Hormonal-sympathetic interactions in long-term regulation of arterial pressure: an hypothesis

Mechanism-based strategies to prevent salt sensitivity and salt-induced hypertension

High-salt diets are a major cause of hypertension and cardiovascular (CV) disease. Many governments are interested in using food salt reduction programs to reduce the risk for salt-induced increases in blood pressure and CV events. It is assumed that reducing the salt concentration of processed foods will substantially reduce mean salt intake in the general population. However, contrary to expectations, reducing the sodium density of nearly all foods consumed in England by 21% had little or no effect on salt intake in the general population. This may be due to the fact that in England, as in other countries including the U.S.A., mean salt intake is already close to the lower normal physiologic limit for mean salt intake of free-living populations. Thus, mechanism-based strategies for preventing salt-induced increases in blood pressure that do not solely depend on reducing salt intake merit attention. It is now recognized that the initiation of salt-induced increases in blood pressure often involves a combination of normal increases in sodium balance, blood volume and cardiac output together with abnormal vascular resistance responses to increased salt intake. Therefore, preventing either the normal increases in sodium balance and cardiac output, or the abnormal vascular resistance responses to salt, can prevent salt-induced increases in blood pressure. Suboptimal nutrient intake is a common cause of the hemodynamic disturbances mediating salt-induced hypertension. Accordingly, efforts to identify and correct the nutrient deficiencies that promote salt sensitivity hold promise for decreasing population risk of salt-induced hypertension without requiring reductions in salt intake.

Long-Term High Salt Intake Involves Reduced SK Currents and Increased Excitability of PVN Neurons with Projections to the Rostral Ventrolateral Medulla in Rats

Evidence indicates that high salt (HS) intake activates presympathetic paraventricular nucleus (PVN) neurons, which contributes to sympathoexcitation of salt-sensitive hypertension. The present study determined whether 5 weeks of HS (2% NaCl) intake alters the small conductance Ca²⁺-activated potassium channel (SK) current in presympathetic PVN neurons and whether this change affects the neuronal excitability. In whole-cell voltage-clamp recordings, HS-treated rats had significantly decreased SK currents compared to rats with normal salt (NS, 0.4% NaCl) intake in PVN neurons. The sensitivity of PVN neuronal excitability in response to current injections was greater in HS group compared to NS controls. The SK channel blocker apamin augmented the neuronal excitability in both groups but had less effect on the sensitivity of the neuronal excitability in HS group compared to NS controls. In the HS group, the interspike interval (ISI) was significantly shorter than that in NS controls. Apamin significantly shortened the ISI in NS controls but had less effect in the HS group. This data suggests that HS intake reduces SK currents, which contributes to increased PVN neuronal excitability at least in part through a decrease in spike frequency adaptation and may be a precursor to the development of salt-sensitive hypertension.

Child Abuse, Resting Blood Pressure, and Blood Pressure Reactivity to Psychosocial Stress

OBJECTIVE

Childhood trauma is associated with hypertension in adults. It is unknown whether childhood trauma predicts elevated blood pressure earlier in development. We investigated whether the trauma of child abuse was associated with blood pressure in adolescents.

METHODS

The sample included 145 adolescents aged 13-17 years, 40% with exposure to child abuse. The mean age of participants was 14.93 years (SD = 1.33); 58% were female. The majority self-identified as non-Hispanic White (43%), with the remainder identifying as non-Hispanic Black (17%), Hispanic (17%), or other/mixed race (23%). We used established age/sex/height-specific cutoffs to determine the prevalence of prehypertension and hypertension in the sample. We used two-sample t tests to examine associations of abuse with resting systolic blood pressure (SBP) and diastolic blood pressure (DBP) and blood pressure reactivity to the Trier Social Stress Test and a frustration task. We used linear regression to adjust for potential confounders including sociodemographic variables, body mass index, smoking, and psychopathology.

RESULTS

Mean resting SBP and DBP were 114.07 mmHg and 61.35 mmHg in those with a history of abuse and 111.39 mmHg and 56.89 mmHg in those without a history of abuse. This difference was significant for DBP only. Twelve percent of participants met criteria for prehypertension or hypertension based on resting blood pressure values; this did not differ between those with and without an abuse history. Child abuse was associated with lower DBP and SBP reactivity to laboratory stress tasks and reduced DBP reactivity to frustration. These associations were robust to adjustment for potential confounders.

CONCLUSIONS

Child abuse is associated with higher resting DBP and blunted DBP and SBP reactivity to laboratory stress in adolescence. These findings suggest a potential pathway by which child abuse leads to hypertension.

Brain mineralocorticoid receptors in cognition and cardiovascular homeostasis

Mineralocorticoid receptors (MR) mediate diverse functions supporting osmotic and hemodynamic homeostasis, response to injury and inflammation, and neuronal changes required for learning and memory. Inappropriate MR activation in kidneys, heart, vessels, and brain hemodynamic control centers results in cardiovascular and renal pathology and hypertension. MR binds aldosterone, cortisol and corticosterone with similar affinity, while the glucocorticoid receptor (GR) has less affinity for cortisol and corticosterone. As glucocorticoids are more abundant than aldosterone, aldosterone activates MR in cells co-expressing enzymes with 11β-hydroxydehydrogenase activity to inactivate them. MR and GR co-expressed in the same cell interact at the molecular and functional level and these functions may be complementary or opposing depending on the cell type. Thus the balance between MR and GR expression and activation is crucial for normal function. Where 11β-hydroxydehydrogenase 2 (11β-HSD2) that inactivates cortisol and corticosterone in aldosterone target cells of the kidney and nucleus tractus solitarius (NTS) is not expressed, as in most neurons, MR are activated at basal glucocorticoid concentrations, GR at stress concentrations. An exception may be pre-autonomic neurons of the PVN which express MR and 11β-HSD1 in the absence of hexose-6-phosphate dehydrogenase required to generate the requisite cofactor for reductase activity, thus it acts as a dehydrogenase. MR antagonists, valuable adjuncts to the treatment of cardiovascular disease, also inhibit MR in the brain that are crucial for memory formation and exacerbate detrimental effects of excessive GR activation on cognition and mood. 11β-HSD1 inhibitors combat metabolic and cognitive diseases related to glucocorticoid excess, but may exacerbate MR action where 11β-HSD1 acts as a dehydrogenase, while non-selective 11β-HSD1&2 inhibitors cause injurious disruption of MR hemodynamic control. MR functions in the brain are multifaceted and optimal MR:GR activity is crucial. Therefore selectively targeting down-stream effectors of MR specific actions may be a better therapeutic goal.

Roles of the subfornical organ and area postrema in arterial pressure increases induced by 48-h water deprivation in normal rats

In rats, water deprivation (WD) increases arterial blood pressure (BP) in part due to actions of elevated osmolality in the brain to increase vasopressin levels and sympathetic activity. However, the osmoreceptors that mediate this response have not been identified. To test the hypothesis that osmoregulatory circumventricular organs are involved, BP and heart rate (HR) were continuously recorded telemetrically during 48 h of WD in normal rats with lesions (x) or sham lesions (sham) of the subfornical organ (SFO) or area postrema (AP). Although WD increased BP in SFOx and SFOsham rats, no significant difference in the hypertensive response was observed between groups. HR decreased transiently but similarly in SFOx and SFOsham rats during the first 24 h of WD. When water was reintroduced, BP and HR decreased rapidly and similarly in both groups. BP (during lights off) and HR were both lower in APx rats before WD compared to APsham. WD increased BP less in APx rats, and the transient bradycardia was eliminated. Upon reintroduction of drinking water, smaller falls in both BP and HR were observed in APx rats compared to APsham rats. WD increased plasma osmolality and vasopressin levels similarly in APx and APsham rats, and acute blockade of systemic V1 vasopressin receptors elicited similar depressor responses, suggesting that the attenuated BP response is not due to smaller increases in vasopressin or osmolality. In conclusion, the AP, but not the SFO, is required for the maximal hypertensive effect induced by WD in rats.

Role of the Median Preoptic Nucleus in Arterial Pressure Regulation and Sodium and Water Homeostasis during High Dietary Salt Intake

Changes in the osmolality and level of angiotensin II (ANG II) are important peripheral signals modulating appropriate central sympathetic output and maintaining a normal arterial pressure during high salt intake. The median preoptic nucleus (MnPO) receives reciprocal inputs from the subfornical organ (SFO) and organum vasculosum of the lamina terminalis (OVLT), the circumventricular organs that have been shown to be necessary in multiple central effects of changes in the osmolality and circulating ANG II directed toward the maintenance of sodium and water homeostasis. We, therefore, hypothesized that the MnPO is a crucial part of the central neuronal mechanisms mediating the blood pressure control by altered osmolality and/or ANG II signaling during chronic high dietary salt intake. Male Sprague-Dawley rats were randomly assigned to either sham (operation), or electrolytic lesion of the MnPO. After a 7-day recovery, rats were instrumented with radiotelemetric transducers and aortic flow probes for the measurement of the mean arterial pressure + heart rate (HR) and cardiac output (CO), respectively. Femoral venous catheters were also implanted to collect blood for the measurements of plasma osmolality and sodium concentration, as well as plasma renin activity. Rats were given another 10 days to recover and then were subjected to a 28-day-long study protocol that included a 7-day control period (1.0% NaCl diet), followed by 14 days of high salt (4.0% NaCl), and a 7-day recovery period (1.0% NaCl). The data showed, that despite a slight increase in the MAP observed in both MnPO- (n = 12) and sham-lesioned (n = 8) rats during the high-salt period, there were no significant differences between the MAP, HR, and CO in the two groups throughout the study protocol. These findings do not support the hypothesis that the MnPO is necessary to maintain normal blood pressure during high dietary salt intake. However, MnPO-lesioned rats showed less sodium balance than sham-lesioned rats during the first 4 days of high salt intake. Although, these results may be explained partly by the plasma hyperosmolarity and hypernatremia observed in MnPO-lesioned rats; they also shed light on the role of the MnPO in central neuronal control of renal sodium handling during chronic high dietary salt intake.

Evidence and consequences of the central role of the kidneys in the pathophysiology of sympathetic hyperactivity

Chronic elevation of the sympathetic nervous system has been identified as a major contributor to the complex pathophysiology of hypertension, states of volume overload – such as heart failure – and progressive kidney disease. It is also a strong determinant for clinical outcome. This review focuses on the central role of the kidneys in the pathogenesis of sympathetic hyperactivity. As a consequence, renal denervation may be an attractive option to treat sympathetic hyperactivity. The review will also focus on first results and the still remaining questions of this new treatment option.

Importance of rostral ventrolateral medulla neurons in determining efferent sympathetic nerve activity and blood pressure

Accentuated sympathetic nerve activity (SNA) is a risk factor for cardiovascular events. In this review, we investigate our working hypothesis that potentiated activity of neurons in the rostral ventrolateral medulla (RVLM) is the primary cause of experimental and essential hypertension. Over the past decade, we have examined how RVLM neurons regulate peripheral SNA, how the sympathetic and renin-angiotensin systems are correlated and how the sympathetic system can be suppressed to prevent cardiovascular events in patients. Based on results of whole-cell patch-clamp studies, we report that angiotensin II (Ang II) potentiated the activity of RVLM neurons, a sympathetic nervous center, whereas Ang II receptor blocker (ARB) reduced RVLM activities. Our optical imaging demonstrated that a longitudinal rostrocaudal column, including the RVLM and the caudal end of ventrolateral medulla, acts as a sympathetic center. By organizing and analyzing these data, we hope to develop therapies for reducing SNA in our patients. Recently, 2-year depressor effects were obtained by a single procedure of renal nerve ablation in patients with essential hypertension. The ablation injured not only the efferent renal sympathetic nerves but also the afferent renal nerves and led to reduced activities of the hypothalamus, RVLM neurons and efferent systemic sympathetic nerves. These clinical results stress the importance of the RVLM neurons in blood pressure regulation. We expect renal nerve ablation to be an effective treatment for congestive heart failure and chronic kidney disease, such as diabetic nephropathy.

Regulation of central angiotensin type 1 receptors and sympathetic outflow in heart failure

Angiotensin type 1 receptors (AT(1)Rs) play a critical role in a variety of physiological functions and pathophysiological states. They have been strongly implicated in the modulation of sympathetic outflow in the brain. An understanding of the mechanisms by which AT(1)Rs are regulated in a variety of disease states that are characterized by sympathoexcitation is pivotal in development of new strategies for the treatment of these disorders. This review concentrates on several aspects of AT(1)R regulation in the setting of chronic heart failure (CHF). There is now good evidence that AT(1)R expression in neurons is mediated by activation of the transcription factor activator protein 1 (AP-1). This transcription factor and its component proteins are upregulated in the rostral ventrolateral medulla of animals with CHF. Because the increase in AT(1)R expression and transcription factor activation can be blocked by the AT(1)R antagonist losartan, a positive feedback mechanism of AT(1)R expression in CHF is suggested. Oxidative stress has also been implicated in the regulation of receptor expression. Recent data suggest that the newly discovered catabolic enzyme angiotensin-converting enzyme 2 (ACE2) may play a role in the modulation of AT(1)R expression by altering the balance between the octapeptide ANG II and ANG- (1-7). Finally, exercise training reduces both central oxidative stress and AT(1)R expression in animals with CHF. These data strongly suggest that multiple central and peripheral influences dynamically alter AT(1)R expression in CHF.

Central acetylcholinesterase inhibition improves hemodynamic counterregulation to severe blood loss in alcohol-intoxicated rats

Acute alcohol intoxication results in impaired hemodynamic counterregulation to blood loss and is associated with an attenuated hemorrhage-induced release of catecholamines and AVP. We speculated that restoration of the neuroendocrine response to hemorrhage would improve mean arterial blood pressure (MABP) recovery during acute alcohol intoxication. Previously, we demonstrated that intracerebroventricular (i.c.v.) choline, a precursor of acetylcholine, transiently increases sympathetic nervous system (SNS) outflow but is not capable of improving neuroendocrine and hemodynamic compensation to hemorrhage in alcohol-treated rats. We hypothesized that prolongation of the observed effect via i.c.v. neostigmine, an acetylcholinesterase inhibitor, would enhance SNS outflow, restore the neuroendocrine response, and in turn improve hemodynamic responses to hemorrhage during acute alcohol intoxication. I.c.v. neostigmine (1 microg) increased MABP, catecholamines, and AVP within 5 min and reversed hypotension due to 40% hemorrhage and intragastric alcohol (30% wt/vol, 2.5 g/kg) administration in chronically catheterized male Sprague-Dawley rats (225-250 g body wt). Acute alcohol intoxication before 50% hemorrhage decreased basal MABP, accentuated hypotension midhemorrhage, suppressed the hemorrhage-induced release of norepinephrine and AVP, and prevented restoration of MABP to basal levels after fluid resuscitation with lactated Ringer solution. I.c.v. neostigmine (0.5 microg) produced a sustained increase in MABP beginning at 30 min of hemorrhage that persisted throughout fluid resuscitation in control and alcohol-treated animals. I.c.v. neostigmine enhanced epinephrine responses and restored the hemorrhage-induced release of norepinephrine and AVP in alcohol-treated rats. These results demonstrate that inhibition of acetylcholinesterase in the central nervous system enhances SNS outflow, restores the neuroendocrine response to severe blood loss, and thereby improves hemodynamic counterregulation during acute alcohol intoxication. This study provides evidence for a central (and not peripheral) role of alcohol in impairing hemodynamic stability during hemorrhagic shock.

Acute effects of sodium ingestion on thirst and cardiovascular function

Sweating during exercise, especially during exercise in the heat, leads to sodium and water losses, and the quantity of these losses depends upon the intensity and duration of the activity, genetic predisposition and conditioning of the individual, and environmental factors. In athletes, adequate sodium intake is necessary to maintain fluid balance during training and competition. To ensure the precise regulation of volume and osmolality of body fluids, a number of integrated neural and hormonal systems have evolved to control thirst and sodium appetite. These systems respond to stimuli that arise from a deficit of fluid arising in both the intracellular and extracellular fluid compartments or to systemic hypertonicity. Thirst is highly sensitive to increases in plasma sodium concentration and osmolality, requiring only a 2%-3% increase to induce feelings of thirst. A larger change in plasma volume (10%) is required to induce thirst if there is no concomitant change in plasma sodium concentration. If plain water is used to replenish body water, plasma volume is preferentially restored over the interstitial and intracellular fluid space, suppressing plasma sodium concentration and removing the dipsogenic drive long before total body fluid has been restored. During or after dehydrating exercise, sodium ingestion helps to maintain and restore plasma volume and osmolality by continuing thirst sensation (thus drinking) and also by increasing body fluid retention. A high sodium meal or intravascular hypertonic saline infusion may cause transient osmotically mediated blood pressure increases, but in healthy people, acute sodium ingestion does not cause sustained hypertension. The purpose of this review is to provide evidence that acute increases in sodium are an intrinsic part of the thirst response during and after exercise, and that blood pressure increases associated with hypertonicity appear to be short lived.

Circulating angiotensin II and dietary salt: converging signals for neurogenic hypertension

Circulating angiotensin II (Ang II) combined with high salt intake increases sympathetic nerve activity (SNA) in some forms of hypertension. Ang II-induced increases in SNA are modest, delayed, and specific to certain vascular beds. The brain targets for circulating Ang II are neurons in the area postrema (AP), subfornical organ (SFO), and possibly other circumventricular organs. Ang II signaling is integrated with sodium-sensitive neurons in the SFO and/or organum vasculosum of the lamina terminalis (OVLT) and drives sympathetic premotor neurons in the rostral ventrolateral medulla (RVLM) via the paraventricular nucleus (PVN). It is likely that, over time, new patterns of gene expression emerge within neurons of the SFO-PVN-RVLM pathway that transform their signaling properties. This transformation is critical in maintaining increased SNA. Identification of a novel gene supporting this process may provide new targets for treatment of neurogenic hypertension.

An intact median preoptic nucleus is necessary for chronic angiotensin II-induced hypertension

The median preoptic nucleus (MnPO) receives afferent input from the subfornical organ, a circumventricular organ that has been shown to be necessary in mediating the full chronic hypertensive response to angiotensin II (ANG II) administration. In addition, intravenous ANG II infusion has been shown to cause activation of a number of neurons in both the dorsal and ventral part of MnPO. Taken together, we hypothesized that the MnPO is necessary for the full hypertensive response observed during chronic ANG II-induced hypertension. To test this hypothesis, male Sprague-Dawley rats were subjected to either sham (SHAM) or electrolytic lesion of both the dorsal and ventral part of the MnPO (MnPOx). During the same surgery, rats were instrumented with venous catheters, and radiotelemetric transducers for the intravenous administration of ANG II and the measurement of blood pressure and heart rate, respectively. Rats were then given a week recovery period. After 3 days of saline control infusion, ANG II was intravenously infused (10 ngxkg(-1).min(-1)) in both sham and MnPOx animals for 10 consecutive days, and followed by 3 recovery days. By day 7 of ANG II infusion, MAP had increased 38+/-3 mm Hg in sham lesion rats (n=6), but MAP of MnPOx rats (>90% MnPO ablated; n=5) had only increased 18+/-2 mm Hg. This trend continued through day 10 of ANG II treatment. These results support the hypothesis that the MnPO is necessary for the chronic hypertensive response to ANG II administration.

Renal sympathetic nerve activity in the development of hypertension

With increasing evidence that the sympathetic nervous system plays a critical role in the development of hypertension, focus is turning to how these signals translate to a chronic increase in arterial pressure. The kidney's role in the control of salt and water homeostasis makes it an obvious target for such investigations. However, to date many studies have been restricted to experiments that last only a few hours, or at most, a few days, whereas others may use indirect methods of assessing sympathetic activity rather than direct recordings. We review current approaches used to determine the effects of renal sympathetic nerve activity (SNA) on arterial pressure and suggest possible avenues of future investigation. We propose that although afferent inputs, such those as from chemoreceptors and baroreceptors, are important for the short-term control of blood pressure via regulation of SNA to multiple organs, it is highly likely that alternative signals are important for setting the long-term level of renal SNA. Emerging evidence indicates that circulating angiotensin II is a hormone that may act on the central nervous system to regulate renal SNA, renal function, and, therefore, blood pressure. Future studies on the genesis of hypertension should focus more on determining the mediators of long-term levels of renal SNA.

Fos-Related Antigen Immunoreactivity After Acute and Chronic Angiotensin II–Induced Hypertension in the Rabbit Brain

Several brain regions are proposed as contributing to chronic sympatho-excitatory effects of elevated circulating angiotensin II. However, earlier c-Fos studies have been limited to acute angiotensin II exposure. This study aims to determine brain regions responding with chronic elevated angiotensin II. Rabbits were administered angiotensin II (50 ng/kg per minute) or saline for 3 hours, 3 days, or 14 days. Basal mean arterial pressure was 71±2 mm Hg and increased 23±2 mm Hg, 32±4 mm Hg, and 22±2 mm Hg for 3 hours, 3 days, and 14 days, respectively, with angiotensin II infusion. Neuronal activation was detected using Fos-related antigens, which recognizes all of the known members of the Fos family. Neurons located in the amygdala and area postrema were activated transiently after acute infusion of angiotensin II but were not responsive by days 3 or 14. Neurons located in the nucleus of the solitary tract, caudal ventrolateral medulla, and lateral parabrachial nucleus were activated for ≤3 days after infusion of angiotensin II but were not responsive by day 14, which is consistent with their role in response to baroreceptor pathways that reset with sustained hypertension. The vascular organ of the lamina terminalis and subfornical organ showed sustained but diminishing activation over the 14-day period. However, the downstream hypothalamic nuclei that receive inputs from these nuclei, the paraventricular, supraoptic, and arcuate nuclei, showed marked sustained activation. These findings suggest that there is desensitization of circumventricular organs but sensitization of neurons in hypothalamic regions to long-term angiotensin II infusion.

What initiates the pressor effect of salt in salt-sensitive humans? Observations in normotensive blacks

We tested the traditional hypothesis that an abnormally enhanced renal reclamation of dietary NaCl alone initiates its pressor effect ("salt sensitivity"). Under metabolically controlled conditions, we grouped 23 normotensive blacks as either salt-sensitive (SS) or salt-resistant (SR), depending on whether or not dietary NaCl loading did or did not increase mean arterial blood pressure (MAP) by >or=5 mm Hg. We determined whether dietary NaCl loading induces greater increases in external Na(+) balance, plasma volume, and cardiac output in SS, compared with any in SR subjects, and differential changes in systemic vascular resistance (SVR) that could account for the pressor differences between SS and SR subjects. Using impedance cardiography, we measured cardiac output and SVR daily at 4-hour intervals throughout the last 3 days of a 7-day period of low NaCl intake (30 mmol per day) and throughout a subsequent 7-day period of NaCl loading (250 mmol per day). In the 11 SS subjects, compared with the 12 SR subjects, NaCl loading induced no greater increases in Na(+) balance, body weight, plasma volume, and cardiac output. Yet, from days 2 to 7 of NaCl loading, changes of MAP in SS diverged progressively from those in SR. From days 2 to 4, progressive increases of MAP in SS subjects reflected importantly impaired decreases of SVR, as judged from "normal" decreases of SVR in SR subjects. In SS and SR subjects combined, changes in both MAP and SVR on day 2 strongly predicted changes in MAP on day 7. In many normotensive blacks, vascular dysfunction is critical to the initiation of a pressor response to dietary NaCl.

Time course of synergistic interaction between DOCA and salt on blood pressure: roles of vasopressin and hepatic osmoreceptors

In DOCA-salt rats, the time course of the synergistic interaction between osmolality and DOCA to produce hypertension is unknown. Therefore, in rats 2 wk after implantation of subcutaneous silicone pellets containing DOCA (65 mg) or no drug (sham), we determined blood pressure (BP) and heart rate (HR) responses, using telemetric pressure transducers, during 2 wk of excess salt ingestion (1% NaCl in drinking water). BP was unaltered in sham rats after increased salt, but in DOCA rats BP increased within 4 h. The initial hypertension of 30–35 mmHg stabilized within 2 days, followed ∼5 days later by a further increment of ∼30 mmHg. HR first decreased during the dark phase; the second phase was linked to an abrupt increase in HR and BP variability and decreased HR variability. Pressor responses to acute intravenous hypertonic saline infusion were doubled in DOCA-treated rats via vasopressin and nonvasopressin mechanisms. Only in DOCA-treated rats, portal vein hypertonic saline infusion increased BP, which was prevented by V1 vasopressin blockade. After 2 wk of DOCA-salt, oral ingestion of water rapidly decreased BP. Intraportal infusion of water did not lower BP in DOCA-salt rats, suggesting that hepatic osmoreceptors were not involved. In summary, the hypertension of DOCA-treated rats consuming excess salt exhibits multiple phases and can be rapidly reversed. Hypertonicity-induced vasopressin and nonvasopressin pressor mechanisms that are augmented by DOCA, and hepatic osmoreceptors may contribute to the initial developmental phase. With time, combined DOCA-salt induces marked changes in the regulation of the autonomic nervous system, which may favor hypertension development.

Autonomic Cardiovascular Responses to Heme Oxygenase Inhibition in Conscious Rats

Carbon monoxide (CO) is produced in the course of heme degradation from biliverdin by heme oxygenase (HO) in various tissues, including the central nervous system. Recent studies suggest the inhibition of HO activity increases arterial pressure mediated by the autonomic nervous system. The present study was designed to investigate the autonomic regulation of cardiovascular responses to inhibition of endogenous CO production by the HO inhibitor Zinc deuteroporphyrin 2, 4-bis glycol (ZnDPBG) by using direct sympathetic nerve recordings in conscious, chronically instrumented rats. ZnDPBG induced increases in mean arterial pressure (MAP) (P<0.05) and renal sympathetic nerve activity (RSNA) (P<0.05) but no significant change in heart rate (P>0.05) in intact rats. In atropine-treated rats, ZnDPBG also induced increases in MAP (P<0.05) and RSNA (P<0.05) but no change in heart rate (P>0.05). In sinoaortic denervated rats, ZnDPBG induced increases in MAP (P<0.05), heart rate (P<0.05), and RSNA (P<0.05). ZnDPBG shifted the baroreflex curve for RSNA upward and to the right, which was characterized by increases in the maximum and minimum response and midpoint pressure without altering the maximum gain. These results indicate that inhibition of HO activity within the central nervous system causes sympathoexcitation, resulting in an increase in arterial pressure. We conclude that the CO/HO system plays an important role in cardiovascular regulation by modulating sympathetic tone.

Enhanced sensitivity of Kv channels to hypoxia in the rabbit carotid body in heart failure: role of angiotensin II

Angiotensin II (Ang II) plays an important role in the enhanced chemoreflex function that occurs in congestive heart failure (CHF), but the mechanism of this effect within the carotid body (CB) is not known. We investigated the sensitivity of Ca2+-independent, voltage-gated K+ (Kv) channels to hypoxia in CB glomus cells from CHF rabbits, and whether endogenous angiotensin II (Ang II) modulates this action. Using the conventional whole-cell patch clamp technique, we found that Kv currents (IK) under normoxic conditions were blunted in the CB glomus cells from CHF rabbits compared with sham rabbits. In addition, the inhibition of IK and the decrease of resting membrane potential (RMP) induced by hypoxia were greater in CHF versus sham glomus cells. Ang II, at 100 pM, had no direct effect on IK at constant normoxic PO2, but increased the sensitivity of IK and RMP to hypoxia in sham glomus cells. In CHF glomus cells, an AT1 receptor (AT1R) antagonist, L-158 809 (1 microM), alone did not affect IK at normoxia, but it decreased the sensitivity of IK and RMP to hypoxia. At higher concentrations, Ang II dose dependently (0.1-100 nM) reduced IK under constant normoxic conditions in sham and CHF glomus cells, with threshold concentrations of about 900 and 600 pM, respectively. Immunocytochemical and Western blot assessments demonstrated the down-expression of Kv3.4 but not Kv4.3 channels in CHF glomus cells. These results indicate that: (1) Ang II/AT1R signalling increases the sensitivity of Kv channels to hypoxia in CB glomus cells from CHF rabbits; (2) high concentrations of Ang II (> 1 nM) directly inhibit IK in CB glomus cells from sham and CHF rabbits; (3) changes in Kv channel protein expression (Kv3.4 versus Kv4.3) in the CB glomus cell may contribute to the suppression of IK and enhanced sensitivity of IK to hypoxia in CHF.

Angiotensin-II type 1 receptor-mediated hypertension in D4 dopamine receptor-deficient mice

Dopamine receptors are important in systemic blood pressure regulation. D4 receptors are expressed in the kidney and brain, but their role in cardiovascular regulation is unknown. In pentobarbital-anesthetized mice, systolic and diastolic blood pressures were elevated in sixth-generation D4 receptor-deficient (D4(-/-)) mice and in tenth-generation D4(-/-) mice compared with D4 wild-type (D4(+/+)) littermates. The conscious blood pressures measured via a chronic arterial (femoral) catheter or telemetry (carotid) were also higher in D4(-/-) mice than in D4 littermates. Basal renal and plasma renin concentrations were similar in the 2 mouse strains. The protein expression of angiotensin II type 1 receptor was increased in homogenates of kidney (330+/-53%, n=5) and brain (272+/-69%, n=5) of D4(-/-) mice relative to D4(+/+) mice (kidney: 100+/-12%, n=5; brain: 100+/-32%, n=5). The expression of the receptor in renal membrane was also increased in D4(-/-) mice (289+/-28%, n=8) relative to D4(+/+) mice (100+/-14%, n=10). In contrast, the expression in the heart was similar in the 2 strains. Bolus intravenous injection of angiotensin II type 1 receptor antagonist losartan initially decreased mean arterial pressures to a similar degree in D4(-/-) and D4(+/+) littermates. However, the hypotensive effect of losartan dissipated after 10 minutes in D4(+/+) mice, whereas the effect persisted for >45 minutes in D4(-/-) mice. We conclude that the absence of the D(4) receptor increases blood pressure, possibly via increased angiotensin II type 1 receptor expression.

Angiotensin II--nitric oxide interactions in the control of sympathetic outflow in heart failure

Activation of the sympathetic nervous system is a compensatory mechanism which initially provides support for the circulation in the face of a falling cardiac output. It has been recognized for some time that chronic elevation of sympathetic outflow with the consequent increase in plasma norepinephrine, is counterproductive to improving cardiac function. Indeed, therapeutic targeting to block excessive sympathetic activation in heart failure is becoming a more accepted modality. The mechanism(s) by which sympathetic excitation occurs in the heart failure state are not completely understood. Components of abnormal cardiovascular reflex regulation most likely contribute to this sympatho-excitation. However, central mechanisms which relate to the elaboration of angiotensin II (Ang II) and nitric oxide (NO) may also play an important role. Ang II has been shown to be a sympatho-excitatory peptide in the central nervous system while NO is sympatho-inhibitory. Recent studies have demonstrated that blockade of Ang II receptors of the AT(1) subtype augments arterial baroreflex control of sympathetic nerve activity in the heart failure state, thereby predisposing to a reduction in sympathetic tone. Ang II and NO interact to regulate sympathetic outflow. Blockade of NO production in normal conscious rabbits was only capable of increasing sympathetic outflow when accompanied by a background infusion of Ang II. Conversely, providing a source of NO to rabbits with heart failure reduced sympathetic nerve activity when accompanied by blockade of AT(1) receptors. Chronic heart failure is also associated with a decrease in NO synthesis in the brain as indicated by a reduction in the mRNA for the neuronal isoform (nNOS). Chronic blockade of Ang II receptors can up regulate nNOS expression. In addition, exercise training of rabbits with developing heart failure has been shown to reduce sympathetic tone, decrease plasma Ang II, improve arterial baroreflex function and increase nNOS expression in the central nervous system. This review summarizes a large number of studies which have concentrated on the mechanisms of sympatho-excitation in heart failure. It now seems clear that one mechanism which is important in regulating sympathetic outflow in this disease state depends upon a central interaction between Ang II and NO at the cellular and nuclear levels.

Influence of Prolonged Baroreflex Activation on Arterial Pressure in Angiotensin Hypertension

Despite recent evidence indicating sustained activation of the baroreflex during chronic infusion of angiotensin II (Ang II), sinoaortic denervation does not exacerbate the severity of the hypertension. Therefore, to determine whether Ang II hypertension is relatively resistant to the blood pressure-lowering effects of the baroreflex, the carotid baroreflex was electrically activated bilaterally for 7 days in 5 dogs both in the presence and absence of a continuous infusion of Ang II (5 ng/kg per minute) producing high physiological plasma levels of the peptide. Under control conditions, basal values for mean arterial pressure (MAP) and plasma norepinephrine concentration (NE) were 93±1 mm Hg and 99±25 pg/mL, respectively. By day 7 of baroreflex activation, MAP and NE were reduced to 72±4 mm Hg (−21±3 mm Hg) and 56±15 pg/mL, respectively, but PRA was unchanged (control=0.41±0.06 ng ANG I/mL per hour). All values returned to basal levels by the end of a 7-day recovery period. After 7 days of Ang II infusion, MAP increased from 93±3 to 129±3 mm Hg, whereas NE fell from 117±15 to 86±23 pg/mL. During the next 7 days of baroreflex activation/Ang II infusion, further reductions in NE were not statistically significant, and on the final day of baroreflex activation, the reduction in MAP was only 5±1 mm Hg, compared with 21±3 mm Hg in the control normotensive state. These findings indicate that long-term baroreflex-mediated reductions in arterial pressure are markedly diminished, but not totally eliminated, in the presence of hypertension produced by chronic infusion of Ang II.

Recent insights into the interactions between the baroreflex and the kidneys in hypertension

Recent findings in chronically instrumented animals challenge the classic concept that baroreflexes do not play a role in the chronic regulation of arterial pressure. As alterations in renal excretory function are of paramount importance in the chronic regulation of arterial pressure, several of these recent studies have focused on the long-term interactions between the baroreflex and the kidneys during chronic perturbations in arterial pressure and body fluid volumes. An emerging body of evidence indicates that the baroreflex is chronically activated in several experimental models of hypertension, but in most cases, the duration of these studies has not exceeded 2 wk. Although these studies suggest that the baroreflex may play a compensatory role in attenuating the severity of the hypertension, possibly even in primary hypertension with uncertain causes of sympathetic activation, there has been only limited assessment of the quantitative importance of this interaction in the regulation of arterial pressure. In experimental models of secondary hypertension, baroreflex suppression of renal sympathetic nerve activity is sustained and chronically promotes sodium excretion. This raises the possibility that the renal nerves may be the critical efferent link for baroreceptor-induced suppression of central sympathetic output through which long-term compensatory reductions in arterial pressure are produced. This contention is supported by strong theoretical evidence but must be corroborated by experimental studies. Finally, although it is now clear that pressure-induced increases in baroreflex activity persist for longer periods of time than previously suggested, studies using new tools and novel approaches and extending beyond 2 wk of hypertension are needed to elucidate the true role of the baroreflex in the pathogenesis of clinical hypertension.

Effects of angiotensin II on autonomic components of nasopharyngeal stimulation in male conscious rabbits

Angiotensin II (ANG II) is known to activate central sympathetic neurons. In this study we determined the effects of ANG II on the autonomic components of the cardiovascular responses to stimulation of nasopharyngeal receptors with cigarette smoke. Experiments were carried out in conscious New Zealand White rabbits instrumented to record arterial pressure and heart rate. Rabbits were exposed to 50 ml of cigarette smoke before and after subcutaneous osmotic minipump delivery of ANG II at a dose of 50 ng.kg(-1).min(-1) for 1 wk in one group and intracerebroventricular (icv) infusion at a dose of 100 pmol/min for 1 h in a second group. The responses were compared before and after heart rate was controlled by pacing. Autonomic components were evaluated by intravenous administration of atropine methyl bromide (0.2 mg/kg) and prazosin (0.5 mg/kg). ANG II given either systemically or icv significantly blunted the pressor response to smoke (P < 0.05) when the bradycardic response was prevented. This blunted response was not due to an absolute increase in baseline blood pressure after ANG II infusion (71.64 +/- 11.6 vs. 92.1 +/- 19.8 mmHg; P < 0.05) because normalization of blood pressure with sodium nitroprusside to pre-ANG II levels also resulted in a significantly blunted pressor response to smoke. The effect of smoke was alpha(1)-adrenergic receptor-mediated because it was essentially abolished by prazosin in both the pre- and the post-ANG II states (P < 0.05). These results suggest that elevations in central ANG II reduce the sympathetic response to smoke in conscious rabbits. This effect may be due to an augmentation of baseline sympathetic outflow and a reduction in reflex sensitivity similar to the effect of ANG II on baroreflex function.

Acute and chronic increases in osmolality increase excitatory amino acid drive of the rostral ventrolateral medulla in rats

Water deprivation is associated with increased excitatory amino acid (EAA) drive of the rostral ventrolateral medulla (RVLM), but the mechanism is unknown. This study tested the hypotheses that the increased EAA activity is mediated by decreased blood volume and/or increased osmolality. This was first tested in urethane-anesthetized rats by determining whether bilateral microinjection of kynurenate (KYN, 2.7 nmol) into the RVLM decreases arterial pressure less in water-deprived rats after normalization of blood volume by intravenous infusion of isotonic saline or after normalization of plasma osmolality by intravenous infusion of 5% dextrose in water (5DW). Water-deprived rats exhibited decreased plasma volume and elevated plasma osmolality, hematocrit, and plasma sodium, chloride, and protein levels (all P < 0.05). KYN microinjection decreased arterial pressure by 24 ± 2 mmHg ( P < 0.05; n = 17). The depressor response was not altered following isotonic saline infusion but, while still present ( P < 0.05), was reduced ( P < 0.05) to −13 ± 2 mmHg soon after 5DW infusion. These data suggest that the high osmolality, but not low blood volume, contributes to the KYN depressor response. To further investigate the action of increased osmolality on EAA input to RVLM, water-replete rats were also studied after hypertonic saline infusion. Whereas KYN microinjection did not decrease pressure immediately following the infusion, a depressor response gradually developed over the next 3 h. Lumbar sympathetic nerve activity also gradually increased to up to 167 ± 19% of control ( P < 0.05) 3 h after hypertonic saline infusion. In conclusion, acute and chronic increases in osmolality appear to increase EAA drive of the RVLM.

Contribution of the subfornical organ to angiotensin II-induced hypertension

Previous studies clearly demonstrated acute actions of angiotensin II (ANG II) at one of the central circumventricular organs, the subfornical organ (SFO), but studies demonstrating a role for the SFO in the chronic actions of ANG II remain uncertain. The purpose of this study was to examine the role of the SFO in the chronic hypertensive phase of ANG II-induced hypertension. We hypothesized that the SFO is necessary for the full hypertensive response observed during the chronic phase of ANG II-induced hypertension. To test this hypothesis, male Sprague-Dawley rats were subjected to sham operation (sham rats) or electrolytic lesion of the SFO (SFOx rats). After 1 wk, the rats were instrumented with venous catheters and radiotelemetric transducers for intravenous administration of ANG II and measurement of blood pressure and heart rate, respectively. Rats were then allowed 1 wk for recovery. After 3 days of saline control infusion (7 ml of 0.9% NaCl/day), sham and SFOx rats were infused with ANG II at 10 ng.kg(-1).min(-1) i.v. for 10 consecutive days and then allowed to recover for 3 days. A 0.4% NaCl diet and distilled water were provided ad libitum. At day 5 of ANG II infusion, mean arterial pressure increased 11.7 +/- 3.0 mmHg in sham rats (n = 9) but increased only 3.7 +/- 1.4 mmHg in SFOx rats (n = 9). This trend continued through day 10 of ANG II treatment. These results support the hypothesis that the SFO is necessary for the full hypertensive response to chronic ANG II administration.

Excitatory amino acids in rostral ventrolateral medulla support blood pressure during water deprivation in rats

Water deprivation is associated with regional increases in sympathetic tone, but whether this is mediated by changes in brain stem regulation of sympathetic activity is unknown. Therefore, this study tested the hypothesis that water deprivation increases excitatory amino acid (EAA) drive of the rostral ventrolateral medulla (RVLM), by determining whether bilateral microinjection of kynurenate (Kyn; 2.7 nmol) into the RVLM decreases arterial pressure more in water-deprived than water-replete rats. Plasma osmolality was increased in 48-h water-deprived rats (313 ± 1 mosmol/kgH2O; P < 0.05) compared with 24-h water-deprived rats (306 ± 2 mosmol/kgH2O) and water-replete animals (300 ± 2 mosmol/kgH2O). Kyn decreased arterial pressure by 28.1 ± 5.2 mmHg ( P < 0.01) in 48-h water-deprived rats but had no effect in water-replete rats (–5.9 ± 1.3 mmHg). Variable depressor effects were observed in 24-h water-deprived animals (–12.5 ± 2.4 mmHg, not significant); however, in all rats the Kyn depressor response was strongly correlated to the osmolality level ( P < 0.01; r2 = 0.47). The pressor responses to unilateral microinjection of increasing doses (0.1, 0.5, 1.0, and 5.0 nmol) of glutamate were enhanced ( P < 0.05) during water deprivation, but the pressor responses to intravenous phenylephrine injection were smaller ( P < 0.05). These data suggest that water deprivation increases EAA drive to the RVLM, in part by increasing responsiveness of the RVLM to EAA such as glutamate.

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

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

Responsiveness vs. basal activity of plasma ANG II as a determinant of arterial pressure salt sensitivity

Infusion of angiotensin II (ANG II) causes salt-sensitive hypertension. It is unclear whether this is due to the body's inability to suppress ANG II during increased salt intake or, rather, an elevated basal level of plasma ANG II itself. To distinguish between these mechanisms, Sprague-Dawley rats were instrumented with arterial and venous catheters for measurement of arterial pressure and infusion of drugs, respectively. The sensitivity of arterial pressure to salt was measured in four groups with the following treatments: 1) saline control (Con, n = 12); 2) administration of the angiotensin-converting enzyme inhibitor enalapril to block endogenous ANG II (ANG-Lo, n = 10); 3) administration of enalapril and 5 ng.kg(-1).min(-1) ANG II to clamp plasma ANG II at normal levels (ANG-Norm, n = 10); and 4) administration of enalapril and 20 ng.kg(-1).min(-1) ANG II to clamp ANG II at high levels (ANG-Hi, n = 10). Rats ingested a 0.4% NaCl diet for 3 days and then a 4.0% NaCl diet for 11 days. Arterial pressure of rats fed the 0.4% NaCl diet was lower in ANG-Lo (84 +/- 2 mmHg) compared with Con (101 +/- 3 mmHg) and ANG-Norm (98 +/- 4 mmHg) groups, whereas ANG-Hi rats were hypertensive (145 +/- 4 mmHg). Salt sensitivity was expressed as the change in arterial pressure divided by the change in sodium intake on the last day of the 4.0% NaCl diet. Salt sensitivity (in mmHg/meq Na) was lowest in Con rats (0.0 +/- 0.1) and progressed from ANG-Lo (0.8 +/- 0.2) to ANG-Norm (1.5 +/- 0.5) to ANG-Hi (3.5 +/- 0.5) rats. We conclude that the major determinant of salt sensitivity of arterial pressure is the basal level of plasma ANG II rather than the responsiveness of the renin-angiotensin system.

Renal denervation chronically lowers arterial pressure independent of dietary sodium intake in normal rats

The present study was designed to test the hypothesis that renal nerves chronically modulate arterial pressure (AP) under basal conditions and during changes in dietary salt intake. To test this hypothesis, continuous telemetric recording of AP in intact (sham) and renal denervated (RDNX) Sprague-Dawley rats was performed and the effect of increasing and decreasing dietary salt intake on AP was determined. In protocol 1, 24-h AP, sodium, and water balances were measured in RDNX (n = 11) and sham (n = 9) rats during 5 days of normal (0.4% NaCl) and 10 days of high (4.0% NaCl) salt intake, followed by a 3-day recovery period (0.4% NaCl). Protocol 2 was similar with the exception that salt intake was decreased to 0.04% NaCl for 10 days after the 5-day period of normal salt (0.04% NaCl) intake (RDNX; n = 6, sham; n = 5). In protocol 1, AP was lower in RDNX (91 +/- 1 mmHg) compared with sham (101 +/- 2 mmHg) rats during the 5-day 0.4% NaCl control period. During the 10 days of high salt intake, AP increased <5 mmHg in both groups so that the difference between sham and RDNX rats remained constant. In protocol 2, AP was also lower in RDNX (93 +/- 2 mmHg) compared with sham (105 +/- 4 mmHg) rats during the 5-day 0.4% NaCl control period, and AP did not change in response to 10 days of a low-salt diet in either group. Overall, there were no between-group differences in sodium or water balance in either protocol. We conclude that renal nerves support basal levels of AP, irrespective of dietary sodium intake in normal rats.

Role of the subfornical organ in the chronic hypotensive response to losartan in normal rats

Angiotensin II is known to act at a unique set of brain regions known as the circumventricular organs. These structures lack the normal blood-brain barrier and are therefore thought to participate in the central nervous system processing of neuroendocrine signals. We have reported that chronic treatment with the angiotensin type 1 (AT1) receptor antagonist, losartan, decreases arterial pressure in normotensive rats. Furthermore, this hypotension is attenuated in area postrema-lesioned rats, suggesting a role of endogenous angiotensin II at this circumventricular organ. Another circumventricular organ, the subfornical organ (SFO), has also been shown to mediate actions of angiotensin II. The present study tested the hypothesis that the SFO is a central site of action of endogenous angiotensin II at AT1 receptors. Adult male Sprague-Dawley rats were anesthetized and placed in a stereotaxic apparatus, and the SFO was sham or electrolytically lesioned. One week later, rats were instrumented with venous catheters and radiotelemetry pressure transducers for continuous infusion and monitoring of mean arterial pressure, respectively. After 3 days of control, losartan was administered intravenously (10 mg x kg(-1) x d(-1)) for 10 days in both SFO-lesioned and sham rats. By day 4 of losartan administration, mean arterial pressure had decreased to 75+/-2 mm Hg in sham rats (n=9) but had only fallen to 83+/-2 mm Hg in lesioned rats (n=10). This attenuated hypotensive response in SFO-lesioned rats continued through day 10 of losartan treatment. These results support the hypothesis that the SFO mediates part of the hypotensive effects of chronic AT1 receptor blockade in the normotensive rat.

Overexpression of eNOS in RVLM improves impaired baroreflex control of heart rate in SHRSP. Rostral ventrolateral medulla. Stroke-prone spontaneously hypertensive rats

We previously demonstrated that the overexpression of endothelial nitric oxide synthase (eNOS) in the rostral ventrolateral medulla (RVLM) decreases blood pressure, heart rate (HR), and sympathetic nerve activity and that these effects are enhanced in stroke-prone spontaneously hypertensive rats (SHRSP). The aim of this study was to determine if an increase in NO production in the RVLM caused by the overexpression of eNOS improves the impaired baroreflex control of HR in SHRSP. We transfected adenovirus vectors encoding eNOS (AdeNOS) into the RVLM of SHRSP or Wistar-Kyoto rats (WKY). Mean arterial pressure and HR were measured by a radio-telemetry system in the conscious state. Reflex changes in HR were elicited by intravenous infusion of either phenylephrine, sodium nitroprusside, or hydralazine at day 7 after the gene transfer. The maximum gain of the baroreflex control of HR was significantly decreased in SHRSP compared with WKY. Overexpression of eNOS in the RVLM of SHRSP improved the impaired maximum gain of the baroreflex control of HR. After treatment with atropine, the maximum gain was still significantly greater in SHRSP in the AdeNOS-transfected group than in the nontransfected group, although it was decreased in both groups. In contrast, after treatment with metoprolol, the maximum gain did not differ between the two groups. These results indicate that an increase in NO production in the RVLM improves the impaired baroreflex control of HR in SHRSP and that these effects may have resulted from a cardiac sympathoinhibitory effect of NO in the RVLM of SHRSP.

Acute sympathoexcitatory action of angiotensin II in conscious baroreceptor-denervated rats

Angiotensin II (ANG II) has complex actions on the cardiovascular system. ANG II may act to increase sympathetic vasomotor outflow, but acutely the sympathoexcitatory actions of exogenous ANG II may be opposed by ANG II-induced increases in arterial pressure (AP), evoking baroreceptor-mediated decreases in sympathetic nerve activity (SNA). To examine this hypothesis, the effect of ANG II infusion on lumbar SNA was measured in unanesthetized chronic sinoaortic-denervated rats. Chronic sinoaortic-denervated rats had no reflex heart rate (HR) responses to pharmacologically evoked increases or decreases in AP. Similarly, in these denervated rats, nitroprusside-induced hypotension had no effect on lumbar SNA; however, phenylephrine-induced increases in AP were still associated with transient decreases in SNA. In control rats, infusion of ANG II (100 ng x kg(-1) x min(-1) iv) increased AP and decreased HR and SNA. In contrast, ANG II infusion increased lumbar SNA and HR in sinoaortic-denervated rats. In rats that underwent sinoaortic denervation surgery but still had residual baroreceptor reflex-evoked changes in HR, the effect of ANG II on HR and SNA was variable and correlated to the extent of baroreceptor reflex impairment. The present data suggest that pressor concentrations of ANG II in rats act rapidly to increase lumbar SNA and HR, although baroreceptor reflexes normally mask these effects of ANG II. Furthermore, these studies highlight the importance of fully characterizing sinoaortic-denervated rats used in experiments examining the role of baroreceptor reflexes.

Sympathetic-renal interaction in chronic arterial pressure control

The effects of neonatal sympathectomy of donors or recipients on posttransplantation arterial pressure were investigated in spontaneously hypertensive rats (SHR) by renal transplantation experiments. Conscious mean arterial pressure (MAP) and renal vascular resistance were 136 +/- 1 mmHg and 15.5 +/- 1.2 mmHg x ml(-1) x min x g in sympathectomized SHR (n = 8) vs. 158 +/- 4 mmHg (P < 0.001) and 20.8 +/- 1.1 mmHg x ml(-1) x min x g (P < 0.05) in controls (n = 10). Seven weeks after transplantation of a kidney from neonatally sympathectomized SHR donors, MAP in SHR recipients (n = 10) was 20 mmHg lower than in controls transplanted with a kidney from hydralazine-treated SHR (n = 10) (P < 0.05) associated with reduced sodium sensitivity of MAP. Neonatal sympathectomy also lowered MAP in F1-hybrids (F1H; SHR x Wistar-Kyoto rats). Within 6 wk after transplantation, renal grafts from untreated SHR increased MAP by 20 mmHg in sympathectomized F1H (n = 10) and by 35 mmHg in sham-treated F1H (n = 8) (P < 0.05). Neonatal sympathectomy induces chronic changes in SHR kidney function leading to a MAP reduction even when extrarenal sympathetic tone is restored. Generalized reduction in sympathetic tone resets the kidney-fluid system to reduced MAP and blunts the extent of arterial pressure rise induced by an SHR kidney graft.

Losartan reduces central and peripheral sympathetic nerve activity in a rat model of neurogenic hypertension

We have developed a new model of neurogenic hypertension in the rat, in which hypertension is caused by injecting 50 microL of 10% phenol in the lower pole of one kidney. Administration of phenol in the kidney causes an immediate and persistent rise in blood pressure (BP), norepinephrine (NE) secretion from the posterior hypothalamic nuclei (PH), and renal sympathetic nerve activity (RSNA). Because angiotensin II (Ang II) is known to stimulate central and peripheral sympathetic nervous system (SNS) activity, we have tested the hypothesis that losartan, a specific Ang II AT1 receptor antagonist, may lower BP, at least in part, by SNS inhibition. To this end, we studied the effects of losartan on BP and SNS activity following intrarenal phenol injection. Central SNS activity was measured by NE secretion from the PH using a microdialysis technique, and peripheral SNS activity was measured by direct recording of renal nerve activity. At the end of the experiments, brains were isolated and interleukin (IL)-1beta and nitric oxide synthase (NOS) mRNA gene expression was measured by RT-PCR in the PH, paraventricular nuclei (PVN), and locus ceruleus (LC). The intrarenal injection of phenol raised BP, as well as central and renal SNS activity, but reduced the abundance of IL-1beta and neuronal NOS (nNOS) mRNA in the PH, PVN, and LC. Whether injected intravenously or in the lateral ventricle, losartan caused a significant (P<0.01) and dose-dependent inhibition of the effects of phenol on BP, NE secretion from the PH, and RSNA. Losartan also caused a significant (P<0.01) and dose-dependent rise in IL-1beta and nNOS-mRNA gene expression in the PH, PVN, and LC of phenol-injected rats. In conclusion, these studies have shown that the intrarenal injection of phenol causes a rise in central and renal SNS activity and a decrease in IL-1beta and nNOS-mRNA in the PH, PVN, and LC. Losartan prevented the rise in BP and SNS activity, as well as the decrease in IL-1beta and nNOS mRNA abundance caused by phenol. These studies have demonstrated that the antihypertensive action of losartan in the phenol renal injury model is largely mediated by inhibition of central and peripheral SNS activity and suggest that activation of IL-1beta and nNOS, 2 important modulators of central SNS activity, mediates the inhibitory action of losartan on SNS activity.

Downregulation of neuronal nitric oxide synthase and interleukin-1beta mediates angiotensin II-dependent stimulation of sympathetic nerve activity

There is substantial evidence that angiotensin II (Ang II) enhances sympathetic nervous system (SNS) activity. We recently observed that nitric oxide and interleukin-1beta (IL-1beta) exert a tonic inhibitory action on central SNS activity. Moreover, in 2 rat models of neurogenic hypertension, one caused by intrarenal injection of phenol and the other by 5/6 nephrectomy, we observed that losartan, an Ang II type 1 receptor blocker, inhibits SNS activity and increases the abundance of IL-1beta and the neuronal isoform of nitric oxide synthase (nNOS) in the posterior hypothalamic nuclei (PH), paraventricular nuclei (PVN), and locus ceruleus (LC). This raises the possibility that the stimulatory effects of Ang II on central SNS activity may be mediated by inhibition of nNOS and IL-1beta. To test this hypothesis, we studied the effect of an intracerebroventricular (ICV) infusion of Ang II on blood pressure (BP), norepinephrine (NE) secretion from the PH, renal SNS activity (RSNA), and abundance of IL-1beta and nNOS mRNA in the PH, PVN, and LC of normal Sprague-Dawley rats. Finally, we measured the concentration of nitrite/nitrate in the dialysate collected from the PH after Ang II or vehicle. ICV infusion of Ang II (100 ng/kg body wt dissolved in 10 microL of artificial cerebrospinal fluid) raised BP, RSNA, and NE secretion from the PH compared with control rats. Ang II reduced the abundance of IL-1beta and nNOS mRNA in the PH, PVN, and LC. Pretreatment with losartan (10 microg/kg body wt dissolved in 10 microL of aCSF) given ICV 20 minutes before Ang II abolished the effects of Ang II on BP, RSNA, and NE secretion from the PH and IL-1beta and nNOS mRNA. Ang II also decreased the secretion of NO from the PH. In conclusion, these studies suggest that Ang II inhibits the expression of IL-1beta and nNOS in the brain. Because locally produced NO exerts a tonic inhibitory action on SNS activity, the decrease in NO expression caused by Ang II results in greater SNS activity.

FGF2 Signaling Is Required for the Development of Neuronal Circuits Regulating Blood Pressure

Fibroblast growth factor 2 (FGF2) signaling is involved in angiogenesis, vascular contractility, and cardiac hypertrophy. Mice lacking a functional FGF2 gene (FGF2 −/− ) are hypotensive, but the primary physiological role of FGF2 in cardiovascular homeostasis remained unknown. Using a chicken FGF2 ( cFGF2 ) transgene under control of the Wnt-1 promotor, we selectively re-expressed FGF2 in the developing nervous system of FGF2 −/− (transgenic FGF2 mutant) embryos. Expression of the cFGF2 transgene in the developing nervous system, including its autonomic region, was limited to the period between embryonic day 9.5 and 14.5. Significantly, no FGF2 re-expression was detected in developing heart and blood vessels. Pharmacological analysis revealed a normalization of the blood pressure response to isoproterenol-induced vasodilation in adult transgenic FGF2 mutant mice. In addition, the hypotensive phenotype was rescued in 1 line (of 2) transgenic FGF2 mutant adult mice having expressed higher levels of cFGF2 proteins during nervous system development. These genetic studies indicate that FGF2 signaling is essential for complete development of the neural circuitry required for central regulation of blood pressure, whereas it appears dispensable for blood pressure control in the healthy adult. The full text of this article is available at http://www.circresaha.org.

Long-term control of renal blood flow: what is the role of the renal nerves?

We have developed a system for long-term continuous monitoring of cardiovascular parameters in rabbits living in their home cage to assess what role renal sympathetic nerve activity (RSNA) has in regulating renal blood flow (RBF) in daily life. Blood pressure, heart rate, locomotor activity, RSNA, and RBF were recorded continuously for 4 wk. Beginning 4-5 days after surgery a circadian rhythm, dependent on feeding time, was observed. When averaged over all days RBF to the innervated and denervated kidneys was not significantly different. However, control of RBF around these mean levels was dependent on the presence of the renal sympathetic nerves. In particular we observed episodic elevations in heart rate and other parameters associated with activity. In the denervated kidney, during these episodic elevations, the increase in renal resistance was closely related to the increase in arterial pressure. In the innervated kidney the renal resistance response was significantly more variable, indicating an interaction of the sympathetic nervous system. These results indicate that whereas overall levels of RSNA do not set the mean level of RBF the renal vasculature is sensitive to episodic increases in sympathetic nerve activity.

Nervous kidney. Interaction between renal sympathetic nerves and the renin-angiotensin system in the control of renal function

Increases in renal sympathetic nerve activity regulate the functions of the nephron, the vasculature, and the renin-containing juxtaglomerular granular cells. Because increased activity of the renin-angiotensin system can also influence nephron and vascular function, it is important to understand the interactions between the renal sympathetic nerves and the renin-angiotensin system in the control of renal function. These interactions can be intrarenal, for example, the direct (by specific innervation) and indirect (by angiotensin II) contributions of increased renal sympathetic nerve activity to the regulation of renal function. The effects of increased renal sympathetic nerve activity on renal function are attenuated when the activity of the renin-angiotensin system is suppressed or antagonized with ACE inhibitors or angiotensin II-type AT(1)-receptor antagonists. The effects of intrarenal administration of angiotensin II are attenuated after renal denervation. These interactions can also be extrarenal, for example, in the central nervous system, wherein renal sympathetic nerve activity and its arterial baroreflex control are modulated by changes in activity of the renin-angiotensin system. In addition to the circumventricular organs, whose permeable blood-brain barrier permits interactions with circulating angiotensin II, there are interactions at sites behind the blood-brain barrier that depend on the influence of local angiotensin II. The responses to central administration of angiotensin II-type AT(1)-receptor antagonists into the ventricular system or microinjected into the rostral ventrolateral medulla are modulated by changes in activity of the renin-angiotensin system produced by physiological changes in dietary sodium intake. Similar modulation is observed in pathophysiological models wherein activity of both the renin-angiotensin and sympathetic nervous systems is increased (eg, congestive heart failure). Thus, both renal and extrarenal sites of interaction between the renin-angiotensin system and renal sympathetic nerve activity are involved in influencing the neural control of renal function.

Sympathetic activity in early renal posttransplantation hypertension in rats

The contribution of elevated sympathetic activity to the development of renal posttransplantation hypertension was investigated. F1 hybrids (F1H) from spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY) were transplanted with either an SHR or an F1H kidney and bilaterally nephrectomized. Three weeks after transplantation, sympathetic activity was assessed by measuring adrenal tyrosine hydroxylase (TH) mRNA content and recording splanchnic nerve activity (SNA) in conscious animals. To investigate the dependence of arterial pressure on sympathetic activity, animals were treated with the alpha(2)-adrenoceptor agonist guanabenz intracerebroventricularly. Mean arterial pressure (MAP) was 143 +/- 4 mmHg in recipients of an SHR kidney (n = 15) versus 110 +/- 3 mmHg in recipients of an F1H kidney (n = 10; P < 0.001). Adrenal TH mRNA content was 1.93 +/- 0.15 fmol/microg total RNA in recipients of an SHR kidney versus 1.96 +/- 0.17 fmol/microg total RNA in recipients of an F1H kidney (not significant). SNA did not differ significantly between recipients of an SHR kidney (n = 8) and recipients of an F1H kidney (n = 7) in terms of frequency and amplitude of synchronized nerve discharges. In response to cumulative intracerebroventricular administration of 10 and 20 microg guanabenz, SNA fell to 51 +/- 5% of control in recipients of an SHR kidney versus 44 +/- 6% of control in recipients of an F1H kidney (not significant) accompanied by a slight fall in MAP in either group. The results suggest that elevated sympathetic activity is not a major contributor to the development of renal posttransplantation hypertension.

Chronic exercise reduces sympathetic nerve activity in rabbits with pacing-induced heart failure: A role for angiotensin II

BACKGROUND

Chronic exercise (EX) improves the quality of life and increases the survival of patients with chronic heart failure (CHF). Because sympathetic nerve activity is elevated in the CHF state, it is possible that EX is beneficial in this disease due to a decrease in sympathetic outflow.

METHODS AND RESULTS

We evaluated arterial baroreflex function and resting renal sympathetic nerve activity (RSNA) in EX normal and CHF rabbits before and after angiotensin II type 1 (AT(1)) receptor blockade. Four groups of rabbits were studied: a normal non-EX group, a normal EX group, a CHF non-EX group, and a CHF EX group. EX lowered resting RSNA in rabbits with CHF but not in normal rabbits. In addition, EX increased arterial baroreflex sensitivity in the CHF group (heart rate slope: CHF 1. 7+/-0.3 bpm/mm Hg, EX CHF 4.9+/-0.3 bpm/mm Hg; P:<0.01; RSNA slope: CHF 2.2+/-0.2%max/mm Hg, EX CHF 5.7+/-0.4%max/mm Hg; P:<0.01. AT(1) receptor blockade enhanced baroreflex sensitivity in the non-EX CHF rabbits but had no effect in EX CHF rabbits. Concomitant with this effect, EX lowered the elevated plasma angiotensin II concentration in the CHF group. A significant positive correlation was observed between sympathetic nerve activity and plasma angiotensin II.

CONCLUSIONS

These data strongly suggest that EX reduces the sympathoexcitatory state in the setting of CHF. Enhanced arterial baroreflex sensitivity may contribute to this reduction. In addition, EX lowers plasma angiotensin II concentration in CHF. These data further suggest that the lowering of angiotensin II may contribute to the decrease in sympathetic nerve activity after EX in the CHF state.

Baroreflexes prevent neurally induced sodium retention in angiotensin hypertension

Recent studies indicate that renal sympathetic nerve activity is chronically suppressed during ANG II hypertension. To determine whether cardiopulmonary reflexes and/or arterial baroreflexes mediate this chronic renal sympathoinhibition, experiments were conducted in conscious dogs subjected to unilateral renal denervation and surgical division of the urinary bladder into hemibladders to allow separate 24-h urine collection from denervated (Den) and innervated (Inn) kidneys. Dogs were studied 1) intact, 2) after thoracic vagal stripping to eliminate afferents from cardiopulmonary and aortic receptors [cardiopulmonary denervation (CPD)], and 3) after subsequent denervation of the carotid sinuses to achieve CPD plus complete sinoaortic denervation (CPD + SAD). After control measurements, ANG II was infused for 5 days at a rate of 5 ng. kg(-1). min(-1). In the intact state, 24-h control values for mean arterial pressure (MAP) and the ratio for urinary sodium excretion from Den and Inn kidneys (Den/Inn) were 98 +/- 4 mmHg and 1.04 +/- 0.04, respectively. ANG II caused sodium retention and a sustained increase in MAP of 30-35 mmHg. Throughout ANG II infusion, there was a greater rate of sodium excretion from Inn vs. Den kidneys (day 5 Den/Inn sodium = 0.51 +/- 0.05), indicating chronic suppression of renal sympathetic nerve activity. CPD and CPD + SAD had little or no influence on baseline values for either MAP or the Den/Inn sodium, nor did they alter the severity of ANG II hypertension. However, CPD totally abolished the fall in the Den/Inn sodium in response to ANG II. Furthermore, after CPD + SAD, there was a lower, rather than a higher, rate of sodium excretion from Inn vs. Den kidneys during ANG II infusion (day 5 Den/Inn sodium = 2.02 +/- 0.14). These data suggest that cardiac and/or arterial baroreflexes chronically inhibit renal sympathetic nerve activity during ANG II hypertension and that in the absence of these reflexes, ANG II has sustained renal sympathoexcitatory effects.

Effects of central infusion of ANG II and losartan on the cardiac baroreflex in rabbits

The effect of chronic activation or inhibition of central ANG II receptors on cardiac baroreflex function in conscious normotensive rabbits was examined. Animals received a fourth ventricular (4V) infusion of ANG II (30 and 100 ng/h), losartan (3 and 30 microg/h), or Ringer solution (2 microl/h) for 2 wk. After 1 and 2 wk, ANG II (100 ng/h) decreased cardiac baroreflex gain by 20 and 37%, respectively (P = 0.015), whereas losartan (30 microg/h) increased baroreflex gain by 24 and 58%, respectively (P = 0.02). Within 1 wk of the end of the infusions, cardiac baroreflex gain had returned to control. Ringer solution or the lower doses of ANG II or losartan did not modify the cardiac baroreflex function. Blood pressure and heart rate were not altered by any treatment, nor was their variability affected. These data demonstrate a novel long-term modulation of cardiac baroreflexes by endogenous ANG II that is independent of blood pressure level.

Participation of arterial baroreceptors input and peripheral vasopressin in the suppression of renal sympathetic nerve activity induced by central salt loading in conscious rats

We examined whether renal sympathetic nerve activity (RSNA) is suppressed in response to intracerebroventricular (i.c.v.) administration of hypertonic saline (HS) in conscious rats. RSNA was suppressed by i.c.v. administration of HS (0.3 M, 0.67 M, and 1.0 M, 1 microl/min for 20 min) in a concentration-dependent manner, which was attenuated under pentobarbital anesthesia. To elucidate mechanisms responsible for central HS-induced decrease in RSNA, possible involvement of arterial baroreceptors and peripheral arginine vasopressin (AVP) secreted from the posterior pituitary gland was examined using sinoaortic denervated (SAD) rats and non-peptide vasopressin receptor antagonists. The maximum suppression of RSNA (-81.5 +/- 5.5%) in control rats was significantly attenuated to -32.5 +/- 6.7% in SAD rats and to -55.8 +/- 5.7% in rats pretreated with intravenous vasopressin V1 receptor antagonist, OPC-21268 (5 mg/kg, i.v.). However, in SAD rats, pretreatment with vasopressin V1 receptor antagonist did not further affect the RSNA inhibition induced by central salt loading. The results suggest that the suppression of RSNA during central salt loading is mainly dependent on the arterial baroreceptors input and the 'additive' role of peripheral vasopressin.

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

BACKGROUND

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

METHODS AND RESULTS

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

CONCLUSIONS

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

Sodium intake, angiotensin II receptor blockade, and baroreflex function in conscious rats

The hypothesis that endogenous angiotensin II (Ang II) chronically supports baroreflex control of lumbar sympathetic nerve activity (LSNA) and heart rate (HR) via AT1 but not AT2 receptors was tested in conscious, normotensive rats. Rats were fed either a sodium deficient diet (LS) to increase circulating Ang II or a high-sodium diet (HS) for 2 to 3 weeks. One to two days after surgery to implant catheters and nerve electrodes, baroreflex curves were produced before and 40 minutes after intravenous administration of the AT1 antagonist losartan (10 mg/kg) or the AT2 antagonist PD123319 (500 micrograms/kg + 50 micrograms.kg-1.min-1). Mean arterial pressure (MAP) after losartan was maintained at basal levels with methoxamine. Forty minutes after losartan in LS rats, LSNA (46 +/- 5 to 22 +/- 1% max) and HR (414 +/- 7 to 387 +/- 8 bpm) were decreased (P < .05). Losartan decreased reflex control of LSNA more in LS than in HS rats (P < .05), as indicated by reductions in maximum LSNA (98 +/- 2 to 78 +/- 3% max) and minimum LSNA (42 +/- 5 to 21 +/- 5% max). Losartan also shifted reflex control of LSNA to a lower pressure in both groups, but the effect was larger in LS rats (-21 +/- 3 [LS] versus -9 +/- 2 [HS] mm Hg at basal LSNA; P < .05). Maximum gain was unaltered in either group. Similarly, losartan reduced maximum HR (534 +/- 6 to 495 +/- 9 bpm) and shifted the HR curve leftward (114 +/- 5 to 105 +/- 4 mm Hg) in LS but not in HS rats. In general, no changes were observed in MAP or baroreflex control of LSNA and HR after PD123319 in LS rats. These results suggest that in conscious, normotensive LS rats, endogenous Ang II supports LSNA and HR over a wide MAP range via AT1 but not AT2 receptors.

Hypotensive response to losartan in normal rats. Role of Ang II and the area postrema

We have reported that the angiotensin II (Ang II) AT1 receptor antagonist losartan markedly lowers arterial pressure in sodium-replete, normotensive rats. We hypothesized that this action of losartan was mediated by its blocking the effects of endogenous Ang II. To test this hypothesis, rats were instrumented with arterial and venous catheters for measurement of arterial pressure and infusion of losartan, respectively. After 3 days of control measurements, losartan was infused for 10 days (10 mg/kg/d) in rats on a normal daily sodium intake (NNa; approximately 2 mmol/d, n=6) and rats on a high daily sodium intake (HNa; approximately 15 mmol/d, n=7) to suppress endogenous Ang II. Although basal plasma renin activity was markedly suppressed in HNa rats (0.9 +/- 0.4 ng Ang I/ mL/h) compared with NNa rats (4.0 +/- 0.3 ng Ang I/mL/h), control arterial pressure was not different between NNa (113 +/- 4 mm Hg) and HNa (113 +/- 2 mm Hg) rats. Losartan decreased arterial pressure from control levels in NNa rats on the first day of infusion (-12 +/- 2 mm Hg) but had no effect on arterial pressure in HNa rats (+4 +/- 4 mm Hg). Furthermore, by day 10 of losartan infusion, arterial pressure had decreased further from control levels in NNa rats (-32 +/- 2 mm Hg) but remained unchanged compared with control in HNa rats (+5 +/- 6 mm Hg). A second study was conducted to test the hypothesis that the area postrema, a circumventricular organ proposed to mediate the long-term neurogenic pressor activity of Ang II is a site of action for losartan. After 3 control days, losartan was administered for 10 days to area postrema-lesioned rats (APx; n=11) or sham-lesioned rats (n=10) consuming an NNa diet. Control arterial pressure was similar in sham (95 +/- 3 mm Hg) and APx (96 +/- 2 mm Hg) rats. Basal plasma renin activity was not different between groups (sham, 4.1 +/- 1.5 versus APx, 5.3 +/- 1.6 mm Hg Ang I/mL/h). On day 1 of losartan treatment, arterial pressure decreased to a significantly lower level in sham (80 +/- 2 mm Hg) compared with APx (90 +/- 3 mm Hg) rats. This trend continued through day 4 of losartan infusion, in which arterial pressure in sham rats (72.2 +/- 2 mm Hg) was significantly lower than in APx rats (83 +/- 4 mm Hg). However, during the remainder of the losartan infusion, there were no significant differences between groups with the exception of day 8 (sham, 72 +/- 2 mm Hg; APx, 84 +/- 2 mm Hg). Taken together, these results support the hypothesis that the hypotensive actions of losartan in sodium-replete, normotensive rats are due to blockade of the physiological effects of endogenous Ang II. Furthermore, an intact area postrema is essential for full expression of the hypotensive actions of losartan in normal rats.

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

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