a nephrology perspective because the nondipping BP increases the risk of proteinuria, GFR reduction, and CKD progression. A literature search of PubMed, Google, EMBASE, and Google Scholar was conducted. Checks of selected papers and relevant reviews complemented the electronic search. With improved BP measurement methods, the physiology of BP profile variations is readily detectable during the day and night. A nondipping BP profile is a distinct BP pattern that may have significant end-organ damage effects and therapeutic importance for nephrologists. The pathophysiology of the nondipping BP variant must be clarified to prevent complications, and further investigations are required. Furthermore, there is debate about the best BP index to utilize: systolic BP, diastolic BP, mean arterial pressure, or a mixture of all. All these areas are important and need new research projects.

Dexamethasone attenuates the modulatory effect of the insular cortex on the baroreflex in anesthetized rat

The arterial baroreflex (BR) is an important neural mechanism for the stabilization of arterial pressure (AP). It is known that the insular cortex (IC) and other parts of the central autonomic network (CAN) are able to modulate the BR arc, altering baroreflex sensitivity (BRS). In addition, the sensitivity of the BR changes under the influence of hormones, in particular glucocorticoids (GC). It has been suggested that GC may influence BRS by altering the ability of the IC to modulate the BR. This hypothesis has been tested in experiments on rats anesthetized with urethane. It was found that microelectrostimulation of the visceral area in the left IC causes a short-term drop in AP, which is accompanied by bradycardia, and impairs BRS. The synthetic GC dexamethasone (DEX) did not significantly affect the magnitude of depressor responses but increased BRS and impaired the effect of IC stimulation on the BR. The results obtained confirm the hypothesis put forward and suggest that GC can attenuate the inhibitory effects of the IC on the BR arc, thereby enhancing the sensitivity of the BR.

Activation of the cholinergic antiinflammatory reflex by occipitoatlantal decompression and transcutaneous auricular vagus nerve stimulation

CONTEXT

The parasympathetic-mediated inflammatory reflex inhibits excessive proinflammatory cytokine production. Noninvasive techniques, including occipitoatlantal decompression (OA-D) and transcutaneous auricular vagus nerve stimulation (taVNS), have been demonstrated to increase parasympathetic tone.

OBJECTIVES

To test the hypothesis that OA-D and taVNS increase parasympathetic nervous system activity and inhibit proinflammatory cytokine mobilization and/or production.

METHODS

Healthy adult participants were randomized to receive OA-D (5 min of OA-D followed by 10 min of rest; n=8), taVNS (15 min; n=9), or no intervention (15 min, time control; n=10) on three consecutive days. Before and after these interventions, saliva samples were collected for determination of the cytokines interleukin-1β (IL-1β), interleukin-6 (IL-6), interleukin-8 (IL-8), and tumor necrosis factor α (TNF-α). Arterial blood pressure and the electrocardiogram were recorded for a 30-min baseline, throughout the intervention, and during a 30-min recovery period to derive heart rate and blood pressure variability markers as indices of vagal and sympathetic control.

RESULTS

OA-D and taVNS increased root mean square of successive RR interval differences (RMSSD) and high frequency heart rate variability, which are established markers for parasympathetic modulation of cardiac function. In all three groups, the experimental protocol was associated with a significant increase in salivary cytokine concentrations. However, the increase in IL-1β was significantly less in the taVNS group (+66 ± 13 pg/mL; p<0.05) than in the time control group (+142 ± 24 pg/mL). A similar trend was observed in the taVNS group for TNF-α (+1.7 ± 0.3 pg/mL vs. 4.1 ± 1.3 pg/mL; p<0.10). In the OA-D group baseline IL-6, IL-8, and TNF-α levels on the third study day were significantly lower than on the first study day (IL-6: 2.3 ± 0.4 vs. 3.2 ± 0.6 pg/mL, p<0.05; IL-8: 190 ± 61 vs. 483 ± 125 pg/mL, p <0.05; TNF-α: 1.2 ± 0.3 vs. 2.3 ± 0.4 pg/mL, p<0.05). OA-D decreased mean blood pressure from the first (100 ± 8 mmHg) to the second (92 ± 6 mmHg; p<0.05) and third (93 ± 8 mmHg; p<0.05) study days and reduced low frequency spectral power of systolic blood pressure variability (19 ± 3 mmHg² after OA-D vs. 28 ± 5 mmHg² before OA-D; p<0.05), a marker of sympathetic modulation of vascular tone. OA-D also increased baroreceptor-heart rate reflex sensitivity from the first (13.7 ± 3.0 ms/mmHg) to the second (18.4 ± 4.3 ms/mmHg; p<0.05) and third (16.9 ± 4.2 ms/mmHg; p<0.05) study days.

CONCLUSIONS

Both OA-D and taVNS elicited antiinflammatory responses that were associated with increases in heart rate variability-derived markers for parasympathetic function. These findings suggest that OA-D and taVNS activate the parasympathetic antiinflammatory reflex. Furthermore, an antihypertensive effect was observed with OA-D that may be mediated by reduced sympathetic modulation of vascular tone and/or increased baroreceptor reflex sensitivity.

Central endothelin ETB receptor activation reduces blood pressure and catecholaminergic activity in the olfactory bulb of deoxycorticosterone acetate-salt hypertensive rats

Endothelins regulate catecholaminergic activity in the olfactory bulb (OB) in normotensive and hypertensive animals. Administration of an endothelin ET_A receptor antagonist decreases blood pressure in deoxycorticosterone acetate-salt (DOCA-salt) rats along with a reduction in tyrosine hydroxylase (TH) activity and expression. In the present work, we sought to establish the role of brain endothelin ET_B receptor on blood pressure regulation and its relationship with the catecholaminergic system within the OB of DOCA-Salt rats. Sprague-Dawley male rats were divided into control and DOCA-Salt groups. Blood pressure, heart rate and TH activity as well as neuronal nitric oxide synthase (nNOS) expression were assessed following IRL-1620 (selective endothelin ET_B receptor agonist) applied to be brain. IRL-1620 significantly reduced systolic, diastolic, and mean arterial pressure in DOCA-Salt hypertensive rats. It also decreased TH activity, TH total and phosphorylated forms expression as well as its mRNA in the OB of hypertensive animals. The expression of phospho-Ser1417-nNOS, which reflects nNOS activation, was significantly decreased in the of OB of DOCA-salt rats, but it was enhanced by IRL-1620. These findings suggest that DOCA-Salt hypertension depends on endogenous central endothelin ET_A receptor activity, rather than on ET_B, and that low endothelin ET_B stimulation is essential for blood pressure elevation in this animal model. The effect of endothelin ET_A receptor antagonism may also result from endothelin ET_B receptor overstimulation. The present study shows that endothelin receptors are involved in the regulation of TH in the OB and that such changes are likely implicated in the hemodynamic control and sympathetic outflow.

Role of Neuroendocrine, Immune, and Autonomic Nervous System in Anorexia Nervosa-Linked Cardiovascular Diseases

Anorexia nervosa represents a severe mental disorder associated with food avoidance and malnutrition. In patients suffering from anorexia nervosa, cardiovascular complications are the main reason leading to morbidity and mortality. However, the origin and pathological mechanisms leading to higher cardiovascular risk in anorexia nervosa are still unclear. In this aspect, the issue of exact pathological mechanisms as well as sensitive biomarkers for detection of anorexia nervosa-linked cardiovascular risk are discussed. Therefore, this review synthesised recent evidence of dysfunction in multiple neuroendocrine axes and alterations in the immune system that may represent anorexia nervosa-linked pathological mechanisms contributing to complex cardiovascular dysregulation. Further, this review is focused on identification of non-invasive biomarkers for the assessment of increased cardiovascular risk in anorexia nervosa that can be linked to a clinical application. Complex non-invasive assessment of cardiovascular autonomic regulation—cardiac vagal control (heart rate variability), sympathetic vascular activity (blood pressure variability), and cardiovascular reflex control (baroreflex sensitivity)—could represent a promising tool for early diagnosis, personalized therapy, and monitoring of therapeutic interventions in anorexia nervosa particularly at a vulnerable adolescent age.

Impaired Baroreflex Sensitivity after Bilateral Convulsive Seizures in Patients with Focal Epilepsy

Background

Sudden unexpected death in epilepsy (SUDEP) is probably due to an autonomic failure in the early postictal phase after bilateral convulsive seizures (BCS) in the majority of cases. The baroreflex sensitivity (BRS) is an established and reliable biomarker of autonomic function and sudden cardiac death.

Objective

To investigate whether postictal BRS depends on seizure type.

Methods

Beat-to-beat systemic blood pressure and heart rate were continuously and non-invasively recorded with the ccNexfin^® device in patients with focal epilepsy undergoing video-EEG monitoring. BRS was calculated using the sequence as well as the spectral method. A random mixed linear model was applied to analyze the influence of seizure type on BRS during three different time periods of 15-min length each (interictal, preictal, and postictal). In addition, the possible effects of other factors (hypertension, hemispheric lateralization of ictal activity, epilepsy type, body position, vigilance state) were explored. Data are given as median with interquartile range.

Results

A total of 26 seizures of 26 patients were analyzed. In BCS (n = 7), BRS significantly dropped from a preictal value of 15.0 ms/mm Hg (13.0–19.4) and an interictal value of 15.6 ms/mm Hg (12.0–20.4) to 3.1 ms/mm Hg (2.7–10.5) during the postictal period (p < 0.0001) according to the sequence method. This finding was replicated with the spectral method. In contrast, focal seizures (n = 19) did not lead to significant alterations of BRS in the postictal phase.

Conclusion

Postictal BRS depends on the seizure type and is markedly impaired after BCS. The present study provides further evidence for a disturbed autonomic function following BCS. These findings might be related to cardiovascular failure in the context of SUDEP.

A mathematical model of salt-sensitive hypertension: the neurogenic hypothesis

Salt sensitivity of arterial pressure (salt-sensitive hypertension) is a serious global health issue. The causes of salt-sensitive hypertension are extremely complex and mathematical models can elucidate potential mechanisms that are experimentally inaccessible. Until recently, the only mathematical model for long-term control of arterial pressure was the model of Guyton and Coleman; referred to as the G-C model. The core of this model is the assumption that sodium excretion is driven by renal perfusion pressure, the so-called 'renal function curve'. Thus, the G-C model dictates that all forms of hypertension are due to a primary shift of the renal function curve to a higher operating pressure. However, several recent experimental studies in a model of hypertension produced by the combination of a high salt intake and administration of angiotensin II, the AngII-salt model, are inconsistent with the G-C model. We developed a new mathematical model that does not limit the cause of salt-sensitive hypertension solely to primary renal dysfunction. The model is the first known mathematical counterexample to the assumption that all salt-sensitive forms of hypertension require a primary shift of renal function: we show that in at least one salt-sensitive form of hypertension the requirement is not necessary. We will refer to this computational model as the 'neurogenic model'. In this Symposium Review we discuss how, despite fundamental differences between the G-C model and the neurogenic model regarding mechanisms regulating sodium excretion and vascular resistance, they generate similar haemodynamic profiles of AngII-salt hypertension. In addition, the steady-state relationships between arterial pressure and sodium excretion, a correlation that is often erroneously presented as the 'renal function curve', are also similar in both models. Our findings suggest that salt-sensitive hypertension is not due solely to renal dysfunction, as predicted by the G-C model, but may also result from neurogenic dysfunction.

Blood pressure is maintained during dehydration by hypothalamic paraventricular nucleus-driven tonic sympathetic nerve activity

Resting sympathetic nerve activity (SNA) consists primarily of respiratory and cardiac rhythmic bursts of action potentials. During homeostatic challenges such as dehydration, the hypothalamic paraventricular nucleus (PVN) is activated and drives SNA in support of arterial pressure (AP). Given that PVN neurones project to brainstem cardio-respiratory regions that generate bursting patterns of SNA, we sought to determine the contribution of PVN to support of rhythmic bursting of SNA during dehydration and to elucidate which bursts dominantly contribute to maintenance of AP. Euhydrated (EH) and dehydrated (DH) (48 h water deprived) rats were anaesthetized, bilaterally vagotomized and underwent acute PVN inhibition by bilateral injection of the GABA-A receptor agonist muscimol (0.1 nmol in 50 nl). Consistent with previous studies, muscimol had no effect in EH rats (n = 6), but reduced mean AP (MAP; P < 0.001) and integrated splanchnic SNA (sSNA; P < 0.001) in DH rats (n = 6). Arterial pulse pressure was unaffected in both groups. Muscimol reduced burst frequency of phrenic nerve activity (P < 0.05) equally in both groups without affecting the burst amplitude-duration integral (i.e. area under the curve). PVN inhibition did not affect the amplitude of the inspiratory peak, expiratory trough or expiratory peak of sSNA in either group, but reduced cardiac rhythmic sSNA in DH rats only (P < 0.001). The latter was largely reversed by inflating an aortic cuff to restore MAP (n = 5), suggesting that the muscimol-induced reduction of cardiac rhythmic sSNA in DH rats was an indirect effect of reducing MAP and thus arterial baroreceptor input. We conclude that MAP is largely maintained in anaesthetized DH rats by a PVN-driven component of sSNA that is neither respiratory nor cardiac rhythmic.

Chronic blood pressure control

Chronic blood pressure is maintained within very narrow limits around an average value. However, the multitude of physiologic processes that participate in blood pressure control present a bewildering array of possibilities to explain how such tight control of arterial pressure is achieved. Guyton and Coleman and colleagues addressed this challenge by creating a mathematical model that integrated the short- and long-term control systems for overall regulation of the circulation. The hub is the renal-body fluid feedback control system, which links cardiac function and vascular resistance and capacitance with fluid volume homeostasis as the foundation for chronic blood pressure control. The cornerstone of that system is renal sodium excretory capability, which is defined by the direct effect of blood pressure on urinary sodium excretion, that is, "pressure natriuresis." Steady-state blood pressure is the pressure at which pressure natriuresis balances sodium intake over time; therefore, renal sodium excretory capability is the set point for chronic blood pressure. However, this often is misinterpreted as dismissing, or minimizing, the importance of nonrenal mechanisms in chronic blood pressure control. This article explains the renal basis for the blood pressure set point by focusing on the absolute dependence of our survival on the maintenance of sodium balance. Two principal threats to sodium balance are discussed: (1) a change in sodium intake or renal excretory capability and (2) a change in blood pressure. In both instances, circulatory homeostasis is maintained because the sodium balance blood pressure set point is reached.

Upregulation of brain-derived neurotrophic factor expression in nodose ganglia and the lower brainstem of hypertensive rats

Hypertension leads to structural and functional changes at baroreceptor synapses in the medial nucleus tractus solitarius (NTS), but the underlying molecular mechanisms remain unknown. Our previous studies show that brain-derived neurotrophic factor (BDNF) is abundantly expressed by rat nodose ganglion (NG) neurons, including baroreceptor afferents and their central terminals in the medial NTS. We hypothesized that hypertension leads to upregulation of BDNF expression in NG neurons. To test this hypothesis, we used two mechanistically distinct models of hypertension, the spontaneously hypertensive rat (SHR) and the deoxycorticosterone acetate (DOCA)-salt rat. Young adult SHRs, whose blood pressure was significantly elevated compared with age-matched Wistar-Kyoto (WKY) control rats, exhibited dramatic upregulation of BDNF mRNA and protein in the NG. BDNF transcripts from exon 4, known to be regulated by activity, and exon 9 (protein-coding region) showed the largest increases. Electrical stimulation of dispersed NG neurons with patterns that mimic baroreceptor activity during blood pressure elevations led to increases in BDNF mRNA that were also mediated through promoter 4. The increase in BDNF content of the NG in vivo was associated with a significant increase in the percentage of BDNF-immunoreactive NG neurons. Moreover, upregulation of BDNF in cell bodies of NG neurons was accompanied by a significant increase in BDNF in the NTS region, the primary central target of NG afferents. A dramatic increase in BDNF in the NG was also detected in DOCA-salt hypertensive rats. Together, our study identifies BDNF as a candidate molecular mediator of activity-dependent changes at baroafferent synapses during hypertension.

Relationship of Resting Baroreflex Activity to 24-Hour Blood Pressure and Mood in Healthy People

Objectives: Previous research suggests that mechanisms related to afferent and efferent components of the baroreflex are associated with blood pressure level and mood, and that mood as a component of subjective well-being may be mediated by changes in blood pressure. This study examines these mechanisms in 213 healthy women and men. Methods: Evaluation of spontaneous baroreflex activity using the sequence technique under resting conditions in the laboratory was followed by 2 days of 24-h ambulatory blood pressure recording and diary ratings of mood. Results: Patterns of components of the baroreflex loop were significant predictors of 24-h ambulatory blood pressure mean level, its circadian variability, and daily ratings of mood. Three mechanisms were found to be involved in the regulation: (1) interaction between gains of afferent and efferent cardiac components of the baroreflex, (2) gain of afferent signals related to fast cardiac responses to baroreceptor activation, and (3) gain of afferent signals related to late cardiac responses to baroreceptor inhibition. The interaction of afferent/efferent components of baroreflex loop appears to independently determine mean levels of systolic blood pressure and positive (happy) mood. The effect of the baroreceptor activation mechanism on subjective well-being (indexed by happy and alert moods) was mediated by the wake/sleep systolic blood pressure difference. Additionally, the baroreceptor inhibitory mechanism effect on subjective well-being (indexed by stressed and sleepy moods) was mediated by the wake/sleep diastolic blood pressure difference. Conclusions: Evaluation of the baroreflex components and their interaction provides important information about blood pressure and mood beyond that provided by baroreflex sensitivity and potential for the evaluation of risk for hypertension and mood disturbance in healthy people.

Current computational models do not reveal the importance of the nervous system in long-term control of arterial pressure

Arterial pressure is regulated over long periods of time by neural, hormonal and local control mechanisms, which ultimately determine the total blood volume and how it is distributed between the various vascular compartments of the circulation. A full understanding of the complex interplay of these mechanisms can be greatly facilitated by the use of mathematical models. In 1967, Guyton and Coleman published a model for long-term control of arterial pressure that focused on renal control of body sodium and water and thus total blood volume. The central point of their model is that the long-term level of arterial pressure is determined exclusively by the 'renal function curve', which relates arterial pressure to urinary excretion of salt and water. The contribution of the sympathetic nervous system to setting the long-term level of arterial pressure in the model is limited. In light of the overwhelming evidence for a major role of the sympathetic nervous system in long-term control of arterial pressure and the pathogenesis of hypertension, new mathematical models for long-term control of arterial pressure may be necessary. Despite the prominence and general acceptance of the Guyton-Coleman model in the field of hypertension research, we argue here that it overestimates the importance of renal control of body fluids and total blood volume in blood pressure regulation. Furthermore, we suggest that it is possible to construct an alternative model in which sympathetic nervous system activity plays an important role in long-term control of arterial pressure independent of its effects on total blood volume.

Baroreflex contribution to blood pressure and heart rate oscillations: time scales, time-variant characteristics and nonlinearities

The aim of this paper is to highlight the aspects of the baroreflex control of the cardiovascular system that could be relevant to the analysis and modelling of cardiovascular oscillations and regulation. In particular, complex and/or controversial issues of the baroreflex control are addressed on the basis of results obtained in previous studies by others as well as by our group. Attention has been focused on time-variant and nonlinear characteristics of the baroreflex function and on the influence of this physiological mechanism on different frequency regions of blood pressure and heart rate spectra.

Dynamic transcriptomic response to acute hypertension in the nucleus tractus solitarius

Baroreceptor afferents project to the cardiovascular region of the nucleus tractus solitarius (cvNTS), and their cvNTS target neurons may play a role in governing the sensitivity and operating range of the arterial baroreceptor reflex (baroreflexes). Recent studies have shown differential gene and protein expression in the cvNTS in response to changed arterial pressure. However, the extent of these responses is unknown. Therefore, we collected differential global gene expression data in a time series following acute hypertension in awake, freely moving rats. To acquire statistically significant results and place them in functional context, we overcame several quality control requirements and developed novel analytical approaches. The physiologically new findings from the study are that acute hypertension causes very extensive, time-varying gene regulatory changes, many involving neuronal function-specific genes and systems of genes. We use standard genomic analysis methods to manage the large data sets and to develop results such as heat maps to examine patterns and clusters in the gene regulation. We used the Gene Ontology categories to provide functional context. To place our findings in the context of the relevant literature, we developed two graphical representations of the networks implicated, linking receptors and channels to signaling pathways. The results point to the multivariate complexity of the response and implicate a group of receptors as candidates for mediating nucleus tractus solitarius baroreflex function in hypertension by identifying concurrent upregulation of receptor genes. We were able to make transcription factor binding predictions and record dysregulation of heart rate correlated with the transcriptional response.

Prolonged Activation of the Baroreflex Abolishes Obesity-Induced Hypertension

Prolonged electrical activation of the carotid baroreflex produces sustained reductions in sympathetic activity and arterial pressure in normotensive dogs. The main goal of this study was to assess the influence of prolonged baroreflex activation on arterial pressure and neurohormonal responses in 6 dogs with obesity-induced hypertension. After control measurements, the diet was supplemented with cooked beef fat for 6 weeks, whereas sodium intake was held constant. After 4 weeks of the high-fat diet, there were increments in body weight from 25.8+/-0.7 to 38.6+/-1.0 kg, mean arterial pressure from 97+/-2 to 110+/-3 mm Hg, heart rate from 67+/-3 to 91+/-4 bpm, and plasma norepinephrine concentration from 141+/-35 to 280+/-52 pg/mL. Plasma glucose and insulin concentrations were elevated, but increases in plasma renin activity during the initial weeks of the high-fat diet were not sustained. During week 5, baroreflex activation resulted in sustained reductions in mean arterial pressure, heart rate, and plasma norepinephrine concentration; at the end of week 5, these values were 87+/-2 mm Hg, 77+/-4 bpm, and 166+/-45 pg/mL, respectively. These suppressed values returned to week 4 levels during a 7-day recovery period after baroreflex activation. There were no changes in plasma glucose or insulin concentrations, or plasma renin activity during prolonged baroreflex activation. These findings indicate that baroreflex activation can chronically suppress the sympathoexcitation associated with obesity and abolish the attendant hypertension while having no effect on hyperinsulinemia or hyperglycemia.

To Dip or Not to Dip

That sleep is accompanied by a blood pressure decrease is well known; however, the underlying physiology deserves further investigation. The present study examines in healthy subjects 2 main questions: is this dipping actively evoked? and what are the consequences of nondipping for daytime blood pressure? Nocturnal blood pressure was extrinsically elevated in 12 sleeping subjects to mean daytime values by continuously infused phenylephrine. This nondipping significantly lowered morning blood pressure during rest and 3 hours after resuming physical activity compared with a control condition (isotonic saline). Neither muscle sympathetic nerve activity nor sensitivity of α-adrenoceptors was reduced. However, the set point for initiation of regulatory responses through the baroreflex was clearly shifted toward lower blood pressure levels. Our results support the hypothesis of an actively regulated central mechanism for blood pressure resetting and set point consolidation of the baroreflex during nighttime sleep. This is suggested by the fact that extrinsically induced nondipping induces sustained decrease in blood pressure during the following morning through an actively lowered baroreflex set point.

Effect of aging on baroreflex function in humans

Arterial blood pressure (BP) is regulated via the interaction of various local, humoral, and neural factors. In humans, the major neural pathway for acute BP regulation involves the baroreflexes. In response to baroreceptor activation/deactivation, as occurs during transient changes in BP, key determinants of BP, such as cardiac period/heart rate (via the sympathetic and parasympathetic nervous system) and vascular resistance (via the sympathetic nervous system), are modified to maintain BP homeostasis. In this review, the effects of aging on both the parasympathetic and sympathetic arms of the baroreflex are discussed. Aging is associated with decreased cardiovagal baroreflex sensitivity (i.e., blunted reflex changes in R-R interval in response to a change in BP). Mechanisms underlying this decrease may involve factors such as increased levels of oxidative stress, vascular stiffening, and decreased cardiac cholinergic responsiveness with age. Consequences of cardiovagal baroreflex impairment may include increased levels of BP variability, an impaired ability to respond to acute challenges to the maintenance of BP, and increased risk of sudden cardiac death. In contrast, baroreflex control of sympathetic outflow is not impaired with age. Collectively, changes in baroreflex function with age are associated with an impaired ability of the organism to buffer changes in BP. This is evidenced by the reduced potentiation of the pressor response to bolus infusion of a pressor drug after compared to before systemic ganglionic blockade in older compared with young adults.

Vasomotor sympathetic neural control is maintained during sustained upright posture in humans

Vasomotor sympathetic activity plays an important role in arterial pressure maintenance via the baroreflex during acute orthostasis in humans. If orthostasis is prolonged, blood pressure may be supported additionally by humoral factors with a possible reduction in sympathetic baroreflex sensitivity. We tested the hypothesis that baroreflex control of muscle sympathetic nerve activity (MSNA) decreases during prolonged upright posture. MSNA and haemodynamics were measured supine and during 45 min 60 deg upright tilt in 13 healthy individuals. Sympathetic baroreflex sensitivity was quantified using the slope of the linear correlation between MSNA and diastolic pressure during spontaneous breathing. It was further assessed as the relationship between MSNA and stroke volume, with stroke volume derived from cardiac output (C2H2 rebreathing) and heart rate. Total peripheral resistance was calculated from mean arterial pressure and cardiac output. We found that MSNA increased from supine to upright (17+/-8 (S.D.) versus 38+/-12 bursts min-1; P<0.01), and continued to increase to a smaller degree during sustained tilt (39+/-11, 41+/-12, 43+/-13 and 46+/-15 bursts min-1 after 10, 20, 30 and 45 min of tilt; between treatments P<0.01). Sympathetic baroreflex sensitivity increased from supine to upright (-292+/-180 versus -718+/-362 units beat-1 mmHg-1; P<0.01), but remained unchanged as tilting continued (-611+/-342 and -521+/-221 units beat-1 mmHg-1 after 20 and 45 min of tilt; P=0.49). For each subject, changes in MSNA were associated with changes in stroke volume (r=0.88+/-0.13, P<0.05), while total peripheral resistance was related to MSNA during 45 min upright tilt (r=0.82+/-0.15, P<0.05). These results suggest that the vasoconstriction initiated by sympathetic adrenergic nerves is maintained by ongoing sympathetic activation during sustained (i.e. 45 min) orthostasis without obvious changes in vasomotor sympathetic neural control.

Glucocorticoids act in the dorsal hindbrain to modulate baroreflex control of heart rate

Systemic corticosterone (Cort) modulates arterial baroreflex control of both heart rate and renal sympathetic nerve activity. Because baroreceptor afferents terminate in the dorsal hindbrain (DHB), an area with dense corticosteroid receptor expression, we tested the hypothesis that prolonged activation of DHB Cort receptors increases the midpoint and reduces the gain of arterial baroreflex control of heart rate in conscious rats. Small (3-4 mg) pellets of Cort (DHB Cort) or Silastic (DHB Sham) were placed on the surface of the DHB, or Cort was administered systemically by placing a Cort pellet on the surface of the dura (Dura Cort). Baroreflex control of heart rate was determined in conscious male Sprague Dawley rats on each of 4 days after initiation of treatment. Plots of arterial pressure vs. heart rate were analyzed using a four-parameter logistic function. After 3 days of treatment, the arterial pressure midpoint for baroreflex control of heart rate was increased in DHB Cort rats (123 +/- 2 mmHg) relative to both DHB Sham (108 +/- 3 mmHg) and Dura Cort rats (109 +/- 2 mmHg, P < 0.05). On day 4, baseline arterial pressure was greater in DHB Cort (112 +/- 2 mmHg) compared with DHB Sham (105 +/- 2 mmHg) and Dura Cort animals (106 +/- 2 mmHg, P < 0.05), and the arterial pressure midpoint was significantly greater than mean arterial pressure in the DHB Cort group only. Also on day 4, maximum baroreflex gain was reduced in DHB Cort (2.72 +/- 0.12 beats x min(-1) x mmHg(-1)) relative to DHB Sham and Dura Cort rats (3.51 +/- 0.28 and 3.37 +/- 0.27 beats x min(-1) x mmHg(-1), P < 0.05). We conclude that Cort acts in the DHB to increase the midpoint and reduce the gain of the heart rate baroreflex function.