Arterial baroreceptor resetting in hypertension (the J. W. McCubbin memorial lecture)

The Origin and Advancement of Cardiovascular Physiology in Brazil: The Contribution of Eduardo Krieger to Research Groups

Since 1996, symposia devoted to the discussion of advances in cardiovascular physiology have been alternately organized by Brazilian research groups, most of which were created or joined by Ph.D. trainees of Eduardo M Krieger. Therefore, as Frontiers in Physiology is publishing a topic devoted to the celebration of the 20th edition of the Brazilian Symposium of Cardiovascular Physiology, it is a great opportunity to talk about the contributions of Eduardo Krieger to the development of cardiovascular physiology. In this historical mini-review, first, the influence of the Argentinian group of Bernardo Houssay and Braun Menéndez on cardiovascular physiology in Brazil is discussed. Second, the contribution of Eduardo Krieger to the creation of several of those groups and to the development of science and technology is reviewed. Finally, the origin and consolidation of the group of Vitoria is highlighted as an example of a research group that was influenced by the University of Sao Paulo-Faculty of Medicine of Ribeirao Preto and has trained hundreds of Master and Ph.D. students in the area of cardiovascular research.

Baroreflex activation therapy for patients with drug-resistant hypertension

Uncontrolled or resistant hypertension is still a major problem facing many physicians daily in the clinic. Several new therapies are being developed to help those patients whose blood pressure does not respond sufficiently to regular antihypertensive medication. One of these promising therapies is electrical activation of the carotid sinus baroreflex. In this overview, the authors predominantly summarize the background, efficacy and safety of this promising treatment with its latest achievements in patients with resistant hypertension. The authors also discuss certain issues that need further clarification before this therapy can be added to the common treatment guidelines of hypertension.

Medial prefrontal cortex N-methyl-D-aspartate receptor/nitric oxide/cyclic guanosine monophosphate pathway modulates both tachycardic and bradycardic baroreflex responses

Neural reflex mechanisms, such as the baroreflex, are involved in regulating cardiovascular system activity. Previous results showed that the ventral portion of the medial prefrontal cortex (vMPFC) is involved in modulation only of the cardiac baroreflex bradycardic component. Moreover, vMPFC N-methyl-D-aspartate (NMDA) receptors modulate the bradycardia baroreflex, but the baroreflex tachycardic component has not been investigated. Furthermore, glutamatergic neurotransmission into the vMPFC is involved in activation of the cardiac sympathetic and parasympathetic nervous system. Finally, it has been demonstrated that glutamatergic neurotransmission into the vMPFC can be modulated by the endocannabinoid system and that activation of the CB1 cannabinoid receptor by anandamide, an endocannabinoid, can decrease both cardiac baroreflex bradycardic and tachycardic responses. Thus, there is the possibility that glutamatergic neurotransmission into the vMPFC does not modulate only the cardiac bradycardic component of the baroreflex. Therefore, the present study investigated whether glutamatergic neurotransmission into the vMPFC modulates both cardiac baroreflex bradycardic and tachycardic responses. We found that vMPFC bilateral microinjection of the NMDA receptor antagonist AP7 (4 nmol/200 nl), of a selective inhibitor of neuronal nitric oxide (NO) synthase N-propyl (0.08 nmol/200 nl), of the NO scavenger carboxy-PTIO (2 nmol/200 nl), or of the NO-sensitive guanylate cyclase ODQ (2 nmol/200 nl) decreased the baroreflex activity in unanesthetized rats. Therefore, our results demonstrate the participation of NMDA receptors, production of NO, and activation of guanylate cyclase in the vMPFC in the modulation of both cardiac baroreflex bradycardic and tachycardic responses.

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.

Cardiac mechanoreceptor function implicated during premature ventricular contraction

In a premature ventricular contraction (PVC), a systolic blood pressure peak is missing during the affected cardiac cycle, leading to a prolonged reduction in blood pressure which is then followed by a large burst of sympathetic outflow. In a normal ventricular contraction, it is generally believed that peak carotid and aortic distensions associated with systolic pressure is the neural feedback that terminates sympathetic outflow through a baroreflex mechanism. Yet, the characteristically large sympathetic burst following a PVC is terminated without a systolic pressure and evidently without this mechanism. To address this anomaly, we examined the possible role of cardiac receptors in providing an alternative mechanism for the termination of sympathetic outflow in a PVC. For this purpose, recordings of electrocardiogram (ECG), arterial blood pressure (ABP), and muscle sympathetic neural activity (MSNA) were made in a human subject during repeated PVC episodes. The time intervals, or "latencies", from key events within the PVC to the peak of the associated MSNA burst were calculated and compared with the latency in a normal ventricular contraction which is associated with central baroreceptor function. It was found that the only event in a PVC that corresponds with a physiologically plausible latency is that which marks the end of ventricular filling. We conclude with the hypothesis that in the unique circumstances of a PVC, where the systolic pressure peak required to trigger arterial baroreceptors to terminate sympathetic outflow is absent, mechanoreceptors in the heart appear to "step in" to perform this sympathoinhibitory function.

Role of the Multidomain Protein Spinophilin in Blood Pressure and Cardiac Function Regulation

Spinophilin controls intensity/duration of G protein-coupled receptor signaling and thereby influences synaptic activity. We hypothesize that spinophilin affects blood pressure through central mechanisms. We measured blood pressure and heart rate in SPL-deficient (SPL −/− ), heterozygous SPL-deficient (SPL +/− ), and wild-type (SPL +/+ ) mice by telemetry combined with fast Fourier transformation. We also assessed peripheral vascular reactivity and performed echocardiography. SPL −/− had higher mean arterial pressure than SPL +/− and SPL +/+ (121±2, 112±1, and 113±1 mm Hg). Heart rate was inversely related to spinophilin expression (SPL −/− 565±0.4, SPL +/− 541±5, SPL +/+ 525±8 bpm). The blood pressure response to prazosin, trimethapane, and the heart rate response to metoprolol were stronger in SPL −/− than SPL +/+ mice, whereas heart rate response to atropine was attenuated in SPL −/− . Mesenteric artery vasoreactivity after angiotensin II, phenylephrine, and the thromboxane mimetic (U46619) as well as change in heart rate, stroke volume, and cardiac output after dobutamine were similar in SPL −/− and SPL +/+ . Baroreflex sensitivity was attenuated in SPL −/− compared with SPL +/− and SPL +/+ , which was confirmed by pharmacological testing. Heart rate variability parameters were attenuated in SPL −/− mice. We suggest that an increase in central sympathetic outflow participates in blood pressure and heart rate increases in SPL −/− mice. The elevated blood pressure in SPL −/− mice was associated with attenuated baroreflex sensitivity and decreased parasympathetic activity. Our study is the first to show a role for the spinophilin gene in blood pressure regulation.

Baroreflex sensitivity and oxidative stress in the LDL receptor knockout mice

This study aims at observing the effect of low-density lipoprotein (LDL) receptor deficiency in cholesterol blood levels, baroreflex sensitivity (BRS), nitric oxide (NO) bioavailability, and oxidative stress. The lack of LDL receptors in mice significantly increased the cholesterol blood levels (179+/-35 vs. 109+/-13mg/dL) in the knockout (KO) mice compared to control. There was no difference in basal mean arterial pressure and heart rate between the groups. However, in KO mice the BRS was significantly attenuated and the antioxidant enzyme activities, measured in erythrocytes and heart, were significantly decreased. On the other hand, the oxidative damage measured by chemiluminescence and carbonyls was increased, while total plasma nitrate levels were lower in KO mice, indicating a decrease in NO availability. In conclusion, these results indicate that the lack of LDL receptor increased cholesterol blood levels, induced oxidative stress and decreased BRS.

Mechanism of cardioventilatory coupling: insights from cardiac pacing, vagotomy, and sinoaortic denervation in the anesthetized rat

Cardioventilatory coupling (CVC), a temporal alignment between the heartbeat and inspiratory activity, is a major determinant of breath-to-breath variation in observed respiratory rate (f(o)). The cardiac-trigger hypothesis attributes this to adjustments of respiratory timing by baroreceptor afferent impulses to the central respiratory pattern generator. A mathematical model of this hypothesis indicates that apparent CVC in graphical plots of ECG R wave vs. inspiratory time is dependent on the heart rate (HR), the rate of the intrinsic respiratory oscillator (f(i)), and the strength of the hypothetical cardiovascular afferent impulse. Failure to account for HR and f(i) may explain the inconsistent results from previous attempts to identify the neural pathways involved in CVC. Cognizant of these interactions, we factored in the HR-to-f(i) ratio in our examination of the role of the vagus nerve and arterial baroreceptors in CVC by cardiac pacing 29 anesthetized Sprague-Dawley rats and incrementally changing the HR. With the assumption of a relatively constant f(i), CVC could be examined across a range of HR-to-f(o) ratios before and after vagotomy, sinoaortic denervation, and vagotomy + sinoaortic denervation. We confirmed the relation between CVC, HR-to-f(o) ratio, and breath-to-breath respiratory period variability and demonstrated the loss of these relations after baroreceptor elimination. Sham experiments (n = 8) showed that these changes were not due to surgical stress. Our data support the notion that inspiratory timing can be influenced by cardiac afferent activity. We conclude that the putative cardiovascular input arises from the arterial baroreceptors and that the vagus nerve is not critical for CVC.

RESETTING OF AORTIC BARORECEPTORS IN RESPONSE TO HYPOTENSION DOES NOT ALTER GAIN SENSITIVITY

1. In chronic hypertension, the baroreceptors reset to hypertensive levels with a decrease in gain sensitivity, but only a few studies have evaluated baroreceptor resetting during chronic hypotension and, under these conditions, no consistent information is available concerning changes in baroreceptor gain sensitivity. Therefore, in the present study, the aortic baroreceptor function curve and the baroreflex control of heart rate (HR) were evaluated in chronic hypotension produced by myocardial infarction (MI) with no heart failure. 2. Aortic baroreceptor function curves were studied in anaesthetized three groups of rats: (i) MI-7, six rats 7 days after MI; (ii) MI-30, nine rats 30 days after MI; and (iii) five control animals (SHAM). The pressure-nerve activity relationship was measured during rapid changes in blood pressure by integrating the whole-nerve activity of the baroreceptors in a computerized beat-to-beat analysis. 3. Both long-term periods (7 or 30 days) of hypotension were accompanied by complete resetting of the baroreceptor in rats (the leftward displacement of the baroreceptor curve matched the decrease in blood pressure). Moreover, the resetting of the baroreceptor function curve was not accompanied by changes in gain sensitivity (1.47, 1.64 and 1.67%/mmHg for SHAM, MI-7 and MI-30 groups, respectively) and the baroreflex control of HR was normal comparing SHAM and MI-30 groups (bradycardic 1.62 +/- 0.18 vs 1.99 +/- 0.52 b.p.m./mmHg, respectively; tachycardic 3.6 +/- 0.5 vs 4.1 +/- 0.4 b.p.m./mmHg for, respectively). 4. The data indicate that the resetting of baroreceptors in chronic hypotension is stable and is not accompanied by changes in gain sensitivity, as observed in hypertension. This may account for the normal baroreflex control of HR observed in non-anaesthetized rats.

Interactions between the cardiovascular and pain regulatory systems: an updated review of mechanisms and possible alterations in chronic pain

Endogenous pain regulatory system dysfunction appears to play a role in the maintenance of chronic pain. An important component of the pain regulatory process is the functional interaction between the cardiovascular and pain regulatory systems, which results in an association between elevated resting blood pressure (BP) and diminished acute pain sensitivity. This BP/pain sensitivity relationship is proposed to reflect a homeostatic feedback loop helping restore arousal levels in the presence of painful stimuli. Evidence is emerging that this normally adaptive BP/pain sensitivity relationship is significantly altered in chronic pain conditions, affecting responsiveness to both acute and chronic pain stimuli. Several mechanisms that may underlie this adaptive relationship in healthy individuals are overviewed, including endogenous opioid, noradrenergic, and baroreceptor-related mechanisms. Theoretical models are presented regarding how chronic pain-related alterations in the mechanisms above and increased pain facilatory system activity (central sensitization) may contribute to altered BP/pain sensitivity interactions in chronic pain. Clinical implications are discussed.

Effect of a perinatal high-salt diet on blood pressure control mechanisms in young Sprague-Dawley rats

In the present investigation we sought to determine if a perinatal high-salt treatment affects blood pressure at an early age (30 days), and if so, to determine the mechanisms responsible for the hypertension. Pregnant dams were given an 8% NaCl diet [high-salt (HS) rats] during the final one-third of gestation and throughout the suckling period. After weaning, the pups continued to receive the high-salt diet until testing at age 30 days. Control groups received a normal-salt diet (NS rats). In HS rats, mean arterial pressure (MAP) was significantly increased (110 ± 5 vs. 96 ± 3 mmHg) compared with NS rats. Blockade of brain AT1 receptors with intracerebroventricular losartan decreased MAP in HS but not NS rats. Blockade of α-adrenergic receptors with intravenous phentolamine or ganglionic transmission with intravenous chlorisondamine produced a greater decrease in MAP in HS rats. Baroreflex control of heart rate was assessed using a four-parameter logistics function. The mid-range MAP (p3) was significantly increased in the HS rats. No other baroreflex parameters were affected. Specific binding of 125I-[Sar1,Ile8]ANG II to AT1 receptors was increased in the subfornical organ (SFO) of the HS rats. Expression of AT1a receptor mRNA was greater in both SFO and PVN of the HS rats. These data suggest that even at an early age, Sprague-Dawley rats treated with a perinatal high-salt diet are hypertensive. The elevated blood pressure appears to be caused by increased sympathetic nervous activity, resulting, in part, from increased brain AT1 receptor activation.

What sets the long-term level of sympathetic nerve activity: is there a role for arterial baroreceptors?

Much of our knowledge of the influence of the sympathetic nervous system on the control of blood pressure is built on experimental approaches that focus very much on time scales <24 h. Although direct recordings of sympathetic nerve activity (SNA) over short time scales provide important information, it is difficult to place their relevance over the longer term where the development of chronic changes in blood pressure are likely to be a mixture of hormonal, renal, and neural influences. Recently new experimental approaches are now revealing a possible role for arterial baroreceptors in the chronic regulation of SNA. These studies reveal that chronic increases in blood pressure are associated with chronic changes in SNA that may be due to nonresetting of the blood pressure-SNA baroreflex relationship. This review discusses the implications of such information, highlighting new technologies for long-term recording of SNA that appear to hold much promise for revealing the role of SNA to the kidney for the long-term control of blood pressure.

Laboratory demonstration of baroreflex control of heart rate in conscious rats

We have developed a laboratory exercise that demonstrates arterial baroreflex control of heart rate (HR) in the conscious unrestrained rat, incorporating graduate level physiological topics as well as a hands-on exposure to conscious animal research. This demonstration utilizes rats chronically instrumented to measure cardiac output (CO), HR, and arterial blood pressure in response to agents that raise or lower blood pressure. The HR response to progressive increases or decreases in blood pressure is recorded, and a baroreflex curve is generated by plotting mean arterial blood pressure (MABP) vs. HR. Observation of altered CO allows for discussion of the relationship between MAP, CO, HR, stroke volume, and total peripheral resistance. Administration of arginine vasopressin demonstrates the ability of this hormone to alter the sensitivity of the baroreflex. Throughout the demonstration, students answer questions from a handout about general cardiovascular physiology, specific pathways of agonists, and the baroreflex system, encouraging group and individual critical analysis of the results. Interpretation of the data reemphasizes lecture material and allows students to observe the baroreflex response in a physiological setting.

Pregnancy and acute baroreflex resetting in conscious rabbits

To test the hypothesis that acute resetting of baroreflex control of heart rate (HR) is enhanced during pregnancy, we determined whether the rightward shift in the baroreflex relationship between arterial pressure and HR after arterial pressure is raised [~25 mmHg for 30 min, due to infusion of phenylephrine (PE) or methoxamine (Meth)] is greater in late pregnant compared with nonpregnant conscious rabbits. Baroreflex function was assessed by monitoring HR responses to both stepwise steady-state changes (n = 14) and rapid ramp changes (n = 10) in arterial pressure. Pregnancy decreased reflex gain, increased reflex minimum HR, and shifted the curves to a lower pressure level, when either the steady-state or ramp method was used (all changes, P < 0.05). When PE was used to increase pressure, resetting of steady-state curves was observed both before and during pregnancy, but the magnitude of the resetting was less in the pregnant rabbits. Further inspection of the data revealed that the size of the shift in pregnant rabbits was inversely related to the dose of PE. Because the pressure rise was the same in all experiments, PE appears to nonspecifically counteract acute resetting. When Meth was used instead to increase pressure, resetting of steady-state curves was similar in pregnant and nonpregnant rabbits and was unrelated to dose. Similarly, when reflex curves were generated using the ramp method, and either Meth or low doses of PE were used to increase pressure, no differences in the degree of resetting were observed between pregnant and nonpregnant rabbits. In summary, high doses of PE counteract acute resetting of baroreflex control of HR. More importantly, while baroreflex function is depressed, the ability of the baroreflex to reset appears to be preserved during pregnancy.

Baroreflex depression persists in the early phase after hypertension reversal

The baroreflex control of heart rate (HR) was evaluated in conscious chronic renal hypertensive rats (RHR; 1K-1C, 2 mo) under control conditions and after reversal of hypertension by unclipping the renal artery or sodium nitroprusside infusion. Unclipping and nitroprusside infusion were both followed by significant decreases in the mean arterial pressure (unclipping: from 199 +/- 4 to 153 +/- 8 mmHg; nitroprusside infusion: from 197 +/- 9 to 166 +/- 6 mmHg) as well as slight and significant increases, respectively, in the baroreflex bradycardic response index (unclipping: from 0.2 +/- 0.04 to 0.6 +/- 0.1 beats x min(-1) x mmHg(-1); nitroprusside infusion: from 0.1 +/- 0.04 to 0.5 +/- 0.1 beats x min(-1) x mmHg(-1)). However, this index was still depressed compared with that for normotensive control rats (2.1 +/- 0.2 beats x min(-1) x mmHg(-1)). The index for the baroreflex tachycardic response was also depressed under control conditions and remained unchanged after hypertension reversal. RHR possessed markedly attenuated vagal tone as demonstrated by pharmacological blockade of parasympathetic and sympathetic control of HR with methylatropine and propranolol, respectively. A reduced bradycardic response was also observed in anesthetized RHR during electrical stimulation of the vagus nerve or methacholine chloride injection, indicating impairment of efferent vagal influence over the HR. Together, these data indicate that 2 h after hypertension reversal in RHR, the previously described normalization of baroreceptor gain occurs independent of the minimal or lack of recovery of baroreflex control over HR.

The interaction of angiotensin II and osmolality in the generation of sympathetic tone during changes in dietary salt intake. An hypothesis

At rest, sympathetic nerves exhibit tonic activity which contributes to arterial pressure maintenance. Significant evidence suggests that the absolute level of sympathetic tone is altered in a number of physiologic and pathophysiologic states. However, the mechanisms by which such changes in sympathetic tone occur are incompletely understood. The purpose of this review is to present evidence that humoral factors are essential in these changes and to detail specifically an hypothesis for the mechanisms that underlie the changes in sympathetic tone that are produced during increases or decreases in dietary salt intake. It is proposed that the net effect of changes in dietary salt on sympathetic activity is determined by the balance between simultaneous and parallel sympathoinhibitory and sympathoexcitatory humoral mechanisms. A key element of the sympathoinhibitory mechanism is the chronic sympathoexcitatory effects of angiotensin II (ANG II). When salt intake increases, ANG II levels fall, and the sympathoexcitatory actions of ANG II are lost. Simultaneously, a sympathoexcitatory pathway is triggered, possibly via increases in osmolality which activate osmoreceptors or sodium receptors. In normal individuals, the sympathoinhibitory effects of increased salt predominate, sympathetic activity decreases, and arterial pressure remains normal despite salt and water retention. However, in subjects with salt-sensitive hypertension, it appears that the sympathoexcitatory effects of salt predominate, possibly due to an inability to adequately suppress the levels or actions of ANG II. The net result, therefore, is an inappropriate increase in sympathetic activity during increased dietary salt which may contribute to the hypertensive process.

Role of the locus ceruleus in baroreceptor regulation of supraoptic vasopressin neurons in the rat

The goal of this study was to identify the source of baroreceptor-related noradrenergic innervation of the diagonal band of Broca (DBB). Male Sprague-Dawley rats underwent sinoaortic denervation (SAD, n = 13) or sham SAD surgery (n = 13). We examined Fos expression produced by baroreceptor activation and dopamine-beta-hydroxylase immunofluorescence in hindbrain regions that contain noradrenergic neurons. Baroreceptors were stimulated by increasing blood pressure >40 mmHg with phenylephrine (10 microgram. kg(-1). min(-1) iv) in sham SAD and SAD rats. Controls were infused with 0.9% saline. Only the locus ceruleus (LC) demonstrated a baroreceptor-dependent increase in Fos immunoreactivity in dopamine-beta-hydroxylase-positive neurons. In a second experiment, normal rats received rhodamine-labeled microsphere injections in the DBB (n = 12) before phenylephrine or vehicle infusion. In these experiments, only the LC consistently contained Fos-positive cells after phenylephrine infusion that were retrogradely labeled from the DBB. Finally, we lesioned the LC with ibotenic acid and obtained extracellular recordings from identified vasopressin neurons in the supraoptic nucleus. LC lesions significantly reduced the number of vasopressin neurons that were inhibited by acute baroreceptor stimulation. Together, these results suggest that noradrenergic neurons in the LC participate in the baroreflex activation of the DBB and may thus be important in the baroreflex inhibition of vasopressin-releasing neurons in the supraoptic nucleus.

Nociceptin modulates renal sympathetic nerve activity through a central action in conscious rats

Nociceptin, an endogenous agonist of the opioid receptor-like(1) receptor, is expressed in the hypothalamus, where it is implicated in autonomic nervous system control. However, the central actions of nociceptin on sympathetic nerve activity have not been studied. We investigated the effect of intracerebroventricularly administered nociceptin (2-10 nmol) on blood pressure, heart rate (HR), and renal sympathetic nerve activity (RSNA) in conscious rats and sinoaortic-denervated (SAD) rats. Intracerebroventricularly administered nociceptin resulted in a dose-dependent decrease in mean arterial pressure (MAP) and HR in intact rats. RSNA decreased 31.5 +/- 2.1 and 19.9 +/- 5.0% at a dose of 2 and 5 nmol, respectively. In SAD rats, MAP, HR, and RSNA decreased in a dose-dependent manner, and the maximum responses were larger than those in intact rats. The decrease in HR induced by nociceptin was blocked by propranolol but not by atropine, which indicates that nociceptin is acting by inhibiting cardiac sympathetic outflow. These nociceptin-induced depressor and bradycardic responses were not antagonized by pretreatment with naloxone and nocistatin. These findings suggest that central nociceptin may have a functional role in regulating cardiovascular and sympathetic nervous systems.

Comparison of three methods for the determination of baroreflex sensitivity in conscious rats

Baroreflex sensitivity was studied in the same group of conscious rats using vasoactive drugs (phenylephrine and sodium nitroprusside) administered by three different approaches: 1) bolus injection, 2) steady-state (blood pressure (BP) changes produced in steps), 3) ramp infusion (30 s, brief infusion). The heart rate (HR) responses were evaluated by the mean index (mean ratio of all HR changes and mean arterial pressure (MAP) changes), by linear regression and by the logistic method (maximum gain of the sigmoid curve by a logistic function). The experiments were performed on three consecutive days. Basal MAP and resting HR were similar on all days of the study. Bradycardic responses evaluated by the mean index (-1.5 +/- 0.2, -2.1 +/- 0.2 and -1.6 +/- 0.2 bpm/mmHg) and linear regression (-1.8 +/- 0.3, -1.4 +/- 0.3 and -1.7 +/- 0.2 bpm/mmHg) were similar for all three approaches used to change blood pressure. The tachycardic responses to decreases of MAP were similar when evaluated by linear regression (-3.9 +/- 0.8, -2.1 +/- 0.7 and -3.8 +/- 0.4 bpm/mmHg). However, the tachycardic mean index (-3.1 +/- 0.4, -6.6 +/- 1 and -3.6 +/- 0.5 bpm/mmHg) was higher when assessed by the steady-state method. The average gain evaluated by logistic function (-3.5 +/- 0.6, -7.6 +/- 1.3 and -3.8 +/- 0.4 bpm/mmHg) was similar to the reflex tachycardic values, but different from the bradycardic values. Since different ways to change BP may alter the afferent baroreceptor function, the MAP changes obtained during short periods of time (up to 30 s: bolus and ramp infusion) are more appropriate to prevent the acute resetting. Assessment of the baroreflex sensitivity by mean index and linear regression permits a separate analysis of gain for reflex bradycardia and reflex tachycardia. Although two values of baroreflex sensitivity cannot be evaluated by a single symmetric logistic function, this method has the advantage of better comparing the baroreflex sensitivity of animals with different basal blood pressures.

Arterial baroreceptor denervation impairs long-term regulation of arterial pressure during dietary salt loading

Experiments were performed to examine the contribution of arterial baroreceptors to long-term regulation of mean arterial pressure (MAP) during changes in dietary salt intake. Normotensive Sprague-Dawley rats were subjected to either sinoaortic denervation (SAD; n = 8) or Sham surgery (n = 6) and instrumented 1 wk later with radiotelemetry transmitters for continuous minute-to-minute monitoring of MAP and heart rate (HR) over the 8-wk protocol. Rats consumed three levels of dietary NaCl: 0.4% NaCl (week 1), 4.0% NaCl (weeks 2-4), and 8.0% NaCl (weeks 5-7). Rats returned to a 0.4% NaCl diet during the eighth week of the experiment. During week 1 (0.4% NaCl), there were no differences between Sham and SAD groups for 24-h averages of MAP or HR. However, by the third week of 4.0% NaCl, 24-h MAP was elevated significantly from baseline in SAD (10 +/- 2 mmHg) but not Sham (1 +/- 1 mmHg) rats. By the end of the third week of 8.0% NaCl diet, 24-h MAP was elevated 15 +/- 2 mmHg above control in SAD rats compared with a 4 +/- 1 mmHg increase in Sham rats (P < 0.05). Hourly analysis of the final 72 h of each level of dietary salt revealed a marked effect of dietary NaCl on MAP in SAD rats, particularly during the dark cycle. MAP increased approximately 20 and 30 mmHg in SAD rats over the 12-h dark cycle for 4.0 and 8.0% NaCl diets, respectively. In contrast, increased dietary NaCl had no effect on MAP during any phase of the light or dark period in Sham rats. These data support the hypothesis that arterial baroreceptors play a critical role in long-term regulation of MAP under conditions of altered dietary salt intake. Finally, hourly analysis of MAP revealed that the majority of the hypertensive response to increased NaCl occurs during the dark cycle in SAD rats. Hence, previous investigations may have underestimated the magnitude of the hypertensive response to increased dietary NaCl in animals with baroreceptor dysfunction.

Baroreflex control of sympathetic activity in experimental hypertension

The arterial baroreceptor reflex system is one of the most powerful and rapidly acting mechanisms for controlling arterial pressure. The purpose of the present review is to discuss data relating sympathetic activity to the baroreflex control of arterial pressure in two different experimental models: neurogenic hypertension by sinoaortic denervation (SAD) and high-renin hypertension by total aortic ligation between the renal arteries in the rat. SAD depresses baroreflex regulation of renal sympathetic activity in both the acute and chronic phases. However, increased sympathetic activity (100%) was found only in the acute phase of sinoaortic denervation. In the chronic phase of SAD average discharge normalized but the pattern of discharges was different from that found in controls. High-renin hypertensive rats showed overactivity of the renin angiotensin system and a great depression of the baroreflexes, comparable to the depression observed in chronic sinoaortic denervated rats. However, there were no differences in the average tonic sympathetic activity or changes in the pattern of discharges in high-renin rats. We suggest that the difference in the pattern of discharges may contribute to the increase in arterial pressure lability observed in chronic sinoaortic denervated rats.

Circadian rhythms of cardiovascular functions are modulated by the baroreflex and the autonomic nervous system in the rat

BACKGROUND

We assessed the hypothesis that the baroreflex and the autonomic nervous system are important in the control of the circadian rhythms of cardiovascular functions.

METHODS AND RESULTS

We continuously measured blood pressure (BP), heart rate (HR), and locomotor activity in sinoaortic denervated (SAD), sympathectomized, and atropine-injected rats by use of a radiotelemetry system. The circadian rhythm of mean blood pressure (MBP) was selectively disrupted in SAD rats under 12-hour light-dark (LD12:12) cycles as a result of an increase in MBP during the light period and disappeared under constant darkness (DD). The locomotor activity and HR were not remarkably affected by SAD. The circadian rhythm of MBP was suppressed in sympathectomized rats by a decrease in the MBP during the dark period, and the abrupt changes in MBP when the lighting was altered were not seen under LD. Under DD, an MBP rhythm similar to that observed under LD was obtained. Sympathectomized rats also showed lower HR levels during the dark period than intact rats under LD cycles. In atropine-injected rats, the MBP and HR increased, especially during the light period, resulting in a reduction of light-dark differences in MBP and HR. The locomotor activity showed an apparent 24-hour variation in the sympathectomized and atropine-injected rats.

CONCLUSIONS

The disruption of the baroreflex selectively eliminates the circadian rhythm of BP, and the circadian rhythms of BP and HR are modulated by the autonomic nervous system in rats. The circadian rhythms of BP and HR are regulated by different mechanisms involving the autonomic nervous system.

Interactions between angiotensin II and the sympathetic nervous system in the the long-term control of arterial pressure

1. The role of the renin-angiotensin system in long-term control of sympathetic activity and arterial pressure is reviewed. 2. There is evidence that favours a necessary role for the sympathetic nervous system in long-term arterial pressure regulation. First, appropriate changes in sympathetic activity appear to be produced in response to chronic changes in blood volume or blood pressure. Second, prevention of the normal homeostatic decrease in sympathetic activity in response to an increase in sodium intake produces hypertension. 3. Long-term changes in sympathetic activity cannot be mediated by the baroreceptor reflex, because it adapts to sustained changes in pressure. Therefore, an hypothesis is presented that evokes a key role for angiotensin II (AngII) in determining the chronic level of sympathetic activity. The key feature of this model is that the role of AngII is non-adaptive: chronic changes in extracellular fluid volume produce sustained reciprocal changes in AngII, and long-term increases in AngII produce sustained increases in sympathetic activity. 4. Evidence is reviewed that suggests that a lack of the normal suppression in AngII and/or sympathetic activity in response to an increase in sodium intake produces salt-sensitive hypertension.

Baroreceptor function is restored by antihypertensive therapy through lowering of blood pressure in adult SHR

1. We investigated the effects of antihypertensive treatment (8 weeks) with four different agents (trichlormethiazide, atenolol, nicardipine and enalapril) on baroreceptor function in 28 week old spontaneously hypertensive rats (SHR) to measure aortic depressor nerve (ADN) activity. 2. Threshold pressure (Pth) of ADN activity was elevated and the gain sensitivity of the pressure-activity curve, as determined by the maximum gain (Gmax) of a logistic function curve, was depressed in untreated SHR compared to those in untreated Wistar-Kyoto (WKY) rats. 3. Treatment with the four agents similarly reduced blood pressure in SHR. Each of the four agents induced a decrease in Pth and an increase in Gmax to a similar extent in SHR. 4. These findings suggest that antihypertensive therapy in chronic hypertension augments baroreceptor function through the lowering of blood pressure but not through specific pharmacological actions.

Chronic infusion of angiotensin II resets baroreflex control of heart rate by an arterial pressure-independent mechanism

The purpose of this study was to test the hypothesis that chronic infusion of angiotensin II (Ang II) in rabbits resets the cardiac baroreflex to a higher arterial pressure level by a pressure-independent mechanism. This hypothesis was tested by determining whether the resetting would be reversed soon after the Ang II infusion was stopped even if the hypertension was maintained by infusion of another vasoconstrictor. Relationships between arterial pressure and heart rate were determined by infusion of increasing doses of nitroprusside to decrease pressure and increase heart rate, followed by increasing doses of phenylephrine to increase pressure and decrease heart rate. After 9 to 10 days of Ang II infusion (20 ng.kg-1.min-1) arterial pressure was increased from 62 +/- 2 to 94 +/- 3 mm Hg (P < .001), and heart rate was unchanged from control values of 126 +/- 7 beats per minute. The baroreflex relationship between arterial pressure and heart rate was shifted to a higher pressure level after 3 to 4 and 9 to 10 days of Ang II infusion. On these same days the Ang II infusion was replaced with phenylephrine (5.0 +/- 0.4 micrograms.kg-1.min-1), and 30 minutes later arterial pressure decreased slightly (P < .05); however, despite the relative hypotension, heart rate was decreased (P < .005) from 126 +/- 5 to 98 +/- 7 beats per minute (days 3 to 4) and from 132 +/- 4 to 103 +/- 7 beats per minute (days 9 to 10). Moreover, the cardiac baroreflex relationships were shifted back to a lower pressure level (P < .05).(ABSTRACT TRUNCATED AT 250 WORDS)

Differential modulation of baroreceptor sensitivity by long-term antihypertensive treatment

We investigated the effects of long-term oral treatment with four different classes of antihypertensive drugs (a thiazide diuretic [trichlormethiazide, 10 mg/kg per day]; a beta-blocker [atenolol, 90 mg/kg per day]; a calcium channel antagonist [nicardipine, 150 mg/kg per day]; and an angiotensin-converting enzyme inhibitor [enalapril maleate, 10 mg/kg per day]) on aortic baroreceptor activity in spontaneously hypertensive rats with chronic hypertension (36 weeks of age). Treatment with each of the four drugs, given from 10 to 36 weeks of age, similarly decreased arterial pressure (171 +/- 2 to 144 +/- 1 mm Hg, P < .01) and similarly decreased the threshold pressure for baroreceptors (116 +/- 3 to 103 +/- 1 mm Hg, P < .05). The four antihypertensive drugs also potentiated the maximal gain of the pressure-activity relation in these rats (untreated, 1.08 +/- 0.05% maximum/mm Hg); however, nicardipine and enalapril (1.77 +/- 0.04% and 1.70 +/- 0.06% maximum/mm Hg, respectively) augmented the maximal gain to a greater extent (P < .05 to .01) than did trichlormethiazide or atenolol (1.49 +/- 0.05% and 1.42 +/- 0.02% maximum/mm Hg, respectively). When the initiation of treatment was delayed to 28 weeks of age, no differences were found in the effects on either threshold pressure (104 +/- 1 mm Hg) or maximal gain (1.36 +/- 0.03% maximum/mm Hg) for all four drugs.(ABSTRACT TRUNCATED AT 250 WORDS)

Reversibility of baroreceptor hyposensitivity during reversal of hypertension

The extent and characteristics of reversal of baroreceptor resetting after pressure normalization were studied in rats with renal hypertension of 2 months' duration. During the control period, the displacement of the entire baroreceptor function curve was accompanied by a decrease slope, indicating that the gain sensitivity was depressed by 36% in the renal hypertensive rats. In response to changes of +10 and -10 mm Hg in the control pressure, the gain sensitivity was attenuated by 56% and 42%, respectively. Two minutes after unclipping and bleeding when necessary, mean arterial pressure decreased from 171 +/- 11 to 134 +/- 11 mm Hg and remained at approximately the same level for the 2-hour period of observation. The extent of reversal of the mean pressure threshold for activation of the baroreceptors was approximately constant (approximately 60%) in the time range of 2-120 minutes. The extent of reversal was slightly higher when the changes in systolic pressure threshold divided by the total change in control diastolic pressure were calculated (maximal of 83%). During the first 20 minutes, the displacements of the curves were parallel with no change in the depressed gain sensitivity. Complete normalization of gain sensitivity was observed after 90-120 minutes. The data indicate that, within the first 2 hours of pressure normalization of chronic renal hypertensive rats, 1) reversal of the resetting of pressure threshold is pronounced (60-80%) but still incomplete and 2) gain sensitivity returns completely to normal.

Baroreceptor resetting and other determinants of baroreflex properties in hypertension

1. Rapid resetting of the arterial baroreceptor threshold in the normal circulation extends the pressure range over which baroreflexes operate at high gain. During sustained falls and rises in resting blood pressure (BP), changes in reflex threshold may be greater or less than those of the receptors, through interactions with other sources of afferent drive (e.g. cardiac baroreceptors). In chronic hypertension the magnitude of the reflex resetting again corresponds to that of the arterial baroreceptors, probably because of the resetting of the threshold of the cardiac receptors. 2. 'Baroreflexes' in intact animals are compound reflexes with input from both arterial and non-arterial baroreceptors (e.g. cardiac/pulmonary baroreceptors). The steady-state responses can be characterized by BP-autonomic output function curves, which are often sigmoidal, with a well-defined effector response range and gain. Both sets of input contribute to the high gain component close to resting, with the arterial baroreceptors the major source of reflex drive; the non-arterial baroreceptors also contribute over this part of the reflex and their role increases considerably at high and low BP. 3. In chronic mild/moderate hypertension the changes in baroreflex properties are similar to those of moderate acute rises in BP or in cardiac load; heart rate range of the vagal component of the cardiac baroreflex is depressed, gain is slightly enhanced and the Valsalva-total peripheral resistance (TPR) reflex is unaltered. In severe hypertension: (i) vagal heart rate range and gain are further depressed; and (ii) there is depression of the Valsalva-TPR reflex, much as observed in constrictor reflexes during acute hypertension in normal animals. Circulatory disturbances produce engagement of non-arterial baroreceptors more readily in hypertensives than in normotensives; depression of baroreflexes in hypertension is due partly to enhanced drive from these receptors and partly due to reduction in the gain of the arterial baroreceptors. 4. The reflex vagal depression and that of neural constrictor reflexes can be considered as important homeostatic mechanisms that limit the effects of circulatory perturbations on cardiac filling pressures and on excessive rises in vascular resistance.