Aldosterone infusion into the rat and dose-dependent changes in blood pressure and arterial ionic transport

Mineralocorticoid Receptor-Dependent Impairment of Baroreflex Contributes to Hypertension in a Mouse Model of Primary Aldosteronism

Primary aldosteronism (PA) is the most common cause of secondary hypertension. The paucity of good animal models hinders our understanding of the pathophysiology of PA and the hypertensive mechanism of PA remains incompletely known. It was recently reported that genetic deletion of TWIK-related acid-sensitive potassium-1 and potassium-3 channels from mice (TASK^−/−) generates aldosterone excess and mild hypertension. We addressed the hypertensive mechanism by assessing autonomic regulation of cardiovascular activity in this TASK^−/− mouse line that exhibits the hallmarks of PA. Here, we demonstrate that TASK^−/− mice were hypertensive with 24-h ambulatory arterial pressure. Either systemic or central blockade of the mineralocorticoid receptor (MR) markedly reduced elevated arterial pressure to normal level in TASK^−/− mice. The response of heart rate to the muscarinic cholinergic receptor blocker atropine was similar between TASK^−/− and wild-type mice. However, the responses of heart rate to the β-adrenergic receptor blocker propranolol and of arterial pressure to the ganglion blocker hexamethonium were enhanced in TASK^−/− mice relative to the counterparts. Moreover, the bradycardiac rather than tachycardiac gain of the arterial baroreflex was significantly attenuated and blockade of MRs to a large degree rescued the dysautonomia and baroreflex gain in TASK^−/− mice. Overall, the present study suggests that the MR-dependent dysautonomia and reduced baroreflex gain contribute to the development of hyperaldosteronism-related hypertension.

Aldosterone infusion into the 4th ventricle produces sodium appetite with baroreflex attenuation independent of renal or blood pressure changes

Aldosterone infusion into the 4th ventricle (4th V), upstream the nucleus of the solitary tract (NTS), produces strong 0.3 M NaCl intake. In the present study, we investigated whether aldosterone infusion into the 4th V activates HSD2 neurons, changes renal excretion, or alters blood pressure and cardiovascular reflexes. Chronic infusion of aldosterone (100 ng/h) into the 4th V increased daily 0.3 M NaCl intake (up to 44 ± 10, vs. vehicle: 5.6 ± 3.4 ml/24 h) and also c-Fos expression in HSD2 neurons in the NTS and in non-HSD2 neurons in the NTS. Natriuresis, diuresis and positive sodium balance were present in rats that ingested 0.3 M NaCl, however, renal excretion was not modified by 4th V aldosterone in rats that had no access to NaCl. 4th V aldosterone also reduced baroreflex sensitivity (-2.8 ± 0.5, vs. vehicle: -5.1 ± 0.9 bpm/mmHg) in animals that had sodium available, without changing blood pressure. The results suggest that sodium intake induced by aldosterone infused into the 4th V is associated with activation of NTS neurons, among them the HSD2 neurons. Aldosterone infused into the 4th V in association with sodium intake also impairs baroreflex sensitivity, without changing arterial pressure.

α-Lipoic acid reduces neurogenic hypertension by blunting oxidative stress-mediated increase in ADAM17

We previously reported that type 2 angiotensin-converting enzyme (ACE2) compensatory activity is impaired by the disintegrin and metalloprotease 17 (ADAM17), and lack of ACE2 is associated with oxidative stress in neurogenic hypertension. To investigate the relationship between ADAM17 and oxidative stress, Neuro2A cells were treated with ANG II (100 nM) 24 h after vehicle or α-lipoic acid (LA, 500 μM). ADAM17 expression was increased by ANG II (120.5 ± 9.1 vs. 100.2 ± 0.8%, P < 0.05) and decreased after LA (69.0 ± 0.3 vs. 120.5 ± 9.1%, P < 0.05). In another set of experiments, LA reduced ADAM17 (92.9 ± 5.3 vs. 100.0 ± 11.2%, P < 0.05) following its overexpression. Moreover, ADAM17 activity was reduced by LA in ADAM17-overexpressing cells [109.5 ± 19.8 vs. 158.0 ± 20.0 fluorescence units (FU)·min(-1)·μg protein(-1), P < 0.05], in which ADAM17 overexpression increased oxidative stress (114.1 ± 2.5 vs. 101.0 ± 1.0%, P < 0.05). Conversely, LA-treated cells attenuated ADAM17 overexpression-induced oxidative stress (76.0 ± 9.1 vs. 114.1 ± 2.5%, P < 0.05). In deoxycorticosterone acetate (DOCA)-salt hypertensive mice, a model in which ADAM17 expression and activity are increased, hypertension was blunted by pretreatment with LA (119.0 ± 2.4 vs. 131.4 ± 2.2 mmHg, P < 0.05). In addition, LA improved dysautonomia and baroreflex sensitivity. Furthermore, LA blunted the increase in NADPH oxidase subunit expression, as well as the increase in ADAM17 and decrease in ACE2 activity in the hypothalamus of DOCA-salt hypertensive mice. Taken together, these data suggest that LA might preserve ACE2 compensatory activity by breaking the feedforward cycle between ADAM17 and oxidative stress, resulting in a reduction of neurogenic hypertension.

Angiotensin type 1 receptor mediates chronic ethanol consumption-induced hypertension and vascular oxidative stress

OBJECTIVES

We hypothesized that chronic ethanol intake enhances vascular oxidative stress and induces hypertension through renin-angiotensin system (RAS) activation.

METHODS AND RESULTS

Male Wistar rats were treated with ethanol (20% v/v). The increase in blood pressure induced by ethanol was prevented by losartan (10mg/kg/day; p.o. gavage), a selective AT1 receptor antagonist. Chronic ethanol intake increased plasma renin activity (PRA), angiotensin converting enzyme (ACE) activity, plasma angiotensin I (ANG I) and angiotensin II (ANG II) levels and serum aldosterone levels. No differences on plasma osmolality and sodium or potassium levels were detected after treatment with ethanol. Ethanol consumption did not alter ACE activity, as well as the levels of ANG I and ANG II in the rat aorta or mesenteric arterial bed (MAB). Ethanol induced systemic and vascular oxidative stress (aorta and MAB) and these effects were prevented by losartan. The decrease on plasma and vascular nitrate/nitrite (NOx) levels induced by ethanol was prevented by losartan. Ethanol intake did not alter protein expression of ACE, AT1 or AT2 receptors in both aorta and MAB. Aortas from ethanol-treated rats displayed decreased ERK1/2 phosphorylation and increased protein expression of SAPK/JNK. These responses were prevented by losartan. MAB from ethanol-treated rats displayed reduced phosphorylation of p38MAPK and ERK1/2 and losartan did not prevent these responses.

CONCLUSIONS

Our study provides novel evidence that chronic ethanol intake increases blood pressure, induces vascular oxidative stress and decreases nitric oxide (NO) bioavailability through AT1-dependent mechanisms.

Increased microparticle production and impaired microvascular endothelial function in aldosterone-salt-treated rats: protective effects of polyphenols

We aimed to characterize circulating microparticles in association with arterial stiffness, inflammation and endothelial dysfunction in aldosterone-salt-induced hypertension in rats and to investigate the preventive effects of red wine polyphenols. Uninephrectomized male Sprague-Dawley rats were treated with aldosterone-salt (1 µg.h⁻¹), with or without administration of either red wine polyphenols, Provinols™ (20 mg.kg⁻¹.day⁻¹), or spironolactone (30 mg.kg⁻¹.day⁻¹) for 4 weeks. Microparticles, arterial stiffness, nitric oxide (NO) spin trapping, and mesenteric arterial function were measured. Aldosterone-salt rats showed increased microparticle levels, including those originating from platelets, endothelium and erythrocytes. Hypertension resulted in enhanced aortic stiffness accompanied by increased circulating and aortic NO levels and an upregulation of aortic inducible NO-synthase, NFκB, superoxide anions and nitrotyrosine. Flow-induced dilatation was reduced in mesenteric arteries. These effects were prevented by spironolactone. Provinols™ did not reduce arterial stiffness or systolic hypertension but had effects similar to those of spironolactone on endothelial function assessed by flow-mediated vasodilatation, microparticle generation, aortic NO levels and oxidative stress and apoptosis in the vessel wall. Neither the contractile response nor endothelium-dependent relaxation in mesenteric arteries differed between groups. The in vivo effects of Provinols™ were not mediated by mineralocorticoid receptors or changes in shear stress. In conclusion, vascular remodelling and endothelial dysfunction in aldosterone-salt-mediated hypertension are associated with increased circulating microparticles. Polyphenols prevent the enhanced release of microparticles, macrovascular inflammation and oxidative stress, and microvascular endothelial dysfunction independently of blood pressure, shear stress and mineralocorticoid receptor activation in a model of hyperaldosteronism.

Subchronic treatment with aldosterone induces depression-like behaviours and gene expression changes relevant to major depressive disorder

The potential role of aldosterone in the pathophysiology of depression is unclear. The aim of this study was to test the hypothesis that prolonged elevation of circulating aldosterone induces depression-like behaviour accompanied by disease-relevant changes in gene expression in the hippocampus. Subchronic (2-wk) treatment with aldosterone (2 μg/100 g body weight per day) or vehicle via subcutaneous osmotic minipumps was used to induce hyperaldosteronism in male rats. All rats (n = 20/treatment group) underwent a modified sucrose preference test. Half of the animals from each treatment group were exposed to the forced swim test (FST), which served both as a tool to assess depression-like behaviour and as a stress stimulus. Affymetrix microarray analysis was used to screen the entire rat genome for gene expression changes in the hippocampus. Aldosterone treatment induced an anhedonic state manifested by decreased sucrose preference. In the FST, depressogenic action of aldosterone was manifested by decreased latency to immobility and increased time spent immobile. Aldosterone treatment resulted in transcriptional changes of genes in the hippocampus involved in inflammation, glutamatergic activity, and synaptic and neuritic remodelling. Furthermore, aldosterone-regulated genes substantially overlapped with genes affected by stress in the FST. This study demonstrates the existence of a causal relationship between the hyperaldosteronism and depressive behaviour. In addition, aldosterone treatment induced changes in gene expression that may be relevant to the aetiology of major depressive disorder. Subchronic treatment with aldosterone represents a new animal model of depression, which may contribute to the development of novel targets for the treatment of depression.

Animal models for the study of arterial hypertension

Hypertension is one of the leading causes of disability or death due to stroke, heart attack and kidney failure. Because the etiology of essential hypertension is not known and may be multifactorial, the use of experimental animal models has provided valuable information regarding many aspects of the disease, which include etiology, pathophysiology, complications and treatment. The models of hypertension are various, and in this review, we provide a brief overview of the most widely used animal models, their features and their importance.

Central neuronal activation and pressor responses induced by circulating ANG II: role of the brain aldosterone-“ouabain” pathway

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·kg−1·min−1 gradually increased blood pressure up to 60 mmHg at the highest dose. ANG II at 500 ng·kg−1·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·kg−1·min−1 were abolished by an intracerebroventricular infusion of the AS inhibitor. Pressor responses to ANG II at 500 ng·kg−1·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.

Microribonucleic acid-21 increases aldosterone secretion and proliferation in H295R human adrenocortical cells

MicroRNAs (miRNAs) are endogenous small noncoding RNAs that decrease the expression levels of specific genes by translational repression, sequestration, and degradation of their mRNAs. Angiotensin II is an important modulator of adrenal zona glomerulosa cell physiology, including steroidogenesis and proliferation among many other physiological processes. Because each miRNA may regulate the expression levels of multiple genes, thereby resembling the transcription regulatory networks triggered by transcription factors, we hypothesize that specific miRNAs may be involved in angiotensin II-mediated adrenocortical cell physiology. The human adrenocortical cell line H295R is the only adrenal cell line available with a steroid secretion pattern and regulation similar to freshly isolated adrenocortical cells. We screened for miRNAs regulated by angiotensin II in H295R cells and found that miRNA-21 expression levels were specifically modulated by angiotensin II. Angiotensin II time dependently increased miRNA-21 expression reaching a 4.4-fold induction after 24 h. Angiotensin II-mediated miRNA-21 expression resulted in biologically active miRNA-21, determined using a fusion mRNA reporter system carrying miRNA-21 target sequences in its 3' untranslated region. Up-regulation of miRNA-21 intracellular levels increased aldosterone secretion but not cortisol. Elevation of miRNA-21 levels also increased cell proliferation in H295R cells. In summary, miRNA-21 is an endogenously expressed miRNA in human adrenal cells. miRNA-21 expression is up-regulated by angiotensin II, and its overexpression caused an increase in aldosterone secretion and cell proliferation. Alterations in miRNA-21 expression levels or function may be involved in dysregulation of angiotensin II signaling and abnormal aldosterone secretion by adrenal glands in humans.

Gene expression profile in rat adrenal zona glomerulosa cells stimulated with aldosterone secretagogues

The mineralocorticoid aldosterone, mainly produced by the adrenal gland, is essential for life, but an abnormally excessive secretion causes severe pathological effects including hypertension and target organ injury in the heart and kidney. The aim of this study was to determine the gene regulatory network triggered by aldosterone secretagogues in a nontransformed cell system. Freshly isolated rat adrenal zona glomerulosa cells were stimulated with the two main aldosterone secretagogues, angiotensin II and potassium, for 2 h and subjected to whole genome expression studies using multiple biological and bioinformatics tools. Several genes were differentially expressed by ANG II (n = 133) or potassium (n = 216). Genes belonging to the nucleic acid binding and transcription factor activity categories were significantly enriched. A subset of the most regulated genes was confirmed by real-time RT-PCR, and then their expression was analyzed in time curve studies. Differentially expressed genes were grouped according to their time response expression pattern, and their promoter regions were analyzed for common regulatory transcription factor binding sites. Finally, data mining with gene promoters, transcription factors, and literature databases was performed to generate gene interaction networks for either ANG II or potassium. This paper provides for the first time a complete study of the genes that are regulated, and the interaction between them, by aldosterone secretagogues in rat adrenal cells. Increasing our knowledge of adrenal physiology and gene regulation in nontransformed cell systems could lead us to a better approach for the discovery of candidate genes involved in pathological conditions of the adrenal cortex.

Enhancement of receptor-operated cation current and TRPC6 expression in arterial smooth muscle cells of deoxycorticosterone acetate-salt hypertensive rats

OBJECTIVES

In deoxycorticosterone acetate (DOCA)-salt hypertensive rats, altered reactivity of blood vessels to vasoactive agonists is frequently associated with an elevation in blood pressure. Canonical transient receptor potential (TRPC) channels are believed to encode receptor-operated cation channels (ROC), the activation of which is involved in smooth muscle depolarization and vasoconstriction. The aims of the present study were to investigate whether the ROC current is increased in DOCA-hypertensive rats and determine whether aldosterone directly enhances the expression of TRPC.

METHODS

The nystatin-perforated patch-clamp technique was used for the recording of receptor-stimulated ion currents in mesenteric arterial smooth muscle cells, which were enzymatically dispersed from sham-operated and DOCA-salt hypertensive rats. Expressions of TRPCs were evaluated by reverse transcriptase-polymerase chain reaction (RT-PCR) and by Western blot analysis.

RESULTS

Receptor-stimulated currents activated by 5-hydroxytryptamine (serotonin) and norepinephrine were increased significantly in the mesenteric arterial smooth muscle cells of DOCA-salt hypertensive rats compared to sham-operated rats. Ion-substitution experiments revealed that the enhanced currents were cation currents (ROC currents). Enhanced expression of TRPC6 in mesenteric arteries from DOCA-salt hypertensive rats was demonstrated by real-time RT-PCR. Up-regulation of TRPC6 by aldosterone treatment in vitro was also observed in A7r5 cells by RT-PCR and in western blots.

CONCLUSION

These results suggest that aldosterone enhances TRPC6 expression and ROC currents in vascular smooth muscle cells, and that this may in turn contribute to altered vascular reactivity and to hypertension.

Disabled-2 is expressed in adrenal zona glomerulosa and is involved in aldosterone secretion

The differentiation of the adrenal cortex into functionally specific zones is probably due to differential temporal gene expression during fetal growth, development, and adulthood. In our search for adrenal zona glomerulosa-specific genes, we found that Disabled-2 (Dab2) is expressed in the zona glomerulosa of the rat adrenal gland using a combination of laser capture microdissection, mRNA amplification, cDNA microarray hybridization, and real-time RT-PCR. Dab2 is an alternative spliced mitogen-regulated phosphoprotein with features of an adaptor protein and functions in signal transduction, endocytosis, and tissue morphogenesis during embryonic development. We performed further studies to analyze adrenal Dab2 localization, regulation, and role in aldosterone secretion. We found that Dab2 is expressed in the zona glomerulosa and zona intermedia of the rat adrenal cortex. Low-salt diet treatment increased Dab2-long isoform expression at the mRNA and protein level in the rat adrenal gland, whereas high-salt diet treatment did not cause any significant modification. Angiotensin II infusion caused a transient increase in both Dab2 isoform mRNAs in the rat adrenal gland. Dab2 overexpression in H295R human adrenocortical cells caused an increase in aldosterone synthase expression and up-regulated aldosterone secretion under angiotensin II-stimulated conditions. In conclusion, Dab2 is an adrenal gland zona glomerulosa- and intermedia-expressed gene that is regulated by aldosterone secretagogues such as low-salt diet or angiotensin II and is involved in aldosterone synthase expression and aldosterone secretion. Dab2 may therefore be a modulator of aldosterone secretion and be involved in mineralocorticoid secretion abnormalities.

Serotonin-induced ion channel modulations in mesenteric artery myocytes from normotensive and DOCA-salt hypertensive rats

Although serotonin (5-hydroxytryptamine, 5-HT) has been found to be a potent vasoconstrictor, a pivotal role of 5-HT in the control of appetite and mood control by the modulation of neuronal synapse has also been proposed. Selective 5-HT reuptake inhibitors (SSRIs) are frequently used to suppress appetite and treat depressive disorder, and the target protein of SSRIs is the 5-HT transporter (5-HTT) in the neuronal synapse. However, SSRIs may increase the free 5-HT concentration in circulating blood because platelets and vascular smooth muscles express functional 5-HTT. In addition, enhanced vasoactive action of 5-HT and alterations in 5-HT receptor subtypes have been reported in some types of hypertension. Therefore, we can infer that the use of drugs such as SSRIs in some hypertensive patients is potentially risky. Altered functional expression of ion channels in vascular smooth muscle is suggested to be a mechanism for the enhanced vasoconstriction by vasoactive agonists, including 5-HT. In this brief review, we compared the electrophysiological properties of mesenteric artery myocytes and their modulation by 5-HT between sham-operated control and deoxycorticosterone acetate (DOCA)-salt hypertensive rats.

Brain sodium channels and ouabainlike compounds mediate central aldosterone-induced hypertension

Central nervous system (CNS) effects of mineralocorticoids participate in the development of salt-sensitive hypertension. In the brain, mineralocorticoids activate amiloride-sensitive sodium channels, and we hypothesized that this would lead to increased release of ouabainlike compounds (OLC) and thereby sympathetic hyperactivity and hypertension. In conscious Wistar rats, intracerebroventricular infusion of aldosterone at 300 or 900 ng/h in artificial cerebrospinal fluid (aCSF) with 0.145 M Na+ for 2 h did not change baseline mean arterial pressure (MAP), renal sympathetic nerve activity (RSNA), or heart rate (HR). Intracerebroventricular infusion of aCSF containing 0.16 M Na+ (versus 0.145 M Na+ in regular aCSF) did not change MAP or RSNA, but significant increases in MAP, RSNA, and HR were observed after intracerebroventricular infusion of aldosterone at 300 ng/h for 2 h. Intracerebroventricular infusion of aCSF containing 0.3 M Na+ increased MAP, RSNA, and HR significantly more after intracerebroventricular infusion of aldosterone versus vehicle. After intracerebroventricular infusion of aldosterone, the MAP, RSNA, and HR responses to intracerebroventricular infusion of aCSF containing 0.16 M Na+ were blocked by blockade of brain OLC with intracerebroventricular infusion of Fab fragments or of brain sodium channels with intracerebroventricular benzamil. Chronic intracerebroventricular infusion of aldosterone at 25 ng/h in aCSF with 0.15 M Na+ for 2 wk increased MAP by 15-20 mmHg and increased hypothalamic OLC by 30% and pituitary OLC by 60%. Benzamil blocked all these responses to aldosterone. These findings indicate that in the brain, mineralocorticoids activate brain sodium channels, with small increases in CSF Na+ leading to increases in brain OLC, sympathetic outflow, and blood pressure.

Increased carotid wall elastic modulus and fibronectin in aldosterone-salt-treated rats: effects of eplerenone

BACKGROUND

Previous studies have demonstrated the development of cardiac fibrosis in aldosterone (Aldo)-salt hypertensive rats. Our aim was to determine the effects of Aldo and the Aldo receptor antagonist eplerenone (Epl) on in vivo mechanical properties of the carotid artery using echo-tracking system.

METHODS AND RESULTS

Aldo was administered (1 microg/h) in uninephrectomized Sprague-Dawley rats (SD) receiving a high-salt diet from 8 to 12 weeks of age. Uninephrectomized control SD rats received a normal salt diet without Aldo. Three groups of Aldo-salt rats were treated with 1, 10, or 30 mg/kg(-1) x d(-1) of Epl by gavage. Elasticity was measured by elastic modulus (Einc)-wall stress curves using medial cross-sectional area (MCSA). The structure of the arterial wall was analyzed by histomorphometry (elastin and collagen), immunohistochemistry (EIIIA fibronectin, Fn), and Northern blot (collagens I and III). Aldo produced increased systolic arterial pressure, pulse pressure, Einc, MCSA, and EIIIA Fn with no change in wall stress or elastin and collagen densities compared with controls without Aldo. No differences in collagen mRNA levels were detected between groups. Epl blunted the increase in pulse pressure in Aldo rats and normalized Einc-wall stress curves, MCSA, and EIIIA Fn. These effects were dose dependent and not accompanied by a reduction in wall stress.

CONCLUSIONS

Aldo is able to increase arterial stiffness associated with Fn accumulation, independently of wall stress. The preventive effects of Epl suggest a direct role for mineralocorticoid receptors in mechanical and structural alterations of large vessels in rat hyperaldosteronism.

Effect of angiotensin II blockade on renal injury in mineralocorticoid-salt hypertension

Kidney function and structure were compared in control rats (group 1) and in 3 groups of rats made hypertensive by administration of aldosterone and saline for 8 weeks (groups 2, 3, and 4). Group 2 rats received only aldosterone and saline, while group 3 also received losartan and group 4 also received enalapril. Rats in all groups were subjected to uninephrectomy before beginning the experiment. Hypertension and proteinuria in rats given aldosterone and saline were not affected by losartan or enalapril (8-week values for blood pressure in mm Hg: 135+/-3 group 1, 193+/-4 group 2, 189+/-4 group 3, 189+/-5 group 4; P<0.05 groups 2, 3, and 4 versus 1; 8-week values for proteinuria in mg/d: 44+/-8 group 1, 278+/-34 group 2, 267+/-37 group 3, 289+/-36 group 4; P<0.05 groups 2, 3, and 4 versus 1). Vascular, glomerular, and tubulointerstitial injury accompanied hypertension and proteinuria at 8 weeks. Losartan and enalapril did not prevent vascular injury, which was characterized by thickening of arterial and arteriolar walls and by fibrinoid necrosis and thrombotic microangiopathy. Likewise, losartan and enalapril did not reduce the prevalence of glomerular segmental sclerosis (1+/-1% group 1, 10+/-2% group 2, 11+/-2% group 3, 13+/-2% group 4; P<0.05 groups 2, 3, and 4 versus 1) or limit tubulointerstitial injury as reflected by the volume fraction of the cortical interstitium (15+/-1% group 1, 20+/-1% group 2, 21+/-1% group 3, 21+/-1% group 4; P<0.05 groups 2, 3, and 4 versus 1). These findings suggest that local angiotensin II activity does not contribute to the development of renal injury in mineralocorticoid-salt hypertension.

Role of aldosterone in renal vascular injury in stroke-prone hypertensive rats

Stroke-prone spontaneously hypertensive rats (SHRSP) on 1% NaCl drinking solution and Stroke-Prone Rodent Diet develop severe hypertension and glomerular and vascular lesions characteristic of thrombotic microangiopathy seen in malignant nephrosclerosis. We recently reported that spironolactone, a mineralocorticoid receptor antagonist, markedly reduced proteinuria and malignant nephrosclerotic lesions in these animals. This observation, together with our previous findings that angiotensin-converting enzyme inhibitors prevent the development of vascular damage, suggests that mineralocorticoids, as part of the renin-angiotensin-aldosterone system, play a pathophysiological role in this model. In the present study, we examined whether chronic (2-week) infusion of aldosterone can reverse the renal vascular protective effects of captopril in SHRSP. SHRSP received vehicle (n=8); captopril alone (50 mg. kg-1. d-1, orally) (n=10); aldosterone infusion alone (40 microg. kg-1. d-1, SC) (n=7); or captopril and aldosterone at 20 (n=6) or 40 (n=7) microg. kg-1. d-1. Systolic blood pressure was markedly elevated in all groups. Vehicle- and aldosterone-infused SHRSP developed severe proteinuria and comparable degrees of renal injury (21+/-3% and 29+/-3%, respectively) manifested as thrombotic and proliferative lesions in the arterioles and glomeruli. Captopril treatment reduced plasma aldosterone levels concomitant with marked reductions in proteinuria and the absence of histologic lesions of malignant nephrosclerosis. Aldosterone substitution at 20 or 40 microg. kg-1. d-1 in captopril-treated SHRSP resulted in the development of severe renal lesions (16+/-3% and 21+/-2%, respectively) and proteinuria comparable with that observed in SHRSP given either aldosterone or vehicle alone. These findings support a major role for aldosterone in the development of malignant nephrosclerosis in saline-drinking SHRSP, independent of the effects of blood pressure.

Adrenal-dependent modulation of the catalytic subunit isoforms of the Na+-K+-ATPase in aorta

Na+-K+-ATPase gene expression and activity were studied in aortas from adrenalectomized (ADX) rats and ADX rats with deoxycorticosterone supplement (ADX-DOCA). Northern analysis of RNA from ADX rats revealed a significant decrease in alpha2-mRNA levels (38.5 +/- 8.3% of control, P < 0.01) that was prevented by DOCA (P < 0.05). A decrease to 55.8 +/- 7.7% in alpha2-isoform protein was observed 8 days after adrenal removal (P < 0.05); DOCA reversed this effect (90.8 +/- 10.5%). Adrenalectomy induced a decrease of 68.5 +/- 4.5% in beta1-mRNA (P < 0.01) and 52.7 +/- 8.3% in ADX-DOCA rats (P < 0.01). Also, a reduction in beta1-isoform protein that was not prevented by DOCA was detected after adrenalectomy (47.1 +/- 11%, P < 0.01). In contrast, no differences in alpha1-mRNA or -protein levels were observed. Vascular sodium pump activity was reduced to 59.8 +/- 4.6% of control values after adrenalectomy (P < 0.01); this reduction was reversed by DOCA. Our data indicate that corticosteroids regulate Na+-K+-ATPase isoform expression and activity in vascular tissue in vivo, suggesting a mineralocorticoid-dependent modulation of alpha2-Na+-K+-ATPase gene expression in aorta, with beta1-isoform expression dependent on the presence of glucocorticoids.

ANG II-related myocardial damage: role of cardiac sympathetic catecholamines and beta-receptor regulation

The objectives of this study were 1) to determine whether ANG II-induced myocardial damage (ANG Dam) is mediated via the beta1-adrenergic receptor, 2) to elucidate whether adrenal medulla or cardiac sympathetic neuron catecholamines are responsible for ANG Dam, and 3) to determine whether the lack of damage after 3 days of elevated ANG II levels is due to beta1-receptor downregulation. To this end, ANG II was administered to rats 1) that were treated with a beta-receptor blocker, 2) after adrenal medullectomy and/or cardiac sympathectomy, and 3) for 3 or 8 days. ANG II caused both myocyte necrosis and coronary vascular damage after adrenal medullectomy but not after cardiac sympathectomy. There was a 38 and 55% decrease in beta-receptor density after 3 and 8 days, respectively, of ANG II infusion, and an upregulation to control levels 5 days after a 3-day ANG II infusion was stopped. We conclude that cardiac sympathetic neuron catecholamines are responsible for ANG Dam and that the acute nature of this damage is associated with a downregulation of beta1-adrenergic receptors.

Differential regulation of angiotensin II receptor subtypes in the adrenal gland: role of aldosterone

It has been shown that aldosterone potentiates the action of angiotensin II (Ang II) in cultured rat vascular smooth muscle cells solely by increasing the number of Ang II receptors. The mechanisms responsible for aldosterone-Ang II interactions in the adrenal gland are unknown. The present study was designed to investigate the effect of aldosterone on expression of Ang II receptor subtypes (AT1 and AT2) in the adrenal gland. Seven-week-old male Wistar rats were treated for 2 weeks with either aldosterone (0.05 microg/h, n=14) or vehicle (n=14) by use of implanted osmotic minipumps. Systolic blood pressure was not altered by aldosterone treatment. Plasma aldosterone levels were higher in aldosterone-treated rats (181+/-53 pg/mL) compared with vehicle-treated rats (33+/-21 pg/mL, P<0.05). Northern blot analysis and radioligand binding assay showed that adrenal AT1 mRNA levels and AT1 receptor density in aldosterone-treated rats were not statistically different from those of vehicle-treated rats. However, immunohistochemical studies showed that the highest adrenal AT1 receptor expression was shifted from the zona glomerulosa to the zona fasciculata after aldosterone treatment. In contrast, adrenal AT2 mRNA and AT2 receptor density in aldosterone-treated rats were decreased by approximately 50% and 40%, respectively, compared with vehicle-treated rats (P<0.05). Aldosterone-induced decrease in adrenal AT2 receptor expression occurred mainly in the medulla. Thus, aldosterone differentially modulates the expression of AT1 and AT2 receptors in the adrenal gland. Although the function of the AT2 receptor in the adrenal gland is largely unknown, our data indicate that aldosterone may modulate the effect of Ang II by altering the location of AT1 receptors and by reducing the number of AT2 receptors in the adrenal gland.

Differential regulation of Na+-K+-ATPase gene expression by corticosteriods in vascular smooth muscle cells

To determine whether gluco- and mineralocorticoids have specific actions on Na+-K+-ATPase gene expression in vascular tissue, we used Northern blot analysis to compare the effects of dexamethasone (Dex) and aldosterone (Aldo) on Na+-K+-ATPase alpha1 and beta1-subunit mRNA expression in cultured vascular smooth muscle cells from rat aortae. Dex at 10(-6)M increased alpha1 -mRNA level 2.5-fold at 24 h and beta1-mRNA level 9.9-fold at 12 h. Aldo at 10(-6)M increased alpha1-mRNA 2.7-fold at 48 h and beta1-mRNA level 10.9-fold at 6 h. The half-maximal stimulation of both alpha1 and beta1-mRNA levels occurred at a concentration of 5-7 X 10(-9)M Dex, whereas it occurred at a concentration of 2-3 X 10(-9)M Aldo. The glucocorticoid receptor antagonist RU-38486 inhibited both Dex- and Aldo-mediated induction of beta1-mRNA. The mineralocorticoid receptor antagonist spironolactone inhibited Aldo-mediated induction of beta1-mRNA, whereas it had no effect on Dex-mediated induction of beta1-mRNA. Removal of Na+ from the extracellular medium (isosmotic replacement with choline) caused no effect on Dex-mediated induction beta1-mRNA, whereas it inhibited Aldo-mediated induction of beta1-mRNA. Addition of a specific inhibitor of the Na+/H+ exchange, ethylisopropylamiloride, had no effect on Dex-mediated induction of beta1-mRNA, whereas it resulted in a significant inhibition of Aldo-mediated induction of beta1-mRNA. We conclude that 1) both Dex and Aldo induce Na+-K+-ATPase alpha1- and beta1-mRNA expression in a time- and dose-dependent manner; 2) Dex-mediated induction of beta1-mRNA occurs only through glucocorticoid receptors, whereas Aldo-mediated induction of beta1-mRNA occurs through both gluco- and mineralocorticoid receptors; and 3) Dex-mediated induction of beta1-mRNA occurs through Na+-independent mechanisms, whereas Aldo-mediated induction of beta1-mRNA, at least in part, occurs through Na+-dependent mechanisms, including stimulation of the Na+/H+ exchange.

Aldosterone (ALDO) increases transmembrane influx of Na+ in vascular smooth muscle (VSM) cells through increased synthesis of Na+ channels

We have previously reported our studies on glucocorticoid (GC) effects on Na+ influx in vascular smooth muscle (VSM) cells. We now report a parallel study on the effect of mineralocorticoid (MC) on Na+ influx in VSM cells. Unidirectional influx of Na+ was measured in cultured cells of rabbit aortic media with 22Na as tracer. Cells were treated with near physiologic (5 nM) or supraphysiologic (50 nM) aldosterone (ALDO) for 24 or 48 hours, or for 7 to 10 days, in the presence of competitive inhibitors of MC-receptor binding, K-prorenoate (PRN), or GC-receptor binding, RU 486. ALDO at 5 nM increased Na+ influx by 98% +/- 12%, but only after 7-10 days of treatment. This effect was inhibited by PRN, but not by RU 486, and blocked by amiloride but not by ethylisopropyl-amiloride or by dichlorobenzamil (DCB). In VSM cell membranes from aortae of rabbits treated in vivo with ALDO (2 mg/day) for 4 weeks. Na+ channels were quantified by determination of specific [3H]amiloride binding in the presence of excess of DCB and EIPA to exclude tracer binding from the Na+/Ca2+ exchanger and the Na+/H+ antiporter. ALDO doubled the number of of Na+ channels in such isolated cell membranes, as determined by Bmax per mg membrane protein. We propose that this vascular effect of ALDO may constitute an important pathogenetic mechanism of hypertension induced by chronic excess of MC, in addition to the well known renal mechanism.

Central effects of mineralocorticoid antagonist RU-28318 on blood pressure of DOCA-salt hypertensive rats

The role of brain mineralocorticoid receptor (MR) sites in the pathogenesis of mineralocorticoid hypertension was studied after an intracerebroventricular injection of the MR antagonist RU-28318. Male Wistar rats received subcutaneously implanted deoxycorticosterone acetate (DOCA) pellets and were maintained on 0.9% saline as drinking solution. Under these conditions hypertension developed in approximately 5 wk as assessed in conscious rats by means of the tail-cuff technique. During the development of this hypertension (after 3 wk of DOCA-salt treatment) a single intracerebroventricular injection of the specific MR antagonist RU-28318 reduced systolic blood pressure (SBP) as measured with the tail-cuff method. A decrease in SBP was observed 2-24 h after this intracerebroventricular injection, with the lowest SBP values occurring at 8 h. In these animals (3 wk after DOCA implantation) continuous direct blood pressure recording via chronic cannulation revealed, on the day of the intracerebroventricular injection of RU-28318, a slight increase in arterial pressure during the light phase, followed by a decrease during the dark phase. In the established hypertensive rats (5 wk after DOCA RU-28318 on the arterial pressure or heart rate was detectable. It is concluded that central MR blockade during the development of the DOCA-salt hypertension reduces blood pressure within 24 h assessed with 1) the indirect method at certain time points after exposure to warming and stress and 2) the direct method during the dark phase of the diurnal cycle.

Studies on the mechanisms of glucocorticoid hypertension in humans

Hypertension is a common feature of both Cushing's syndrome, which is relatively rare, and iatrogenic steroid administration, which is much more common. Cardiovascular mortality and morbidity are very significant in patients with both naturally occurring and iatrogenic disease. The mechanism of glucocorticoid induced hypertension in man remains undefined. Contrary to previous notions, it does not reflect urinary sodium retention or volume expansion. Increased pressor responsiveness may be an important contributor to the rise in blood pressure.

Mechanism of the effects of glucocorticoids and mineralocorticoids on vascular smooth muscle contractility

We have previously demonstrated that receptors to both mineralocorticoids (MC) and glucocorticoids (GC) exist in the arterial wall and that treatment with GC markedly increases Na+ and Ca2+ influx in cultured aortic vascular smooth muscle (VSM) cells, whereas treatment with MC increases only Na+ influx. We now report the results of the study aimed at the elucidation of the mechanism(s) of these effects. Unidirectional influx of Na+ and Ca2+ was measured in cultured cells of rabbit aortic media, using 22Na and 45Ca as tracers, in the presence of ouabain. The cells were treated for different periods with dexamethasone (DEX) or aldosterone (ALDO) in physiologic or supraphysiologic concentrations, in the presence or absence of competitive inhibitors of GC-receptor binding, RU 486, or MC-receptor binding, K-prorenoate. DEX in 50 nM concentration increased Na+ influx by 98 +/- 18% and Ca2+ influx by 100 +/- 20%, and the maximum effect was seen after 48 hour cell-treatment. ALDO in 5 nM concentration increased Na+ influx by 90 +/- 12% and had no effect on Ca2+ influx, and the maximum effect was seen after 7-10 days of cell-treatment. The enhancing effect of both DEX and ALDO on the influx rate of Na+ was prevented by actinomycin D and by cycloheximide. RU 486 completely inhibited DEX from exercising its enhancing effect on Na+ influx, but diminished influx rate of Na+ increased by ALDO only by 25%. Prorenoate (PRN) did not have any effect on DEX-increased Na+ influx, but completely inhibited ALDO from exercising its effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Different effects of three aldosterone treatments on plasma aldosterone and salt intake

Adrenalectomized male rats received a nominal dose of 47.6 micrograms/day aldosterone for 14 days by daily injections, osmotic minipumps, or controlled-release pellets. Plasma aldosterone concentrations were barely detectable (< 20 pg/ml) 24 h after rats received aldosterone by injection, remained constant at 200 pg/ml in rats with osmotic minipumps, and dropped from > 500 to 75 pg/ml during the first week after implantation of controlled-release pellets. For the most part, the effects of the different treatments on NaCl intake were related to their effects on plasma aldosterone levels according to a U-shaped function. However, NaCl intake was dissociated from plasma aldosterone levels when treatment first began or was discontinued. NaCl intake may be a function of the amount of aldosterone delivered but not necessarily plasma aldosterone concentration.

Antifibrotic effects of spironolactone in preventing myocardial fibrosis in systemic arterial hypertension

Activation of the renin-angiotensin-aldosterone system in arterial hypertension can lead to remodeling of the myocardial collagen network, with progressive collagen accumulation in the cardiac interstitium. This reactive myocardial fibrosis, which is not secondary to myocyte necrosis, appears to be an important determinant of diastolic dysfunction and thus of pathologic hypertrophy. To examine the effects of the aldosterone antagonist spironolactone on myocardial fibrosis, we analyzed interstitial fibrosis in 7 different models of arterial hypertension in rats: 2 kidney, 1 clip model of renovascular hypertension (RHT); continuous subcutaneous aldosterone (0.75 micrograms/hr) infusion; RHT and aldosterone models treated with 20 mg/kg per day of subcutaneous spironolactone; uninephrectomized rats on a high sodium diet; and age- and sex-matched controls with or without spironolactone treatment. Systolic arterial pressure was comparably elevated in RHT and aldosterone models; it was modestly lowered but not normalized by 8 weeks of spironolactone treatment at the low doses used. Such treatment also failed to prevent left ventricular hypertrophy (LVH) in all experimental groups with hypertension. Spironolactone, however, was able to prevent myocardial fibrosis in RHT and aldosterone models of acquired arterial hypertension irrespective of the development of LVH and the presence of hypertension. These findings provide further evidence that elevated aldosterone levels play an important role in the adverse remodeling of the myocardium in arterial hypertension. The antifibrotic effects of spironolactone, if confirmed in human studies, may be a valuable strategy in treating hypertensive heart disease.

Role of mechanical and hormonal factors in cardiac remodeling and the biologic limits of myocardial adaptation

Patients with chronic congestive heart failure manifest > or = 1 of the following abnormalities: diastolic dysfunction, systolic dysfunction, and arrhythmias. Diastolic dysfunction, one of the first symptoms to occur during hypertensive cardiopathy, depends on both active relaxation of the cardiac muscle and passive ventricular compliance. The ability of the ventricles to relax depends on normal calcium metabolism and adenosine triphosphate concentration. Ability to extrude intracellular calcium is depressed in the hypertrophied, overloaded heart as compared with the normal myocardium. Myocardial fibrosis is the major cause of increased diastolic ventricular stiffness. Left ventricular (LV) hypertrophy and myocardial fibrosis also greatly increase the likelihood of ventricular arrhythmias, in particular by prolonging the QRS interval and facilitating the occurrence of reentry arrhythmias. Findings in animal studies have indicated that such fibrosis, which involves excessive collagen deposition, is independent of LV hypertrophy and that LV hypertrophy does not necessarily result in myocardial fibrosis. Instead, the development of myocardial fibrosis is sensitive to circulating levels of both angiotensin II and aldosterone, and the fibrotic response to each of these substances is independent. The aldosterone antagonist spironolactone prevents myocardial fibrosis in several animal models, thus confirming the importance of aldosterone in the genesis of excessive collagen deposition.

Myocardial fibrosis: role of angiotensin II and aldosterone

In this report we review the replacement (i.e., scarring) and reactive (i.e., perivascular and interstitial fibrosis) fibrous tissue responses found in the myocardium in response to effector hormones of the renin-angiotensin-aldosterone system. Experimental data are presented to indicate: a) endogenous or exogenous elevations in plasma angiotensin II are associated with acute cardiac myocyte necrosis and subsequent microscopic scarring; b) chronic elevations in plasma aldosterone (ALDO), relative to Na+ intake, are associated with a perivascular and interstitial fibrosis of the coronary and systemic circulations and are also seen in response to chronic administration of the mineralocorticoid hormone deoxycorticosterone (DOC); and c) chronic mineralocorticoid excess, due to ALDO or DOC, is associated with enhanced urinary K+ excretion, cardiac myocyte necrosis and scarring. Pharmacologic agents which interfere with these effector hormones (e.g., ACE inhibition and ALDO receptor antagonism) protect the myocardium against this pathologic structural remodeling created by the reactive and replacement (reparative) fibrosis. Evidence is also presented to indicate that chronic ACE inhibition is associated with a regression in reactive myocardial fibrosis. Based on these experimental findings we would suggest that clinical trials are indicated to address the prevention and regression of myocardial fibrosis--an important determinant of pathologic structural remodeling and abnormal myocardial stiffness.

Vascular responsiveness in rats resistant to aldosterone-salt hypertension

Wistar-Furth rats have been shown to be resistant to mineralocorticoid-salt hypertension, but the mechanism for this resistance is unknown. In the current experiments, adult male Wistar and Wistar-Furth rats were given a subcutaneous aldosterone infusion (0.15 microgram/hr) for 4 weeks, and changes in blood pressure and vascular reactivity were studied. Rats received a 1% NaCl, 0.2% KCl solution to drink. After 4 weeks of aldosterone infusion, systolic blood pressure measured using a tail-cuff technique had increased by 60 mm Hg in Wistar rats but was unchanged in Wistar-Furth rats. Hypokalemia occurred in both strains in response to the aldosterone infusion. Isolated, helically cut strips of common carotid artery and aorta were prepared for isometric force recording. Cumulative concentration-response curves to norepinephrine, serotonin, KCl, calcium, nitroprusside, and acetylcholine were performed in carotid artery strips, and concentration-response curves to ouabain were performed in aortic strips. Increased vascular contractile sensitivity to KCl, ouabain, norepinephrine, and serotonin was observed in vessels from Wistar rats treated with aldosterone and salt. The same treatment in Wistar-Furth rats produced only increased vascular sensitivity to ouabain and serotonin, and these changes were of smaller magnitude than those seen in Wistar rats. Aldosterone-salt treatment produced decreased vascular sensitivity to acetylcholine and nitroprusside in both Wistar and Wistar-Furth rats. These results support the hypothesis that resistance of Wistar-Furth rats to aldosterone-salt hypertension is due to resistance to the effects of aldosterone-salt treatment that normally result in increased vasoconstrictor sensitivity.

Cardiac myocyte necrosis induced by angiotensin II

Although the role of angiotensin II (Ang II) in the pathogenesis and progression of the failing heart is uncertain, previous reports have suggested that myocyte injury may be a component in this process. In this study, we investigated this possibility in more detail. Cardiotoxic effects of nonacutely hypertensive doses of Ang II were examined in 90 rats, including those receiving an angiotensin infusion (200 ng/min i.p.) and those with renovascular hypertension, where endogenous stimulation of Ang II occurred. Myocyte injury and wound healing resulting from these treatments were evaluated by 1) immunofluorescence after in vivo monoclonal antibody labeling of myosin to detect abnormal sarcolemmal permeability, 2) [3H]thymidine incorporation into DNA, to detect fibroblast proliferation, and 3) light microscopic evidence of myocytolysis and subsequent scar formation. We found that exogenous Ang II produced multifocal antimyosin labeling of cardiac myocytes and myocytolysis, which were maximal on days 1-2 of the infusion. Subsequently, DNA synthesis rates were increased, with fibroblast proliferation reaching peak levels on day 2 (Ang II-treated rats, 90.0 +/- 18.6 cpm/micrograms DNA; control rats, 11.4 +/- 2.3 cpm/micrograms DNA; p less than 0.05); microscopic scarring was found on day 14 and represented 0.12 +/- 0.02% of the myocardium. Concurrent treatment with both propranolol (30 mg/kg/day s.c.) and phenoxybenzamine (5 mg/kg/day i.m.) did not attenuate Ang II-induced antimyosin labeling. Increased endogenous Ang II, resulting from renal ischemia after abdominal aortic constriction, produced both antimyosin labeling and increased rates of DNA synthesis like that observed with Ang II infusion. Both myocyte injury and fibroplasia were prevented with captopril (65 mg/day p.o.), but this protective effect was not seen with reserpine pretreatment. Infrarenal aortic banding without renal ischemia, on the other hand, produced hypertension without necrosis. We conclude that pathophysiological levels of endogenous as well as low-dose exogenous Ang II were associated with altered sarcolemmal permeability and myocytolysis with subsequent fibroblast proliferation and scar formation. Myocyte injury was unrelated to the hypertensive or enhanced adrenergic effects of Ang II or to hypertension per se. Captopril was effective in preventing myocyte injury in renovascular hypertension. The mechanism(s) responsible for Ang II-induced necrosis will require further study.

Cardiac content of brain natriuretic peptide in DOCA salt-hypertensive rats

The cardiac content of immunoreactive rat brain natriuretic peptide (ir-rBNP) in deoxycorticosterone acetate (DOCA)-salt hypertensive rats was measured by radioimmunoassay (RIA). The atrial content of ir-rBNP was significantly lower in the DOCA-salt group than in the control group (p less than 0.01). However, the ventricular content of ir-rBNP was markedly increased in the DOCA-salt group as compared to the other groups. Ir-rBNP level in the atria was negatively correlated with other groups. Ir-rBNP level in the atria was negatively correlated with blood pressure (r = -0.49, p less than 0.01), while that in the ventricle was positively correlated with blood pressure (r = 0.79, p less than 0.001). A significant correlation was observed between tissue levels of ir-rBNP and ir-rat atrial natriuretic peptide (rANP) both in atrium and ventricle (atrium, r = 0.63, p less than 0.001; ventricle, r = 0.95, p less than 0.001). These results raise the possibility that rBNP as well as rANP functions as a cardiac hormone, the production of which probably changes in response to increased of body fluid and blood pressure.

Altered phospholipase activities related to alpha 1-adrenergic receptor supersensitivity of aortas from aldosterone-salt hypertensive rats

Many of the concepts presented in this paper are summarized in Fig. 7. Some aspects are well supported while others are speculative. The operation of PLC in VSM is well established, and in some hypertensive models (AHR, SHRSP) PLC assays exhibited altered activation. Currently this pathway leading to the production of IP3 and DAG is considered to be the major regulator of Ca release from sarcoplasmic reticulum (SR) and Ca entry by channels (CaC). Regulation of PKC by [Ca]i and DAG is thought to play a major role in controlling Ca entry. PKC has also been proposed to regulate PLA2 as well as PLD in conjunction with elevated [Ca]i. An important issue to be resolved is whether receptor regulation of other lipases occurs independently of the PLC-[Ca]i-PKC axis. Currently information supporting receptor regulation is lacking for VSM, but few studies have been conducted. Our observation that NE stimulation of PLD activity occurs in VSM indicates that the control of VSM by biochemical messengers is much more complicated than previously proposed. This seemingly redundant pathway may allow VSM to use alternate substrates for producing PA and DAG than are readily available to PLC. It also allows PA to be produced directly without phosphorylation of DAG. Although the role of PA in the regulation of Ca entry was proposed earlier, definitive studies establishing this linkage are still required. Any PLD activity on PIP2 would produce biochemical messengers (PA, DAG) which could stimulate Ca entry without producing the messenger, IP3, associated with Ca release (inactive IP2 would be produced). If PLC and PLD were independently regulated by receptor-guanine nucleotide-regulatory protein (G-protein) complexes, this would offer the potential for some agonists to excite VSM by Ca release and Ca entry mechanisms while others may excite by Ca entry alone. This system would also circumvent the problem of limited substrate for cellular regulation of [Ca]i if PIP2 were the primary substrate. This limitation does not exist with other phospholipids such as phosphatidylcholine which is a preferred substrate for PLD. The presence of multiple phospholipases under separate receptor regulation allows for a wider range of tissue responses to various agonists, than a system which is linked only through the PLC-[Ca]i-PKC axis. The presence of a PLD pathway also reopens the interpretation of previous studies which demonstrated a resetting between receptor occupancy and production of second messengers by PLC.(ABSTRACT TRUNCATED AT 400 WORDS)

Remodeling of the rat right and left ventricles in experimental hypertension

Pathological left ventricular hypertrophy in renovascular hypertension is associated with the accumulation of fibrillar collagen within the extracellular space and around intramyocardial coronary arteries. Even though the angiotensin converting enzyme inhibitor captopril was previously found to attenuate this interstitial and perivascular fibrosis, the relative importance of arterial and ventricular systolic pressures versus circulating angiotensin II (AII) and aldosterone (AL) in promoting hypertrophy and collagen accumulation in renovascular hypertension is uncertain. By drawing on the in-parallel arrangement of the right and left ventricles, with respect to their coronary circulation, and the in-series mechanical alignment of the ventricles, with a pressure-overloaded left and a normotensive right ventricle, this study sought to address this uncertainty. Three models of experimental hypertension, each having a different circulating AII and AL profile, were examined and compared with their controls: renovascular hypertension, where both AII and AL are increased; infrarenal aorta banding, where AII and AL are normal; and a chronic infusion of AL, where AII is suppressed or normal and AL is increased. In renovascular hypertension, as well as with AL, we found a significant rise in the interstitial collagen volume fraction and perivascular collagen area of the pressure-overloaded, hypertrophied left ventricle as well as the normotensive, nonhypertrophied right ventricle. This remodeling was not seen in either ventricle with infrarenal aorta banding despite comparable systemic hypertension and left ventricular hypertrophy. Thus, in experimental arterial hypertension in the rat, myocyte and nonmyocyte compartments of the myocardium are under separate controls: myocyte hypertrophy is most closely related to ventricular loading while circulating AII and AL, acting alone or in concert with other humoral factors, regulate the accumulation of collagen within the right and left ventricles.

Effects of antimineralocorticoid RU 26752 on steroid-induced hypertension in rats

The effect of mineralocorticoid antagonist RU 26752 on the development and maintenance of hypertension produced by long-term administration of mineralocorticoid agonist aldosterone has been investigated. Uninephrectomized, saline-drinking male Sprague-Dawley rats were subcutaneously implanted with either placebo (control) pellets or pellets containing 100 micrograms aldosterone, 50 mg RU 26752, or 100 micrograms aldosterone plus 50 mg RU 26752. Aldosterone treatment resulted in an increase in blood pressure to 165 +/- 5 mmHg over the control value of 105 +/- 2 mmHg within 3 wk of experimental period. RU 26752 given alone had no observable hypertensinogenic effect. However, RU 26752 administered with aldosterone significantly prevented the hypertension produced by aldosterone alone. RU 26752 when given with aldosterone was able to prevent the aldosterone-induced increase in saline consumption, increase urine output, and reduce urinary Na+ excretion. The results presented suggest that long-term administration of antimineralocorticoid RU 26752 in vivo to Sprague-Dawley rats prevents the aldosterone-induced hypertension.

Intracerebroventricular infusion of RU28318 blocks aldosterone-salt hypertension

The chronic intracerebroventricular (icv) infusion of aldosterone in rats and dogs elevates the blood pressure within 10-14 days at doses far below those that produce hypertension systemically. The effect in rats is dose dependent and blocked by the concomitant icv infusion of the antimineralocorticoid, prorenone. The effect of the icv infusion of RU28318, another specific spironolactone mineralocorticoid antagonist, on the hypertension produced by chronic subcutaneous (sc) administration of aldosterone in sensitized rats was reported. Miniosmotic pumps were used to deliver 1 micrograms/h aldosterone sc and 1.1 micrograms/h RU8318 icv. Over a 24-day period the indirect systolic blood pressure of the control, RU28318 icv, and aldosterone sc plus RU28318 icv groups increased from 105 to 123 mmHg and were not significantly different from each other, whereas the aldosterone sc group increased to 156 mmHg. RU28318, icv or sc, did not alter the increase in urine volume produced by aldosterone sc, and there was no significant differences in weight between the groups. This study provides evidence of the importance of the central nervous system in the pathogenesis of hypertension produced by systemic mineralocorticoid excess.

Prostanoids and aldosterone-induced mild experimental hypertension in rats

The goal of this study was to determine the role of prostanoids in a new model of mineralocorticoid-dependent hypertension induced by the subcutaneous infusion of aldosterone (1 micrograms/hr) to normal male Sprague-Dawley rats. This regimen caused a mild and gradual increase in systolic pressure over a period of 4 weeks (113 +/- 1 vs. 137 +/- 3 mm Hg) and was associated with an increase in the in vivo formation of prostaglandins I2 and E2 and of thromboxane A2 in the kidney. High sodium intake induced a fall in the urinary levels of prostaglandin E2 and a rise in the arterial pressure of control rats (126 +/- 1 vs. 113 +/- 1 mm Hg) but did not influence aldosterone-induced hypertension. Indomethacin (3.0 mg/kg/day) caused a profound inhibition of the in vivo synthesis of prostaglandin I2 and thromboxane A2 without modifying the renal production of prostaglandin E2. Although indomethacin exerted no effect on aldosterone-induced hypertension in rats fed a normal diet, it caused a further rise in systolic pressure in aldosterone-treated rats fed a high sodium diet (157 +/- 6 vs. 140 +/- 4 mm Hg). The results of this study in a model of aldosterone-induced mild hypertension in the rat indicate that 1) aldosterone exerts a stimulatory effect on the renal synthesis of prostanoid, particularly prostaglandin E2; 2) thromboxane A2 and prostaglandin I2 do not seem to play a role in aldosterone-induced hypertension under conditions of normal dietary salt intake, whereas the role of prostaglandin E2 is unclear; 3) there is enough sodium in a normal diet to allow for the maximal expression of the hypertensive effect of aldosterone; 4) prostaglandin I2 seems to play a significant role in modulating the cardiovascular impact of a high sodium diet in aldosterone-treated rats; and 5) the renal biosynthesis of prostaglandin E2 is particularly resistant to the inhibitory effect of indomethacin in vivo.

Calcium antagonists inhibit elevated potassium efflux from aorta of aldosterone-salt hypertensive rats

The purpose of this study was to evaluate the effect of calcium antagonists on basal tension and the elevated 42K efflux in aorta from aldosterone-salt hypertensive rats. Diltiazem decreased the basal tension (2.0 +/- 0.4 g) as well as the phasic contractile activity and returned the elevated 42K efflux (0.018 +/- 0.002/min) toward control values (0.010 +/- 0.001/min, p less than 0.001). The diltiazem median inhibitory concentration (IC50) for basal tension (0.04 +/- 0.02 microM), however, was sevenfold less than the IC50 for basal 42K efflux (0.22 +/- 0.08 microM, p less than 0.01). The basal 45Ca influx in aorta from aldosterone-salt hypertensive rats (120 +/- 4 microM/l cell H2O/min) was also decreased by diltiazem in a concentration-dependent manner, whereas the 45Ca influx in aorta from control-salt rats (135 +/- 3 microM/l cell H2O/min) was not altered. Similarly, the dihydropyridine nisoldipine eliminated the basal tension (2.7 +/- 0.5 g) and returned the elevated basal 42K efflux from the hypertensive aorta toward control levels (0.010 +/- 0.0003/min, p less than 0.001). The nisoldipine IC50 for basal tension (0.016 +/- 0.01 nM) was 160-fold less than the IC50 for basal 42K efflux (1.8 +/- 1.2 nm, p less than 0.001). Neither diltiazem nor nisoldipine nisoldipine altered the basal 42K efflux or contractile activity of aorta from control-salt rats. These results suggest that the basal tension and elevated 42K efflux in aorta from aldosterone-salt hypertensive rats are supported by the entry of extracellular calcium into the tissue through potential-operated calcium channels.

Studies on spirolactone steroid antagonists in ACTH-induced hypertension in sheep

This study investigated the anti-mineralocorticoid potency and haemodynamic effects of a series of mineralocorticoid antagonists of the spirolactone type (RU 28318, spironolactone, K-prorenoate, K-canrenoate and canrenone), for their ability to prevent the development of ACTH-induced hypertension in conscious sheep. In vivo bioassay, using aldosterone dependent changes in parotid salivary [Na+]/[K+] of sodium depleted adrenalectomized sheep, showed spironolactone was the most potent anti-mineralocorticoid tested. Infusions of the antagonists at equal doses alone for 4 days demonstrated that none affected mean arterial pressure, except for K-prorenoate which exhibited slight pressor activity. All the antagonists produced a natriuresis. Some of the steroid antagonists of the spirolactone group blocked the development of ACTH hypertension in sheep, spironolactone being the most effective. This study provides additional evidence for an essential mineralocorticoid component in ACTH-induced hypertension.

Nisoldipine inhibition of sodium influx into aorta from aldosterone-salt-hypertensive rats

The purpose of this study was to determine whether increased sodium (Na) influx into the aorta was associated with aldosterone-salt hypertension in the rat and, if present, to determine what mechanisms contributed to the increase. Basal 24Na influx was elevated in aorta from the hypertensive rats (2.21 +/- 0.10 mmol/l cell H2O/min, n = 25) compared with control-salt rats (1.75 +/- 0.04 mmol/l cell H2O/min, n = 24). The calcium (Ca) antagonist nisoldipine inhibited the Na influx into aorta from hypertensive rats in a concentration-dependent manner. At 10 nM nisoldipine, the Na influx in hypertensive rats (1.52 +/- 0.14 mmol/l cell H2O/min, n = 10) was similar to control rats (1.66 +/- 0.18 mmol/l cell H2O/min, n = 7). The basal Na influx in aorta from hypertensive rats was not altered by dichlorobenzamil or ethylisopropylamiloride, selective inhibitors of Na-Ca and Na-H exchange, respectively. The Na influx was 2.21 +/- 0.10, 2.03 +/- 0.24, and 2.11 +/- 0.19 mmol/l cell H2O/min for basal (n = 25), dichlorobenzamil (n = 4), and ethylisoproisopropylamiloride (n = 11), respectively. Inhibition of Na influx in hypertensive rats by 0.1 microM nisoldipine (delta Na influx = -0.72 +/- 0.18 mmol/l cell H2O/min, n = 9) was not significantly altered when applied with dichlorobenzamil (-0.72 +/- 0.21 mmol/l cell H2O/min, n = 4) or ethylisopropylamiloride (-0.55 +/- 0.15 mmol/l cell H2O/min, n = 11). These agents did not alter Na influx in control aorta.(ABSTRACT TRUNCATED AT 250 WORDS)

Digoxin enhances the pressor response to aldosterone administration in conscious sheep

1. This study examined the hypothesis that inhibition of Na,K ATPase with digoxin would enhance the pressor response to aldosterone infusion in conscious sheep. 2. While intravenous infusion of digoxin (10 micrograms/kg per day for 5 days) had no effect on blood pressure and aldosterone infusion (6 micrograms/kg per day for 5 days) increased blood pressure by 7 mmHg, combined infusion of digoxin and aldosterone increased blood pressure by 17 mmHg. 3. The metabolic effects of the combined digoxin and aldosterone infusion were similar to those for aldosterone alone, suggesting that digoxin did not enhance the mineralocorticoid action of aldosterone. 4. The results of this study suggest that changes in Na influx (aldosterone-dependent) and efflux (digoxin-dependent) are important in the genesis of aldosterone-induced hypertension.

Central administration of aldosterone increases blood pressure in rats

Experiments were designed to determine whether hypertension in rats caused by a central infusion of aldosterone requires supplemental sodium and uninephrectomy. Group 1 was uninephrectomized and received an intracerebroventricular (i.c.v.) infusion of aldosterone (9 ng/h) plus 1M NaCl, dissolved in 0.01% ethyl alcohol-artificial cerebrospinal fluid (vehicle). Group 2 received the same infusion but was not uninephrectomized. Group 3 received an i.c.v. infusion of aldosterone alone in vehicle. Group 4 received an i.c.v. infusion of vehicle with intravenous (i.v.) infusion of aldosterone plus NaCl. All rats received a diet of standard Purina rat chow and tap water ad libitum. Systolic blood pressure of groups 1 and 2 was significantly increased. Rats treated with i.c.v. aldosterone alone also showed a significant increase in blood pressure on day 21. However, i.v. infusion of the same dose of aldosterone did not change blood pressure. The results show that hypertension induced with chronic central infusion of aldosterone does not require uninephrectomy. We conclude that aldosterone may act directly within the central nervous system to increase blood pressure.

Altered biochemical and functional responses in aorta from hypertensive rats

Factors that lead to supersensitivity of vascular smooth muscle to norepinephrine during aldosterone-salt-induced hypertension in rats appear to reside beyond ligand-alpha-adrenergic receptor binding, which we have shown previously to be normal. The objective of this study was to determine whether significant shifts occur in the coupling between receptors and the production of putative second messengers. Measures of [3H]myo-inositol phosphates in aorta (endothelium removed) exhibited a concentration-dependent increase to norepinephrine, with the 50% response shifted significantly to the left in the hypertensive group (7.0 +/- 0.9 X 10(-7) M in 8 control rats vs 1.1 +/- 0.2 X 10(-7) M in 8 hypertensive rats; p less than 0.001). The production of [32P]phosphatidic acid was also shifted (6.5 +/- 2.5 X 10(-7) M in 16 control vs 1.9 +/- 0.8 X 10(-7) M in 12 hypertensive rats; p less than 0.05). The functional responses of 42K efflux and contraction to norepinephrine were also significantly shifted threefold to 15-fold in the hypertensive group (p less than 0.001), but the 50% response typically occurred at a 10 to 100 times lower concentration than that for the production of myo-inositol phosphates and phosphatidic acid. The amplification between receptor occupancy and functional responses apparently occurs beyond the production of phosphoinositide metabolites. The fivefold shift in the 50% response of biochemical end points for the hypertensive group accounted for most of the shift (sixfold) in the functional end points.(ABSTRACT TRUNCATED AT 250 WORDS)

Receptors and steroid-dependent hypertension

1. Repeated observations indicate that ACTH administration causes hypertension. 2. Development of hypertension requires 17 alpha-hydroxyprogesterone and 17 alpha,20 alpha-dihydroxy-4-pregnene-3-one to be present in association with other steroids. 3. The hypertensinogenic activity of corticosteroids is distinct from their glucocorticoid and mineralocorticoid effects. 4. The location of central and peripheral receptors for this hypertensinogenic activity is not clear. 5. The physiological mechanisms that mediate the response are unknown, though a number of potential mediating effects has been demonstrated. 6. The overall importance of unusual steroids and steroid actions in human essential hypertension still requires elucidation.

Effect of adrenocorticoid receptors on potassium and sodium flux in rat C6 glioma cells

C6 glioma cells contain two types of receptors for adrenocorticoids. Glucocorticoid (Type II) receptors are present at higher density and mediate increases in glycerol phosphate dehydrogenase and glutamine synthetase activity. The function of mineralocorticoid (Type I) receptors present at low density in C6 cells is unknown. Since mineralocorticoid (Type I) receptors in renal epithelial cells regulate cation transport, we sought to determine whether adrenocorticoid receptors located in glioma cells are similarly linked to electrolyte transporting activity. Occupation of mineralocorticoid receptors in C6 glioma by adrenocorticoids did not alter Na+ or K+ transport, in contrast to their effects on renal epithelial and vascular smooth muscle cells. Occupation of glucocorticoid receptors produced a 20-25% decrease in K+ uptake into C6 cells, but did not alter Na+ influx. Stimulation of Na+ influx with the ionophore monensin produced a large ouabain-sensitive increase in glucose utilization, as measured by 2-deoxyglucose uptake. However, mineralocorticoid receptor occupation did not alter glucose utilization, providing further evidence that these receptors do not influence Na+ transport in C6 cells. These studies provide evidence that mineralocorticoid receptors in glioma cells do not regulate Na+ or K+ transport. Glial glucocorticoid receptors have an inhibitory effect on glial K+ influx, which may contribute to glucocorticoid hormone effects on brain excitability.