To assess the possible contribution of the circulatory and cardiac renin-angiotensin system (RAS) to the cardiac hypertrophy induced by a beta-agonist, the present study evaluated the effects of isoproterenol, alone or combined with an angiotensin I-converting enzyme inhibitor or AT(1) receptor blocker, on plasma and LV renin activity, ANG I, and ANG II, as well as left ventricular (LV) and right ventricular (RV) weight. Male Wistar rats received isoproterenol by osmotic minipump subcutaneously and quinapril or losartan once daily by gavage. Plasma and LV ANGs were measured by radioimmunoassay after separation by HPLC. Isoproterenol alone decreased blood pressure, more markedly when combined with losartan or quinapril. Isoproterenol significantly increased LV and RV weight and total collagen. Neither losartan nor quinapril inhibited the increases in LV or RV weight. Losartan prevented the increase in RV collagen but enhanced the increase in LV collagen. Isoproterenol increased plasma renin, ANG I, and ANG II three- to fourfold. Isoproterenol combined with losartan or quinapril, caused marked further increases except for a significant decrease in plasma ANG II with quinapril. Isoproterenol alone did not increase LV ANG II and, combined with losartan or quinapril, actually decreased LV ANG II. These results indicate that isoproterenol-induced cardiac hypertrophy is associated with clear increases in plasma ANG II, but not in LV ANG II. Both losartan and quinapril lower LV ANG II below control levels, but do not prevent the isoproterenol-induced cardiac hypertrophy. These findings do not support a role for the circulatory or cardiac RAS in the cardiac trophic responses to beta-receptor stimulation.

An increase in plasma ANG II causes neuronal activation in hypothalamic nuclei and a slow pressor response, presumably by increasing sympathetic drive. We evaluated whether the activation of a neuromodulatory pathway, involving aldosterone and "ouabain," is involved in these responses. In Wistar rats, the subcutaneous infusion of ANG II at 150 and 500 ng x kg(-1) x min(-1) gradually increased blood pressure up to 60 mmHg at the highest dose. ANG II at 500 ng x kg(-1) x min(-1) increased plasma ANG II by 4-fold, plasma aldosterone by 25-fold, and hypothalamic aldosterone by 3-fold. The intracerebroventricular infusion of an aldosterone synthase (AS) inhibitor prevented the ANG II-induced increase in hypothalamic aldosterone without affecting the increase in plasma aldosterone. Neuronal activity, as assessed by Fra-like immunoreactivity, increased transiently in the subfornical organ (SFO) but progressively in the paraventricular nucleus (PVN) and supraoptic nucleus (SON). The central infusion of the AS inhibitor or a mineralocorticoid receptor blocker markedly attenuated the ANG II-induced neuronal activation in the PVN but not in the SON. Pressor responses to ANG II at 150 ng x kg(-1) x min(-1) were abolished by an intracerebroventricular infusion of the AS inhibitor. Pressor responses to ANG II at 500 ng x kg(-1) x min(-1) were attenuated by the central infusion of the AS inhibitor or the mineralocorticoid receptor blocker by 70-80% and by Digibind (to bind "ouabain") by 50%. These results suggest a novel central nervous system mechanism for the ANG II-induced slow pressor response, i.e., circulating ANG II activates the SFO, leading to the direct activation of the PVN and SON, and, in addition, via aldosterone-dependent amplifying mechanisms, causes sustained activation of the PVN and thereby hypertension.

) 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 ϩ ] 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 ϳ35% and corticosterone tended to increase in Dahl S rats, whereas both steroids decreased by ϳ65% 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 ϩ ]. 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; corticosterone; high-salt diet IN DAHL SALT-SENSITIVE (S) rats, high salt intake increases cerebrospinal fluid (CSF) Na ϩ concentration ([Na ϩ ]) and causes sympathetic hyperactivity and hypertension (19). Saltinduced sympathetic hyperactivity and hypertension can be prevented by central blockade of steroid biosynthesis by the steroid synthase 3␤-hydroxysteroid dehydrogenase (HSD) inhibitor trilostane (14) or central blockade of mineralocorticoid receptors (MR) (13).The agonist binding to the MR in the central nervous system (CNS) has not been defined, nor has its origin, i.e., whether it is derived from the circulation or locally produced, been determined. Inasmuch as central infusion of trilostane has antihypertensive effects (14), it appears that local production of a steroid in the CNS plays a major role. However, trilostane blocks the synthesis not only of the mineralocorticoid hormone aldosterone but also of the glucocorticoid hormone corticosterone (14, 24). Aldosterone and corticosterone can be synthesized in the brain (11,12,24). Aldosterone and corticosterone bind MR with equal affinity (3), but the presence of the corticosterone-inactivating enzyme 11␤-HSD-2 enhances aldosterone selectivity of the MR in brain regions where it is expressed, such as the paraventricular nucleus of the hypothalamus (34).There is no evidence that, in Dahl S rats, high salt intake actually increases aldosterone or corticosterone production and release in brain regions implicated in sympathetic regulation or, alternatively, causes changes in MR or 11␤-HSD-2 expression. FAD286 is a specific aldosterone synthase (CYP11B2) blocker that appears to have no effects on corticosterone synthesis (25). Oral administration...

tional studies indicate that the sympathoexcitatory and pressor responses to an increase in cerebrospinal fluid (CSF) [Na ϩ ] by central infusion of Na ϩ -rich artificial cerebrospinal fluid (aCSF) in Wistar rats are mediated in the brain by mineralocorticoid receptor (MR) activation, ouabain-like compounds (OLC), and AT1-receptor stimulation. In the present study, we examined whether increasing CSF [Na ϩ ] by intracerebroventricular infusion of Na ϩ -rich aCSF activates MR and thereby increases OLC and components of the renin-angiotensin system in the brain. Male Wistar rats received via osmotic minipump an intracerebroventricular infusion of aCSF or Na ϩ -rich aCSF, in some groups combined with intracerebroventricular infusion of spironolactone (100 ng/h), antibody Fab fragments (to bind OLC), or as control ␥-globulins. After 2 wk of infusion, resting blood pressure and heart rate were recorded, OLC and aldosterone content in the hypothalamus were assessed by a specific ELISA or radioimmunoassay, and angiotensin-converting enzyme (ACE) and AT1-receptor binding densities in various brain nuclei were measured by autoradiography using 125 I-labeled 351 A and 125 I-labeled ANG II. When compared with intracerebroventricular aCSF, intracerebroventricular Na ϩ -rich aCSF increased CSF [Na ϩ ] by ϳ5 mmol/l, mean arterial pressure by ϳ20 mmHg, heart rate by ϳ65 beats/min, and hypothalamic content of OLC by 50% and of aldosterone by 33%. Intracerebroventricular spironolactone did not affect CSF [Na ϩ ] but blocked the Na ϩ -rich aCSF-induced increases in blood pressure and heart rate and OLC content. Intracerebroventricular Na ϩ -rich aCSF increased ACE and AT 1-receptor-binding densities in several brain nuclei, and Fab fragments blocked these increases. These data indicate that in Wistar rats, a chronic increase in CSF [Na ϩ ] may increase hypothalamic aldosterone and activate CNS pathways involving MR, and OLC, leading to increases in AT 1-receptor and ACE densities in brain areas involved in cardiovascular regulation and hypertension. brain mineralocorticoid receptor; ouabain; angiotensin-converting enzyme; AT 1-receptor NEURAL MECHANISMS play a major role in the development of salt-induced hypertension in Dahl salt-sensitive (S) rats (3). In Dahl S rats but not salt-resistant (R) rats, high salt intake increases cerebrospinal fluid (CSF) [Na ϩ ] (20), associated with increases in hypothalamic ouabain-like compounds (OLC) (41) and in angiotensin-converting enzyme (ACE) activity (46) and AT 1 -receptors (44). Functional studies indicate that high saltinduced sympathetic hyperactivity and hypertension can be prevented by CNS blockade of aldosterone synthesis (15), mineralocorticoid receptors (MR) (14, 32), epithelial sodium channels (ENaC) (42), OLC, or of AT 1 -receptors (19). We proposed (23) that in Dahl S rats on high salt, genetically determined enhancement in the activity of MR-ENaC activates central pathways involving OLC and AT 1 -receptors resulting in sympathetic hyperactivity and hypertension.Recent studies...

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.

These results indicate that high salt intake increases blood pressure, ACE expression and activity in the hypothalamus and pons of Dahl S rats without a parallel increase in angiotensin II levels. Effects of high salt intake on ACE mRNA and activity appear to be secondary to activation of brain 'ouabain'.

We evaluated in rats the time course of changes in cardiac versus plasma ANG I and II postmyocardial infarction (MI) and the effects of nephrectomy and angiotensin-converting enzyme (ACE) inhibitors on the early changes post-MI. Acute coronary artery ligation was induced in conscious rats using the two-stage model, and plasma and cardiac tissue were obtained shortly (6 h, 1 and 3 days) and chronically (1, 4, and 8–9 wk) after MI. In an additional group of rats, bilateral nephrectomy was performed 18 h before the coronary artery ligation, and samples were obtained at 6 h post-MI. Furthermore, in two additional groups of rats, treatment with enalapril and quinapril was started 3 days before the ligation, and samples were obtained at 1 or 3 days post-MI. In these groups of rats, plasma and left ventricular (LV) (infarct and infarct free) ANG I and II were measured by RIA after separation on HPLC. In control rats, plasma ANG I and II showed a clear increase at 6 h post-MI but subsequently only minor increases were observed. In contrast, LV ANG II showed major increases at 6 h and 1 day post-MI, which had returned to normal by 3 days in the infarct-free LV and after 1(–2) wk in the infarct LV. LV ANG I showed a more gradual increase and remained elevated in the infarct up to 8–9 wk. Nephrectomy preceding the MI lowered ANG I and II in plasma but enhanced their increases in the heart at 6 h post-MI. Both ACE inhibitors decreased plasma ANG II associated with large increases in plasma ANG I. They also inhibited the increases in LV ANG II in both the infarct and infarct-free LV at 1 and 3 days post-MI with however no significant increase in LV ANG I. In conclusion, induction of a MI in conscious rats leads to rapid and marked, but only short-lived, increases in cardiac tissue ANG II in both the infarct and infarct-free parts of the LV. Pretreatment with ACE inhibitors, but not nephrectomy, blocks this increase. Local production appears to play a major role in the increases in cardiac ANG II post-MI.

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