Aging and baroreflex control of RSNA and heart rate in rats

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

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

Epidemiological evidence of increased waist circumference, but not body mass index, associated with impaired baroreflex sensitivity

Although an inverse relationship between body mass index (BMI) and baroreflex sensitivity (BRS) was found, the effect of waist circumference (WC) on BRS is still inconclusive. The contradictory results of previous studies may be related to the heterogeneity and relatively small sample size of the subjects examined. The aim of this population-based study was to investigate whether the influence of increased WC is more detrimental to BRS than BMI. A total of 760 community dwellers were recruited and they were classified into Q1 (n = 189), Q2 (n = 183), Q3 (n = 192) and Q4 (n = 196) groups, based on WC quartiles. Spontaneous BRS was determined by spectral α coefficient method. Valsalva ratio was the longest RR interval after release of Valsalva maneuver divided by the shortest RR interval during maneuver. Cardiac autonomic function was calculated by power spectrum of heart rate in low and high frequency (LF, 0.04-0.15 Hz; HF, 0.15-0.40 Hz), and LF/HF ratio in supine position for five minutes. There were significant differences in spontaneous BRS and Valsalva ratio among different WC groups. In multivariate analysis, obesity was inversely associated with spontaneous BRS and Valsalva ratio. However, these inverse relationships became insignificant after further adjustment for WC quartiles. In contrast, Q4 vs. Q1, Q3 vs. Q1 and Q2 vs. Q1 of WC were inversely related to spontaneous BRS. Q4 vs. Q1 and Q3 vs. Q1 of WC were negatively associated with the Valsalva ratio. In conclusion, increased and even high-normal WC had a stronger adverse effect on BRS than BMI, independent of cardio-metabolic risk factors.

Cardiac autonomic modulation in healthy elderly after different intensities of dynamic exercise

Purpose

To investigate the heart rate (HR) and its autonomic modulation at baseline and during dynamic postexercise (P_EX) with intensities of 40% and 60% of the maximum HR in healthy elderly.

Methods

This cross-sectional study included ten apparently healthy people who had been submitted to a protocol on a cycle ergometer for 35 minutes. Autonomic modulation was evaluated by spectral analysis of HR variability (HRV).

Results

A relevant increase in HR response was observed at 15 minutes postexercise with intensities of 60% and 40% of the maximum HR (10±2 bpm versus 5±1 bpm, respectively; P=0.005), and a significant reduction in HRV was also noted with 40% and 60% intensities during the rest period, and significant reduction in HRV (RR variance) was also observed in 40% and 60% intensities when compared to the baseline, as well as between the post-exercise intensities (1032±32 ms versus 905±5 ms) (P<0.001). In the HRV spectral analysis, a significant increase in the low frequency component HRV and autonomic balance at 40% of the maximum HR (68±2 normalized units [nu] versus 55±1 nu and 2.0±0.1 versus 1.2±0.1; P<0.001) and at 60% of the maximum HR (77±1 nu versus 55±1 nu and 3.2±0.1 versus 1.2±0.1 [P<0.001]) in relation to baseline was observed. A significant reduction of high frequency component at 40% and 60% intensities, however, was observed when compared to baseline (31±2 nu and 23±1 nu versus 45±1 nu, respectively; P<0.001). Moreover, significant differences were observed for the low frequency and high frequency components, as well as for the sympathovagal balance between participants who reached 40% and 60% of the maximum HR.

Conclusion

There was an increase in the HR, sympathetic modulation, and sympathovagal balance, as well as a reduction in vagal modulation in the elderly at both intensities of the P_EX.

Involvement of nitric oxide and caveolins in the age-associated functional and structural changes in a heart under osmotic stress

Previous work done in our laboratory showed that water restriction during 24 and 72h induced changes in cardiovascular NOS activity without altering NOS protein levels in young and adult animals. These findings indicate that the involvement of NO in the regulatory mechanisms during dehydration depends on the magnitude of the water restriction and on age. Our aim was to study whether a controlled water restriction of 1 month affects cardiac function, NO synthase (NOS) activity and NOS, and cav-1 and -3 protein levels in rats during aging. Male Sprague-Dawley rats aged 2 and 16 months were divided into 2 groups: (CR) control restriction (WR) water restriction. Measurements of arterial blood pressure, heart rate, oxidative stress, NOS activity and NOS/cav-1 and -3 protein levels were performed. Cardiac function was evaluated by echocardiography. The results showed that adult rats have greater ESV, EDV and SV than young rats with similar SBP. Decreased atria NOS activity was caused by a reduction in NOS protein levels. Adult animals showed increased cav-1. Water restriction decreased NOS activity in young and adult rats associated to an increased cav-1. TBARS levels increased in adult animals. Higher ventricular NOS activity in adulthood would be caused by a reduction in both cav. Water restriction reduced NOS activity and increased cav in both age groups. In conclusion, our results indicated that dehydration modifies cardiac NO system activity and its regulatory proteins cav in order to maintain physiological cardiac function. Functional alterations are induced by the aging process as well as hypovolemic state.

Dysautonomic responses during percutaneous carotid intervention: principles of physiology and management

Percutaneous carotid artery stenting (CAS) has emerged as a less invasive alternative to carotid endarterectomy for the treatment of carotid atherosclerotic disease. The main risk of CAS is the occurrence of neuro-vascular complications; however, carotid artery stenting-related dysautonomia (CAS-D) (hypertension, hypotension, and bradycardia) is the most frequently reported problem occurring in the periprocedural period. Alterations in autonomic homeostasis result from baroreceptor stimulation, which occurs particularly at the time of balloon inflation in the region of the carotid sinus. The response can be profound enough to induce asystole or even complete cessation of postganglionic sympathetic nerve activity. Frequency and factors predisposing a patient to CAS-D have been investigated in several studies; however, there are significant discrepancies in results among reports. Lack of consistent findings may arise from using different methods and definitions, as well as other factors discussed in detail in this review. Furthermore, a correlation of CAS-D with short and long-term outcomes has been investigated only in small and mostly retrospective studies, explaining why its prognostic significance remains uncertain. In this manuscript, we have focused on risk factors, pathophysiology and management of periprocedural autonomic dysfunction. As there is no standardized approach to the treatment of CAS-D, we present an algorithm for the periprocedural management of patients undergoing CAS. The proposed algorithm was developed based on our procedural experience as well as data from the available literature. The Yale Algorithm was successfully implemented at our institution and we are currently collecting data for short- and long-term safety. © 2014 Wiley Periodicals, Inc.

Age-related declines in thirst and salt appetite responses in male Fischer 344×Brown Norway rats

The F344×BN strain is the first generational cross between Fischer 344 (F344) and Brown Norway (BN) rats. The F344×BN strain is widely used in aging studies as it is regarded as a model of "healthy" aging (Sprott, 1991). In the present work, male F344×BN rats aged 4mo (young, n=6) and 20mo (old, n=9) received a series of experimental challenges to body fluid homeostasis to determine their thirst and salt appetite responses. Corresponding urinary responses were measured in some of the studies. Following sodium depletion, old rats ingested less saline solution (0.3M NaCl) than young rats on a body weight basis, but both ages drank enough saline solution to completely repair the accrued sodium deficits. Following intracellular dehydration, old rats drank less water than young rats, again on a body weight basis, and were less able than young rats to drink amounts of water proportionate to the osmotic challenge. Compared with young rats, old rats drank less of both water and saline solution after combined food and fluid restriction, and also were refractory to the stimulatory effects of low doses of captopril on water drinking and sodium ingestion. Age differences in urinary water and sodium excretion could not account for the age differences in accumulated water and sodium balances. These results extend observations of diminished behavioral responses of aging animals to the F344×BN rat strain and support the idea that impairments in behavior contribute more to the waning ability of aging animals to respond to body fluid challenges than do declines in kidney function. In addition, the results suggest that behavioral defense of sodium homeostasis is less diminished with age in the F344×BN strain compared to other strains so far studied.

Preventive role of exercise training in autonomic, hemodynamic, and metabolic parameters in rats under high risk of metabolic syndrome development

High fructose consumption contributes to metabolic syndrome incidence, whereas exercise training promotes several beneficial adaptations. In this study, we demonstrated the preventive role of exercise training in the metabolic syndrome derangements in a rat model. Wistar rats receiving fructose overload in drinking water (100 g/l) were concomitantly trained on a treadmill (FT) or kept sedentary (F) for 10 wk. Control rats treated with normal water were also submitted to exercise training (CT) or sedentarism (C). Metabolic evaluations consisted of the Lee index and glycemia and insulin tolerance test (kITT). Blood pressure (BP) was directly measured, whereas heart rate (HR) and BP variabilities were evaluated in time and frequency domains. Renal sympathetic nerve activity was also recorded. F rats presented significant alterations compared with all the other groups in insulin resistance (in mg · dl(-1) · min(-1): F: 3.4 ± 0.2; C: 4.7 ± 0.2; CT: 5.0 ± 0.5 FT: 4.6 ± 0.4), mean BP (in mmHG: F: 117 ± 2; C: 100 ± 2; CT: 98 ± 2; FT: 105 ± 2), and Lee index (in g/mm: F = 0.31 ± 0.001; C = 0.29 ± 0.001; CT = 0.27 ± 0.002; FT = 0.28 ± 0.002), confirming the metabolic syndrome diagnosis. Exercise training blunted all these derangements. Additionally, FS group presented autonomic dysfunction in relation to the others, as seen by an ≈ 50% decrease in baroreflex sensitivity and 24% in HR variability, and increases in sympathovagal balance (140%) and in renal sympathetic nerve activity (45%). These impairments were not observed in FT group, as well as in C and CT. Correlation analysis showed that both Lee index and kITT were associated with vagal impairment caused by fructose. Therefore, exercise training plays a preventive role in both autonomic and hemodynamic alterations related to the excessive fructose consumption.

Alagebrium in combination with exercise ameliorates age-associated ventricular and vascular stiffness

Advanced glycation end-products (AGEs) initiate cellular inflammation and contribute to cardiovascular disease in the elderly. AGE can be inhibited by Alagebrium (ALT), an AGE cross-link breaker. Moreover, the beneficial effects of exercise on aging are well recognized. Thus, we investigated the effects of ALT and exercise (Ex) on cardiovascular function in a rat aging model. Compared to young (Y) rats, in sedentary old (O) rats, end-systolic elastance (Ees) decreased (0.9±0.2 vs 1.7±0.4mmHg/μL, P<0.05), dP/dt(max) was attenuated (6054±685 vs 9540±939mmHg/s, P<0.05), ventricular compliance (end-diastolic pressure-volume relationship (EDPVR)) was impaired (1.4±0.2 vs 0.5±0.4mmHg/μL, P<0.05) and diastolic relaxation time (tau) was prolonged (21±3 vs 14±2ms, P<0.05). In old rats, combined ALT+Ex (4weeks) increased dP/dt(max) and Ees (8945±665 vs 6054±685mmHg/s, and 1.5±0.2 vs 0.9±0.2 respectively, O with ALT+Ex vs O, P<0.05 for both). Diastolic function (exponential power of EDPVR and tau) was also substantially improved by treatment with Alt+Ex in old rats (0.4±0.1 vs 0.9±0.2 and 16±2 vs 21±3ms, respectively, O with ALT+EX vs O, P<0.05 for both). Pulse wave velocity (PWV) was increased in old rats (7.0±0.7 vs 3.8±0.3ms, O vs Y, P<0.01). Both ALT and Ex alone decreased PWV in old rats but the combination decreased PWV to levels observed in young (4.6±0.5 vs 3.8±0.3ms, O with ALT+Ex vs Y, NS). These results suggest that prevention of the formation of new AGEs (with exercise) and breakdown of already formed AGEs (with ALT) may represent a therapeutic strategy for age-related ventricular and vascular stiffness.

Effects of β-adrenergic receptor agonists on drinking and arterial blood pressure in young and old rats

These experiments examined water-drinking and arterial blood pressure responses to β-adrenergic receptor activation in young (4 mo), "middle-aged" adult (12 mo), and old (29 mo) male rats of the Brown-Norway strain. We used isoproterenol to simultaneously activate β(1)- and β(2)-adrenergic receptors, salbutamol to selectively activate β(2)-adrenergic receptors, and the combination of isoproterenol and the β(2)-adrenergic receptor antagonist ICI 118,551 to stimulate only β(1)-adrenergic receptors. Animals received one of the drug treatments, and water drinking was measured for 90 min. About 1 wk later, animals received the same drug treatment for measurement of arterial blood pressure responses for 90 min. In some rats, levels of renin and aldosterone secretion in response to isoproterenol or salbutamol were measured in additional tests. Old and middle-aged rats drank significantly less after isoproterenol than did young rats and also had greater reductions in arterial blood pressure. Old and middle-aged rats drank significantly less after salbutamol than did young rats, although reductions in arterial blood pressure were equivalent across the ages. The β(2)-adrenergic antagonist ICI 118,551 abolished drinking after isoproterenol and prevented most of the observed hypotension. Renin secretion after isoproterenol and salbutamol was greater in young rats than in middle-aged rats, and wholly absent in old rats. Aldosterone secretion was reduced in old rats compared with young and middle-aged rats after treatment with isoproterenol, but not after treatment with salbutamol. In conclusion, there are age-related differences in β-adrenergic receptor-mediated drinking that can be explained only in part by age-related differences in renin secretion after β-adrenergic receptor stimulation.

Drinking and arterial blood pressure responses to ANG II in young and old rats

We investigated water drinking and arterial blood pressure responses to intravenous infusions of ANG II in young (4 mo), middle-aged adult (12 mo), and old (29 mo) male Brown Norway rats. Infusions of ANG II began with arterial blood pressure either at control levels or at reduced levels following injection of the vasodilator minoxidil. Under control conditions, mean arterial pressure (MAP) in response to ANG II rose to the same level for all groups, and middle-aged and old rats drank as much or more water in response to ANG II compared with young rats, depending on whether intakes were analyzed using absolute or body weight-adjusted values. When arterial blood pressure first was reduced with minoxidil, MAP in response to ANG II stabilized at significantly lower levels compared with control conditions for all groups. Young rats drank significantly more water under reduced pressure conditions compared with control conditions, while middle-aged and old rats did not. Urine volume in response to ANG II was lower, while water balance was higher, under conditions of reduced pressure compared with control conditions. Baroreflex control of heart rate was substantially reduced in old rats compared with young and middle-aged animals. In summary, young rats appear to be more sensitive to the inhibitory effects of increased arterial blood pressure on water drinking than are older animals.

Hypertensive effects of central angiotensin II infusion and restraint stress are reduced with age

OBJECTIVE

We investigated the effect of age on cardiovascular responses mediated by central angiotensin II (AngII) after intracerebroventricular infusion of AngII, and during restraint stress.

METHODS

Blood pressure (BP) and heart rate (HR) of young (5-month-old) and old (27-month-old) male Fischer-344 x Brown-Norway rats were measured using radiotelemetry. AngII was infused intracerebroventricularly using osmotic minipumps (10 ng/0.5 microl/h for 11 days). BP and HR responses to stress were evaluated by placing animals in restrainers for 20 min before and after intracerebroventricular infusion of the AngII-type-1 receptor inhibitor losartan (15 microg/microl per h for 3 days).

RESULTS

Resting BP was significantly elevated and HR was significantly lower in old rats compared with young. AngII-induced BP increase was markedly reduced in old rats, but HR responses were similar. Diurnal variation of both BP and HR was lower in old animals, and AngII reduced the amplitude of BP variation in young rats, but not in old. Restraint stress-induced BP and HR elevations were reduced with age. BP responses were diminished by central losartan infusion in both young and old, but this effect was more significant in young rats. In addition, expression of CuZn-superoxide dismutase and catalase declined significantly with age in the hypothalamus, whereas baseline oxidative stress increased. In contrast, AngII-induced increase in hypothalamic oxidative stress decreased with age.

CONCLUSION

This study demonstrates that the role of central AngII diminishes with age in the regulation of BP both during baseline conditions and during stress, whereas the involvement of AngII in the regulation of HR remains unaffected.

Animal aging and regulation of sympathetic nerve discharge

Studies completed in human subjects have made seminal contributions to understanding the effects of age on sympathetic nervous system (SNS) regulation. Numerous experimental constraints limit the design of studies involving human subjects; therefore, completion of studies in animal models of aging would be expected to provide additional insight regarding mechanisms mediating age-related changes in sympathetic nerve discharge (SND) regulation. The present review assesses the current state of the literature regarding contributions from animal studies on the effects of advancing age on SND regulation, focusing primarily on studies that have used direct recordings of sympathetic nerve outflow. Few studies using direct SND recordings have been completed in animal models of aging, regardless of the fundamental component of SND regulation reviewed (basal levels, acute responsiveness, relationships between the discharges in sympathetic nerves, central neural regulation). SNS responsiveness to various acute stressors is altered in aged compared with young animals; however, mechanisms remain virtually unexplored. There is a marked dearth of studies that have used central neural microinjection techniques in conjunction with SND recordings in aged animals, making it difficult to develop an evidence-based framework regarding potential age-associated effects on central regulation of SND. Determination of age-related changes in mechanisms regulating SND is important for understanding relationships between chronic disease development and changes in SNS function; however, this can only be achieved by substantially extending the current knowledge base regarding the effects of age on SND regulation in animal studies.

Status epilepticus produces chronic alterations in cardiac sympathovagal balance

PURPOSE

Status epilepticus (SE) activates the autonomic nervous system, increasing sympathetic nervous system control of cardiac function during seizure activity. However, lasting effects of SE on autonomic regulation of the heart, which may contribute to mortality following seizure activity, are unknown. Therefore, autonomic control of cardiac function was assessed following SE.

METHODS

Using Sprague-Dawley rats after 1-2 weeks of recovery from lithium-pilocarpine-induced SE or control procedures, we tested overall sympathovagal control of the heart, the individual contributions of the sympathetic and parasympathetic components of the autonomic nervous system, and baroreflex sensitivity.

RESULTS

SE induced a chronic shift in sympathovagal balance toward sympathetic dominance resulting from decreased parasympathetic activity. Baroreflex sensitivity to increased blood pressure was also decreased, likely resulting from diminished vagal activation.

DISCUSSION

Chronic alterations in autonomic regulation of cardiac function, characterized by increased sympathetic dominance, occur following SE and likely contribute to subsequent increased cardiac risk and mortality.

Hypotension- and osmotically induced thirst in old Brown Norway rats

Compared to young cohorts, old rats drink less water in response to several thirst-inducing stimuli. In these experiments, we characterized water drinking in response to hypotension and cellular dehydration in young (4 mo), middle-aged adult (12 mo) and old (29-30 mo) male Brown Norway rats. We injected the vasodilator, minoxidil as an intravenous bolus in a range of doses (0-20 mg/kg), so that drinking responses could be compared at equivalent reductions of arterial pressure. Old rats had greatly diminished reflex tachycardia and became significantly more hypotensive after minoxidil compared with young and middle-aged rats. When compared at equivalent reductions of arterial pressure, old rats drank one-third as much as middle-aged rats, and one-fifth as much as young rats. In addition, there were age-related deficits in drinking in response to a range of administered loads of sodium (0.15-2 M NaCl, 2 ml/100 g body wt). Urinary excretion of water and sodium in response to the loads was equivalent across ages. Both middle-aged and old rats were less able than young rats to repair their water deficits after sodium loading, attributable almost entirely to their reduced drinking responses compared with young rats. Lastly, age-related declines in drinking appeared to be more severe in response to hypotension than in response to cellular dehydration.

Delayed peripheral administration of the N-terminal tripeptide of IGF-1 (GPE) reduces brain damage following microsphere induced embolic damage in young adult and aged rats

We have previously reported that peripheral administration of GPE prevents neuronal injury after ischemic reperfusion injury in young adult rats. This study examined the ameliorating effects of GPE-treatment after embolic injury induced by microsphere injection in young adult and aged male rats. Unilateral injury was induced by injecting microspheres into the right internal carotid artery in both young adult (3-4 months) and aged (16-17 months) male rats. Either GPE (12mg/kg) or the vehicle was infused intravenously over 1h starting 3h after embolic injury and the degree of brain injury, astrocytosis and vascular remodeling were examined using histological and immunohistochemical analysis 8 days later. Changes in core temperature, blood glucose concentration, oxygen saturation and heart rate were monitored. Microsphere injection induced multiple sites of focal damage in the ipsilateral subcortical regions. Massive numbers of microglia accumulated within the core of the tissue damage whereas astrocytes were located in the penumbra. There was no difference in the degree of brain injury between the young and aged control rats. However the aged rats showed less injury-induced astrocytosis and greater vascular remodeling. Intravenous infusion of GPE 3h after the injury reduced overall damage scores in both young (p<0.01) and aged rats (p<0.05). GPE-treatment reduced astrocytosis in young, but not aged animals and did not significantly alter the vascular remodeling in either age group. The data suggested that the neuroprotection of the tripeptide is independent of cerebral reperfusion and is not age selective.

A brief review of chronic exercise intervention to prevent autonomic nervous system changes during the aging process

The aging process is associated with alterations in the cardiovascular and autonomic nervous systems. Autonomic changes related to aging involve parasympathetic and sympathetic alterations leading to a higher incidence of cardiovascular disease morbidity and mortality. Several studies have suggested that physical exercise is effective in preventing deleterious changes. Chronic exercise in geriatrics seems to be associated with improvement in the cardiovascular system and seems to promote a healthy lifestyle. In this review, we address the major effects of aging on the autonomic nervous system in the context of cardiovascular control. We examine the use of chronic exercise to prevent cardiovascular changes during the aging process.

Aging alters regulation of visceral sympathetic nerve responses to acute hypothermia

Hypothermia produced by acute cooling prominently alters sympathetic nerve outflow. Skin sympathoexcitatory responses to skin cooling are attenuated in aged compared with young subjects, suggesting that advancing age influences sympathetic nerve responsiveness to hypothermia. However, regulation of skin sympathetic nerve discharge (SND) is only one component of the complex sympathetic nerve response profile to hypothermia. Whether aging alters the responsiveness of sympathetic nerves innervating other targets during acute cooling is not known. In the present study, using multifiber recordings of splenic, renal, and adrenal sympathetic nerve activity, we tested the hypothesis that hypothermia-induced changes in visceral SND would be attenuated in middle-aged and aged compared with young Fischer 344 (F344) rats. Colonic temperature (Tc) was progressively reduced from 38°C to 31°C in young (3 to 6 mo), middle-aged (12 mo), and aged (24 mo) baroreceptor-innervated and sinoaortic-denervated (SAD), urethane-chloralose anesthetized, F344 rats. The following observations were made. 1) Progressive hypothermia significantly ( P < 0.05) reduced splenic, renal, and adrenal SND in young baroreceptor-innervated F344 rats. 2) Reductions in splenic, renal, and adrenal SND to progressive hypothermia were less consistently observed and, when observed, were generally attenuated in baroreceptor-innervated middle-aged and aged compared with young F344 rats. 3) Differences in splenic, renal, and adrenal SND responses to reduced Tc were observed in SAD young, middle-aged, and aged F344 rats, suggesting that age-associated attenuations in SND responses to acute cooling are not the result of age-dependent modifications in arterial baroreflex regulation of SND. These findings demonstrate that advancing chronological age alters the regulation of visceral SND responses to progressive hypothermia, modifications that may contribute to the inability of aged individuals to adequately respond to acute bouts of hypothermia.

Effects of long-term angiotensin converting enzyme inhibition on cardiovascular variability in aging rats

We studied the effects of chronic (4 weeks) angiotensin converting enzyme inhibition with captopril on arterial pressure (AP) and heart rate (HR) variability, as well as on cardiac baroreflex sensitivity (BRS), in aged (20 months) rats. Series of basal RR interval (RRi) and systolic AP (SAP) were studied by autoregressive spectral analysis with oscillations quantified in low (LF: 0.2-0.8 Hz) and high frequency (HF: 0.8-2.5 Hz). BRS was measured by linear regression between HR and MAP changes. Captopril did not affect the spectra of RRi or SAP in young rats. Aged rats presented a reduction in variance (time domain) and in LF and HF oscillations of RRi and SAP. Captopril induced, in aged rats, a decrease in absolute and normalized LF oscillations and in LF/HF ratio of RRi. Captopril also reduced the variance, without changing its LF or HF components of SAP. Reflex tachycardia was reduced in aged as compared to young rats (-1.1+/-0.2 versus -3.4+/-0.5 bpm/mm Hg) and captopril did not affect it. Reflex bradycardia was also reduced in aged rats (-0.7+/-0.5 versus -2.0+/-0.4 bpm/mm Hg), but captopril prevented this attenuation in aged rats (-2.3+/-0.3 versus -0.7+/-0.5 bpm/mm Hg). These data indicate that there is a reduction in HR and SAP variability during aging, suggesting impairment of cardiovascular autonomic control. Captopril was able to change the power of oscillatory components of RRi, suggesting a shift in cardiac sympatho/vagal balance toward parasympathetic predominance. In addition, blockage of ACE improved the reflex bradycardia, but not the reflex tachycardia in aged rats.

Long-term effects of streptozotocin-induced diabetes on the electrocardiogram, physical activity and body temperature in rats

In vivo biotelemetry studies have demonstrated that short-term streptozotocin (STZ)-induced diabetes is associated with a reduction in heart rate (HR) and heart rate variability (HRV) and prolongation of QT and QRS intervals. This study investigates the long-term effects of STZ-induced diabetes on the electrocardiogram (ECG), physical activity and body temperature. Transmitter devices were surgically implanted in the peritoneal cavity of young adult male Wistar rats. Electrodes from the transmitter were arranged in Einthoven bipolar lead II configuration. ECG, physical activity and body temperature data were continuously recorded with a telemetry system before and following the administration of STZ (60 mg kg(-1)) for a period of 22 weeks. HR, physical activity and body temperature declined rapidly 3-5 days after the administration of STZ. The effects became conspicuous with time reaching a new steady state approximately 1-2 weeks after STZ treatment. HR at 4 weeks was 268 +/- 5 beats min(-1) in diabetic rats compared to 347 +/- 12 beats min(-1) in age-matched controls. HRV at 4 weeks was also significantly reduced after STZ treatment (18 +/- 3 beats min(-1)) compared to controls (33 +/- 3 beats min(-1)). HR and HRV were not additionally altered in either diabetic rats (266 +/- 5 and 20 +/- 4 beats min(-1)) or age-matched controls (316 +/- 6 and 25 +/- 4 beats min(-1)) at 22 weeks. Reduced physical activity and/or body temperature may partly underlie the reductions in HR and HRV. In addition, the increased power spectral low frequency/high frequency ratio from 4 weeks after STZ treatment may indicate an accompanying disturbance in sympathovagal balance.

Decreased alpha-adrenergic constriction of renal preglomerular arteries occurs with age and is gender-specific in the rat

Age and/or gender appear to moderate alpha-adrenergic mediated constrictor mechanisms found in the interlobar arteries of the Munich Wistar rat. We have determined the extent of constriction to alpha-adrenergic receptor stimulation using norepinephrine, phenylephrine and A61603 (α1A-adrenergic receptor agonist) as a function of age and gender. Norepinephrine produced less constriction in male-derived arteries at ages greater than eight months as compared to the younger adult male (four to six months). The arteries derived from females did not demonstrate altered constriction until greater than 15 months of age. Similarly, arteries derived from the male demonstrated weaker constrictions to phenylephrine (10(-6) to 10(-3) M) at ages greater than eight months while arteries from females showed differences at greater than 15 months. In contrast, the effective concentration of norepinephrine to cause a 50% maximal constriction (EC50) was significantly less in the four to five-month-old male rats compared to the pooled data from older groups. Interestingly, four to five month old males had A61603 EC50 values similar to the 8 to 12-month and 15+ old females. These studies conclude that an age related loss of sympathetic α-adrenergic constriction of renal interlobar arteries is present in Munich Wistar rats. Furthermore, this loss, while similar along longitudinal aspects of age, is also different as a function of gender with the loss of α-adrenergic constrictor function delayed in the female when compared to the male.

The Effects of Aging on Somatocardiac Reflexes in Anesthetized Rats

Nociceptive cutaneous stimulation produces a reflex tachycardiac response that is mediated through the activation of cardiac sympathetic efferents. This response includes reflex components of both supraspinal and spinal origin, depending on which segmental afferent area is stimulated (for a review see Sato et al.: Rev Physiol Biochem Pharmacol 130: 1-328, 1997). We herein examined the effects of aging on these supraspinal and spinal reflexes in anesthetized rats that were 4-7 (young adult), 24-27 (old), and 32-36 (very old) months of age. In central nervous system (CNS)-intact animals, we found that the supraspinal tachycardiac response induced by the pinching of a hindpaw was well preserved in old rats but was significantly attenuated in very old rats, while pinching-induced increases in cardiac sympathetic nerve activity were well maintained in both of these rat populations. In spinalized animals, spinal-mediated changes in heart rate (HR) and cardiac sympathetic nerve activity induced by the pinching of the chest skin were well preserved in both old and very old rats. There were no significant differences in resting HR among the 3 age groups studied, though the maximum HR induced by the beta-adrenergic agonist isoproterenol was reduced with age. The beta-receptor-mediated maximum HR was greater than that induced by pinching in young adult and old rats, CNS-intact and spinalized rats, and very old spinalized rats, while the maximum HR was nearly the same as the pinching-induced HR in CNS-intact very old rats. These results suggest that both supraspinal and spinal neural reflex pathways involved in the cardiac sympathetic response to cutaneous pinching are well preserved in older animals. They also suggest that the decline in the responsiveness of the heart to beta-adrenergic stimulation results in a reduced pinching-induced supraspinal tachycardiac response in very old rats.

Attenuation of baroreflex sensitivity after domoic acid lesion of the nucleus ambiguus of rats

The nucleus ambiguus (NA) and the dorsal motor nucleus of the vagus (DmnX) innervate distinct populations of cardiac ganglionic principal neurons. This anatomic evidence suggests that these two nuclei play different roles (Cheng Z and Powley TL, Soc Neurosci Abstr 26: 1189, 2000). However, lesion of the DmnX does not attenuate baroreflex sensitivity (Cheng Z, Guo SZ, Lipton AJ, and Gozal D, J Neurosci 22: 3215-3226, 2002). The present study tested the functional role of the NA in baroreflex control of heart rate (HR). Domoic acid (DA) was injected into the left NA of Sprague-Dawley rats to lesion the NA. The neuronal loss was assessed using retrograde labeling and confocal microscopy. HR changes induced by phenylephrine and sodium nitroprusside administration and after electrical stimulation of the left vagal trunk were measured at 15 days, and HR responses to left NA microinjection of L-glutamate were determined at 180 days postlesion. Compared with vehicle injections, DA lesions significantly reduced the population of NA motor neurons by approximately 68% (P < 0.01) and attenuated baroreflex sensitivity by approximately 83% (P < 0.01) at 15 days. Similarly, electrical stimulation of the vagal trunk of DA-lesioned animals led to attenuated decreases in HR responses. NA neuronal counts were reduced by approximately 81% (P < 0.01) and mean HR responses to l-glutamate injection into the lesioned NA were attenuated by approximately 65% (P < 0.01) at 180 days. Therefore, the NA plays a major role in baroreflex control of HR, and the integrity of the NA is critically important for the normal baroreflex control. In addition, NA lesions produce long-term anatomic and functional dysfunction of the nucleus, and thus it may provide an useful model for functional assessment of respective roles of the NA and DmnX.

Arterial baroreflex function and cardiovascular variability: interactions and implications

The arterial baroreflex contributes importantly to the short-term regulation of blood pressure and cardiovascular variability. A number of factors (including reflex, humoral, behavioral, and environmental) may influence gain and effectiveness of the baroreflex, as well as cardiovascular variability. Many central neural structures are also involved in the regulation of the cardiovascular system and contribute to the integrity of the baroreflex. Consequently, brain injuries or ischemia may induce baroreflex impairment and deranged cardiovascular variability. Baroreflex dysfunction and deranged cardiovascular variability are also common findings in cardiovascular disease. A blunted baroreflex gain and impaired heart rate variability are predictive of poor outcome in patients with heart failure and myocardial infarction and may represent an early index of autonomic activation in left ventricular dysfunction. The mechanisms mediating these relationships are not well understood and may in part be the result of cardiac structural changes and/or altered central neural processing of baroreflex signals.

Sympathetic nerve regulation to heating is altered in senescent rats

Renal and splanchnic sympathetic nerve discharge (SND) responses to increased (38-41 degrees C) internal temperature were determined in anesthetized young (3-6 mo old), mature (12 mo old), and senescent (24 mo old) Fischer 344 (F344) rats. We hypothesized that SND responses would be altered in senescent and mature rats as demonstrated by attenuated sympathoexcitatory responses to heating and by the absence of hyperthermia-induced SND pattern changes. The following observations were made. 1) Renal and splanchnic SND responses were significantly increased during heating in young and mature but not in senescent rats. 2) At 41 degrees C, renal and splanchnic SND responses were higher in young compared with senescent rats, and renal SND was higher in mature than in senescent rats. 3) Heating changed the SND bursting pattern in young, but not in mature or senescent, rats. 4) SND responses to heating did not differ between baroreceptor-innervated (BRI) and sinoaortic-denervated (SAD) senescent rats but were higher in SAD compared with BRI young rats. These results demonstrate an attenuated responsiveness of sympathetic neural circuits to heating in senescent F344 rats.