Relationship between baroreceptor reflex function and end-organ damage in spontaneously hypertensive rats

Central α7 and α4β2 nicotinic acetylcholine receptors offset arterial baroreceptor dysfunction in endotoxic rats

Cardiac autonomic neuropathy is a prominent feature of endotoxemia. Given the defensive role of the cholinergic pathway in inflammation, we assessed the roles of central homomeric α7 and heteromeric α4β2 nAChRs in arterial baroreceptor dysfunction caused by endotoxemia in rats. Endotoxemia was induced by i.v. administration of lipopolysaccharides (LPS, 10 mg/kg), and baroreflex activity was measured by the vasoactive method, which assesses reflex chronotropic responses to increments (phenylephrine, PE) or decrements (sodium nitroprusside, SNP) in blood pressure. Shifts caused by LPS in PE/SNP baroreflex curves and associated decreases in baroreflex sensitivity (BRS) were dose-dependently reversed by nicotine (25-100 μg/kg, i.v.). The nicotine effect disappeared after intracisternal administration of methyllycaconitine (MLA) or dihydro-β-erythroidine (DHβE), selective blockers of α7 and α4β2 receptors, respectively. The advantageous effect of nicotine on BRSPE was replicated in rats treated with PHA-543613 (α7-nAChR agonist) or 5-iodo-A-85380 (5IA, α4β2-nAChRs agonist) in dose-dependent fashions. Conversely, the depressed BRSSNP of endotoxic rats was improved after combined, but not individual, treatments with PHA and 5IA. Central α7 and α4β2 nAChR activation underlies the nicotine counteraction of arterial baroreflex dysfunction induced by endotoxemia. Moreover, the contribution of these receptors depends on the nature of the reflex chronotropic response (bradycardia vs. tachycardia).

Chronic stimulation of group II metabotropic glutamate receptors in the medulla oblongata attenuates hypertension development in spontaneously hypertensive rats

Baroreflex dysfunction is partly implicated in hypertension and one responsible region is the dorsal medulla oblongata including the nucleus tractus solitarius (NTS). NTS neurons receive and project glutamatergic inputs to subsequently regulate blood pressure, while G-protein-coupled metabotropic glutamate receptors (mGluRs) play a modulatory role for glutamatergic transmission in baroreflex pathways. Stimulating group II mGluR subtype 2 and 3 (mGluR2/3) in the brainstem can decrease blood pressure and sympathetic nervous activity. Here, we hypothesized that the chronic stimulation of mGluR2/3 in the dorsal medulla oblongata can alleviate hypertensive development via the modulation of autonomic nervous activity in young, spontaneously hypertensive rats (SHRs). Compared with that in the sham control group, chronic LY379268 application (mGluR2/3 agonist; 0.40 μg/day) to the dorsal medulla oblongata for 6 weeks reduced the progression of hypertension in 6-week-old SHRs as indicated by the 40 mmHg reduction in systolic blood pressure and promoted their parasympathetic nervous activity as evidenced by the heart rate variability. No differences in blood catecholamine levels or any echocardiographic indices were found between the two groups. The improvement of reflex bradycardia, a baroreflex function, appeared after chronic LY379268 application. The mRNA expression level of mGluR2, but not mGluR3, in the dorsal medulla oblongata was substantially reduced in SHRs compared to that of the control strain. In conclusion, mGluR2/3 signaling might be responsible for hypertension development in SHRs, and modulating mGluR2/3 expression/stimulation in the dorsal brainstem could be a novel therapeutic strategy for hypertension via increasing the parasympathetic activity.

Nonlinear identification of the total baroreflex arc: chronic hypertension model

The total baroreflex arc is the open-loop system relating carotid sinus pressure (CSP) to arterial pressure (AP). Its linear dynamic functioning has been shown to be preserved in spontaneously hypertensive rats (SHR). However, the system is known to exhibit nonlinear dynamic behaviors. The aim of this study was to establish nonlinear dynamic models of the total arc (and its subsystems) in hypertensive rats and to compare these models with previously published models for normotensive rats. Hypertensive rats were studied under anesthesia. The vagal and aortic depressor nerves were sectioned. The carotid sinus regions were isolated and attached to a servo-controlled piston pump. AP and sympathetic nerve activity were measured while CSP was controlled via the pump using Gaussian white noise stimulation. Second-order, nonlinear dynamics models were developed by application of nonparametric system identification to a portion of the measurements. The models of the total arc predicted AP 21-43% better (P < 0.005) than conventional linear dynamic models in response to a new portion of the CSP measurement. The linear and nonlinear terms of these validated models were compared with the corresponding terms of an analogous model for normotensive rats. The nonlinear gains for the hypertensive rats were significantly larger than those for the normotensive rats [-0.38 ± 0.04 (unitless) vs. -0.22 ± 0.03, P < 0.01], whereas the linear gains were similar. Hence, nonlinear dynamic functioning of the sympathetically mediated total arc may enhance baroreflex buffering of AP increases more in SHR than normotensive rats.

Arterial pressure and cerebral blood flow variability: friend or foe? A review

Variability in arterial pressure and cerebral blood flow has traditionally been interpreted as a marker of cardiovascular decompensation, and has been associated with negative clinical outcomes across varying time scales, from impending orthostatic syncope to an increased risk of stroke. Emerging evidence, however, suggests that increased hemodynamic variability may, in fact, be protective in the face of acute challenges to perfusion, including significant central hypovolemia and hypotension (including hemorrhage), and during cardiac bypass surgery. This review presents the dichotomous views on the role of hemodynamic variability on clinical outcome, including the physiological mechanisms underlying these patterns, and the potential impact of increased and decreased variability on cerebral perfusion and oxygenation. We suggest that reconciliation of these two apparently discrepant views may lie in the time scale of hemodynamic variability; short time scale variability appears to be cerebroprotective, while mid to longer term fluctuations are associated with primary and secondary end-organ dysfunction.

Inhibition of oxidative stress in rostral ventrolateral medulla improves impaired baroreflex sensitivity in stroke-prone spontaneously hypertensive rats

Reactive oxygen species (ROS) in rostral ventrolateral medulla (RVLM) of brainstem contribute to sympathoexcitation and are critically involved in the pathogenesis of hypertension. Baroreflex sensitivity (BRS) is a valuable prognostic parameter of the autonomic nervous system, and is impaired in hypertension. The aim of the present study was to determine whether or not a chronic reduction of ROS in the RVLM improves impaired BRS in hypertensive rats. We transfected adenovirus vectors encoding either manganese superoxide dismutase (AdMnSOD) or β-galactosidase (AdLacZ) into the RVLM of stroke-prone spontaneously hypertensive rats (SHRSP). We measured BRS using the spontaneous sequence method. BRS was significantly lower in SHRSPs than in Wistar-Kyoto rats. In the AdMnSOD-transfected SHRSP, blood pressure, heart rate, and sympathetic nervous system activation were significantly decreased from day 5 after the gene transfer. BRS in the AdMnSOD-transfected SHRSP was significantly increased from day 4 after the gene transfer with the reduction of ROS in the RVLM. Furthermore, in the AdMnSOD-transfected SHRSP, intravenous infusion of atropine dramatically decreased BRS. In contrast, in the AdLacZ-transfected SHRSP, atropine did not decrease BRS. These results suggest that chronic reduction of ROS in the local RVLM improves the impaired BRS in SHRSP through inhibition of the sympathetic component.

Angiotensin-converting enzyme and Angiotensin-converting enzyme 2 are involved in sinoaortic denervation-induced cardiovascular hypertrophy in rats

The balance of angiotensin-converting enzyme (ACE) and angiotensin-converting enzyme 2 (ACE2) in high blood pressure variability (BPV) induced cardiovascular hypertrophy remains elusive. The aim of the present work was to investigate expression and activity of ACE and ACE2 in the heart and aorta of sinoaortic denervation (SAD) rats with high BPV and normal BP, and explore the potential role of ACE and ACE2 in high BPV-induced cardiovascular damage. Hemodynamics, cardiovascular hypertrophy, angiotensin II (Ang II) concentrations, ACE and ACE2 activity were determined. Cardiac-tissue ACE and ACE2 expression were assayed by real-time polymerase chain reaction and Western blot. Compared with sham-operated rats, systolic BPV and diastolic BPV increased and baroreflex sensitivity decreased significantly in SAD rats. SAD rats presented with obvious cardiovascular hypertrophy characterized by increased ratio of left ventricle weight to body weight and aortic weight to the length of aorta. There was no difference in plasma Ang II concentration between sham-operated and SAD rats. The cardiac and aortic ACE expression, aortic ACE2 expression and ACE activity were elevated in SAD rats. There was no significant difference in cardiac ACE2 expressions between sham-operated and SAD rats. The present work demonstrated that cardiac and aortic ACE expression, aortic ACE2 expression and ACE activity were increased in SAD rats. It is the tissue rather than the circulating renin-angiotensin system that contributes to high BPV-induced cardiovascular hypertrophy.

Synergism of irbesartan and amlodipine on hemodynamic amelioration and organ protection in spontaneously hypertensive rats

AIM

To investigate the synergism of low-doses of amlodipine and irbesartan on reduction of blood pressure variability (BPV), amelioration of baroreflex sensitivity (BRS) and organ protection in spontaneously hypertensive rats (SHR).

METHODS

The rats were administered amlodipine (1 mg·kg(-1)·d(-1)) alone, irbesartan (10 mg·kg(-1)·d(-1)) alone, or the combination of the two drugs for 4 months. The drugs were mixed into the rat chow. Blood pressure (BP) was continuously monitored in conscious animals. After the determination of BRS, the rats were killed for morphological evaluation of organ damages.

RESULTS

The combination of low-dose irbesartan and amlodipine had statistically significant synergism on reduction of BP and BPV, amelioration of BRS and organ protection in SHR. Multiple regression analysis showed that the decrease in left ventricular hypertrophy was associated with the decrease in systolic BPV (r=0.665, P<0.01); the decrease in aortic hypertrophy was associated with the increase in BRS (r=0.656, P<0.01); and the amelioration in renal lesion was associated with the increase in BRS (r=0.763, P<0.01) and the decrease in systolic BPV (r=0.706, P<0.01).

CONCLUSION

Long-term treatment with a combination of low-doses of amlodipine and irbesartan showed significant synergism on reduction of BP and BPV, restoration of BRS and organ protection in SHR. Besides BP reduction, the enhancement of BRS and reduction of BPV might contribute to the organ protection.

Effects of allisartan, a new AT(1) receptor blocker, on blood pressure and end-organ damage in hypertensive animals

AIM

To investigate the effects of allisartan, a new angiotensin II type 1 (AT(1)) receptor antagonist, on blood pressure (BP) and end-organ damage (EOD) in hypertensive rats and dogs.

METHODS

First, a single dose of allisartan was given intragastrically to evaluate the BP reduction in spontaneously hypertensive rats (SHRs), two kidney-one clip (2K1C) renovascular hypertensive rats and dogs, and Beagle dogs with angiotensin II-induced hypertension. Second, allisartan was mixed in rat chow for long-term treatment. After 4 months of drug administration, rats were instrumented to determine BP and baroreflex sensitivity (BRS). Observation of morphologic changes was used to estimate EOD. Third, the acute toxicity of allisartan was compared with that of losartan in mice.

RESULTS

BP was significantly decreased after intragastric administration of allisartan in SHRs, 2K1C rats, 2K1C dogs and Beagle dogs with angiotensin II-induced hypertension. Compared with the control, SHRs that received long-term treatment with allisartan exhibited an improved BRS and organ protective effects. Mice who were administered allisartan experienced less acute toxicity than those treated with losartan.

CONCLUSION

Allisartan is highly effective for BP reduction and organ protection with low toxicity.

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

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

Baroreceptor denervation prevents sympathoinhibition during angiotensin II-induced hypertension

Arterial baroreflexes are well established to provide the basis for short-term control of arterial pressure; however, their role in long-term pressure control is more controversial. We proposed that if the sustained decrease in renal sympathetic nerve activity (RSNA) we observed previously in response to angiotensin II-induced hypertension is baroreflex mediated, then the decrease in RSNA in response to angiotensin II would not occur in sinoaortic-denervated (SAD) animals. Arterial pressure and RSNA were recorded continuously via telemetry in sham and SAD rabbits living in their home cages before, during, and after a 7-day infusion of angiotensin II (50 ng . kg(-1) . min(-1)). The arterial pressure responses in the 2 groups of rabbits were not significantly different (82+/-3 mm Hg sham versus 83+/-3 mm Hg SAD before angiotensin II infusion, and 101+/-6 mm Hg sham versus 100+/-4 mm Hg SAD day 6 of angiotensin II). In sham rabbits, there was a significant sustained decrease in RSNA (53+/-7% of baseline on day 2 and 65+/-7% on day 6 of the angiotensin II). On ceasing the angiotensin II, all variables recovered to baseline. In contrast, RSNA did not change in SAD rabbits with the angiotensin II infusion (RSNA was 98+/-8% of baseline on day 2 and 98+/-8% on day 6 of the angiotensin II infusion). These results support our hypothesis that the reduction in RSNA in response to a pressor dose of angiotensin II is dependent on an intact arterial baroreflex pathway.

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.

High-level apoptosis is persistent in myocardiocytes of sinoaortic-denervated rats

BACKGROUND

Arterial baroreflex plays an important part in the regulation of cardiovascular activity. Sinoaortic denervation (SAD) produces organ damage in rats. A previous study suggested that apoptosis in myocardium is involved in the organ damage induced by SAD.

OBJECTIVE

To study the time course of cardiomyocyte apoptosis in SAD rats by evaluating apoptotic cells and expression of apoptosis-related genes in the left ventricles of SAD rats, 4, 8, 16 and 32 weeks after SAD operation.

METHODS

Male Sprague-Dawley rats underwent SAD or sham operation at the age of 10 weeks. Four, 8, 16 or 32 weeks after operation, blood samples and heart tissues were taken for the following studies: determination of angiotensin II in plasma and heart, pathological evaluations, terminal deoxynucleotidyl transferase (TdT)-mediated dUTP nick-end labelling and immunohistochemistry. Some observations were also made in rats 1 and 2 weeks after denervation.

RESULTS

Loss of body weight gain, cardiac hypertrophy, an increase in left ventricular collagen volume and an increase in cardiac angiotensin II content were observed in SAD rats from 2 to 32 weeks after SAD operation. The apoptotic myocardiocytes were increased in SAD rats compared with sham-operated rats. The expression of Bcl-2 protein, an inhibiting factor of apoptosis, was markedly decreased in the myocardiocytes of SAD rats. In contrast, the expression of Bax, Fas and Fas-L proteins, promoting factors of apoptosis, was significantly increased in the myocardiocytes of SAD rats. All these modifications were persistent from 4 to 32 weeks after SAD operation.

CONCLUSION

These findings demonstrate that a high level of apoptosis is persistent in myocardiocytes in SAD rats. We propose that apoptosis may be one of the mechanisms underlying the cardiac damage induced by SAD.

Daily exercise reduces measures of heart rate and blood pressure variability in hypertensive rats

This study was designed to test the hypothesis that daily spontaneous running (DSR) reduces measures of heart rate and blood pressure variability in spontaneously hypertensive rats (SHR). After 8 weeks of DSR or sedentary control, rats were chronically instrumented with arterial catheters. Daily exercise reduced most measures of heart rate (HR) and blood pressure variability. Specifically DSR decreased heart rate, Low Frequency Power (LF: 0.19-0.61 Hz), and Low Frequency/High Frequency (HF: 1.2-2.5 Hz) ratio of HR. Furthermore, Total Power (TP), LF power, and LF/HF ratio of systolic blood pressure were reduced by daily spontaneous running. Finally, TP, LF and HF powers and LF/HF ratios of diastolic blood pressure were reduced by daily spontaneous running. These data demonstrate that daily exercise reduces sympathetic activity and possibly increases cardiac reserve in hypertensive animals.