A pressor substance in the cerebrospinal fluid of normotensive and hypertensive patients

Reduction of angiotensin II in the cerebrospinal fluid of patients with multiple sclerosis

We previously demonstrated that angiotensin II acts as a crucial neuroprotective factor after neural injury through angiotensin II type-2 (AT2) receptor signaling. Although the pathway is known to play an important role in the development of experimental autoimmune encephalomyelitis, cerebrospinal fluid (CSF) angiotensin II levels in patients with multiple sclerosis (MS) have never been studied. To clarify the significance of angiotensin II in MS, we assayed angiotensin II concentrations using an established enzyme-linked immunoabsorbent assay in CSF samples from patients with MS ( n = 21), patients with inflammatory neuropathies (IN) ( n = 23) and control individuals who did not have either of the neurological diseases or any other disease that might affect the angiotensin II levels in the CSF (control) ( n = 24). Angiotensin II levels in the CSF were 3.79 ± 1.54 pg/ml in the MS group, 5.13 ± 2.27 pg/ml in the IN group and 6.71 ± 2.65 pg/ml in the control group. The angiotensin II levels in the CSF of the MS group were significantly lower than in the control group ( p = 0.00057). Angiotensin II concentration in the CSF tended to have a negative correlation with the Kurtzke’s Expanded Disability Status Scale scores during MS relapse ( p = 0.0847). These findings suggest that reduced levels of intrathecal angiotensin II may be related to the abnormal neural damage and repair processes in MS.

Hypertensive mechanisms and converting enzyme inhibitors

The introduction of angiotensin-converting enzyme (ACE) inhibitors marks a new era in the understanding and treatment of high blood pressure. Although the benefits of therapy with ACE inhibitors are documented, it is more difficult to isolate the principal mechanisms that account for their effective actions in the treatment of hypertension. Recent data suggest that these agents affect both the pressor and depressor mechanisms that regulate vascular tone and cardiac function. For example, ACE inhibitors decrease angiotensin II-mediated vasoconstriction, reduce adrenal medullary catecholamine release, restore baroreceptor activity, and normalize vasomotor sympathetic activity. Experimental work indicates that ACE inhibitors also act on hypertensive mechanisms via actions on the central nervous system. Cardiovascular centers in the brain have receptor sites for angiotensin II and contain the proteins required for local synthesis of angiotensins. Vasomotor neurons possess such receptors and exhibit ACE activity. In addition, angiotensin II receptors involved in the regulation of baroreceptor activity are present in neuronal elements of the baroreflex arc. It is suggested that ACE inhibitors reach the brain via circumventricular organs to reduce sympathetic activity and enhance baroreceptor sensitivity. New studies suggest that depressor actions of ACE inhibitors include enhanced biosynthesis of vasodilator prostaglandins. From animal experiments it is deduced that enhanced production of angiotensin-(1-7) after inhibition of ACE stimulates release of vasodilator prostaglandins. These investigations clarify the function of tissue renin-angiotensin systems in the control of blood pressure in both normal and hypertensive states.

Atrial natriuretic peptide, angiotensin, norepinephrine and electrolyte in cerebrospinal fluid of essential hypertension

We determined concentrations of atrial natriuretic peptide (ANP), angiotensin (Ang), norepinephrine (NE) and electrolyte in plasma and cerebrospinal fluid (CSF) to study possible roles of these substances within the brain in human hypertension. Blood and CSF samples were obtained from 10 patients with mild to moderate essential hypertension (EHT) aged 40-65 y and 10 age-matched normotensive subjects (NT) on a regular salt diet (8 g/day). Levels of ANP, NE, Na, K, Ca and Cl in CSF and plasma were comparable between EHT and NT. Plasma renin activity, plasma and CSF Ang II were lower in EHT than NT. CSF Ang III tended to be lower in EHT. There was no correlation between CSF and plasma ANP, or between CSF and plasma Ang II. Our results indicate that CSF levels of ANP may not be altered in middle aged patients with mild to moderate hypertension. It is also suggested that Ang II, NE and sodium in the central nervous system may not have important roles in hypertension of those patients.

Saralasin blocks the effect of angiotensin II and extracellular fluid saline expansion on the Na-K-ATPase inhibitor release in rats

A low molecular weight substance which behaves like ouabain as inhibitor of brain membrane Na-K-ATPase and 3H-ouabain binding was found in plasma after saline expansion of extracellular fluid or angiotensin II infusion into the third brain ventricle in the rat. Intracerebroventricular infusion of angiotensin II antagonist, saralasin, blocks the increase of the Na-K-ATPase inhibitor produced by infusion of angiotensin II into the third ventricle or extracellular fluid saline expansion.

Angiotensinogen levels in the brain and cerebrospinal fluid of the genetically hypertensive rat

The present experiments were designed to document changes in the regional distribution of angiotensinogen in the rat brain with the development of hypertension in spontaneously hypertensive rats (SHR) relative to age-matched normotensive Wistar-Kyoto rats (WKY). Levels of angiotensinogen were measured in discrete brain nuclei and cerebrospinal fluid from rats at 4, 7, and 16 weeks of age and in cerebrospinal fluid obtained by cisternal puncture at 7 and 16 weeks. Age-dependent changes in angiotensinogen were found, with levels higher in both strains at 4 weeks of age compared with 7 or 16 weeks. In contrast, plasma levels of angiotensinogen were essentially the inverse of the brain levels, low at 4 weeks and higher at 7 and 16 weeks. Levels in a number of regions adjacent to the rostral third ventricle from the 4-week-old SHR (prehypertensive phase) were significantly elevated relative to the WKY (p less than 0.05), while levels in the amygdala and posterior hypothalamus were significantly lower in the SHR (p less than 0.05). In 7-week-old rats (evolving phase), levels in nine brain regions were significantly elevated in the SHR relative to the WKY and included the nucleus tractus solitarii (p less than 0.01). Unlike the prehypertensive and evolving phases, in 16-week-old rats (maintenance phase) only two brain areas, the nucleus of the diagonal band and the lateral hypothalamus, had significantly elevated levels in the SHR (p less than 0.05). Cerebrospinal fluid levels of angiotensinogen did not correlate well with brain levels of angiotensinogen.(ABSTRACT TRUNCATED AT 250 WORDS)

Low activity of angiotensin-converting enzyme in cerebral microvessels of young spontaneously hypertensive rats

Angiotensin-converting enzyme (ACE) activity was measured in microvessels prepared from cerebral cortices of 4-week-old spontaneously hypertensive rats (SHR). The Vmax value of the ACE activity in the cerebral microvessels of SHR was lower than that of Wistar Kyoto controls of the same age by 25% without difference in Km value for substrate. The low activity of ACE in the cerebral microvessels of young SHR indicates that in this animal model of hypertension the function of ACE is genetically altered in the cerebral microvessels, which may be correlated with the alteration of the cerebral microcirculation and pathogenesis of hypertension.

Angiotensin-converting enzyme activity is reduced in brain microvessels of spontaneously hypertensive rats

Angiotensin -converting enzyme (ACE) activity in brain microvessels of spontaneously hypertensive rats (SHR) and Wistar Kyoto (WKY) controls was measured. Cerebral microvessels, prepared from the cerebral cortices by the albumin flotation and glass bead filtration technique, were free of neuronal and glial elements. ACE activity in brain microvessels of SHR was lower than that of WKY. A Woolf - Augustinsson -Hofstee plot showed that the reduction of the enzyme activity in SHR was due to a 30% decrease in Vmax, without any change in Km for substrate. The decrease of ACE activity in brain microvessels of SHR may indicate an impairment of the central renin-angiotensin system and may be related to cerebral microvascular dysfunctions occurring in hypertension.

Catecholamines, angiotensin II and sodium concentrations in cerebrospinal fluid in young men with borderline hypertension

To evaluate the role of central nervous mechanisms and their relationships to the peripheral sympathetic nervous system in borderline hypertension, we measured catecholamines, angiotensin II (AII) and sodium (Na) concentrations in cerebrospinal fluid (CSF) with plasma catecholamines concomitantly in 12 young men with borderline hypertension and 7 age-matched healthy normotensive men on ordinary salt intake. Plasma norepinephrine (NE) and epinephrine (E) were higher in the borderline hypertensives than in the normotensives (NE: 239 +/- 15 vs 190 +/- 11 pg/ml, p less than 0.05, E: 83 +/- 9 vs 43 +/- 6 pg/ml, p less than 0.01). NE levels in CSF were also higher in the borderline hypertensives than in the normotensives (200 +/- 15 vs 150 +/- 18 pg/ml, p less than 0.05). In most of the subjects, CSF E and plasma and CSF dopamine levels were below the sensitivity of the assay. CSF NE correlated positively with both plasma NE (p less than 0.01) and mean blood pressure (p less than 0.05) in all subjects. Immunoreactive AII and Na concentrations in CSF did not differ between the borderline hypertensives and normotensives. These results suggest that peripheral sympathoadrenal overactivity in young subjects with borderline hypertension may be related to an altered function of central noradrenergic neurons. AII and Na in the central nervous system do not appear to have an important role in borderline hypertension.

Evidence for the existence of a family of biologically active angiotensin I-like peptides in the dog central nervous system

A family of angiotensin I-like peptides has been derived from endogenous precursors present in dog cerebrospinal fluid after incubation with species homologous renin. These peptides are immunologically and pharmacologically similar to [Ile5]angiotensin I, and have molecular weights ranging between 1300 and 2200 daltons. The presence of precursors in the cerebrospinal fluid able to generate various biologically active angiotensin I-like peptides dissimilar to plasma angiotensin I supports the concept of a local angiotensin I-forming system in the brain.

Exaggerated salt appetite of spontaneously hypertensive rats is decreased by central angiotensin-converting enzyme blockade

Injection of angiotensin II (AII) into the cerebral ventricles at doses as low as 1 pmol h-1 results in a marked stimulation of salt and water ingestion in the rat. Evidence that AII is produced in the central nervous system independently of the circulating renin-angiotensin system (RAS) raises the possibility that endogenous brain AII is involved in the physiological regulation of thirst. The role of brain AII in salt appetite is still unclear. Here we confirm that the spontaneously hypertensive rat (SHR), believed to have elevated levels of brain AII, possesses an exaggerated salt appetite compared with its normotensive controls. We also show that this exaggerated salt appetite is reduced by chronic central treatment with the angiotensin-converting enzyme inhibitor, captopril, while that of the normotensive controls is unaffected. Our study suggests that a central neuropeptide, probably AII, is involved in the maintenance of the exaggerated salt appetite in this model of hypertension.

Angiotensin-like immunoreactivity in the brain of the spontaneously hypertensive rat

Evidence for the brain renin-angiotensin system being involved in the hypertension of the spontaneously hypertensive rat (SHR) includes central administration of angiotensin II (AII) antagonists and converting enzyme inhibitors that lower blood pressure in SHR. Using the unlabeled antibody enzyme method, we have found a significant difference in the distribution of brain angiotensin in SHR and Wistar-Kyoto controls (WKY). Six rats of each group were perfused with buffered picric acid-paraformaldehyde, and their brains sectioned at 50 and 100 mu. The sections were reacted with a 1:1000 dilution of AII antiserum for 36 hours followed by goat antirabbit immunoglobulin G and rabbit peroxidase antiperoxidase. For controls, preabsorption with AII, arginine vasopressin or preimmune serum were evaluated. The results showed over twice as many cells and fibers staining for AII-like immunoreactivity in SHR. The AII immunoreactive cell bodies were localized, in the order of their relative preponderance, in supraoptic and paraventricular nuclei of the hypothalamus, hippocampus, and cortex. The most prominent demonstration of AII-like immunoreactivity was observed in fiber profiles containing densely stained varicosities, which were present in many neuroanatomical subdivisions of the brain and brain stem including anterior and middle hypothalamus, basal ganglia, thalamus, locus coeruleus, nucleus of the solitary tract, limbic structures, and reticular formation. The increased fiber staining in the SHR was particularly evident in the frontal hypothalamic region, medial preoptic, and stria terminalis. We conclude that the results support the hypothesis of brain AII involvement in hypertension.

Hypotensive action of captopril and saralasin in intact and anephric spontaneously hypertensive rats

Intravenous injection of the converting enzyme inhibitor SQ14,225 (captopril, 2 mg/kg) reduced the blood pressure of anesthetized, spontaneously hypertensive rats (SHR) progressively over a 3-hour period. An indistinguishable fall in blood pressure occurred in SHR that were bilaterally nephrectomized 1 hour prior to injection of the converting enzyme inhibitor. In the nephrectomized animals, plasma renin activity (PRA) had fallen to less than 30% of its initial values at the time of injection. Injection of the vehicle alone had no effect on blood pressure in either anephric or intact SHR. The converting enzyme inhibitor produced no significant change in the blood pressure of either intact or anephric normotensive Wistar-Kyoto (NT-WK) rats. Infusions of Sar1-Ala8-angiotensin II (saralasin, 10 micrograms/kg-1/min-1) similarly reduced blood pressure of both intact and anephric SHR. These results indicate that captopril and saralasin lower blood pressure in the SHR by some mechanism(s) independent of the kidneys, circulating renin, or bradykinin potentiation. It is suggested that angiotensin II, locally produced at some critical tissue site(s), is involved in the maintenance of raised blood pressure in SHR.

Independence of the central nervous and the peripheral renin-angiotensin systems in the dog

What regulates the activity of the central nervous renin-angiotensin system is not known. To define whether control of this central system is linked to that in the periphery, simultaneous blood and cerebrospinal fluid (CSF) samples for measurement of immunoreactive angiotensin II were drawn from anesthetized dogs during hemorrhage, furosemide-induced volume depletion, insulin-hypoglycemia, beta-adrenergic blockade and saline infusion. Despite vigorous increments or decrements in plasma innunoreactive angiotensin II, CSF levels remained stable. Since immunoreactive angiotensin II in dog CSF is claimed to be mainly the heptapeptide des-Asp1-angiotensin II (angiotensin III), the possibility that the level of this peptide within CSF simply reflects plasma concentrations was assessed by infusing angiotensin III (2.5 and 25 ng/kg/min intravenously, each for 60 minutes) and monitoring plasma and CSF peptide levels. Whereas plasma immunoreactive angiotensin II levels increased appropriately across the infusions, no change in CSF levels was observed. These studies indicate the angiotensin III does not cross the blood-CSF barrier, at least in the short term.

Angiotensin regulates release and synthesis of serotonin in brain

Angiotensin II released serotonin from neuron terminals and accelerated synthesis of the serotonin. This increase in synthesis depended on the activation of tryptophan hydroxylase. A biphasic effect was observed: at high doses the stimulatory effect depended on conversion of angiotensin II to angiotensin III. At low doses an inhibitory effect was found, possible dependent on an angiotensin II metabolite. These actions represent a subtle regulation of the open-loop serotonin system.

Reduction of plasma renin activity by centrally-administered angiotensin II in anesthetized cats

Cerebroventricular administration of angiotensin II elevated arterial pressure and reduced plasma renin activity (PRA) in renal venous blood in anesthetized cats. Further, there was an increase in the concentration of urinary sodium, as well as the rate of Na+ excretion. Acute renal denervation significantly reduced the effects of central angiotensin II on the PRA and on the natriuresis. The data suggested involvement of a neurogenic component in these effects. Further, it appears that there may be a reciprocal relationship between CSF renin activity and PRA.

Increased blood pressure responses to central angiotensin II in spontaneously hypertensive rats

The blood pressure responses following infusions of angiotensin II (ANG II) into the brain ventricles (i.v.t.) have been tested in spontaneously hypertensive (SH) rats and in normotensive Wistar Kyoto (WK) rats. The mean arterial blood pressure increases were significantly higher in SH rats than in WK rats. Propranolol treatment reduced blood pressure increases to i.v.t. ANG II in WK, but not in SH rats. The higher sensitivity to i.v.t. ANG II in SH rats supports a role of central ANG II in the maintenance of high blood pressure in SH rats.

Humoral and neurohormonal aspects of blood pressure regulation: focus on angiotensin

Angiotensin circulates in the blood as a hormone. Its main target organs are vascular smooth muscle, adrenal gland and the kidney. Hormonal angiotensin increases blood pressure by its vasoconstrictor action, by stimulation of aldosterone secretion and subsequent sodium and water retention, and by the stimulation of catecholamine release. Circulating plasma angiotensin also effects brain mechanisms of blood pressure regulation. In addition to this hormonal function, angiotensin is present in the brain as part of an endogenous brain renin-angiotensin system. Brain angiotensin is not secreted into the blood and can be considered a neurohormone with local function. A role of brain angiotensin in the maintenance of high blood pressure of spontaneously hypertensive rats has been demonstrated. Circulating plasma angiotensin appears to influence brain renin levels and vice versa. Stimulation of specific areas in the brain known to be involved in the regulation of the cardiovascular system, stimulate renin secretion from the kidney. The renin-angiotensin system can therefore serve as an example for the intimate interrelationship between humoral and neurohumoral mechanisms of blood pressure regulation.

The ventricular system in neuroendocrine mechanisms. III. Supraependymal neuronal networks in the primate brain

This investigation has utilized a correlative scanning-transmission electron microscopic technique in the analysis of the primate cerebral ventricular system. This approach has demonstrated a complex network of supraependymal cellular elements upon the walls of the third cerebral ventricle in direct contact with the ventricular lumen. Type I neuronal-like cells and type II histiocytic-like cells with potential phagocytic capabilities have been observed in large numbers throughout the third ventricle. Type I neuron-like cells are discussed in the context that they may represent a population of receptor-cells which serve to assess ambient changes in the composition of bioactive peptides in the cerebrospinal fluid and may serve as a supraependymal network that integrates the endocrine hypothalamus with other circumventricular organs which may also be sites of neuroendocrine transduction.