Analysis of the actions of angiotensin on the central nervous system of conscious dogs

Angiotensin II (ANG II) acts on the brain to elevate blood pressure (BP), stimulate drinking, increase the secretion of vasopressin and corticotropin (ACTH), and inhibit the secretion of renin. The present studies were designed to evaluate the possible physiological significance of these effects. The experiments were performed in conscious dogs with small catheters chronically implanted in both carotid and both vertebral arteries. ANG II was infused into both carotid or both vertebral arteries in doses of 0.1, 0.33, 1.0, and 2.5 ng.kg-1.min-1. Intravertebral ANG II produced dose-related increases in BP that were generally accompanied by increases in heart rate. Intracarotid angiotensin also increased BP but did not change heart rate. Intracarotid ANG II stimulated drinking and, at the highest dose only, increased the secretion of vasopressin, ACTH, and corticosteroids. Intravertebral and intracarotid ANG II suppressed plasma renin activity (PRA). In a parallel series of experiments, the effects of intravenous ANG II, in doses of 2, 5, 10, and 20 ng.kg-1.min-1, were studied. These infusions produced dose-related increases in BP and water intake and suppressed PRA. Only the highest dose of ANG II increased vasopressin or corticosteroid secretion. Analysis of these results in terms of calculated or measured changes in plasma ANG II concentration indicate that the central cardiovascular and dipsogenic actions of angiotensin, as well as the suppression of PRA, can be elicited by concentrations of the peptide that are within the physiological range. On the other hand high, probably supraphysiological, levels of ANG II are required to increase vasopressin or ACTH secretion.     

Angiotensin II is a growth factor in the peri-implantation rat embryo

Angiotensin II (ANG II) is increasingly recognised as a growth factor, both in its own right and through interactions with other growth factors. There is a high density of ANG II receptors in the rat fetus, especially the AT2 receptor, the function of which is still uncertain. We have now studied the effects of ANG II on growth and development in the rat embryo in vitro between d 9.5 and 11.5, and characterised the receptor subtype mediating these effects. Embryos were cultured in whole rat serum, a high molecular weight retenate after ultrafiltration of whole rat serum, retenate with angiotensin II and retenate with ANG II and AT1 or AT2 receptor blockers. Growth and development were scored using conventional methods. Culture in retenate was associated with a marked reduction in growth and development by comparison with whole rat serum. This was partly, and significantly ( P <0.001), reversed by angiotensin II. The optimum concentration of angiotensin II was found to be angiotensin II 10⁻¹¹ M , within the physiological range. Angiotensin II had highly significant effects on both somatic ( P <0.001) and yolk sac/allantoic ( P <0.005) development. The latter effects suggest a role for angiotensin II in placentation. The effects of angiotensin II were blocked by PD123319, an AT2 blocker, but not by GR117289, an AT1 blocker. Interestingly, culture in retenate with GR117289 without added angiotensin II was also associated with some increase in growth ( P <0.05). Angiotensin II in low concentrations was measurable in the retenate, presumably arising from the action of endogenous renin on angiotensinogen. We therefore postulate that this effect of GR117289 was due to the action of endogenous angiotensin II on 'uncovered' AT2 receptors. This study has thus demonstrated a direct growth promoting effect of angiotensin II during organogenesis in the whole rat embryo in vitro. This effect is mediated through the AT2 receptors.     

Angiotensin AT1 receptor antagonist losartan and the defence reaction in the anaesthetised rat. Effect on the carotid chemoreflex

Modulation at the level of the nucleus tractus solitarii (NTS) appears to be an effective way of controlling cardiovascular reflexes. Angiotensin II acting on angiotensin AT1 receptors at the central nervous system appears to have an important role in these modulatory processes. The hypothalamic defence area (HDA) is a potential source of descending fibres containing angiotensin II that innervate the NTS. We investigated the effect of AT1 receptor blockade in the NTS on the response to stimulation of HDA in anaesthetised rats treated with the neuromuscular blocking agent pancuronium bromide. The characteristic increase in heart rate, blood pressure and phrenic nerve activity evoked by electrical stimulation of HDA is decreased by the microinjection of the AT1 receptor antagonist losartan into the NTS and the cardiovascular response to carotid body chemical stimulation is also reduced. These results support the hypothesis that AT1 receptors in the NTS play a role in the modulation of cardiovascular reflexes, and modify the influence exerted on the processing of these reflexes by other areas of the central nervous system.     

The effect of angiotensin II and vasopressin on renal haemodynamics

A comparison was made of the vascular actions of two hormones having a renal site of action, angiotensin II and vasopressin, using laser Doppler flowmetry to measure perfusion of the cortical and papillary regions of the kidney. Angiotensin II infusion caused dose-related increases in blood pressure and reductions in cortical perfusion, the latter responses being potentiated in the presence of the converting enzyme inhibitor, cilazapril. However, angiotensin II had no effect on papillary perfusion either before or following cilazapril. The reasons for this differing vasoconstrictor ability of angiotensin II at the cortex and papilla are unclear, but it could be due to medullary generation of prostaglandin or bradykinin. Administration of equipressor doses of vasopressin caused graded reductions in both cortical and papillary perfusions, and subsequent cilazapril significantly enhanced the papillary responses. This study demonstrates that the regulation of blood flow through the different regions of the kidney can be differentially regulated by the peptide hormones angiotensin II and vasopressin.     

Effects of angiotensin II on blood flow in rat submandibular gland

The effect of angiotensin II (Ang II) was studied on blood flow in the submandibular gland and tongue in male rats. Blood flow changes were determined with laser Doppler flowmetry and Ang II was infused into the common carotid artery before and after i.v. doses (18 nmol kg-1) of the angiotensin II antagonist saralasin. Angiotensin II (10-60 pmol min-1) dose-dependently increased blood pressure and tongue blood flow, whereas glandular blood flow decreased at all of the doses used. After saralasin administration the angiotensin II effects on blood pressure, tongue and glandular blood flow were significantly diminished (glandular blood flow reduction was diminished from 29%-3%, P less than 0.005, n = 9). However, the responsiveness of these 3 parameters to local infusions with noradrenaline (0.75-3.0 pmol min-1) was unaffected by saralasin. The dose of saralasin used in the present study did not affect any of the parameters on it's own. Our results show that vascular receptors sensitive to angiotensin II operate in the submandibular gland but not in the tongue.     

Effect of angiotensin II on parotid saliva secretion in conscious sheep.

Intravenous infusion of angiotensin II over the dose range 3-20 microgram/h for 15 min caused a dose-dependent reduction in parotid saliva secretion and increase in arterial blood pressure in conscious sheep. The blood levels of angiotensin II contrived by these infusions were probably within the physiological range for sheep. Infusion of angiotensin II (3 microgram/h) into the carotid artery ipsilateral to the parotid gland under study caused greater reduction in saliva secretion rate than an equivalent infusion of angiotensin II into the contralateral carotid artery. This result suggests a direct effect of angiotensin II at the parotid, possibly by a constrictor action on its vasculature or by altering water and electrolyte transport by the gland. In sodium-deplete sheep, intravenous infusion of the angiotensin antagonist saralasin (1 mg/h for 1 h) caused transient increase of saliva flow for 20-30 min. It is suggested that angiotensin II may have a physiological role in regulating parotid saliva secretion during sodium depletion.     

Effects of angiotensin ll on blood flow in rat submandibular gland

The effect of angiotensin II (Ang II) was studied on blood flow in the submandibular gland and tongue in male rats. Blood flow changes were determined with laser Doppler flowmetry and Ang II was infused into the common carotid artery before and after i.v. doses (18 nmol kg^-l) of the angiotensin II antagonist saralasin. Angiotensin II (10–60 pmol min^-l) dose-dependently increased blood pressure and tongue blood flow, whereas glandular blood flow decreased at all of the doses used. After saralasin administration the angiotensin II effects on blood pressure, tongue and glandular blood flow were significantly diminished (glandular blood flow reduction was diminished from 29%-3%, P < 0.005, n = 9). However, the responsiveness of these 3 parameters to local infusions with noradrenaline (0.75–3.0 pmol min^-1) was unaffected by saralasin. The dose of saralasin used in the present study did not affect any of the parameters on it's own. Our results show that vascular receptors sensitive to angiotensin II operate in the submandibular gland but not in the tongue.     

The effect of angiotensin, noradrenaline and vasopressin on blood flow distribution in the rat kidney

1. The effect of val(5)-angiotensin II amide, noradrenaline and vasopressin, on kidney volume and intrarenal distribution of carbon particles and thioflavine S was examined in the rat.2. Angiotensin produced a dose-dependent shrinkage of the kidney coinciding with the rise in systemic blood pressure. Noradrenaline and vasopressin, however, produced reduction in kidney volume only in much higher doses than were necessary to produce a pressor effect.3. An intravenous infusion of angiotensin sufficient to produce a diuretic response resulted in a striking increase in glomerular content of injected carbon particles, and a marked reduction in filling of the capillary plexuses of the subcortex and outer medulla. The reduction in outer medullary filling was also observed using the thioflavine S technique.4. Noradrenaline infused in amounts sufficient to produce diuresis, aortic constriction above the kidney and vasopressin injection produced no measurable change in carbon particle distribution.5. The reduction in capillary blood flow produced by angiotensin may result in impaired tubular reabsorptive capacity by reducing peritubular removal of reabsorbate, or by reducing oxygen availability. Thus the vasoconstrictor effects of angiotensin may explain its diuretic action.     

Do angiotensin-induced changes in sheep renal venous blood and plasma composition indicate a reversal of glomerular filtration?

Angiotensin II markedly lowers the relative cell content of renal venous blood both with injection into the jugular vein and into the left ventricle of anaesthetised sheep; angiotensin I has a similar effect in this regard when given intravenously but the decreased cellular fraction is much less evident with left ventricular injection when the direct presentation to the kidneys precludes prior intra-pulmonary conversion to angiotensin II. The angiotensins also produce readily demonstrable alterations in renal venous plasma electrolyte, protein, creatinine and urea levels. This pattern of changes is related to associated intra-renal haemodynamics, reverse filtration of proximal tubular fluid, a possible anti-diuretic action of angiotensin I itself on the peritubular capillaries and acute renal failure in man and experimental animals.     

Drinking induced by injections of angiotensin into forebrain and mid-brain sites of the monkey

1. Unilateral and bilateral injections of 1.0 mul. solutions of angiotensin II into specific brain sites produced copious drinking of water in the water-replete rhesus monkey (Macaca mulatta).2. Of six brain regions in seven monkeys into which a total of 368 microinjections of angiotensin II were made, three were sensitive to angiotensin II. In decreasing order of sensitivity, they were (i) a rostral zone that included the septum, the anterior hypothalamus and the preoptic region, (ii) a caudal zone consisting of the mesencephalic central grey, and (iii) the lateral and third ventricles near the foramen of Monro. Of the regions tested, those that were relatively inactive included (i) the mid line thalmus, (ii) the mid-brain reticular formation, and (iii) metencephalic points in the cerebellum, the 4th ventricle and the dorsal aspect of the pons.3. Bilateral microinjections of angiotensin II into the sensitive regions in doses as low as 0.75-6 ng were dipsogenic and, with increasing doses, drinking occurred in a dose-dependent fashion up to 500 ng, after which the amount drunk levelled off or was reduced. The dose-response curve for unilateral microinjections began at 12.5 ng, and at doses higher than 50 ng unilateral and bilateral microinjections were equipotent.4. The onset of drinking (without eating) averaged 2.1-3.2 min following the end of microinjections for all sensitive tissue sites. Injections into the ventricles produced significantly longer drinking latencies.5. Angiotensin I elicited drinking in amounts comparable to angiotensin II at a dose of 100 ng whereas analogues of angiotensin II were weak dipsogens. Of the three analogues tested, Phe(4), Tyr(8)-angiotensin II was the most potent dipsogen, followed by Ile(8)-angiotensin II. The 1-7 heptapeptide, des-Phe(8)-angiotensin II was an ineffective dipsogen. Carbachol microinjected into the most sensitive angiotensin drinking sites had no dipsogenic action in the water-replete monkey.6. Tachyphylaxis to angiotensin II was demonstrated as a reduction in mean water intake of 55 and 74 per cent on the second and third microinjections, respectively. This reduction appeared to be due to dilutional inhibition or signals from the amount of water ingested on the first microinjection of angiotensin II.7. Monkeys drank an amount equal to a normal daily intake following two to three microinjections of angiotensin II in doses of 100-250 ng into sensitive regions. This extra water load caused no reductions in normal daily water intake either for the remainder of the experimental day or 24 hr later.8. Pre-treatments with microinjections of an angiotensin-converting enzyme inhibitor, SQ 20,881, did not reduce the dipsogenic action of angiotensin I, suggesting that this and perhaps other peptide precursors act directly on receptor mechanisms to produce drinking. Attempts to change the polydipsic effects of angiotensin II were unsuccessful with pre-treatments of intracranial microinjections of either haloperidol, Ile(8)-angiotensin II or carbachol.9. Microinjections of angiotensin II dissolved in hypertonic saline solutions had no influence on water intake when compared with the same dose dissolved in distilled water or isotonic saline.10. Yawning was the only other response that appeared to be related directly to intracranial injections of angiotensin II. In some instances, a hyperactive state of the animal followed intraventricular injections of angiotensin II. In other instances, intracranial microinjections of angiotensin II were followed by quietude or e.e.g. and behavioural signs of light sleep.11. This work further confirms the findings of previous research which showed that angiotensin II is the most potent dipsogen in all species tested to date. This endogenous peptide appears to participate in natural thirst by acting on central mechanisms of extracellular thirst.     

Separation of drinking and pressor responses to central angiotensin by monoamines

This study investigated the neurotransmitters involved in the increase in blood pressure and drinking produced when angiotension II is injected intraventricularly (ivt). Using pharmacologic manipulations of the monoamines norepinephrine, dopamine, and serotonin it has been possible to separate the pressor response from dipsogenic responses to angiotension II. Alpha-adrenergic blockade with phentolamine restricted to the brain blocked the pressor response to angiotensin II in a dose-related manner, while drinking remained unaffected. Norepinephrine alone, injected into the ventricles elevated blood pressure, but did not produce drinking. The norepinephrine effect was also blocked by phentolamine by the same ventricular route. Other monoamines were not involved. Dopamine alone did not produce thirst. Cardiovascular effects with dopamine were observed only with large doses. The dopaminergic agonist apomorphine produced no change in blood pressure or drinking. Reduction of central serotonin stores by p-chlorophenylalanine intraperitoneally or 5,7-dihydroxytryptamine intraventricularly had no effect on the pressor or dipsogenic effects of angiotensin II. The serotonin agonist N,N-dimethyl-5-methoxytryptamine ivt did not produce a rise in blood pressure or drinking. It is concluded that the pressor effect of angiotensin II, but not the drinking effect is mediated by noradrenergic stimulation of alpha-receptors. The drinking response does not appear to be mediated by the monoamines.     

Aldosterone

Aldosterone is one the representative cardiovascular hormones involved in the blood pressure and body-fluid homeostasis. Elevation of aldosterone leads to systemic hypertension through its action on the mineralocorticoid receptor (MR) in the kidney. More recent studies demonstrated that aldosterone may produce target organ damage through its direct actions on the non-epithelial MR of the heart in addition to its systemic effects. Clinical experience in primary aldosteronism supports the concept that aldosterone is a risk factor of cardiovascular complications, since concentric type of cardiac hypertrophy is most common in primary aldosteronism among various types of endocrine hypertension. Clinical mega-trial in congestive heart failure (RALES study, EPHESUS study) demonstrated blocking angiotensin II action is not sufficient for cardioprotection unless aldosterone action is equally blocked. An important phenomenon related to this issue is the aldosterone breakthrough which implies a reelevation of plasma aldosterone during chronic administration of ACE inhibitors and Angiotensin receptor antagonists. Normal level of aldosterone could still be a risk factor. Combination of ACE inhibitor or ARB with aldosterone antagonist could result in a better cardioprotection in cardiovascular diseases. Although spironolactone has been the only one aldosterone antagonist, a new antagonist eplerenone has been developed. Eplerenone is specific to MR and is practically devoid of the major side effect gynecomastia of spironolactone. Another topic of aldosterone is its very quick cardiovascular effect presumably via a non-genomic action. All these recent findings support that this adrenocortical steroid hormone is as important as angiotensin II. Determining aldosterone levels is therefore much morel important than before in the diagnosis and treatment of cardiovascular diseases.     

Central Dopamine Receptors Regulate Blood Eosinophilia in the Rat

Dopaminergic agents, dopa and apomorphine, affected biphasically the blood eosinophil count in the rat: low doses of the drug elevated, while high doses lowered it. The response to a high dose of dopa was retained in rats pretreated with an inhibitor of dopamine-beta-hydroxylase, U 10, 157, but prevented by a centrally acting dopa decarboxylase inhibitor, NSD 1015. This indicates that the eosinopenia observed after large doses of dopa is due to the action of dopamine formed from the precursor. As intracerebroventricular injections of Ldopa also produce eosinopenia, the central site of dopamine action is indicated. The eosinopenic response to apomorphine was antagonized by a dopamine receptor blocking agent, haloperidol. This indicates that some central dopamine receptors are involved in the regulation of the eosinophil count in circulating blood. The hypophysis seems to play a crucial role in the phenomenon observed, as no eosinopenia was produced by dopa in hypophysectomized rats.     

The effect of reserpine on the vegetative reflexes

Summary Experimental investigations ascertained the selective nature of reserpine action on the interoceptive reflexes. The reflexes from the carotid sinus and the central section of the vagus were the least resistant to the effect of this preparation. In a series of experiments the reflex from the tibial nerve not only shows no inhibition, but becomes markedly intensified.The experiments also proved that the negetatíve reflexes are inhibited irrespective of the reserpine effect on the efferent pathways of the interoceptive reflexes and receptor formation. Its effect on the vegetative reflexes is evidently caused by the action upon the central apparatuses of the blood circulation.Presented by Active Member AMN SSSR V. V. Zakusov     

Cardiovascular responses in apnoeic asphyxia: role of arterial chemoreceptors and the modification of their effects by a pulmonary vagal inflation reflex

1. In the spontaneously breathing anaesthetized dog, the systemic circulation was perfused at constant blood flow; there was no pulmonary blood flow and the systemic arterial blood P(O2) and P(CO2) were controlled independently by an extracorporeal isolated pump-perfused donor lung preparation. The carotid and aortic bodies were separately perfused at constant pressure with blood of the same composition as perfused the systemic circulation.2. Apnoeic asphyxia, produced by stopping the recipient animal's lung movements and, at the same time, making the blood perfusing the systemic circulation and the arterial chemoreceptors hypoxic and hypercapnic by reducing the ventilation of the isolated perfused donor lungs, caused an increase in systemic vascular resistance.3. While the systemic arterial blood was still hypoxic and hypercapnic, withdrawal of the carotid and aortic body ;drive' resulted in a striking reduction in systemic vascular resistance. Re-establishing the chemoreceptor ;drive' immediately increased the vascular resistance again.4. Apnoeic asphyxia carried out while the carotid and aortic bodies were continuously perfused with oxygenated blood of normal P(CO2) had little or no effect on systemic vascular resistance.5. The systemic vasoconstrictor response produced by apnoeic asphyxia was reduced or abolished by re-establishing the recipient animal's lung movements, and this effect occurred in the absence of changes in the composition of the blood perfusing the systemic circulation and arterial chemoreceptors. This abolition of the vasoconstriction was due to a pulmonary reflex.6. Apnoeic asphyxia slowed the rate of the beating atria due to excitation of the carotid and aortic body chemoreceptors. This response can be over-ridden by an inflation reflex arising from the lungs.7. It is concluded that the cardiovascular responses observed in apnoeic asphyxia are due, at least in part, to primary reflexes from the carotid and aortic body chemoreceptors engendered by arterial hypoxia and hypercapnia. The appearance of these responses is, however, dependent upon there being no excitation of a pulmonary (inflation) vagal reflex.     

Reflex prolongation of stage I of expiration

Experiments were performed on anesthetized cats to test the theory that the interval between phrenic bursts is comprised of two phases, stage I and stage II of expiration. Evidence that these represent two separate neural phases of the central respiratory rhythm was provided by the extent to which stage duration is controlled individually when tested by superior laryngeal, vagus and carotid sinus nerve stimulation. Membrane potential trajectories of bulbar postinspiratory neurons were used to identify the timing of respiratory phases.Stimulation of the superior laryngeal, vagus and carotid sinus nerves during stage I of expiration prolonged the period of depolarization in postinspiratory neurons without significantly changing the durations of either stage II expiratory or inspiratory inhibition, indicating a fairly selective prolongation of the first stage of expiration. Changes in subglottic pressure, insufflation of smoke into the upper airway, application of water to the larynx or rapid inflation of the lungs produced similar effects. Sustained tetanic stimulation of superior laryngeal and vagus nerves arrested the respiratory rhythm in stage I of expiration. Membrane potentials in postinspiratory, inspiratory and expiratory neurons were indicative of a prolonged postinspiratory period. Thus, such an arrhythmia can be described as a postinspiratory apneic state of the central oscillator. The effects of carotid sinus nerve stimulation reversed when the stimulus was applied during stage II expiration. This was accompanied by corresponding changes in the membrane potential trajectories in postinspiratory neurons.The results manifest a ternary central respiratory cycle with two individually controlled phases occurring between inspiratory bursts.     

Blood flows in the maxillocarotid anastomoses and internal carotid artery of conscious dogs

Although the external carotid artery is known to contribute to the cerebral blood flow in anesthetized dogs, quantitative information on the anastomoses and their role in conscious dogs is lacking. This study was carried out to determine blood flows in these anastomoses and the internal carotid artery, and also to examine the functional significance of the anastomoses in conscious dogs. Fifteen-micron radioactive microspheres were injected into common and external carotid arteries of four conscious dogs through chronically implanted catheters. Blood flows were determined by the reference sample method and by comparing microsphere distributions in the brain and the masseter muscle. Blood flows were estimated to be 140 +/- 32, 7.7 +/- 1.4, and 3.3 +/- 1.1 ml/minute (mean +/- SD) in the common carotid artery, internal carotid artery, and anastomoses on each side, respectively. Additional evidence indicated that the anastomotic flow so determined was primarily the flow in the anastomotic artery. Humoral responses to angiotensin II infusions were also studied in conscious dogs. External carotid angiotensin increased plasma 11-hydroxycorticosteroid concentration (used as an index of ACTH secretion) but did not increase plasma vasopressin concentration to the same extent as common carotid infusion. Therefore, the external carotid artery is functionally important in perfusing the brain in conscious dogs.     

Monocyte chemoattractant protein-1 or macrophage inflammatory protein-1alpha deficiency does not affect angiotensin II-induced intimal hyperplasia in carotid artery ligation model.

BACKGROUND: Angiotensin II (Ang II) promotes atherosclerotic vascular diseases, in which proinflammatory and proliferative effects play a major pathogenic role. Ang II up-regulates chemokines, such as monocyte chemoattractant protein (MCP)-1 and macrophage inflammatory protein (MIP)-1alpha, which are important pro-inflammatory factors mediating infiltration of inflammatory cells into atherosclerotic lesion. The aim of the present study was to determine whether the presence of MCP-1 or MIP-1alpha is essential in Ang II-induced intimal hyperplasia in the carotid artery ligation model. METHODS: Six-month-old male C57BL/6-, MCP-1-, or MIP-1alpha-deficient mice underwent ligation of the common left carotid artery and were randomly assigned to receive either vehicle or Ang II (1.4 mg kg(-1) day(-1)) via a subcutaneously implanted osmotic infusion pump (model 2004, Alzet) for 4 weeks. RESULTS: Ang II not only increased MCP-1 and MIP-1alpha production but also enhanced neo-intimal formation, media thickness, and adventitia development in the ligated carotid arteries in C57BL/6 mice. However, MCP-1 or MIP-1alpha deficiency failed to affect intimal hyperplasia in vascular remodeling. CONCLUSION: These results indicate that MCP-1 or MIP-1alpha may not be essential in mediating the proliferative effects of Ang II, a major pathological changes in intimal hyperplasia in the carotid artery ligation model.