Bing S. Huang

Prevention of Sympathetic and Cardiac Dysfunction After Myocardial Infarction in Transgenic Rats Deficient in Brain Angiotensinogen

Abstract— To provide evidence for the role of angiotensin II locally produced in the brain in the development of sympathetic hyperactivity and heart failure after myocardial infarction (MI), transgenic rats (TGR) were used, which express an antisense RNA against angiotensinogen. In TGR and control Sprague-Dawley (SD) rats, an MI was induced by acute coronary artery ligation. At 8 weeks after MI, MI sizes were similar in TGR and SD rats. In the groups with MI ≥25% of left ventricle (LV), LV peak systolic pressure decreased in SD rats but not in TGR. LV end-diastolic pressure increased substantially more in SD-MI than TGR-MI rats (from 2±1 to 15±2 mm Hg, and 2±1 to 8±1 mm Hg, respectively; P <0.05). LV dP/dtmax decreased from ≈5400 to 3573±187 in SD-MI rats, but only to 4353±180 mm Hg/sec in TGR-MI (P <0.05). LV pressure volume curves in vitro showed a marked shift to the right in SD-MI rats. This shift was significantly attenuated by −70% in TGR versus SD rats with MI. Both RV weight and interstitial fibrosis in the LV increased clearly in the SD-MI rats, but not or significantly less in the TGR-MI rats. In SD-MI rats, arterial baroreflex control of heart rate and renal sympathetic nerve activity was markedly impaired but was not affected in the TGR-MI. Plasma angiotensin II levels tended to be higher in SD versus TGR rats, both in sham and MI-groups. This study provides the major new finding that in rats after MI, angiotensin II locally produced in the brain plays a dominant role in the development of LV dysfunction after MI, possibly through its effects on sympathetic function and on circulatory/cardiac renin-angiotensin system.

Brain Ouabain mediates the sympathoexcitatory and hypertensive effects of high sodium intake in Dahl salt-sensitive rats

To assess whether brain ouabain-like activity (OLA) mediates the hypertensive effects of high sodium intake in Dahl salt-sensitive (Dahl S) rats, the effects of blockade of brain OLA on mean arterial pressure (MAP) and renal sympathetic nerve activity (RSNA) were evaluated in conscious Dahl salt-resistant (Dahl R) and Dahl S rats on a regular (120 mumol/g) or high sodium (1370 mumol/g) diet from 4 to 7 weeks of age. Dahl S rats given high sodium showed higher basal MAP and augmented responses of MAP and RSNA to air stress and to intracerebroventricular injection of the alpha 2-adrenergic receptor agonist guanabenz as compared with Dahl R rats or Dahl S rats given regular sodium. In contrast, the sympathoexcitatory and pressor responses to intracerebroventricular injection of ouabain (0.3 and 1.0 microgram) were markedly attenuated in Dahl S rats given high sodium. Intracerebroventricular preinjection of 0.3 microgram ouabain significantly enhanced blood pressure and RSNA responses to air stress and intracerebroventricular guanabenz in Dahl S rats given regular sodium to the levels observed in Dahl S rats given high sodium. Intracerebroventricular digoxin-specific antibody Fab (DAF) fragments (132 micrograms/8 microL for 5 minutes) did not change basal MAP and RSNA during the first 4 hours after administration in Dahl S rats on a high sodium diet for 3 weeks. However, 18 hours after the injection of DAF fragments, basal MAP and RSNA were significantly decreased, reaching values for Dahl S rats on a regular sodium diet. The magnitude of increases or decreases in MAP and RSNA to air stress or intracerebroventricular guanabenz were significantly attenuated by the DAF fragments in Dahl S rats on a high sodium but not regular sodium diet. Concomitant intracerebroventricular infusion of DAF fragments (200 micrograms per day) prevented the development of hypertension after a high sodium diet in Dahl S rats and prevented an augmentation in pressor and sympathoexcitatory responses to air stress. After discontinuing the infusion of DAF fragments, resting MAP gradually increased to the high levels found in Dahl S rats given high sodium treated with gamma-globulins. These results support the concept that high sodium intake may cause hypertension in Dahl S rats by increasing endogenous brain OLA, thereby enhancing sympathetic outflow and basal blood pressure as well as sympathoexcitatory and pressor responses to stress.

Chronic central versus peripheral ouabain, blood pressure, and sympathetic activity in rats.

To assess whether chronic ouabain administration causes hypertension by increasing sympathetic activity, we recorded arterial blood pressure and heart rate at rest and after ganglionic blockade in conscious Wistar rats following 10 to 14 days of central or peripheral administration of ouabain. Intracerebroventricular or intravenous infusion of ouabain (10 micrograms/d for both) as well as subcutaneous ouabain pellets (releasing 25 micrograms ouabain/d per pellet) increased mean arterial pressure by 20 to 30 mm Hg and heart rate by 40 to 60 beats per minute. Ouabain pellets increased blood pressure and heart rate in a dose-related manner. After 2 weeks of all ouabain treatments, ouabainlike activity in plasma was not changed but increased significantly in hypothalamus and adrenals. Ouabainlike activity in the adrenals was increased more by intravenous than subcutaneous or intracerebroventricular ouabain treatment, but the different treatment modes caused similar increases in the hypothalamus. Concomitant central infusion of antibody Fab fragments against ouabain prevented the ouabain pellet-induced increases in blood pressure and heart rate. Ganglionic blockade by intravenous hexamethonium normalized blood pressure and heart rate in ouabain-treated rats. These data suggest that in normotensive rats exogenous ouabain, regardless of the mode of administration, may act centrally to cause sympathoexcitation and thus hypertension.

Role of central nervous system aldosterone synthase and mineralocorticoid receptors in salt-induced hypertension in Dahl salt-sensitive rats

In Dahl salt-sensitive (S) rats, high salt intake increases cerebrospinal fluid (CSF) Na(+) concentration ([Na(+)]) and blood pressure (BP). Intracerebroventricular (ICV) infusion of a mineralocorticoid receptor (MR) blocker prevents the hypertension. To assess the role of aldosterone locally produced in the brain, we evaluated the effects of chronic central blockade with the aldosterone synthase inhibitor FAD286 and the MR blocker spironolactone on changes in aldosterone and corticosterone content in the hypothalamus and the increase in CSF [Na(+)] and hypertension induced by high salt intake in Dahl S rats. After 4 wk of high salt intake, plasma aldosterone and corticosterone were not changed, but hypothalamic aldosterone increased by approximately 35% and corticosterone tended to increase in Dahl S rats, whereas both steroids decreased by approximately 65% in Dahl salt-resistant rats. In Dahl S rats fed the high-salt diet, ICV infusion of FAD286 or spironolactone did not affect the increase in CSF [Na(+)]. ICV infusion of FAD286 prevented the increase in hypothalamic aldosterone and 30 mmHg of the 50-mmHg BP increase induced by high salt intake. ICV infusion of spironolactone fully prevented the salt-induced hypertension. These results suggest that, in Dahl S rats, high salt intake increases aldosterone synthesis in the hypothalamus and aldosterone acts as the main MR agonist activating central pathways contributing to salt-induced hypertension.

Brain ‘Ouabain’ Mediates Sympathetic Hyperactivity in Congestive Heart Failure

In congestive heart failure (CHF), endogenous compounds with ouabainlike activity (OLA) may contribute to the maintenance of the circulatory homeostasis by peripheral as well as central effects. In the present study, we assessed changes in peripheral (plasma and left ventricle) and central (pituitary, hypothalamus, pons, and cortex) OLA in two animal models of CHF and determined whether brain OLA mediates sympathetic hyperactivity in CHF. Cardiomyopathic hamsters with their controls were studied at 9 months of age for tissue OLA. Rats were studied 4 weeks after acute coronary artery ligation for tissue OLA and sympathetic activity. In both models, left ventricular end-diastolic pressure was markedly increased. CHF was associated with significant increases in both plasma and tissue OLA in both models. In the brain, the most marked (twofold to threefold) increases occurred in the hypothalamus. In vitro, all OLA measured could be blocked by antibody Fab fragments (Digibind). Conscious rats with CHF showed elevated plasma catecholamines and enhanced responses of mean arterial pressure (MAP), heart rate (HR), and renal sympathetic nerve activity (RSNA) to air stress and to intracerebroventricular (ICV) injection of the alpha 2-adrenergic receptor agonist guanabenz compared with sham-operated rats. ICV administration of the Fab fragments did not change resting RSNA or responses to air stress at 1 hour. However, 18 hours after injection of the Fab fragments, resting RSNA levels had significantly decreased compared with the control values, and plasma catecholamine levels had decreased to control values.(ABSTRACT TRUNCATED AT 250 WORDS)

The central role of the brain in salt-sensitive hypertension

Purpose of review To integrate recent studies showing that abnormal Na+ transport in the central nervous system plays a pivotal role in genetic models of salt-sensitive hypertension. Recent findings Na+ transport-regulating mechanisms classically considered to reflect renal control of the blood pressure, i.e. aldosterone–mineralocorticoid receptors–epithelial sodium channels–Na+/K+-ATPase, have now been demonstrated to be present in the central nervous system contributing to regulation of cerebrospinal fluid [Na+] by the choroid plexus and to neuronal responsiveness to cerebrospinal fluid/brain [Na+]. Dysfunction of either or both can activate central nervous system pathways involving ‘ouabain’ and angiotensin type 1 receptor stimulation. The latter causes sympathetic hyperactivity and adrenal release of marinobufagenin − a digitalis-like inhibitor of the α1 Na+/K+-ATPase isoform – both contributing to hypertension on high salt intake. Conversely, specific central nervous system blockade of mineralocorticoid receptors or epithelial sodium channels prevents the development of hypertension on high salt intake, irrespective of the presence of a ‘salt-sensitive kidney’. Variants in the coding regions of some of the genes involved in Na+ transport have been identified, but sodium sensitivity may be mainly determined by abnormal regulation of expression, pointing to primary abnormalities in regulation of transcription. Summary Looking beyond the kidney is providing new insights into mechanisms contributing to salt-sensitive hypertension, which will help to dissect the genetic factors involved and to discover novel strategies to prevent and treat salt-sensitive hypertension.

Brain ouabain-like activity and the sympathoexcitatory and pressor effects of central sodium in rats.

Intracerebroventricularly infused hypertonic saline elicits sympathoexcitatory and pressor effects. To clarify the mechanisms mediating these effects, we evaluated blood pressure (BP), heart rate (HR), and renal sympathetic nerve activity (RSNA) responses to intracerebroventricular administration of 0.3 M NaCl, ouabain, and rat hypothalamic and pituitary extracts containing ouabain-like activity (OLA) in conscious Wistar rats, before and after intracerebroventricular preinjection of digoxin-specific antibody Fab (DAF) fragments. To exclude modulatory effects of arginine vasopressin (AVP), treatment with DAF fragments was in all experiments preceded by intravenous injection of the AVP antagonist [beta-mercapto-beta,beta-cyclopentamethylenepropionyl1,o- Me-Tyr2,Arg8]AVP. After AVP antagonist pretreatment, 0.3 M NaCl i.c.v. at 3.8 microliters/min for 10 minutes caused simultaneous increases in BP, RSNA, and HR. After AVP antagonist pretreatment, intracerebroventricular injections of 0.3 and 1.0 microgram/l microliter ouabain or the OLA equivalent to 1 microgram ouabain/2 microliters elicited similar significant increases in BP, HR, and RSNA. After pretreatment with AVP antagonist and DAF fragments (66 micrograms/4 microliters i.c.v.), BP, HR, and RSNA responses to 0.3 M NaCl, ouabain, and OLA were all significantly diminished. In contrast, combined AVP blockade and DAF fragments did not affect the BP response to intracerebroventricular angiotensin II, the BP, HR, and RSNA response to intracerebroventricular carbachol and to air stress, or the HR and RSNA responses to intravenous sodium nitroprusside. Intracerebroventricularly injected gamma-globulins (66 micrograms/4 microliters) did not affect the responses to 0.3 M NaCl, ouabain, or OLA.(ABSTRACT TRUNCATED AT 250 WORDS)

Responses to Central Na+ and Ouabain Are Attenuated in Transgenic Rats Deficient in Brain Angiotensinogen

Studies with angiotensin (Ang) II type 1 receptor blockers suggest that the brain renin-angiotensin system contributes to sodium-induced sympathoexcitation and hypertension. To provide more specific evidence for the involvement of Ang II, locally produced in the brain, transgenic rats were used, which express an antisense RNA against angiotensinogen mRNA specifically in the brain, reducing angiotensinogen levels in the brain by >90%. In freely moving transgenic rats and Sprague-Dawley rats as control animals, blood pressure and heart rate responses to intracerebroventricular infusion (3.8 &mgr;L/min for 10 minutes) of artificial cerebrospinal fluid and Na+-rich artificial cerebrospinal fluid (containing 0.2, 0.3, and 0.45 mol/L Na+) as well as intracerebroventricular injection of ouabain (0.3 and 0.6 &mgr;g/2 &mgr;L) were assessed. Central infusion of Na+-rich artificial cerebrospinal fluid increased blood pressure and heart rate in a dose-related manner. However, the peak increases by each dose of Na+ were attenuated by 50% to 70% in the transgenic versus Sprague-Dawley rats. Increases in blood pressure and heart rate in response to ouabain at both doses were attenuated by 55% to 70% in the transgenic versus Sprague-Dawley rats. In the hypothalamus, Ang I level was markedly lower (31±9 versus 76±13 pg/g, P <0.05) and Ang II level tended to be lower in the transgenic versus Sprague-Dawley rats. These results indicate that the production of angiotensins in the brain is decreased in transgenic rats. The attenuated sympathoexcitatory and pressor responses to ouabain and Na+-rich artificial cerebrospinal fluid in transgenic rats support the concept that the local brain renin-angiotensin system, that is, locally produced Ang II, plays an important role in the sympathoexcitatory effects of ouabain and sodium.

Regulation of hypothalamic renin-angiotensin system and oxidative stress by aldosterone

In rats with salt‐induced hypertension or postmyocardial infarction, angiotensin II type 1 receptor (AT1R) densities and oxidative stress increase and neuronal NO synthase (nNOS) levels decrease in the paraventricular nucleus (PVN). The present study was designed to determine whether these changes may depend on activation of the aldosterone ‐‘ouabain’ neuromodulatory pathway. After intracerebroventricular (i.c.v.) infusion of aldosterone (20 ng h−1) for 14 days, blood pressure (BP) and heart rate (HR) were recorded in conscious Wistar rats, and mRNA and protein for nNOS, endothelial NO synthase (eNOS), AT1R and NADPH oxidase subunits were assessed in brain tissue. Blood pressure and HR were significantly increased by aldosterone. Aldosterone significantly increased mRNA and protein of AT1R, P22phox, P47phox, P67phox and Nox2, and decreased nNOS but not eNOS mRNA and protein in the PVN, as well as increased the angiotensin‐converting enzyme and AT1R binding densities in the PVN and supraoptic nucleus. The increases in BP and HR, as well as the changes in mRNA, proteins and angiotensin‐converting enzyme and AT1R binding densities were all largely prevented by concomitant i.c.v. infusion of Digibind (to bind ‘ouabain’) or benzamil (to block presumed epithelial sodium channels). These data indicate that aldosterone, via ‘ouabain’, increases in the PVN angiotensin‐converting enzyme, AT1R and oxidative stress, but decreases nNOS, and suggest that endogenous aldosterone may cause the similar pattern of changes observed in salt‐sensitive hypertension and heart failure postmyocardial infarction.

Central infusion of aldosterone synthase inhibitor prevents sympathetic hyperactivity and hypertension by central Na+ in Wistar rats

In Wistar rats, increasing cerebrospinal fluid (CSF) Na+ concentration ([Na+]) by intracerebroventricular (ICV) infusion of hypertonic saline causes sympathetic hyperactivity and hypertension that can be prevented by blockade of brain mineralocorticoid receptors (MR). To assess the role of aldosterone produced locally in the brain in the activation of MR in the central nervous system (CNS), Wistar rats were infused ICV with artificial CSF (aCSF), Na+ -rich (800 mmol/l) aCSF, aCSF plus the aldosterone synthase inhibitor FAD286 (100 microg x kg(-1) x day(-1)), or Na+ -rich aCSF plus FAD286. After 2 wk of infusion, rats treated with Na+ -rich aCSF exhibited significant increases in aldosterone and corticosterone content in the hypothalamus but not in the hippocampus, as well as increases in resting blood pressure (BP) and sympathoexcitatory responses to air stress, and impairment of arterial baroreflex function. Concomitant ICV infusion of FAD286 prevented the Na+ -induced increase in hypothalamic aldosterone but not corticosterone and prevented most of the increases in resting BP and sympathoexcitatory and pressor responses to air stress and the baroreflex impairment. FAD286 had no effects in rats infused with ICV aCSF. In another set of rats, 24-h BP and heart rate were recorded via telemetry before and during a 14-day ICV infusion of Na+ -rich aCSF with or without FAD286. Na+ -rich aCSF without FAD286 caused sustained increases ( approximately 10 mmHg) in resting mean arterial pressure that were absent in the rats treated with FAD286. These data suggest that in Wistar rats, an increase in CSF [Na+] may increase the biosynthesis of corticosterone and aldosterone in the hypothalamus, and mainly aldosterone activates MR in the CNS leading to sympathetic hyperactivity and hypertension.