Effect of chronic stress on the cardiac baroreflex in the post-weanling rat

There is increasing evidence that early life stressors may program blood pressure control mechanisms such that the risk for cardiovascular disease in later life is increased. In the present investigation, the effect of repeated restraint/heat stress during the two-week period immediately after weaning on baroreflex function was determined and the contribution of brain angiotensin II (ANG II) to the changes was assessed in young, conscious, freely moving Sprague Dawley rats. In rats two weeks post weaning, basal MAP was significantly higher and basal HR significantly lower than rats tested immediately after weaning. This change in the operating point of HR was not accompanied by any changes in baroreflex function. Treatment with chronic icv infusion of losartan, an AT1 receptor antagonist, during the two-week period prevented the changes in basal MAP and HR. Chronic stress during the two weeks post weaning, whether due to surgical implantation of icv cannulae or due to restraint/heat stress, significantly shifted the set-point of the baroreflex function to a higher pressure. Chronic icv infusion of losartan during the period prevented these effects (at least in the case of stress due to the presence of icv cannulae) suggesting a role for brain ANG II in the change. Changes in the expression of CRH mRNA in the paraventricular nucleus could not explain the stress-related change in baroreflex function. If the rightward shift in the baroreflex persists into adulthood, it could increase the susceptibility to cardiovascular diseases such as hypertension.     

Effect of environmental stress on release of norepinephrine in posterior nucleus of the hypothalamus in awake rats: role of sinoaortic nerves

Norepinephrine(NE) release in posterior nucleus(PH) of the hypothalamus was examined before and during acute shaker (oscillation) stress in sinoaortic denervated(SAD) and sham-operated(SO) rats. NE in PH extracellular fluid of freely moving rats was collected by microdialysis and measured by sensitive radioenzymatic assay. Three days after SAD or SO operation, mean arterial pressure(MAP) and heart rate(HR) were significantly higher in SAD rats than SO rats. Baseline levels of NE in PH dialysate were also significantly elevated in SAD rats. Although five minutes of shaker stress elicited pressor and tachycardic responses coupled with increased NE release in PH of both groups, the increases in MAP and dialysate NE were larger in SAD than SO rats. These findings indicate that noradrenergic neurons in the PH respond to stress-induced stimuli and receive tonic input from baroreflex pathways.     

Enhanced sympathoexcitatory and pressor responses to central Na+ in Dahl salt-sensitive vs. -resistant rats

An enhanced responsiveness to increases in cerebrospinal fluid (CSF) Na+ by high salt intake may contribute to salt-sensitive hypertension in Dahl salt-sensitive (S) rats. To test this hypothesis, sympathetic and pressor responses to acute and chronic increases in CSF Na+ were evaluated. In conscious young (5-6 wk old) and adult (10-11 wk old) Dahl S and salt-resistant (R) rats as well as weight-matched Wistar rats, hemodynamic [blood pressure (BP) and heart rate (HR)] and sympathetic [renal sympathetic nerve activity (RSNA)] responses to 10-min intracerebroventricular infusions of artificial CSF (aCSF) and Na+-rich aCSF (containing 0.2-0.45 M Na+) were evaluated. Intracerebroventricular Na+-rich aCSF increased BP, RSNA, and HR in a dose-related manner. The extent of these increases was significantly larger in Dahl S versus Dahl R or Wistar rats and young versus adult Dahl S rats. In a second set of experiments, young Dahl S and R rats received a chronic intracerebroventricular infusion of aCSF or Na+-rich (0.8 M) aCSF (5 microl/h) for 14 days, with the use of osmotic minipumps. On day 14 in conscious rats, CSF was sampled and BP, HR, and RSNA were recorded at rest and in response to air stress, intracerebroventricular alpha2-adrenoceptor agonist guanabenz, intracerebroventricular ouabain, and intravenous phenylephrine and nitroprusside to estimate baroreflex function. The infusion of Na+-rich aCSF versus aCSF increased CSF Na+ concentration to the same extent but caused severe versus mild hypertension in Dahl S and Dahl R rats, respectively. After central Na+ loading, hypothalamus "ouabain" significantly increased in Dahl S and only tended to increase in Dahl R rats. Moreover, sympathoexcitatory and pressor responses to intracerebroventricular exogenous ouabain were attenuated by Na+-rich aCSF to a greater extent in Dahl S versus Dahl R rats. Responses to air-jet stress or intracerebroventricular guanabenz were enhanced by Na+-rich aCSF in both strains, but the extent of enhancement was significantly larger in Dahl S versus Dahl R. Na+-rich aCSF impaired arterial baroreflex control of RSNA more markedly in Dahl S versus R rats. These findings indicate that genetic control of mechanisms linking CSF Na+ with brain "ouabain" is altered in Dahl S rats toward sympathetic hyperactivity and hypertension.     

Effect of Barodenervation on Cardiovascular Responses Elicited from the Hypothalamic Arcuate Nucleus of the Rat

We have previously reported that chemical stimulation of the hypothalamic arcuate nucleus (ARCN) in the rat elicited increases as well as decreases in blood pressure (BP) and sympathetic nerve activity (SNA). The type of response elicited from the ARCN (i.e., increase or decrease in BP and SNA) depended on the level of baroreceptor activity which, in turn, was determined by baseline BP in rats with intact baroreceptors. Based on this information, it was hypothesized that baroreceptor unloading may play a role in the type of response elicited from the ARCN. Therefore, the effect of barodenervation on the ARCN-induced cardiovascular and sympathetic responses and the neurotransmitters in the hypothalamic paraventricular nucleus (PVN) mediating the excitatory responses elicited from the ARCN were investigated in urethane-anesthetized adult male Wistar rats. Bilateral barodenervation converted decreases in mean arterial pressure (MAP) and greater splanchnic nerve activity (GSNA) elicited by chemical stimulation of the ARCN with microinjections of N-methyl-D-aspartic acid to increases in MAP and GSNA and exaggerated the increases in heart rate (HR). Combined microinjections of NBQX and D-AP7 (ionotropic glutamate receptor antagonists) into the PVN in barodenervated rats converted increases in MAP and GSNA elicited by the ARCN stimulation to decreases in MAP and GSNA and attenuated increases in HR. Microinjections of SHU9119 (a melanocortin 3/4 receptor antagonist) into the PVN in barodenervated rats attenuated increases in MAP, GSNA and HR elicited by the ARCN stimulation. ARCN neurons projecting to the PVN were immunoreactive for proopiomelanocortin, alpha-melanocyte stimulating hormone (alpha-MSH) and adrenocorticotropic hormone (ACTH). It was concluded that increases in MAP and GSNA and exaggeration of tachycardia elicited by the ARCN stimulation in barodenervated rats may be mediated via release of alpha-MSH and/or ACTH and glutamate from the ARCN neurons projecting to the PVN.     

Neuropeptide Y reverses chronic stress-induced baroreflex hypersensitivity in rats

Chronic stress, as a risk factor for cardiovascular diseases, has been reported to result in elevated plasma neuropeptide Y (NPY) and be highly associated with abnormal cardiac autonomic function. This study aimed to explore the effect of NPY on the chronic stress-induced abnormal baroreceptor reflex sensitivity (BRS). Seven types of recognized stressors were used to develop chronic stress rat model. Subcutaneously implanting ALZET mini-osmotic pumps containing NPY were used to evaluate the action of NPY on the stressed male rats. We found that chronic stress showed no influence on baseline systolic blood pressure (SBP) and heart rate (HR), whereas NPY (85 μg for 30 days) could elevate baseline SBP and induce bradycardia in rats intervened by various stimuli. NPY pretreatment could preserve chronic stress-induced decreases in left ventricular systolic pressure (LVSP) and the maximum rate of change in left ventricular pressure in the isovolumic contraction period (+dp/dt(max)) but has shown no effect on left ventricular end diastolic pressure (LVEDP) and the maximum rate of change in left ventricular pressure in the isovolumic relaxation period (-dp/dt(max)). Notably, chronic stress led to baroreflex oversensitivity indicated by the elevated ratio of Δheart rate (HR)/ Δmean arterial blood pressure (MABP) in rats followed by vasoconstrictor (phenylephrine, PE) or vasodilator (sodium nitroprusside, SNP) administration, which was almost completely reversed by NPY pretreatment. The expressions of substance P (SP) and gamma aminobutyric acid A receptor (GABA(A)R) in nucleus tractus solitarius were increased in chronic stress rats, which were counteracted by NPY pretreatment. We conclude that chronic stress-induced baroreflex hypersensitivity could be blocked by NPY pretreatment. Furthermore, the altered expressions of neurotransmitters and receptors in the brainstem might contribute to this process.     

Selective impairment of central mediation of baroreflex in anesthetized young adult Fischer 344 rats after chronic intermittent hypoxia

Baroreflex control of heart rate (HR) is impaired after chronic intermittent hypoxia (CIH). However, the location and nature of this response remain unclear. We examined baroreceptor afferent, vagal efferent, and central components of the baroreflex circuitry. Fischer 344 (F344) rats were exposed to room air (RA) or CIH for 35-50 days and were then anesthetized with isoflurane, ventilated, and catheterized for measurement of mean arterial blood pressure (MAP) and HR. Baroreceptor function was characterized by measuring percent changes of integrated aortic depressor nerve (ADN) activity (Int ADNA) relative to the baseline value in response to sodium nitroprusside- and phenylephrine-induced changes in MAP. Data were fitted to a sigmoid logistic function curve. HR responses to electrical stimulation of the left ADN and the right vagus nerve were assessed under ketamine-acepromazine anesthesia. Compared with RA controls, CIH significantly increased maximum baroreceptor gain or maximum slope, maximum Int ADNA, and Int ADNA range (maximum - minimum Int ADNA), whereas other parameters of the logistic function were unchanged. In addition, CIH increased the maximum amplitude of bradycardic response to vagal efferent stimulation and decreased the time from stimulus onset to peak response. In contrast, CIH significantly reduced the maximum amplitude of bradycardic response to left ADN stimulation and increased the time from stimulus onset to peak response. Therefore, CIH decreased central mediation of the baroreflex but augmented baroreceptor afferent function and vagal efferent control of HR.     

兔肾性高血压时的动脉压力感受器反射

14只雄性家兔在双肾缩扎术后12周,经氨基甲酸乙酯静脉麻醉,分别在缓冲神经完整、切断两侧减压神经或切断两侧窦神经后静注新福林或硝普钠升降血压以改变动脉压力感受器活动,观察其心率、后肢血管阻力和肾交感神经活动的反射性变化,并与正常血压兔的反射效应相比较。主要结果如下:(1) 动物双肾动脉缩扎后12周,平均动脉血压(131±9mmHg)较正常动物血压(95±10mmHg)有显著升高(P<0.001);(2) 缓冲神经完整时,新福林和硝普钠升降血压诱发的心率反射性变化与正常血压动物相比显著减弱(P<0.001),而后肢血管阻力和肾交感神经活动的反射性调节无明显改变,表明肾性高血压动物的心率反射性调节与外周循环的反射性调节机能不相平行;而由股动脉内直接注射新福林或硝普钠时,股动脉灌流压的增减幅度与正常血压动物相比并无明显差异;(3) 切断两侧减压神经或切断两侧窦神经后,在正常动物仅使反射性心率调节作用减弱,而后肢血管阻力和肾交感神经活动的反射性调节无明显改变;但在高血压动物,除心率的反射性调节进一步减弱外,新福林和硝普钠升降血压时后肢血管阻力和肾交感神经活动的反射性调节效应也显著地减弱(P<0.001),提示肾性高血压时动脉压力感受器反射的潜在调节能力降低。由此似表明,肾性高血压时动脉压力感受器反射     

Sympathetic hyperreactivity to air-jet stress in the chromosome 1 blood pressure quantitative trait locus congenic rats

A chromosome 1 blood pressure quantitative trait locus (QTL) was introgressed from the stroke-prone spontaneously hypertensive rats (SHRSP) to Wistar-Kyoto (WKY) rats. This congenic strain (WKYpch1.0) showed an exaggerated pressor response to both restraint and cold stress. In this study, we evaluated cardiovascular and sympathetic response to an air-jet stress and also examined the role of the brain renin-angiotensin system (RAS) in the stress response of WKYpch1.0. We measured mean arterial pressure (MAP), heart rate (HR), and renal sympathetic nerve activity (RSNA) responses to air-jet stress in WKYpch1.0, WKY, and SHRSP. We also examined effects of intracerebroventricular administration of candesartan, an ANG II type 1 receptor blocker, on MAP and HR responses to air-jet stress. Baseline MAP in the WKYpch1.0 and WKY rats were comparable, while it was lower than that in SHRSP rats. Baseline HR did not differ among the strains. In WKYpch1.0, air-jet stress caused greater increase in MAP and RSNA than in WKY. The increase in RSNA was as large as that in SHRSP, whereas the increase in MAP was smaller than in SHRSP. Intracerebroventricular injection of a nondepressor dose of candesartan inhibited the stress-induced pressor response to a greater extent in WKYpch1.0 than in WKY. Intravenous injection of phenylephrine caused a presser effect comparable between WKYpch1.0 and WKY. These results suggest that the chromosome 1 blood pressure QTL congenic rat has a sympathetic hyperreactivity to an air-jet stress, which causes exaggerated pressor responses. The exaggerated response is at least partly mediated by the brain RAS.     

Cardiac effects of hypocretin-1 in nucleus ambiguus

Although recent studies have reported hypocretin 1 (hcrt-1)-like-immunoreactivity (ir) within the region of the nucleus ambiguus (Amb) in the caudal brain stem, the function of hcrt-1 in the Amb on cardiovascular function is not known. Three series of experiments were done in male Wistar rats to investigate the effects of microinjections of hcrt-1 into Amb on heart rate (HR), mean arterial pressure (MAP), and the arterial baroreceptor reflex. In the first series, a detailed mapping of the distribution of hcrt-1- and hcrt-1 receptor (hcrtR-1)-like-ir was obtained of the Amb region. Although hcrt-1-like- and hcrtR-1-like-ir were found throughout the rostrocaudal extent of the Amb and adjacent ventrolateral medullary reticular formation, most of the hcrtR-1-like-ir was observed in the area just ventral to the compact formation of Amb, in the region of the external formation of the nucleus (Ambe). In the second series, the Amb region that contained hcrt-1 and hcrtR-1-ir was explored for sites that elicited changes in HR and MAP in urethane and alpha-chloralose-anesthetized rats. Microinjections of hcrt-1 (0.5-2.5 pmol) into the Ambe elicited a dose-related decrease in HR, with little or no direct change in MAP. The small decreases in MAP were found to be secondary to the HR changes. The largest bradycardia responses were elicited from sites in the Ambe. Administration (iv) of the muscarinic receptor antagonist atropine methyl bromide or ipsilateral vagotomy abolished the HR response, indicating that the HR response was due to activation of vagal cardiomotor neurons. In the final series, microinjections of hcrt-1 into the Ambe significantly potentiated the reflex bradycardia elicited by activation of the baroreflex as a result of the increased MAP after the intravenous injection of phenylephrine. These data suggest that hcrt-1 in the Ambe activates neuronal systems that alter the excitability of central circuits that reflexly control the circulation through the activation of vagal preganglionic cardioinhibitory neurons.     

Baroreceptor reflex modulation by circulating angiotensin II is mediated by AT1 receptors in the nucleus tractus solitarius

Circulating ANG II modulates the baroreceptor reflex control of heart rate (HR), at least partly via activation of ANG II type 1 (AT1) receptors on neurons in the area postrema. In this study, we tested the hypothesis that the effects of circulating ANG II on the baroreflex also depend on AT1 receptors within the nucleus tractus solitarius (NTS). In confirmation of previous studies in other species, increases in arterial pressure induced by intravenous infusion of ANG II had little effect on HR in urethane-anesthetized rats, in contrast to the marked bradycardia evoked by equipressor infusion of phenylephrine. In the presence of a continuous background infusion of ANG II, the baroreflex control of HR was shifted to higher levels of HR but had little effect on the baroreflex control of renal sympathetic activity. The modulatory effects of circulating ANG II on the cardiac baroreflex were significantly reduced by microinjection of candesartan, an AT1 receptor antagonist, into the area postrema and virtually abolished by microinjections of candesartan into the medial NTS. After acute ablation of the area postrema, a background infusion of ANG II still caused an upward shift of the cardiac baroreflex curve, which was reversed by subsequent microinjection of candesartan into the medial NTS. The results indicate that AT1 receptors in the medial NTS play a critical role in modulation of the cardiac baroreflex by circulating ANG II via mechanisms that are at least partly independent of AT1 receptors in the area postrema.     

Microinjection of muscimol into the periaqueductal gray suppresses cardiovascular and neuroendocrine response to air jet stress in conscious rats

Microinjection of the neuronal inhibitor muscimol into the dorsomedial hypothalamus (DMH) suppresses increases in heart rate (HR), mean arterial pressure (MAP), and circulating levels of adrenocorticotropic hormone (ACTH) evoked in air jet stress in conscious rats. Similar injection of muscimol into the caudal region of the lateral/dorsolateral periaqueductal gray (l/dlPAG) reduces autonomic responses evoked from the DMH, leading to the suggestion that neurons in the l/dlPAG may represent a descending relay for DMH-induced increases in HR and MAP. Here, we examined the role of neuronal activity in the caudal l/dlPAG on the increases in MAP, HR, and plasma ACTH seen in air jet stress in rats. Microinjection of muscimol into the caudal l/dlPAG reduced stress-induced increases in HR and MAP, while identical injections into sites just dorsal or into the rostral l/dlPAG had no effect. Microinjection of a combination of the glutamate receptor antagonists 2-amino-5-phosphonopentanoate (AP5) and 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo[f]quinoxaline-2,3-dione (NBQX) into the caudal l/dlPAG decreased stress-induced increases in HR alone only at the end of the 20-min stress period but significantly accelerated return to baseline. Surprisingly, microinjection of muscimol into the caudal l/dlPAG also reduced the stress-induced increase in plasma ACTH by 51%. Compared with unstressed control rats, rats exposed to air jet stress exhibited approximately 3 times the number of Fos-positive neurons in the l/dlPAG. These findings suggest that neurons in the l/dlPAG are activated in air jet stress and that this activity contributes to increases in HR, MAP, and plasma ACTH.     

激动中枢组胺能受体对应激大鼠颈动脉窦压力感受性反射重调定的影响

应激1周的大鼠,麻醉后弧离双侧颈动脉窦区,将不同窦内压（ISP）与其对应的平均动脉压（MAP）值进行Logistic5个参数曲线拟合。根据所得ISP－MAP关系曲线及其特征参数,分别观察侧脑室(i.c.v.）注射和弧束核（NTS）内注射组胺受体拮抗剂对颈动脉窦压力感受性反射（CSR）的影响,并与相应的非应激CSR水平进行比较。结果如下：（1）应激导致ISP－MAP关系曲线显著全面上移,窦内压和增益（ISP-Gain）关系曲线中部明显下移,反射参数中阈压（TP）、饱和压（SP）、调定点（set point）和最大增益时的窦内压（ISPGmax）值增大,MAP反射变动范围（MAPrange）及反射最大增益（Gmax）减小;（2）I.c.v.H1或H2受体拮抗剂氯苯吡胺（CHL）15μg或西咪替丁（CIM）15μg,在20min内均可明显减弱应激对CSR的上述改变,CIM的这种减弱作用不如CHL的显著;（3）NTS内注射CHL（0.5μg）或CIM（1.5μg）,对应激所致CSR变化的影响与i.c.v.CHL或CIM后的相类似;（4）分别i.c.v.和NTS内注射CHL或CIM后,均不能使应激的CSR水平完全恢复到相应的非应激对照水平。以上结果提示,应激引起CSR重调定,反射敏感性下降;其部分机制可能是激活中枢组胺能系统,通过中枢组胺能受体（H1和H2受体）尤其是H1受体介导而发挥作用;下丘脑－NTS的组胺能通路可能是应激导致CSR重调定的下行通路之一。     

A differential action for ethanol on baroreceptor reflex control of heart rate and sympathetic efferent discharge in rats

The acute effects of ethanol (0.33, 0.66, or 1 g/kg) on baroreflex control of heart rate (HR) and sympathetic efferent discharge (SED) were investigated in chloralose-anesthetized rats. The two higher doses of ethanol caused a progressive and significant increase in baseline SED and a slight increase in HR. That these effects were ethanol mediated is suggested by the absence of any change in blood pressure following ethanol injection in any amount used and the finding that equivolume saline had no effect on any of the tested parameters. On the other hand, the baroreflex slope of the MAP-SED relationship after ethanol was similar to the control (preethanol) value in contrast to a significant decrease in the baroreflex slope of MAP-HR under the same conditions. These findings suggest that the sensitivity of the reflex control of SED was preserved whereas that of HR was impaired after acute ethanol administration. Since these findings were obtained in the same animals, our data suggest that acute ethanol has a differential action on reflex control of SED and HR. Further, the significant increase in SED after moderate and high doses of ethanol suggests an increased central sympathetic tone as recordings were made from preganglionic nerve fibers (splanchnic nerve). The absence of an increase in baseline MAP, in spite of a significant increase in baseline SED following acute ethanol injection, could be explained, at least in part, by an ethanol-evoked reduction in pressor responsiveness to phenylephrine, an alpha-adrenergic agonist.     

Nitric oxide at the CVLM is involved in the attenuation of the reflex bradycardia in renovascular hypertensive rats

Hypertension is associated to an increase in central oxidative stress and an attenuation of the baroreflex control of arterial pressure. The present study evaluated the effect of alterations in the levels of nitric oxide (NO) and superoxide anion in the caudal ventrolateral medulla (CVLM), a key area of the brainstem for the baroreflex control of arterial pressure, in renovascular hypertensive rats (2K1C). Baseline mean arterial pressure (MAP), heart rate (HR), and reflex bradycardia were evaluated 30 days after renal artery occlusion in anesthetized (urethane, 1.2 g/kg, i.p.) 2K1C or normotensive (SHAM) rats. The MAP, HR, and baroreflex control of HR were evaluated before and after CVLM microinjections of the non-selective NOS inhibitor L-NAME (10 nmol), the NO precursor L-ARG (50 nmol), or the antioxidant ascorbic acid, Vit C (10 nmol). In both 2K1C and SHAM animals, CVLM microinjection of L-NAME produced a decrease in MAP, whereas L-ARG induced a significant increase in MAP. However, microinjection of Vit C into the CVLM produced a decrease in MAP and HR only in 2K1C and not in SHAM rats. Cardiovascular effects produced by microinjection of l-ARG into the CVLM were abolished by prior microinjection of L-NAME in the CVLM of 2K1C and SHAM rats. Microinjection of L-NAME into the CVLM increased the sensitivity of reflex bradycardia in 2K1C animals. In contrast, the CVLM microinjection of L-ARG reduced reflex bradycardia only in SHAM rats. Vit C in the CVLM did not change reflex bradycardia in either 2K1C or in SHAM rats. These results suggest that increased oxidative stress in the CVLM during hypertension contributes to the reduced baroreflex sensitivity and to maintain hypertension in the 2K1C model.     

侧脑室注射Orexin-1受体拮抗剂对大鼠心血管效应的影响

目的:用侧脑室微量注射和免疫组化方法研究食欲素-1受体(OX1R)拮抗剂SB408124对麻醉大鼠的心血管效应及其作用机制。方法:雄性SD大鼠,侧脑室微量注射SB408124,及甲硝阿托品、六甲溴铵静脉注射预处理后侧脑室注射SB408124,观测动脉血压(MAP)和心率(HR)的变化。然后,用免疫组化方法检测侧脑室注射SB408124对大鼠脑延髓头端腹外侧区(RVLM)内酪氨酸羟化酶(TH)阳性神经元的影响。结果:侧脑室注射SB408124可显著降低麻醉大鼠的MAP和HR,但是HR变化不如MAP变化明显。应用六甲溴铵可完全阻断SB408124的心血管效应,但甲硝阿托品不能阻断SB408124的心血管效应。侧脑室注射SB408124可使鼠脑延髓头端腹外侧区内酪氨酸羟化酶阳性神经元的数量显著降低。结论:侧脑室注射OX1R拮抗剂SB408124可显著降低麻醉大鼠的MAP和HR,其作用主要是通过抑制交感神经系统的活性而实现的。     

Evidence for a role of AT(2) receptors at the CVLM in the cardiovascular changes induced by low-intensity physical activity in renovascular hypertensive rats

In the present study, we evaluated the involvement of the rennin-angiotensin system (RAS) in the control of the blood pressure (BP), baroreceptor-mediated bradycardia and the reactivity of caudal ventrolateral medulla (CVLM) neurons to Ang II and to AT(2) receptor antagonist in sedentary or trained renovascular hypertensive rats. Physical activity did not significantly change the baseline mean arterial pressure (MAP), heart rate (HR) or the sensitivity of the baroreflex bradycardia in normotensive Sham rats. However, in 2K1C hypertensive rats, physical activity induced a significant fall in baseline MAP and HR and produced an improvement of the baroreflex function (bradycardic component). The microinjections of Ang II into the CVLM produced similar decreases in MAP in all groups, Sham and 2K1C, sedentary and trained rats. The hypotensive effect of Ang II at the CVLM was blocked by previous microinjection of the AT(2) receptors antagonist, PD123319, in all groups of rats. Unexpectedly, microinjection of PD123319 at the CVLM produced a depressor effect in 2K1C sedentary that was attenuated in 2K1C trained rats. No significant changes in MAP were observed after PD123319 in Sham rats, sedentary or trained. These data showed that low-intensity physical activity is effective in lowering blood pressure and restoring the sensitivity of the baroreflex bradycardia, however these cardiovascular effects are not accompanied by changes in the responsiveness to Ang II at CVLM in normotensive or hypertensive, 2K1C rats. In addition, the blood pressure changes observed after AT(2) blockade in 2K1C rats suggest that hypertension may trigger an imbalance of AT(1)/AT(2) receptors at the CVLM that may be restored, at least in part, by low-intensity physical activity.     

Chronic blockade of brain "ouabain" prevents sympathetic hyper-reactivity and impairment of acute baroreflex resetting in rats with congestive heart failure

In rats with congestive heart failure (CHF) post myocardial infarction (MI) acute blockade of brain "ouabain" reverses sympathetic hyperactivity and chronic blockade prevents the desensitization of baroreflex function. This study was conducted to determine: i) if chronic blockade of brain "ouabain" maintains normal sympathetic reactivity; and ii) if acute baroreflex resetting (another parameter of baroreflex function) also becomes impaired, and if so, does brain "ouabain" contribute to impairment in acute baroreflex resetting. CHF post MI was induced by acute coronary artery ligation in Wistar rats. Animals were treated with 200 microg x day(-1) i.c.v. or i.v. Fab fragments (which bind brain "ouabain" with high affinity), or treated with 200 microg x day(-1) i.c.v. gamma-globulins (control group). The length of treatment was 0.5-8 weeks or 4-8 weeks post MI. At 8 weeks mean arterial pressure (MAP), central venous pressure (CVP), heart rate (HR), and renal sympathetic nerve activity (RSNA) were recorded in concious rats at rest and in response to: i) air-jet stress, ii) i.c.v. guanabenz (an alpha2-adrenoceptor agonist), and iii) a 30 min i.v. infusion of nitroprusside (NP). Excitatory responses to air stress and inhibitory responses to guanabenz of MAP, HR, and RSNA were significantly enhanced in rats with CHF versus the sham-operated treated group. This enhancement was prevented in the CHF group treated with i.c.v., but not i.v., Fab. Nitroprusside induced a sustained decrease in MAP (approximately 25 mmHg) and a transient decrease in CVP. Heart rate and RSNA increased significantly within 1 min of beginning the infusion. The peak increases as well as the product of changes in MAP-HR and RSNA-HR were significantly smaller in rats with CHF treated with gamma-globulins versus sham rats and versus CHF rats treated with i.c.v. Fab. In sham-operated rats and CHF rats treated with i.c.v. Fab, RSNA and HR began to decrease within 3-4 min of beginning the NP infusion and had returned to baseline by 20 min. In contrast, RSNA and HR remained increased throughout the infusion in the CHF rats treated with gamma-globulins. These data indicate that in rats with CHF acute resetting of the arterial baroreflex in response to a lower BP becomes impaired, and chronic blockade of brain "ouabain" prevents both this change in baroreflex resetting as well as sympathetic hyperactivity.     

Baroreflex regulation of renal sympathetic nerve activity and heart rate in renal wrap hypertensive rats

Despite its usefulness as a nongenetic model of hypertension, little information is available regarding baroreflex function in the Grollman, renal wrap model of hypertension in the rat. Baroreflex regulation of renal sympathetic nerve activity (RSNA) and heart rate (HR) were studied in male, Sprague-Dawley rats hypertensive (HT) for 1 or 4-6 wk after unilateral nephrectomy and figure-8 ligature around the remaining kidney or normotensive (NT) after sham surgery. Rats were anesthetized with Inactin and RSNA, and HR was recorded during intravenous infusions of sodium nitroprusside or phenylephrine to lower or raise mean arterial pressure (MAP). Response curves were analyzed using a logistic sigmoid function. In 1- and 4-wk HT rats the midpoints of RSNA and HR reflex curves were shifted to the right (P < 0.05). Comparing NT to 1- or 4-wk HT rats, the gain of RSNA-MAP curves was no different; however, gain was reduced in the HR-MAP curves at both 1 and 4 wk in HT rats (P < 0.05). In anesthetized rats the HR range was small; therefore, MAP and HR were measured in conscious rats during intravenous injections of three doses of phenylephrine and three doses of sodium nitroprusside. Linear regressions revealed a reduced slope in both 1- and 4-wk HT rats compared with NT rats (P < 0.05). The results indicate that baroreflex curves are shifted to the right, to higher pressures, in hypertension. After 1-4 wk of hypertension the gain of baroreflex regulation of RSNA is not altered; however, the gain of HR regulation is reduced.     

ANG II in median preoptic nucleus and pressor responses to CSF sodium and high sodium intake in SHR

Pressor responses to increases in cerebrospinal fluid (CSF) sodium in Wistar rats and to high salt intake in spontaneously hypertensive rats (SHR) involve both brain ouabainlike activity ("ouabain") and the brain renin-angiotensin system (RAS). Because some of the effects of "ouabain" are mediated by the median preoptic nucleus (MnPO) and this nucleus contains all elements of the RAS, the present study assessed possible interactions of "ouabain" and ANG II in this nucleus. In conscious Wistar rats, injection of ANG II into the MnPO significantly increased mean arterial pressure (MAP) and heart rate (HR). This response was not affected by pretreatment with a subpressor dose of ouabain. MAP and HR increases by ouabain in the MnPO were significantly attenuated by MnPO pretreatment with losartan. In Wistar rats, losartan in the MnPO also abolished pressor and HR responses to intracerebroventricular 0.3 M NaCl and attenuated MAP and HR responses to intracerebroventricular ouabain. Five weeks of a high-salt diet in SHRs resulted in exacerbation of hypertension and increased responses to air-jet stress and intracerebroventricular guanabenz. Losartan injected into the MnPO reversed the salt-sensitive component of the hypertension and normalized the depressor response to guanabenz but did not change responses to air-jet stress. We conclude that in the MnPO, ANG II via AT(1) receptors mediates cardiovascular responses to an acute increase in CSF sodium as well as the chronic pressor responses to high sodium intake in SHR.     

Chronic central infusion of aldosterone leads to sympathetic hyperreactivity and hypertension in Dahl S but not Dahl R rats

Six-week-old Dahl salt-sensitive (S) and -resistant (R) rats received for 2 wk an intracerebroventricular infusion of aldosterone (Aldo) (22.5 ng/h) or vehicle containing artificial cerebrospinal fluid (aCSF) with 0.15 M Na+. At 8 wk, mean arterial pressure (MAP), heart rate (HR), and renal sympathetic nerve activity (RSNA) were recorded in conscious rats at rest, in response to air stress, and to an intracerebroventricular injection of the alpha2-adrenoceptor agonists guanabenz or ouabain. Baroreflex control of RSNA and HR was estimated by using intravenous phenylephrine and nitroprusside. In Dahl S but not Dahl R rats, Aldo raised resting MAP by 20-25 mmHg, doubled sympathoexcitatory and pressor responses to air stress and sympathoinhibitory and depressor responses to guanabenz, and impaired baroreflex function. In Dahl S but not Dahl R rats, Aldo significantly increased content of ouabain-like compounds (OLC) in the hypothalamus and attenuated excitatory responses to ouabain. Aldo did not affect water intake, plasma electrolytes, or OLC in plasma and adrenal glands. In another set of three groups of Dahl S rats, Aldo dissolved in aCSF containing 0.16, 0.15, or 0.14 M Na+ was infused intracerebroventricularly for 2 wk. CSF Na+ concentration ([Na+]) showed only a nonsignificant increase, but resting MAP increased from 111 +/- 3 mmHg in rats with Aldo in 0.14 M Na+ to 131 +/- 3 and 147 +/- 3 mmHg with Aldo in 0.15 and 0.16 M Na+, respectively (P < 0.05 for both). These findings indicate that in Dahl S rats, intracerebroventricular infusion of Aldo causes similar central responses as high salt intake, i.e., increases in brain OLC content, sympathetic hyperreactivity, and hypertension. The extent of the increase in blood pressure (BP) by intracerebroventricular Aldo depends on the [Na+] in the vehicle. In Dahl R rats, intracerebroventricular Aldo did not increase brain OLC, sympathetic reactivity, and BP, suggesting that in this rat strain, a decrease in central responsiveness to mineralocorticoids may contribute to its salt-resistant nature.