Monir Ahmad

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.

Central infusion of aldosterone synthase inhibitor attenuates left ventricular dysfunction and remodelling in rats after myocardial infarction

AIMS Blockade of mineralocorticoid receptors in the central nervous system (CNS) prevents sympathetic hyperactivity and improves left ventricle (LV) function in rats post-myocardial infarction (MI). We examined whether aldosterone produced locally in the brain may contribute to the activation of mineralocorticoid receptors in the CNS. METHODS AND RESULTS Two days after coronary artery ligation, Wistar rats received an intra-cerebroventricular (icv) infusion via osmotic mini-pumps of the aldosterone synthase inhibitor FAD286 at 100 microg/kg/day or vehicle for 4 weeks. LV function was assessed by echocardiography at 2 and 4 weeks, and by Millar catheter at 4 weeks. At 4 weeks post-MI, aldosterone in the hippocampus was increased by 70% and tended to increase in the hypothalamus by 20%. These increases were prevented by FAD286. Across groups, aldosterone in the hippocampus and hypothalamus showed a high correlation. There were no differences in brain corticosterone levels. Compared to sham rats, at both 2 and 4 weeks post-MI rats treated with vehicle showed increased LV dimensions and decreased LV ejection fraction. Icv infusion of FAD286 attenuated these changes in LV dimensions and ejection fraction by approximately 30%. At 4 weeks post-MI, LV peak systolic pressure (LVPSP) and dP/dt(max/min) were decreased and LV end-diastolic pressure (LVEDP) was increased. In rats treated with icv FAD286, LVPSP and dP/dt(min) remained normal and LVEDP and dP/dt(max) were markedly improved. Post-MI increases in cardiac fibrosis and cardiomyocyte diameter were substantially attenuated by icv FAD286. CONCLUSION These data suggest that aldosterone produced locally in the brain acts as the main agonist of mineralocorticoid receptors in the CNS and contributes substantially to the progressive heart failure post MI.

Neuronal Responsiveness to Central Na+ in 2 Congenic Strains of Dahl Salt-Sensitive Rats

Dahl salt-sensitive rats show increased Na+ entry into the brain on high salt intake and increased sympathetic and pressor responses to central Na+. We examined C10QTL2 and C17QTL to test whether they contribute to these phenotypes. In Dahl salt-sensitive, Lewis, and C10S.L16, and C17S.L2 congenic rats on a high salt diet for 8 to 10 days, blood pressure and heart rate were higher in Dahl salt-sensitive versus others and in C10S.L16 and C17S.L2 versus Lewis rats. Cerebrospinal fluid [Na+] increased by ∼5 mmol/L in Dahl salt-sensitive, C10S.L16, and C17S.L2 compared with Lewis rats. In rats on a regular salt diet, 8-minute intracerebroventricular infusions of artificial cerebrospinal fluid with increasing [Na+] caused increases in blood pressure, heart rate, and renal sympathetic nerve activity, which were ≈90% larger in Dahl salt-sensitive and C17S.L2 versus Lewis rats and only 35% to 45% larger in C10S.L16 versus Lewis rats. In another set of rats on regular salt, blood pressure and heart rate were recorded by telemetry before and during intracerebroventricular infusion of Na+-rich cerebrospinal fluid for 14 days. Na+-rich cerebrospinal fluid caused significantly larger increases in blood pressure and heart rate, larger responses to air stress and more impairment of baroreflex in Dahl salt-sensitive and C17S.L2 rats versus Lewis rats. In contrast, responses in C10S.L16 rats were similar to those in Lewis rats. These data suggest that, in Dahl salt-sensitive rats, genetic variants in C10QTL2 but not C17QTL contribute to increased neuronal responsiveness to cerebrospinal fluid [Na+]. However, neither of them contributes to the increase in cerebrospinal fluid [Na+] induced by high salt.

Role of brain aldosterone and mineralocorticoid receptors in aldosterone-salt hypertension in rats

Central blockade of mineralocorticoid receptors (MRs) or angiotensin II type 1 receptors (AT1Rs) attenuates aldosterone (aldo)-salt induced hypertension. We examined the role of the subfornical organ (SFO), aldo synthesized locally in the brain, and MR and AT1R specifically in the paraventricular nucleus (PVN) in aldo-salt hypertension. Wistar rats were treated with subcutaneous aldo (1 μg/h) plus saline as drinking fluid, and gene expression was assessed by real-time qPCR. Other sets of rats received chronic intra-cerebroventricular (icv) infusion of aldo synthase (AS) inhibitor FAD286, MR blocker eplerenone or vehicle, electrolytic or sham lesions of the SFO, or intra-PVN infusion of AAV-MR-siRNA or AAV-AT1aR-siRNA. Infusion of aldo had no effect on 11βHSD2, MR and AT1R mRNA in different nuclei but increased CYP11B2 mRNA in the SFO, and serum and glucocorticoid-kinase 1 (Sgk1) and epithelial sodium channel (ENaC) γ subunit mRNA in the SFO and supraoptic nucleus (SON). MR-siRNA decreased both MR and AT1R mRNA in the PVN by ∼ 60%, but AT1aR-siRNA only decreased AT1R mRNA. SFO lesion, blockade of brain AS or MR, or knockdown of MR or AT1R in the PVN similarly attenuated aldosterone-induced saline intake by ∼ 50% and hypertension by ∼ 70%. These results suggest that an increase in circulating aldosterone may via MR and AT1R in the SFO increase local aldosterone production in hypothalamic nuclei such as the SON and PVN, and via MR enhance AT1R signaling in the PVN. This central aldosterone-MR-AT1R neuro-modulatory pathway appears to play a major role in the progressive hypertension.

Inhibition of brain angiotensin III attenuates sympathetic hyperactivity and cardiac dysfunction in rats post myocardial infarction

AIMS In rats post-myocardial infarction (MI), activation of angiotensinergic pathways in the brain contributes to sympathetic hyperactivity and progressive left ventricle (LV) dysfunction. The present study examined whether angiotensin III (Ang III) is one of the main effector peptides of the brain renin-angiotensin system controlling these effects. METHODS AND RESULTS After coronary artery ligation, Wistar rats were infused intracerebroventricularly for 4 weeks via minipumps with vehicle, the aminopeptidase A (APA) inhibitor RB150 (0.3 mg/day), which blocks the formation of brain Ang III, or losartan (0.25 mg/day). Blood pressure (BP), heart rate, and renal sympathetic nerve activity in response to air stress and acute changes in BP were measured, and LV function was evaluated by echocardiography and Millar catheter. At 4 weeks post-MI, brain APA activity was increased, sympatho-excitatory and pressor responses to air stress enhanced, and arterial baroreflex function impaired. LV end-diastolic pressure (LVEDP) was increased and ejection fraction (EF) and maximal first derivative of change in pressure over time (dP/dt(max)) were decreased. Central infusion of RB150 during 4 weeks post-MI normalized brain APA activity and responses to stress and baroreflex function, and improved LVEDP, EF, and dP/dt(max). Central infusion of losartan had similar effects but was somewhat less effective, and had no effect on brain APA activity. CONCLUSION These results indicate that brain APA and Ang III appear to play a pivotal role in the sympathetic hyperactivity and LV dysfunction in rats post-MI. RB150 may be a potential candidate for central nervous system-targeted therapy post-MI.

Knockdown of mineralocorticoid or angiotensin II type 1 receptor gene expression in the paraventricular nucleus prevents angiotensin II hypertension in rats

Chronic subcutaneous infusion of Ang II causes a progressive increase in blood pressure (BP) associated with significant increases in angiotensin type 1 receptor (AT1R) and mineralocorticoid receptor (MR) expression in hypothalamic nuclei. Intra‐paraventricular nucleus (PVN) infusion of AAV‐ AT1aR‐siRNA or of AAV‐MR‐siRNA markedly knockdown AT1a‐R or MR expression in the PVN but not in the subfornical organ; or supraoptic nucleus, and prevent most of the increase in BP. These findings indicate that increased MR and AT1R activation in the PVN play a critical role in Ang II‐induced hypertension in rats on regular salt intake.

Role of brain corticosterone and aldosterone in central angiotensin II-induced hypertension.

Circulating angiotensin II (Ang II) activates a central aldosterone–mineralocorticoid receptor neuromodulatory pathway, which mediates most of the Ang II–induced hypertension. This study examined whether specific central infusion of Ang II also activates this central aldosterone–mineralocorticoid receptor pathway. Intracerebroventricular infusion of Ang II at 1.0, 2.5, and 12.5 ng/min for 2 weeks caused dose-related increases in water intake, Ang II concentration in the cerebrospinal fluid, and blood pressure. Intracerebroventricular Ang II, at 2.5 and 12.5 ng/min, increased hypothalamic aldosterone and corticosterone, as well as plasma aldosterone and corticosterone without affecting plasma Ang II levels. Intracerebroventricular infusion of the aldosterone synthase inhibitor FAD286—but not the mineralocorticoid receptor blocker eplerenone—inhibited by ≈60% the Ang II–induced increase in hypothalamic aldosterone. Both blockers attenuated by ≈50% the increase in plasma aldosterone and corticosterone with only minimal effects on hypothalamic corticosterone. By telemetry, intracerebroventricular infusion of Ang II maximally increased blood pressure within the first day with no further increase over the next 2 weeks. Intracerebroventricular infusion of FAD286 or eplerenone did not affect the initial pressor responses but similarly prevented 60% to 70% of the chronic pressor responses to intracerebroventricular infusion of Ang II. These results indicate distinctly different patterns of blood pressure increase by circulating versus central Ang II and support the involvement of a brain aldosterone–mineralocorticoid receptor–activated neuromodulatory pathway in the chronic hypertension caused by both circulating and central Ang II.

Chronic central versus systemic blockade of AT1 receptors and cardiac dysfunction in rats post-myocardial infarction

In rats, both central and systemic ANG II type 1 (AT(1)) receptor blockade attenuate sympathetic hyperactivity, but central blockade more effectively attenuates left ventricular (LV) dysfunction post-myocardial infarction (MI). In protocol I, we examined whether functional effects on cardiac load may play a role and different cardiac effects disappear after withdrawal of the blockade. Wistar rats were infused for 4 wk post-MI intracerebroventricularly (1 mg.kg(-1).day(-1)) or injected subcutaneously daily (100 mg x kg(-1) x day(-1)) with losartan. LV dimensions and function were assessed at 4 wk and at 6 wk post-MI, i.e., 2 wk after discontinuing treatments. At 4 and 6 wk post-MI, LV dimensions were increased and ejection fraction was decreased. Intracerebroventricular but not subcutaneous losartan significantly improved these parameters. At 6 wk, LV peak systolic pressure (LVPSP) and maximal or minimal first derivative of change in pressure over time (dP/dt(max/min)) were decreased and LV end-diastolic pressure (LVEDP) was increased. All four indexes were improved by previous intracerebroventricular losartan, whereas subcutaneous losartan improved LVEDP only. In protocol II, we evaluated effects of oral instead of subcutaneous administration of losartan for 4 wk post-MI. Losartan ( approximately 200 mg x kg(-1) x day(-1)) either via drinking water or by gavage similarly decreased AT(1) receptor binding densities in brain nuclei and improved LVEDP but further decreased LVPSP and dP/dt(max). These results indicate that effects on cardiac load by peripheral AT(1) receptor blockade or the pharmacokinetic profile of subcutaneous versus oral dosing do not contribute to the different cardiac effects of central versus systemic AT(1) receptor blockade post-MI.

Angiotensin-converting enzyme inhibitors, inhibition of brain and peripheral angiotensin-converting enzymes, and left ventricular dysfunction in rats after myocardial infarction.

The brain renin-angiotensin system contributes significantly to progressive left ventricular (LV) dysfunction in rats after myocardial infarction (MI). The present study evaluated the effects of central versus peripheral plus central angiotensin-converting enzyme (ACE) blockade on sympathetic activity, and LV anatomy and function after MI. Methods: Wistar rats were treated for 4 weeks after MI with the lipophilic ACE inhibitor trandolapril at 5 mg/kg/day or the hydrophilic blocker lisinopril at 50 mg/kg/day by once daily subcutaneous injection, or with a central infusion of lisinopril at 0.1 mg/kg/day. Results: At 24 hours after the last dose, subcutaneous trandolapril caused 70% to 80% ACE inhibition in both brain and kidneys; lisinopril caused 10% to 20% less. Central infusion of lisinopril caused 70% inhibition of brain ACE and minimal (6%) inhibition in the kidneys. All three treatments similarly improved sympathetic reactivity and arterial baroreflex function. All three treatments lowered cardiac Ang I and II, and similarly attenuated the increases in LV end diastolic pressure, circumference, and fibrosis. Both subcutaneous treatments further decreased LV peak systolic pressure and dP/dtmax, whereas icv lisinopril caused no change. Conclusion: Despite marked differences in the extent of peripheral blockade, all three treatments similarly affected sympathetic activity and decreased cardiac Ang II, preload and remodeling after MI. One may speculate that central and peripheral ACE-mediated mechanisms are sequential and therefore only minor additional effects of peripheral ACE blockade are noted.

Mineralocorticoid and angiotensin II type 1 receptors in the subfornical organ mediate angiotensin II - induced hypothalamic reactive oxygen species and hypertension.

Activation of angiotensinergic pathways by central aldosterone (Aldo)-mineralocorticoid receptor (MR) pathway plays a critical role in angiotensin II (Ang II)-induced hypertension. The subfornical organ (SFO) contains both MR and angiotensin II type 1 receptors (AT1R) and can relay the signals of circulating Ang II to downstream nuclei such as the paraventricular nucleus (PVN), supraoptic nucleus (SON) and rostral ventrolateral medulla (RVLM). In Wistar rats, subcutaneous (sc) infusion of Ang II at 500ng/min/kg for 1 or 2weeks increased reactive oxygen species (ROS) as measured by dihydroethidium (DHE) staining in a nucleus - specific pattern. Intra-SFO infusion of AAV-MR- or AT1aR-siRNA prevented the Ang II-induced increase in AT1R mRNA expression in the SFO and decreased MR mRNA. Both MR- and AT1aR-siRNA prevented increases in ROS in the PVN and RVLM. MR- but not AT1aR-siRNA in the SFO prevented the Ang II-induced ROS in the SON. Both MR- and AT1aR-siRNA in the SFO prevented most of the Ang II-induced hypertension as assessed by telemetry. These results indicate that Aldo-MR signaling in the SFO is needed for the activation of Ang II-AT1R-ROS signaling from the SFO to the PVN and RVLM. Activation of Aldo-MR signaling from the SFO to the SON may enhance AT1R dependent activation of pre-sympathetic neurons in the PVN.