Cardiovascular autonomic control in mice lacking angiotensin AT1a receptors

Sustained hypoxia in mice increases parasympathetic but not sympathetic tone

The autonomic profile of mice submitted to sustained hypoxia (SH) was not yet fully evaluated. Herein, we characterized the cardiovascular and autonomic profile of conscious freely moving mice submitted to SH using two sequential experimental protocols to evaluate the parasympathetic and sympathetic tone to the heart and the sympathetic tone to the vascular resistance. In the first protocol the sequence of antagonists was methyl-atropine followed by propranolol and then by prazosin, while in the second protocol the sequence was propranolol followed by methyl-atropine and then by prazosin. In SH the baseline heart rate was significantly lower than in control mice and the antagonism of the parasympathetic and sympathetic tone to the heart in both experimental protocols indicated an increased parasympathetic tone in SH mice and no changes in the sympathetic tone. Antagonism of the sympathetic tone to the vascular resistance with prazosin produced similar changes in arterial pressure in control and SH mice. Altogether these findings support the concept that mice submitted to SH present a significant increase in the parasympathetic but not in the sympathetic tone, which may explain why the baseline arterial pressure was not increased in SH mice.

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Autonomic profile of awake mice submitted to sustained hypoxia (SH) was evaluated by sequential pharmacological antagonists.

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Baseline heart rate in SH mice was lower and the tachycardic response to methyl-atropine was greater than in control mice.

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Fall in mean arterial pressure in response to prazosin was similar in control and SH mice.

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Parasympathetic tone to the heart of mice submitted to SH is increased while the overall sympathetic tone is not.

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These findings contribute to explain why mice, different of rats, are not hypertensive in response to SH.

Elastase-2, a Tissue Alternative Pathway for Angiotensin II Generation, Plays a Role in Circulatory Sympathovagal Balance in Mice

In vitro and ex vivo experiments indicate that elastase-2 (ELA-2), a chymotrypsin-serine protease elastase family member 2A, is an alternative pathway for angiotensin II (Ang II) generation. However, the role played by ELA-2 in vivo is unclear. We examined ELA-2 knockout (ELA-2KO) mice compared to wild-type (WT) mice and determined whether ELA-2 played a role in hemodynamics [arterial pressure (AP) and heart rate (HR)], cardiocirculatory sympathovagal balance and baroreflex sensitivity. The variability of systolic arterial pressure (SAP) and pulse interval (PI) for evaluating autonomic modulation was examined for time and frequency domains (spectral analysis), whereas a symbolic analysis was also used to evaluate PI variability. In addition, baroreflex sensitivity was examined using the sequence method. Cardiac function was evaluated echocardiographically under anesthesia. The AP was normal whereas the HR was reduced in ELA-2KO mice (425 ± 17 vs. 512 ± 13 bpm from WT). SAP variability and baroreflex sensitivity were similar in both strains. The LF power from the PI spectrum (33.6 ± 5 vs. 51.8 ± 4.8 nu from WT) and the LF/HF ratio (0.60 ± 0.1 vs. 1.45 ± 0.3 from WT) were reduced, whereas the HF power was increased (66.4 ± 5 vs. 48.2 ± 4.8 nu from WT) in ELA-2KO mice, indicating a shift toward parasympathetic modulation of HR. Echocardiographic examination showed normal fractional shortening and an ejection fraction in ELA-2KO mice; however, the cardiac output, stroke volume, and ventricular size were reduced. These findings provide the first evidence that ELA-2 acts on the sympathovagal balance of the heart, as expressed by the reduced sympathetic modulation of HR in ELA-2KO mice.

Complex and interacting influences of the autonomic nervous system on cardiac electrophysiology in conscious mice

Mice may now be the preferred animal model for biomedical research due to its anatomical, physiological, and genetic similarity to humans. However, little is known about accentuated antagonism of chronotropic and dromotropic properties in conscious mice. Accordingly, we describe the complex and interacting influence of the autonomic nervous system on cardiac electrophysiology in conscious mice. Specifically, we report the effects of single and combined cardiac autonomic blockade on measurements of pulse interval (heart rate), atrio-ventricular interval, sinus node recovery time (SNRT), SNRT corrected for spontaneous sinus cycle, and Wenckebach cycle length in conscious mice free of the confounding influences of anesthetics and surgical trauma. Autonomic influences were quantified as the change in parameter induced by its selective blocker (Sympathetic or Parasympathetic Effect) or as the difference between the intrinsic value and the value after a selective blocker (Sympathetic or Parasympathetic Tonus). Sympatho-Vagal Balance (SVB) was assessed as the ratio of control interval to intrinsic interval. SVB suggests slight parasympathetic dominance in the control of cardiac electrophysiology intervals. Furthermore, results documents a complex interaction between the sympathetic and parasympathetic divisions of the autonomic nervous system in the control of cardiac electrophysiology parameters. Specifically, the parasympathetic effect was greater than the parasympathetic tonus in the control of cardiac electrophysiology parameters. In contrast, the sympathetic effect was smaller than the sympathetic tonus in the control of cardiac electrophysiology parameters. Results have important implications because actions of pharmacological agents that alter the autonomic control of cardiac electrophysiology are transformed by these interacting mechanisms.

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

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

Brain angiotensin AT1 receptors as specific regulators of cardiovascular reactivity to acute psychoemotional stress

1. Cardiovascular reactivity, an abrupt rise in blood pressure (BP) and heart rate in response to psychoemotional stress, is a risk factor for heart disease. Pharmacological and molecular genetic studies suggest that brain angiotensin (Ang) II and AT(1) receptors are required for the normal expression of sympathetic cardiovascular responses to various psychological stressors. Moreover, overactivity of the brain AngII system may contribute to enhanced cardiovascular reactivity in hypertension. 2. Conversely, brain AT(1) receptors appear to be less important for the regulation of sympathetic cardiovascular responses to a range of stressors involving an immediate physiological threat (physical stressors) in animal models. 3. Apart from threatening events, appetitive stimuli can induce a distinct, central nervous system-mediated rise in BP. However, evidence indicates that brain AT(1) receptors are not essential for the regulation of cardiovascular arousal associated with positively motivated behaviour, such as anticipation and the consumption of palatable food. The role of central AT(1) receptors in regulating cardiovascular activation elicited by other types of appetitive stimuli remains to be determined. 4. Emerging evidence also indicates that brain AT(1) receptors play a limited role in the regulation of cardiovascular responses to non-emotional natural daily activities, sleep and exercise. 5. Collectively, these findings suggest that, with respect to cardiovascular arousal, central AT(1) receptors may be involved primarily in the regulation of the defence response. Therefore, these receptors could be a potential therapeutic target for selective attenuation of BP hyperreactivity to aversive stressors, without altering physiologically important cardiovascular adjustments to normal daily activities, sleep and exercise.

Endothelial cell-specific aryl hydrocarbon receptor knockout mice exhibit hypotension mediated, in part, by an attenuated angiotensin II responsiveness

Hypotension in aryl hydrocarbon receptor knockout mice (ahr(-/-)) is mediated, in part, by a reduced contribution of angiotensin (Ang) II to basal blood pressure (BP). Since AHR is highly expressed in endothelial cells (EC), we hypothesized that EC-specific ahr(-/-) (ECahr(-/-)) mice would exhibit a similar phenotype. We generated ECahr(-/-) mice by crossing AHR floxed mice (ahr(fx/fx)) to mice expressing Cre recombinase driven by an EC-specific promoter. BP was assessed by radiotelemetry prior to and following an acute injection of Ang II or chronic treatment with an angiotensin converting enzyme inhibitor (ACEi). ECahr(-/-) mice were hypotensive (ECahr(+/+): 116.1±1.4; ECahr(-/-): 107.4±2.0 mmHg, n=11, p<0.05) and exhibited significantly different responses to Ang II and ACEi. While Ang II increased BP in both genotypes, the increase was sustained in ECahr(+/+), whereas the increase in ECahr(-/-) mice steadily declined. Area under the curve analysis showed that Ang II-induced increase in diastolic BP (DBP) over 30 min was significantly lower in ECahr(-/-) mice (ECahr(+/+) 1297±223 mmHg/30 min; ECahr(-/-)(AUC): 504±138 mmHg/30 min, p<0.05). In contrast, while ACEi decreased BP in both genotypes, the subsequent rise in DBP after treatment was significantly delayed in the ECahr(-/-) mice. ECahr(-/-) mice also exhibited reduced vascular and adipose Ang II type 1 receptor (AT1R) expression, and reduced aortic Ang II-dependent vasoconstriction in the presence of vascular adipose. Taken together these data suggest that hypotension in ECahr(-/-) mice results from reduced vascular responsiveness to Ang II that is influenced by AT1R expression and adipose.

Cardiovascular rhythms and cardiac baroreflex sensitivity in AT(1A) receptor gain-of-function mutant mice

A mutant mouse expressing a gain-of-function of the AT(1A) angiotensin II receptor was engineered to study the consequences of a constitutive activation of this receptor on blood pressure (BP). Cardiovascular rhythms and spontaneous cardiac baroreflex sensitivity (BRS) were evaluated using telemetric BP recordings of five transgenic (AT(1A)MUT) and five wild (AT(1A)WT) mice. The circadian rhythms were described with the Chronos-Fit program. The gain of the transfer function between systolic BP (SBP) and pulse intervals used to estimate the spontaneous BRS (ms/mmHg) was calculated in the low frequency (0.15-0.60 Hz) band. Transgenic AT(1A)MUT exhibited higher BP and heart rate (HR) levels compared to controls (SBP AT(1A)MUT 134.6 +/- 5.9 mmHg vs. AT(1A)WT 110.5 +/- 5.9; p < 0.05; HR AT(1A)MUT 531.0 +/- 14.9 vs. AT(1A)WT 454.8 +/- 5.4 beats/min; p = 0.001). Spontaneous BRS was diminished in transgenic mice (AT(1A)MUT 1.23 +/- 0.17 ms/mmHg vs. AT(1A)WT 1.91 +/- 0.18 ms/mmHg; p < 0.05). Motor activity did not differ between groups. These variables exhibited circadian changes, and the differences between the strains were maintained throughout the cycle. The highest values for BP, HR, and locomotor activity were observed at night. Spontaneous BRS varied in the opposite direction, with the lowest gain estimated when BP and HR were elevated (i.e., at night, when the animals were active). It is likely the BP elevation of the mutant mice results from the amplification of the effects of AngII at different sites. Future studies are necessary to explore whether AT(1A) receptor activation at the central nervous system level effectively contributed to the observed differences.

The day-night difference of blood pressure is increased in AT(1A)-receptor knockout mice on a high-sodium diet

BACKGROUND

Abnormal circadian variation of blood pressure (BP) increases cardiovascular risk. In this study, we examined the influence of angiotensin AT(1A) receptors on circadian BP variation, and specifically on its behavioral activity-related and -unrelated components.

METHODS

BP and locomotor activity were recorded by radiotelemetry in AT(1A)-receptor knockout mice (AT(1A)(-/-)) and their wild-type controls (AT(1A)(+/+)) placed on a normal-salt diet (NSD) or high-salt diet (HSD, 3.1% Na).

RESULTS

The 24-h BP was lower in AT(1A)(-/-) than AT(1A)(+/+) mice on a NSD (92 +/- 2 and 118 +/- 2 mm Hg, respectively), whereas the day-night BP difference (DeltaDNBP) was similar between groups (11 +/- 2 and 12 +/- 1 mm Hg, respectively). HSD increased BP by 20 +/- 2 mm Hg and DeltaDNBP by 7 +/- 1 mm Hg in AT(1A)(-/-) mice, without affecting these parameters much in AT(1A)(+/+) mice. The DeltaDNBP increase in AT(1A)(-/-) mice was caused by nondipping BP during the inactive late-dark period. Conversely, BP rise associated with circadian behavioral activation during the early dark period was not altered by HSD in AT(1A)(-/-) mice. The BP change associated with spontaneous ultradian activity-inactivity bouts was also similar between strains on HSD as was the BP rise associated with induced (cage-switch) behavioral activity. Ganglionic or alpha(1)-adrenergic blockade decreased BP in both strains; HSD did not affect this response in AT(1A)(-/-), but abolished it in AT(1A)(+/+) mice.

CONCLUSIONS

AT(1A)-receptor deficiency, when combined with HSD, can increase circadian BP difference in mice. This increase is mediated principally by activity-unrelated factors, such as the nonsuppressibility of basal resting sympathetic tone by HSD, thus suggesting a form of salt-/volume-dependent hypertension.

An activated renin-angiotensin system maintains normal blood pressure in aryl hydrocarbon receptor heterozygous mice but not in null mice

It has been postulated that fetal vascular abnormalities in aryl hydrocarbon receptor null (ahr(-/-)) mice may alter cardiovascular homeostasis in adulthood. We tested the hypothesis that blood pressure regulation in adult heterozygous mice (ahr(+/-)) would be normal, compared to ahr(-/-) mice, since no vascular abnormalities have been reported in the heterozygote animals. Mean arterial blood pressure (MAP) was measured using radiotelemetry prior to and during treatment with inhibitors of the autonomic nervous system, nitric oxide synthase (NOS), angiotensin converting enzyme (ACE), or endothelin-1 A receptor (ET(A)). Also, indices of renin-angiotensin system (RAS) activation were measured. ahr(+/-) and ahr(-/-) mice were normotensive and hypotensive, respectively, compared to wild-type (ahr(+/+)) littermates. Responses of all genotypes to autonomic nervous system inhibition were normal. ahr(+/-) mice responded normally to NOS inhibition, while the responses of ahr(-/-) mice were significantly blunted. In contrast, ahr(+/-) mice were significantly more responsive to inhibition of ACE, an ET(A) antagonist, or both, while ahr(-/-) mice were significantly less responsive to ACE inhibition and more responsive to an ET(A) antagonist. ahr(+/-) mice also exhibited significant increases in plasma renin and ACE activity, plasma sodium, and urine osmolality, indicative of RAS activation. Thus, normotension in ahr(+/-) mice appears to be maintained by increased RAS and ET-1 signaling, while hypotension in ahr(-/-) mice may result from decreased RAS signaling. In conclusion, despite the lack of overt fetal vascular abnormalities in ahr(+/-) mice, the loss of a single ahr allele has a significant effect on blood pressure regulation.

Acrolein consumption induces systemic dyslipidemia and lipoprotein modification

Aldehydes such as acrolein are ubiquitous pollutants present in automobile exhaust, cigarette, wood, and coal smoke. Such aldehydes are also constituents of several food substances and are present in drinking water, irrigation canals, and effluents from manufacturing plants. Oral intake represents the most significant source of exposure to acrolein and related aldehydes. To study the effects of short-term oral exposure to acrolein on lipoprotein levels and metabolism, adult mice were gavage-fed 0.1 to 5 mg acrolein/kg bwt and changes in plasma lipoproteins were assessed. Changes in hepatic gene expression related to lipid metabolism and cytokines were examined by qRT-PCR analysis. Acrolein feeding did not affect body weight, blood urea nitrogen, plasma creatinine, electrolytes, cytokines or liver enzymes, but increased plasma cholesterol and triglycerides. Similar results were obtained with apoE-null mice. Plasma lipoproteins from acrolein-fed mice showed altered electrophoretic mobility on agarose gels. Chromatographic analysis revealed elevated VLDL cholesterol, phospholipids, and triglycerides levels with little change in LDL or HDL. NMR analysis indicated shifts from small to large VLDL and from large to medium-small LDL with no change in the size of HDL particles. Increased plasma VLDL was associated with a significant decrease in post-heparin plasma hepatic lipase activity and a decrease in hepatic expression of hepatic lipase. These observations suggest that oral exposure to acrolein could induce or exacerbate systemic dyslipidemia and thereby contribute to cardiovascular disease risk.

Optimized surgical techniques and postoperative care improve survival rates and permit accurate telemetric recording in exercising mice

Background

The laboratory mouse is commonly used as a sophisticated model in biomedical research. However, experiments requiring major surgery frequently lead to serious postoperative complications and death, particularly if genetically modified mice with anatomical and physiological abnormalities undergo extensive interventions such as transmitter implantation. Telemetric transmitters are used to study cardiovascular physiology and diseases. Telemetry yields reliable and accurate measurement of blood pressure in the free-roaming, unanaesthetized and unstressed mouse, but data recording is hampered substantially if measurements are made in an exercising mouse. Thus, we aimed to optimize transmitter implantation to improve telemetric signal recording in exercising mice as well as to establish a postoperative care regimen that promotes convalescence and survival of mice after major surgery in general.

Results

We report an optimized telemetric transmitter implantation technique (fixation of the transmitter body on the back of the mouse with stainless steel wires) for subsequent measurement of arterial blood pressure during maximal exercise on a treadmill. This technique was used on normal (wildtype) mice and on transgenic mice with anatomical and physiological abnormalities due to constitutive overexpression of recombinant human erythropoietin. To promote convalescence of the animals after surgery, we established a regimen for postoperative intensive care: pain treatment (flunixine 5 mg/kg bodyweight, subcutaneously, twice per day) and fluid therapy (600 μl, subcutaneously, twice per day) were administrated for 7 days. In addition, warmth and free access to high energy liquid in a drinking bottle were provided for 14 days following transmitter implantation. This regimen led to a substantial decrease in overall morbidity and mortality. The refined postoperative care and surgical technique were particularly successful in genetically modified mice with severely compromised physiological capacities.

Conclusion

Recovery and survival rates of mice after major surgery were significantly improved by careful management of postoperative intensive care regimens including key supportive measures such as pain relief, administration of fluids, and warmth. Furthermore, fixation of the blood pressure transmitter provided constant reliable telemetric recordings in exercising mice.

Blood pressure reactivity to emotional stress is reduced in AT1A-receptor knockout mice on normal, but not high salt intake

Pharmacological evidence suggests that angiotensin II type 1 (AT(1)) receptors are involved in the regulation of cardiovascular response to emotional stress and reinforcing effect of dietary salt on this response. In this study, we examined the effect of genetic deletion of AT(1A) receptors on the cardiovascular effects of stress and salt in mice. AT(1A) receptor knockout (AT(1A)(-/-)) and wild-type (AT(1A)(+/+)) mice were implanted with telemetry devices and placed on a normal (0.4%) or high (3.1%) salt diet (HSD). Resting blood pressure (BP) in AT(1A)(-/-) mice (84+/-3 mm Hg) was lower than in AT(1A)(+/+) mice (107+/-2 mm Hg). Negative emotional (restraint) stress increased BP by 33+/-3 mm Hg in AT(1A)(+/+) mice. This response was attenuated by 40% in AT(1A)(-/-) mice (18+/-3 mm Hg). Conversely, the BP increase caused by food presentation and feeding was similar in AT(1A)(-/-) (25+/-3 mm Hg) and AT(1A)(+/+) mice (26+/-3 mm Hg). HSD increased resting BP by 14+/-4 mm Hg in AT(1A)(-/-) mice without affecting it significantly in AT(1A)(+/+) mice. Under these conditions, the pressor response to restraint stress in AT(1A)(-/-) mice (30+/-3 mm Hg) was no longer different from that in wild-type animals (28+/-3 mm Hg). The BP response to feeding was not altered by HSD in either AT(1A)(-/-) or AT(1A)(+/+) mice (25+/-2 and 27+/-3 mm Hg, respectively). These results indicate that AT(1A) receptor deficiency leads to a reduction in BP reactivity to negative emotional stress, but not feeding. HSD can selectively reinforce the cardiovascular response to negative stress in AT(1A)(-/-) mice. However, there is little interaction between AT(1A) receptors, excess dietary sodium and feeding-induced cardiovascular arousal.

Exercise improves cardiovascular control in a model of dislipidemia and menopause

The present study investigated the effects of exercise training on arterial pressure, baroreflex sensitivity, cardiovascular autonomic control and metabolic parameters on female LDL-receptor knockout ovariectomized mice. Mice were divided into two groups: sedentary and trained. Trained group was submitted to an exercise training protocol. Blood cholesterol was measured. Arterial pressure (AP) signals were directly recorded in conscious mice. Baroreflex sensitivity was evaluated by tachycardic and bradycardic responses to AP changes. Cardiovascular autonomic modulation was measured in frequency (FFT) and time domains. Maximal exercise capacity was increased in trained as compared to sedentary group. Blood cholesterol was diminished in trained mice (191+/-8mg/dL) when compared to sedentary mice (250+/-9mg/dL, p<0.05). Mean AP and HR were reduced in trained group (101+/-3mmHg and 535+/-14bpm, p<0.05) when compared with sedentary group (125+/-3mmHg and 600+/-12bpm). Exercise training induced improvement in bradycardic reflex response in trained animals (-4.24+/-0.62bpm/mmHg) in relation to sedentary animals (-1.49+/-0.15bpm/mmHg, p<0.01); tachycardic reflex responses were similar between studied groups. Exercise training increased the variance (34+/-8 vs. 6.6+/-1.5ms(2) in sedentary, p<0.005) and the high-frequency band (HF) of the pulse interval (IP) (53+/-7% vs. 26+/-6% in sedentary, p<0.01). It is tempting to speculate that results of this experimental study might represent a rationale for this non-pharmacological intervention in the management of cardiovascular risk factors in dyslipidemic post-menopause women.

Diabetes and cardiovascular autonomic dysfunction: application of animal models

When diabetes is associated with cardiovascular autonomic dysfunction, there is a poor prognosis and increased morbidity and mortality. Information on the mechanisms of diabetes-associated autonomic dysfunction has been provided by advanced studies using physiological, pharmacological, anatomical and molecular methods in experimental animal models of insulin deficiency and resistance. This has been augmented by new approaches which combine diabetes induction with genetically modified animal models. The aim of this review is to outline and discuss the animal models used for the study of insulin deficiency and insulin resistance with a focus on autonomic neural interactions. The goal is to better understand the clinical relevance of cardiovascular autonomic dysfunction associated with diabetes.

Validation of volume-pressure recording tail-cuff blood pressure measurements

BACKGROUND

The American Heart Association has recommended tail-cuff blood pressure measurement for high-throughput experimental designs, including mutagenesis screens and genetic crosses. However, some tail-cuff methods show good agreement with radiotelemetry and others do not, indicating that each tail-cuff method requires independent validation.

METHODS

We validated the volume-pressure recording (VPR) tail-cuff method by comparison to simultaneous radiotelemetry measurements.

RESULTS

Bland-Altman analysis of 560 cycles from 26 independent measurement sessions showed good agreement between VPR and radiotelemetry measurements, with tail-cuff measurements being 0.25 mm Hg lower than telemetry measurements on average. However, the VPR method was less accurate, compared to radiotelemetry, at extreme high and low (i.e., <110 or >180 mm Hg) systolic blood pressures (SBPs).

CONCLUSIONS

We conclude that the VPR tail-cuff method provides accurate blood pressure measurements over the physiological range of blood pressure in mice.

Blood pressure regulation and cardiac autonomic control in mice overexpressing alpha- and gamma-melanocyte stimulating hormone

Melanocyte stimulating hormones (MSH) derived from pro-opiomelanocortin have been demonstrated to participate in the central regulation of cardiovascular functions. The aim of the present study was to elucidate the chronic effects of increased melanocortin activation on blood pressure regulation and autonomic nervous system function. We adapted telemetry to transgenic mice overexpressing alpha- and gamma-MSH and measured blood pressure, heart rate and locomotor activity, and analyzed heart rate variability (HRV) in the frequency-domain as well as baroreflex function by the sequence technique. Transgenic (MSH-OE) mice had increased systolic blood pressure but their heart rate was similar to wild-type (WT) controls. The 24-h mean of systolic blood pressure was 132+/-7mmHg in MSH-OE and 113+/-4mmHg in WT mice. Locomotor activity was decreased in the MSH-OE mice. Furthermore, MSH-OE mice showed slower adaptation to mild environmental stress in terms of blood pressure changes. The low frequency (LF) power of HRV tended to be higher in MSH-OE mice compared to WT mice, without a difference in overall variability. The assessment of baroreflex function indicated enhanced baroreflex effectiveness and more frequent baroreflex operations in MSH-OE mice. Baseline heart rate, increased LF power of HRV and increased baroreflex activity may all reflect maintenance of baroreflex integrity and an increase in cardiac vagal activity to counteract the increased blood pressure. These results provide new evidence that long-term activation of the melanocortin system elevates blood pressure without increasing heart rate.

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

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

Disfunção autonômica cardiovascular no diabetes mellitus experimental

Muitas informações novas têm sido publicadas nos últimos anos a respeito da fisiopatologia da disfunção autonômica cardiovascular em ratos e camundongos diabéticos. Nosso grupo tem estudado o curso temporal das alterações cardiovasculares associadas ao diabetes experimental há alguns anos, obtendo evidências consistentes de grave disautonomia em modelos animais de diabetes. O objetivo deste trabalho foi revisar a contribuição que estudos envolvendo diferentes modelos de deficiência e resistência à insulina têm fornecido para o entendimento, tratamento e prevenção da disfunção autonômica cardiovascular do diabetes.

Phenotyping the level of blood pressure by telemetry in mice

1. Using telemetry, arterial blood pressure (BP) can be measured directly over long periods in freely behaving animals without recent anaesthesia or surgery. In the present review, we discuss the strengths and limitations of this method and important considerations in using the method to characterize the BP level in mice. 2. A variety of informative statistics can be used to describe the BP level and we have made available a spreadsheet template for their calculation on a routine basis. The BP level is well summarized using the average value for an entire 24 h period or for the individual light and dark phases of the day. Such long-term averages exhibit less statistical variation than those of short recording periods. In addition, averages of the dark and light phases of the day convey information concerning circadian variations of BP. 3. The frequency distribution of BP samples provides additional information concerning the range of BP values recorded over the course of the day and can be described in terms of percentiles of the distribution that correspond with the minimum and maximum BP values and their span. 4. In mice, BP can be markedly affected by locomotor activity cycles that occur frequently throughout both the light and dark phases of the day. In addition, BP is strongly affected by ambient temperature and food intake, as well as potentially by other determinants of energy balance. Consideration of these factors may help improve accuracy and precision when phenotyping the BP level in mice.

Nocturnal hypertension in mice consuming a high fructose diet

OBJECTIVE

To investigate the effect of fructose consumption on the light/dark pattern of blood pressure, heart rate and autonomic neural function in mice.

BACKGROUND

Insulin resistant diabetes is associated with hypertension and autonomic dysfunction. There is evidence that the increasing incidence of diabetes may be related to dietary changes, including consumption of high levels of fructose.

DESIGN/METHODS

C57/BL mice, instrumented with radiotelemetric arterial catheters, were fed a control or high fructose diet (60%). Cardiovascular parameters measured were light/dark pattern of mean arterial pressure (MAP), heart rate (HR) and variability (time and frequency domain). We also measured plasma insulin, glucose, lipids and angiotensin II (Ang II) as well as glucose tolerance. In situ hybridization was used to measure brainstem expression of tyrosine hydroxylase (TH) and Ang AT1a mRNA.

RESULTS

Fructose diet (8 weeks) produced an increase in MAP, variance and low frequency domain (14+/-3 vs. 33+/-4 mm Hg(2), variance and 10+/-2 vs. 26+/-4 mm Hg(2), LF, control vs. fructose, P<0.01). The changes occurred only at night, a period of activity for mice. Glucose tolerance was attenuated in the fructose group. Fructose also increased plasma cholesterol (80+/-1 vs. 126+/-2 mg/dl, control vs. fructose, P<0.05) and plasma Ang II (18+/-5 vs.65+/-12 pg/ml, control vs. fructose, P<0.05). Depressor responses to alpha(1)-adrenergic blockade with prasozin were augmented in fructose-fed mice. Using quantitative in situ hybridization, we found that Ang AT1a receptor and TH mRNA expression were significantly increased in the brainstem locus coeruleus.

CONCLUSION

A high fructose diet in mice produced nocturnal hypertension and autonomic imbalance which may be related to activation of sympathetic and angiotensin systems.

Sarin produces delayed cardiac and central autonomic changes

The aim was to evaluate the acute and delayed effects of low dose sarin exposure on cardiac autonomic and brainstem catecholaminergic function in mice. The rationale was to expand our knowledge of the cardiovascular effects of this neurotoxic, acetylcholinesterase (AChE) inhibitor. C57BL/6 male mice with telemetric arterial catheters were injected with saline or sarin (8 microg/kg, 0.05x LD(50); sc, two injections) with blood pressure (BP) measurements made at 1 and 10 weeks after sarin exposure. BP and pulse interval variability (PI) and low and high frequency spectral oscillations were measured using autoregressive spectral analysis. In situ hybridization (ISH) was used to quantify tyrosine hydroxylase (TH) mRNA expression in brainstem cardiovascular centers. Sarin had no effect on blood AChE activity, heart rate (HR) or BP. There was a biphasic response in PI variance, an early increase (+140%) and a delayed decrease (-62%) at more than 2 months after sarin exposure. There were no changes in BP variance. Assuming that increased PI variance is a positive outcome, the short-term response to sarin should be protective. This is opposite for the delayed decrease in PI variance which is associated with adverse cardiovascular effects. There was an increase in TH mRNA in both locus coeruleus (0.18+/-0.05 vs. 1.4+/-0.2 microCi/g; control vs. sarin) and dorsal vagal complex (0.09+/-0.06 vs. 1.17+/-0.03 microCi/g; control vs. sarin). Results show that a dose of sarin which had no peripheral cholinergic effects caused changes in autonomic modulation, a short-term enhancement followed by a delayed impairment in heart rate variability. Sarin-induced cardiac effects suggest a controversial aspect to the use of pharmacological agents which target AChE for management of cardiovascular risk.

Stress cardiovascular/autonomic interactions in mice

Studies evaluated the role of the autonomic nervous system in the cardiovascular response to stress using radiotelemetric blood pressure (BP) recording coupled with autoregressive spectral analysis. Conscious male C57/BL6 mice with carotid arterial telemetric catheters were exposed to acute episodes of shaker stress before and after administration of cholinergic, beta1-adrenergic and alpha1-adrenergic receptor antagonists. Pulse interval (PI) and systolic arterial pressure (SAP) were analyzed for variance and the low frequency (LF: 0.1-1.0 Hz) and high frequency (HF: 1-5 Hz) spectral components. Stress (5 min) increased BP and heart rate (HR) as well as PI and SAP variability. PI variance increased from 41+/-6 to 75+/-14 ms2 while SAP variance increased from 25+/-5 to 55+/-9 mm Hg2. Autonomic blockade had specific effects on stress-induced changes in PI and SAP and their respective variability. Atropine reduced the tachycardia and abolished the increase in PI variance and its LF component. Data documents that in mice the cholinergic system is fundamental for the maintenance of HR variability. Atropine had no effects on the BP responses, either the increase in SAP or the variance associated with stress. Atenolol blocked the increase in PI and SAP variability induced by stress. Prazosin reduced the tachycardia produced by stress and blocked the increase in PI (only LF) and SAP variability. Using quantitative spectral analysis of telemetrically collected BP data in mice along with pharmacological antagonism, we were able to accurately determine the role of autonomic input in the mediation of the stress response. Data verify the role of sympathetic/parasympathetic balance in stress-induced changes in HR, BP and indices of variance.

Salt consumption increases blood pressure and abolishes the light/dark rhythm in angiotensin AT1a receptor deficient mice

Experiments were performed to study the role of angiotensin (Ang) AT1a receptors in dietary sodium-induced changes in blood pressure (BP). We measured light/dark rhythms in BP, heart rate (HR) and drinking behavior in Ang AT1a deficient (AT1a -/-) and wild type (AT1a +/+) mice with arterial telemetric catheters. Mice were given ad libitum access to a high salt diet (8% NaCl, HSD for 8 days) and tap water. The major finding was that the Ang AT1a -/- mice showed enhanced sodium sensitivity. This was seen by a greater percentage increase in BP (+21% vs. +12%) and an earlier onset of BP change (increase on day 5 vs. day 8) in AT1a -/- vs. AT1a +/+. The normal light/dark BP rhythm was abolished in AT1a -/- after 5 days of HSD. HSD produced an increase in water intake (drinking activity and volume consumed) in both groups with no difference in the percentage increase or the light/dark drinking rhythm. HSD produced no changes in plasma osmolality, hematocrit or body weight in either group. Evidence shows that a deficiency of Ang AT1a receptors results in an enhancement in sodium sensitivity along with a disruption of the normal light/dark BP rhythm. The data combined with previous findings suggests that activation of other components of the renin angiotensin system and/or sympathetic pathways may be responsible for the cardiovascular changes in AT1a deficient mice.

Norepinephrine transporter-deficient mice respond to anxiety producing and fearful environments with bradycardia and hypotension

The study of anxiety and fear involves complex interrelationships between psychiatry and the autonomic nervous system. Altered noradrenergic signaling is linked to certain types of depression and anxiety disorders, and treatment often includes specific transporter blockade. The norepinephrine transporter is crucial in limiting catecholaminergic signaling. Norepinephrine transporter-deficient mice have increased circulating catecholamines and elevated heart rate and blood pressure. We hypothesized, therefore, that reduced norepinephrine clearance would heighten the autonomic cardiovascular response to anxiety and fear. In separate experiments, norepinephrine transporter-deficient (norepinephrine transporter-/-) mice underwent tactile startle and trace fear conditioning to measure hemodynamic responses. A dramatic tachycardia was observed in norepinephrine transporter-/- mice compared with controls following both airpuff or footshock stimuli, and pressure changes were also greater. Interestingly, in contrast to normally elevated home cage levels in norepinephrine transporter-deficient mice, prestimulus heart rate and blood pressure were actually higher in norepinephrine transporter+/+ animals throughout behavioral testing. Upon placement in the behavioral chamber, norepinephrine transporter-deficient mice demonstrated a notable bradycardia and depressor effect that was more pronounced in females. Power spectral analysis indicated an increase in low frequency oscillations of heart rate variability; in mice, suggesting increased parasympathetic tone. Finally, norepinephrine transporter-/- mice exhibited sexual dimorphism in freeze behavior, which was greatest in females. Therefore, while reduced catecholamine clearance amplifies immediate cardiovascular responses to anxiety- or fear-inducing stimuli in norepinephrine transporter-/- mice, norepinephrine transporter deficiency apparently prevents protracted hemodynamic escalation in a fearful environment. Conceivably, chronic norepinephrine transporter blockade with transporter-specific drugs might attenuate recognition of autonomic and somatic distress signals in individuals with anxiety disorders, possibly lessening their behavioral reactivity, and reducing the cardiovascular risk factors associated with persistent emotional arousal.

Physiological regulation of brain angiotensin receptor mRNA in AT1a deficient mice

Experiments were performed to study the physiological regulation of angiotensin (Ang) AT1b receptors using Ang AT1a knockout mice (AT1aKO). Ang AT1b mRNA was analyzed in forebrain, hypothalamus, and brainstem using in situ hybridization (ISH) under baseline and water-restricted conditions. Plasma was analyzed for osmolality, vasopressin, and corticosterone. Dehydration (24 h) increased osmolality and corticosterone and decreased body weight with no difference between groups. Plasma vasopressin was not different between the groups and was not stimulated by dehydration. Under water ad libitum conditions, there were no differences in AT1b mRNA expression in medial periventricular, anterior third ventricle (AV3V), and subfornical organ (SFO) between controls and AT1aKO. In contrast, there was higher expression in the dorsal motor nucleus of the vagus (DMV) of AT1aKO vs. Controls (0.6 +/- 0.1 vs. 0.9 +/- 0.1 microCi/g, Control vs. AT1aKO in water ad libitum group). Dehydration increased AT1b expression in SFO in AT1aKO, but not in controls (0.6 +/- 0.07 vs. 0.9 +/- 0.06 microCi/g; water ad libitum vs. dehydrated). Emulsion autoradiography documents the detailed pattern of AT1b expression in brainstem of controls and AT1aKO. There was labeling in DMV, locus coeruleus, inferior olive, lateral reticular nucleus, and caudalis spinal trigemius. In conclusion, deletion of AT1a receptors produces a compensatory increase in AT1b receptor mRNA expression in brainstem, but not in hypothalamus or rostral forebrain. In addition, AT1aKO mice showed an enhanced response to dehydration in terms of AT1b mRNA expression in SFO.