Intracerebroventricular infusions of angiotensin II increases sodium excretion

Role of oxidative stress in the natriuresis induced by central administration of angiotensin II

Introduction. Central administration of angiotensin II (Ang II) is known to reduce urinary volume and to increase sodium and potassium excretion. Recently, a novel signalling mechanism for Ang II in the periphery has been shown to involve reduced nicotinamide adenine dinucleotide phosphate [NAD(P)H] oxidase-derived reactive oxygen species (ROS).Although ROS are now known to be involved in numerous Ang II-regulated processes in peripheral tissues, and are increasingly implicated in CNS neurodegenerative diseases, the role of ROS in central regulation of Ang II-induced hydromineral metabolism remains unexplored.The hypothesis that ROS are involved in central Ang II signalling and in Ang II-dependent antidiuresis, natriuresis and kaliuresis was tested by the use of selective antagonists of the NAD(P)H oxidase cascade. Materials and methods. In intracerebroventricular (ICV)-cannulated rats,Ang II was injected ICV and urinary sodium and potassium excretion was assessed at 1-, 3-, and 6-hour periods of urine collection. Urine sample was analysed for sodium and potassium concentration using a flame photometer. The role of NAD(P)H oxidase-dependent signalling cascade was evaluated using the selective NAD(P)H oxidase inhibitor, apocynin; the superoxide dismutase mimetic, 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (tempol); and the protein kinase C inhibitor, chelerythrine. Results. ICV administration of Ang II to conscious hydrated rats resulted in a significant decrease in urinary volume in the first hour, and an increased sodium and potassium excretion during the 6-hour period of urine collection, which was most effective during the 3 and 6 h. Interference with the NAD(P)H oxidase signalling by central administration of apocynin, tempol or chelerythrine, blunted the natriuretic and kaliuretic effect induced by central administration of Ang II, without affecting its antidiuretic action.

Conclusion.This study demonstrates that increases of urinary sodium and potassium excretion elicited by central administration of Ang II are mediated by NAD(P)H oxidase- dependent production of superoxide and protein kinase C activation in conscious hydrated rats.

Neurohumoral mechanism in the natriuretic action of intracerebroventricular administration of renin

INTRODUCTION

Intracerebroventricular (i.v.t.) administration of renin (R) decreases urinary volume and increases urinary sodium excretion. We investigated whether i.v.t.-R-induced natriuresis could be associated with the release of atrial natriuretic peptide (ANP), its interaction with renal ANP-A receptors (ANPR-A) and the subsequent increase of urinary cyclic 3-5 guanosine monophosphate (cGMP).

METHODS

In i.v.t. cannulated rats, the left carotid artery was catheterised with PE-50 tubing for blood collection, renin was injected i.v.t. and arterial blood samples were collected at 0, 2, 5, 10 and 15 minutes of injection, and urinary sodium and cGMP excretion at 1-, 3- and 6-hour periods of urine collection. Plasma ANP levels and urinary cGMP were determined by radioimmunoassay, and each urine sample was analysed for sodium concentration using a flame photometer.

RESULTS

Our results demonstrate that i.v.t.-R administration increases plasma ANP levels after two minutes of injection and urinary cGMP concentration at 1-, 3- and 6 hour period of urine collection. The natriuretic action induced by i.v.t.-R was blunted by peripheral administration of anantin, an ANPR-A antagonist. We assessed the role of brain angiotensin II (Ang II) AT1-receptors on the i.v.t.-induced antidiuresis, natriuresis and cGMP urinary excretion, the last as an indirect index of ANP secretion. Blockade of brain Ang II AT1-receptors with losartan (LOS; 120 microg/3 microl, i.v.t.), inhibited the antidiuretic action and blocked the increased urinary sodium and cGMP excretion induced by i.v.t.-R (7.14 mGU/5 microl). The increase in urinary cGMP was independent of nitric oxide synthase stimulation, since L-NAME pre-treatment did not alter the renal actions induced by i.v.t.-R.

CONCLUSIONS

Our results suggest that there is a link between the brain and the kidney. The activation of brain angiotensinergic neurons and stimulation of AT1- receptors may stimulate the release of ANP to the circulation. The released ANP circulates to the kidneys where it acts through renal ANPR-As and the consequent increase in cGMP to produce natriuresis.

Brain angiotensin and body fluid homeostasis

Angiotensinogen, the precursor molecule of the peptides angiotensin I, II, and III, is synthesized in the brain and the liver. Evidence is reviewed that angiotensin II, and possibly angiotensin III, that are generated within the brain act within neural circuits of the central nervous system to regulate body fluid balance. Immunohistochemical studies in the rat brain have provided evidence of angiotensin-containing neurons, especially in the hypothalamic paraventricular nucleus, subfornical organ, periventricular region, and nucleus of the solitary tract, as well as in extensive angiotensin-containing fiber pathways. Angiotensin immunoreactivity is observed by electron microscope in synaptic vesicles in several brain regions, the most prominent of these being the central nucleus of the amygdala. Neurons in many parts of the brain (lamina terminalis, paraventricular and parabrachial nuclei, ventrolateral medulla, and nucleus of the solitary tract) known to be involved in the regulation of body fluid homeostasis exhibit angiotensin receptors of the AT(1) subtype. Pharmacological studies in several species show that intracerebroventricular administration of AT(1) receptor antagonist drugs inhibit homeostatic responses to the central administration of hypertonic saline, intravenous infusion of the hormone relaxin, or thermal dehydration. Responses affected by centrally administered AT(1) antagonists are water drinking, vasopressin secretion, natriuresis, increased arterial pressure, reduced renal renin release, salt hunger, and thermoregulatory adjustments. We conclude that angiotensinergic neural pathways in the brain probably have an important homeostatic function, especially in regard to osmoregulation and thermoregulation, and the maintenance of arterial pressure.

Role of brain angiotensin II in the somatosensory induced antinatriuresis in the anaesthetized rat

1. The present study set out to explore the importance of angiotensin (Ang)II in the brain in allowing the somatosensory system to cause a reflex renal nerve-mediated reduction in renal sodium and water excretion. 2. In chloralose-urethane-anaesthetized rats receiving saline i.c.v. (2 microL + 1 microL/h), the administration of capsaicin (0.5 mg, s.c.) increased blood pressure by 14% (P < 0.001) and, while renal perfusion pressure was regulated at an unchanged level, neither renal blood flow (RBF) nor glomerular filtration rate was changed. However, urine flow and absolute and fractional sodium excretion was reduced between 29 and 38% (P<0.05-0.01). All variables had returned to control levels 30 min later. 3. The administration of captopril (40 microg + 20 microg/h i.c.v.) decreased blood pressure and sodium excretion by 6 and 17%, respectively (both P < 0.05). Under these conditions, capsaicin s.c. increased blood pressure by 9% (P<0.05); however, with renal perfusion pressure regulated at a constant level, neither renal haemodynamics nor water nor sodium excretion were changed. 4. A final group of animals received AngII (100 ng + 50 ng/h) concomitantly with captopril i.c.v., which increased blood pressure, RBF and urine flow, absolute and fractional sodium excretions by 8 (P < 0.05), 22 (P < 0.001 ) and 52-149% (P < 0.05-0.01), respectively. Capsaicin given s.c. under these conditions increased blood pressure by 6% (P < 0.05) and, while renal perfusion pressure was maintained at an unchanged value and renal haemodynamics remained constant, urine flow and absolute and fractional sodium excretion were reduced by 35-38% (all P < 0.05). 5. These data show that for the somatosensory system to induce a reflex increase in renal sympathetic nerve activity sufficient to cause an antinatriuresis and antidiuresis, the presence of AngII is necessary in the brain. How AngII exerts this facilitatory action within the central nervous system remains to be investigated.

Central administration of zinc increases renal sodium and potassium excretion in rats

The aim of the present study was to investigate the effect of acute third ventricle injections of zinc on the brain control of renal sodium and potassium excretion. Adult Wistar male rats received third ventricle injections of zinc acetate in three different doses (0.03, 0.3 and 3.0 nmol/rat). Third ventricle administration of zinc acetate provoked a significant intensification of natriuresis and kaliuresis as compared to sodium acetate-treated controls. When rats were pretreated with losartan, a selective angiotensin II AT1 receptor antagonist (10.8 nmol/rat into the third ventricle 10 min before central zinc injection) the increase in both natriuresis and kaliuresis was abolished. Furthermore, pretreatment with gadolinium, a calcium channel blocker (0.3 nmol/rat into the third ventricle 20 min before central zinc injection), also blunted the increase in renal sodium and potassium excretion seen in animals receiving zinc alone. In a group of rats receiving the same water load used in the previous experiments, the injection of zinc acetate into the third ventricle (3.0 nmol/rat) did not modify arterial blood pressure. It is suggested that zinc in the central nervous system may be involved in the control of renal sodium and potassium excretion by a mechanism unrelated to blood pressure increase. It is also shown that both natriuretic and kaliuretic actions of zinc depend on AT1 receptor activation. Whatever should be the mechanism(s) related to the central effects of zinc here evidenced, the functional integrity of calcium channels is required.

Haemorrhage increases the pressor effect of angiotensin-(1-7) but not of angiotensin II at the rat rostral ventrolateral medulla

OBJECTIVE

To evaluate the effects of angiotensins acting at the rostral ventrolateral medulla (RVLM) on the cardiovascular adjustments following haemorrhage.

DESIGN

Changes in mean arterial pressure (MAP) and heart rate (HR) produced by micro-injections of angiotensin II (Ang II) and angiotensin (Ang)-(1-7) and different angiotensin antagonists into the RVLM of anaesthetized rats submitted to haemorrhage, were determined.

METHODS

Experiments were performed in 79 urethane-anaesthetized male Wistar rats. Ang-(1-7) (2.5 and 25 pmol), Ang II (25 pmol), [Sar1,Thr8]-Ang II (non-selective angiotensin antagonist, 0.2 nmol), A-779 (Ang-(1-7) antagonist, 0.1 nmol), losartan (AT1 Ang II receptor antagonist, 0.2 nmol) or vehicle (200 nl) were bilaterally micro-injected into the RVLM under basal conditions or 30 min after blood withdrawal (0.6 ml/100 g bodyweight). In additional groups, [Sar1,Thr8]-Ang II, A-779, losartan or vehicle were micro-injected into the RVLM 10 min before bleeding to uncover a possible role of endogenous peptides during haemorrhage.

RESULTS

The pressor effect produced by Ang II micro-injection was not altered by haemorrhage. Conversely, haemorrhage significantly increased the magnitude and duration of the pressor effect of Ang-(1-7) at the RVLM. The fall in MAP induced by haemorrhage was similar after micro-injection of vehicle or A-779. However, micro-injection of [Sar1,Thr8]-Ang II significantly reduced the fall in MAP after haemorrhage. A similar finding was obtained with micro-injection of losartan. In addition, while RVLM micro-injection of [Sar1,Thr8]-Ang II or losartan 30 min after blood withdrawn produced MAP changes that were similar to that observed in control animals, micro-injection of A-779 did not significantly alter baseline MAP.

CONCLUSIONS

These results suggest that changes in the RVLM reactivity to Ang-(1-7) but not Ang II may contribute to the haemodynamic adjustments triggered by acute reductions in blood volume. The data obtained with [Sar1,Thr8]-Ang II and losartan suggest a primary inhibitory role for endogenous Ang II at the RVLM during haemorrhage.

Central lead administration induces natriuretic and kaliuretic effects in rats

The aim of the present experiments was to discern whether central acute lead injections affect brain control of renal function. Adult Wistar male rats received third-ventricle injections of lead acetate in three different doses (0.03, 0.3, and 3.0 nmol/rat). Lead acetate induced a significant increase in renal excretion of sodium and potassium. Pretreatment with losartan, a selective angiotensin II AT1 receptor antagonist (10.8 nmol/rat into the third ventricle 10 min before central lead injection), inhibits lead-induced natriuretic and kaliuretic effects. In addition, pretreatment with gadolinium, a calcium-channel blocker (0.3 nmol/rat into the third ventricle 20 min before central lead administration), reversed the increase in renal excretion of sodium and potassium provoked by central lead administration. Taken together, the data presented here suggest that lead injected into the third ventricle increases renal excretion of sodium and potassium by a mechanism that depends on the functional integrity of central angiotensin II AT1 receptors and calcium channels.

Brain angiotensin and circulatory control

1. Components of the renin-angiotensin system (RAS) are found in the brain; both outside and inside the blood-brain barrier. 2. Almost all of the classical actions of the brain RAS are attributable to angiotensin (Ang) II and mediated by AT1 receptors. 3. Circumventricular organs (CVO), which lack the blood-brain barrier, are rich in AngII receptors and monitor circulating AngII levels. In vivo binding studies suggest that the CVO are also accessible to cerebrospinal fluid-derived AngII. 4. The median preoptic nucleus, paraventricular hypothalamic nucleus, supraoptic nucleus, nucleus tractus solitarius and ventrolateral medulla are inside the blood-brain barrier and are sites of action of brain AngII. In these nuclei, AngII seems to act as an excitatory neurotransmitter or neuromodulator. 5. Actions of AngII in the brain, both inside and outside the blood-brain barrier, are implicated in the central regulation of blood pressure and sympathetic outflow, release of hypothalamic and pituitary hormones and renal sodium handling. 6. Alterations in the activity of brain AngII may be involved in the mechanisms of some types of hypertension.

Cardiovascular and renal effects of intracerebroventricular angiotensin II in conscious sheep

Effects on systemic and pulmonary haemodynamics, renal electrolyte excretion, and plasma concentration of vasopressin, catecholamines, electrolytes and proteins in response to intracerebroventricular infusions of [Val5]-angiotensin II (ANG II) at 1, 2 and 4 pmol kg-1 min-1 in isotonic saline for 30 min were studied in conscious sheep (n = 6). Vehicle control infusions were performed in four of the animals. All three doses of ANG II were expected to increase CFS concentration of the peptide above physiological levels. All ANG II infusions were noticed to be dipsogenic, but the animals were not allowed to drink freely until at the end of the experiments (at 120 min post-infusion). The systemic arterial blood pressure increased significantly only in response to 2 and 4 pmol kg-1 min-1, concomitant with an increase of the systemic vascular resistance, whereas the cardiac output and heart rate remained unchanged. The central venous pressure increased only after administration of the highest ANG II dose, while pulmonary artery, and capillary wedge pressures were unaffected during all experiments. The plasma protein and K concentration fell in response to ANG II administration. Also here, the effects were significant only at 2 and 4 pmol kg-1 min-1. The plasma levels of vasopressin, noradrenaline, adrenaline and dopamine did not change significantly in response to any of the infusions. The renal Na excretion increased by 100-400%, but not in a strictly dose-dependent manner. Much smaller and more variable effects were seen on the renal K excretion.(ABSTRACT TRUNCATED AT 250 WORDS)

The renal and vascular effects of central angiotensin II and atrial natriuretic factor in the anaesthetized rat

1. The interaction between atrial natriuretic factor (ANF) and angiotensin II (Ang II) within the brain to influence renal function and blood pressure was studied in Inactin-anaesthetized male Sprague-Dawley rats. 2. Central infusion of ANF produced a diuresis which was associated with a significant decrease in plasma arginine vasopressin (AVP) level. There was no change in sodium excretion rate over the 80 min of intracerebroventricular ANF infusion and ANF produced no detectable change in mean arterial blood pressure. 3. Central Ang II administration produced a significant decrease in urine flow, which was associated with elevated plasma AVP, an increase in sodium excretion and a rise in mean arterial blood pressure. 4. Combined ANF and Ang II infusion produced an antidiuresis, which was associated with increased plasma AVP concentration. Both the natriuretic and vasopressor actions of central Ang II were abolished when ANF was co-administered. 5. It is concluded that ANF and Ang II interact centrally; ANF antagonizes the pressor and natriuretic effects but not the antidiuretic effects of central Ang II. These data suggest the possibility of distinct and separate sites within the brain through which Ang II influences vasopressin release and renal sodium handling and elevates blood pressure.

Atrial natriuretic peptide (ANP) as a neuropeptide: interaction with angiotensin II on volume control and renal sodium handling

1. Angiotensin II (ANG II) and atrial natriuretic peptide (ANP) are functionally antagonistic circulating hormones involved in blood pressure and body fluid regulation. An inappropriate atrial secretion of ANP has been implicated in the pathogenesis of hypertension, but clinical and experimental results on the role of ANP in hypertension are still conflicting. 2. In the brain both peptides have been localized in close proximity, preferentially in areas involved in central cardiovascular, electrolyte and volume control. ANP was shown to inhibit ANG II-induced drinking, release of pituitary hormones and natriuresis, and to induce sodium retention when given alone. 3. These findings suggest that also in the brain ANG II and ANP exert functionally antagonistic effects. However, in contrast to their peripheral effects, ANG II induces natriuresis while ANP appears to cause antinatriuresis in the brain.

Differential effects of central angiotensin II and substance P on sympathetic nerve activity in conscious rats. Implications for cardiovascular adaptation to behavioral responses

The centrally induced effects of angiotensin II and substance P on the cardiovascular system and on neuronal efferent activity of the splanchnic, renal, and adrenal nerves were investigated in chronically instrumented conscious rats. The pressor responses to substance P injected into the lateral brain ventricle were accompanied by marked and short latency increases in heart rate, cardiac output, splanchnic, renal, and adrenal nerve activity, and a rise in plasma noradrenaline and adrenaline. Behaviorally, an arousal-type reaction was observed. In contrast, the pressor responses to intracerebroventricular angiotensin II were associated with initial decreases in heart rate, cardiac output, splanchnic, renal, and adrenal nerve activity, and a fall in plasma noradrenaline at the time of the maximal blood pressure increase. In some but not all animals, a second blood pressure peak associated with increases in heart rate and splanchnic nerve activity was observed after several minutes. Incomplete chronic sinoaortic baroreceptor deafferentiation prevented the angiotensin II-induced fall in heart rate but not the initial fall in splanchnic nerve activity. The decreases in splanchnic nerve activity also occurred in diabetes insipidus rats and persisted in Long Evans rats after vascular vasopressin receptor blockade with d(CH2)5AVP, despite marked reductions of the pressor responses in both groups. Peripheral alpha-adrenoceptor blockade with prazosin or ganglion blockade with hexamethonium inhibited the central angiotensin II pressor responses only in combination with vasopressin receptor blockade. On the other hand, either sympatholytic drug, alone, abolished the pressor responses in the diabetes insipidus rats. This indicates that in intact conscious rats the central pressor effects of angiotensin II are initiated by vasopressin release but become dependent on the sympathetic nervous system when vasopressin is absent or not effective. When rats were allowed to drink in response to angiotensin II, a further sharp rise in blood pressure occurred, together with increases in heart rate and splanchnic nerve activity. The results demonstrate fundamental differences in the mechanisms by which central pressor peptides can influence cardiovascular and autonomic function. It is conceivable that the distinct sympathetic response patterns to central angiotensin II and substance P receptor stimulation form part of a specific cardiovascular adjustment to the individual behavioral reactions, such as drinking, as in the case of angiotensin II, or arousal within the central processing of pain, as in the case of substance P.

Humoral responses to intracerebroventricularly administered angiotensin II in dogs

The mechanism(s) by which intracerebroventricularly administered angiotensin II (ANG II) regulates aldosterone production was investigated in dogs with chronically implanted cannula into a lateral cerebroventricle. In salt-replete and salt-depleted dogs, artificial cerebrospinal fluid (CSF) with or without ANG II (1, 10, 100 ng X kg-1 X min-1) was infused intracerebroventricularly for 2 h under pentobarbital anesthesia. Artificial CSF produced no significant humoral changes. Intracerebroventricular ANG II decreased plasma renin activity and increased both ACTH and plasma cortisol in both groups but decreased plasma aldosterone (PA) only in salt-depleted dogs. Dexamethasone pretreatment during intracerebroventricular ANG II decreased PA further in salt-replete but not in salt-depleted dogs. Moreover, the fall in PA during intracerebroventricular ANG II in salt-depleted dogs was prevented when intravenous infusion of ANG II (10 ng X kg-1 X min-1) was given simultaneously to maintain circulating ANG II levels. We conclude that PA response to intracerebroventricular ANG II is mediated primarily through the renin-angiotensin system in the salt-depleted state; however, in the salt-replete state, ACTH assumes a more important role.