Impact of the renin-angiotensin system on cerebral perfusion following subarachnoid haemorrhage in the rat

Angiotensin-(1-7) as a Potential Therapeutic Strategy for Delayed Cerebral Ischemia in Subarachnoid Hemorrhage

Aneurysmal subarachnoid hemorrhage (SAH) is a substantial cause of mortality and morbidity worldwide. Moreover, survivors after the initial bleeding are often subject to secondary brain injuries and delayed cerebral ischemia, further increasing the risk of a poor outcome. In recent years, the renin-angiotensin system (RAS) has been proposed as a target pathway for therapeutic interventions after brain injury. The RAS is a complex system of biochemical reactions critical for several systemic functions, namely, inflammation, vascular tone, endothelial activation, water balance, fibrosis, and apoptosis. The RAS system is classically divided into a pro-inflammatory axis, mediated by angiotensin (Ang)-II and its specific receptor AT₁R, and a counterbalancing system, presented in humans as Ang-(1-7) and its receptor, MasR. Experimental data suggest that upregulation of the Ang-(1-7)/MasR axis might be neuroprotective in numerous pathological conditions, namely, ischemic stroke, cognitive disorders, Parkinson's disease, and depression. In the presence of SAH, Ang-(1-7)/MasR neuroprotective and modulating properties could help reduce brain damage by acting on neuroinflammation, and through direct vascular and anti-thrombotic effects. Here we review the role of RAS in brain ischemia, with specific focus on SAH and the therapeutic potential of Ang-(1-7).

Central sympathetic nerve activation in subarachnoid hemorrhage

Subarachnoid hemorrhage (SAH) is a life-threatening condition, and although its two main complications-cerebral vasospasm (CVS)/delayed cerebral ischemia (DCI) and early brain injury (EBI)-have been widely studied, prognosis has not improved over time. The sympathetic nerve (SN) system is important for the regulation of cardiovascular function and is closely associated with cerebral vessels and the regulation of cerebral blood flow and cerebrovascular function; thus, excessive SN activation leads to a rapid breakdown of homeostasis in the brain. In the hyperacute phase, patients with SAH can experience possibly lethal conditions that are thought to be associated with SN activation (catecholamine surge)-related arrhythmia, neurogenic pulmonary edema, and irreversible injury to the hypothalamus and brainstem. Although the role of the SN system in SAH has long been investigated and considerable evidence has been collected, the exact pathophysiology remains undetermined, mainly because the relationships between the SN system and SAH are complicated, and many SN-modulating factors are involved. Thus, research concerning these relationships needs to explore novel findings that correlate with the relevant concepts based on past reliable evidence. Here, we explore the role of the central SN (CSN) system in SAH pathophysiology and provide a comprehensive review of the functional CSN network; brain injury in hyperacute phase involving the CSN system; pathophysiological overlap between the CSN system and the two major SAH complications, CVS/DCI and EBI; CSN-modulating factors; and SAH-related extracerebral organ injury. Further studies are warranted to determine the specific roles of the CSN system in the brain injuries associated with SAH.

The Cushing reflex and the vasopressin-mediated hemodynamic response to increased intracranial pressure during acute elevations in intraabdominal pressure

BACKGROUND

Abdominal compartment syndrome has been linked to detrimental hemodynamic side effects that include increased intracranial pressure and diminished renal function, but the mechanisms behind this continue to be elucidated. In this study, we sought to investigate any direct association between acute elevations in intra-abdominal pressure and intracranial hypertension during experimentally induced abdominal compartment syndrome and between acutely elevated intracranial pressure and the hemodynamic response that might be elicited by a vasopressin-induced Cushing reflex affecting urine osmolality and urine output. The aim of this study is to explain the Cushing reflex and the vasopressin-mediated hemodynamic response to intracranial pressure during acute elevations in intra-abdominal pressure.

METHODS

We measured intra-abdominal pressure, intrathoracic pressure, optic nerve sheath diameter as an indirect sign of intracranial pressure, vasopressin levels in blood, urine osmolality, and urine output at 4 time points during surgery in 16 patients undergoing sleeve gastrectomy for morbid obesity. Values for the 4 time points were compared by repeated-measures analysis of variance.

RESULTS

More than 50-fold elevations in serum vasopressin paralleled increases in optic nerve sheath diameter, rising throughout prepneumoperitoneum and tapering off afterward, in conjunction with a marked decrease in urine but not serum osmolality. Mean arterial pressure rose transiently during pneumoperitoneum without elevated positive end-expiratory pressure but was not significantly elevated thereafter.

CONCLUSIONS

These findings support our hypothesis that the oliguric response observed in abdominal compartment syndrome might be the result of the acutely elevated intra-abdominal pressure triggering increased intrathoracic pressure, decreased venous outflow from the central nervous system, increased intracranial pressure, and resultant vasopressin release via a Cushing reflex.

Complications extraneurologiques des hémorragies sous-arachnoïdiennes anévrismales

L’hémorragie sous-arachnoïdienne anévrismale (HSA) est une pathologie rare, touchant principalement la femme jeune en bonne santé. Cette pathologie est bien connue, ainsi que son évolution. Les HSA peuvent se compliquer de nombreuses complications d’ordre neurologique comme l’hydrocéphalie aiguë, le vasospasme, la comitialité, l’hypertension intracrânienne par exemple. Cependant, d’autres complications extracrâniennes peuvent aggraver le pronostic de cette pathologie. Les mécanismes principaux de ces complications extraneurologiques sont un stress catécholaminergique et le syndrome de réponse inflammatoire systémique. Ces complications peuvent être d’ordre cardiovasculaire (défaillance cardiaque, modification de l’ECG…), pulmonaire (œdème pulmonaire neurogénique, PAVM…) et métabolique (anomalies ioniques, hyperglycémie, insuffisance rénale).

The Cushing Reflex: Oliguria as a Reflection of an Elevated Intracranial Pressure

Oliguria is one of the clinical hallmarks of renal failure. The broad differential diagnosis is well known, but a rare cause of oliguria is intracranial hypertension (ICH). The actual knowledge to explain this relationship is scarce. Almost all literature is about animals where authors describe the Cushing reflex in response to ICH. We hypothesize that the Cushing reflex is translated towards the sympathetic nervous system and renin-angiotensin-aldosterone system with a subsequent reduction in medullary blood flow and oliguria. Recently, we were confronted with a patient who had complicated pituitary surgery and displayed multiple times an oliguria while he developed ICH.

Angiotensin II type 1 receptor antagonists in animal models of vascular, cardiac, metabolic and renal disease

We have reviewed the effects of angiotensin II type 1 receptor antagonists (ARBs) in various animal models of hypertension, atherosclerosis, cardiac function, hypertrophy and fibrosis, glucose and lipid metabolism, and renal function and morphology. Those of azilsartan and telmisartan have been included comprehensively whereas those of other ARBs have been included systematically but without intention of completeness. ARBs as a class lower blood pressure in established hypertension and prevent hypertension development in all applicable animal models except those with a markedly suppressed renin-angiotensin system; blood pressure lowering even persists for a considerable time after discontinuation of treatment. This translates into a reduced mortality, particularly in models exhibiting marked hypertension. The retrieved data on vascular, cardiac and renal function and morphology as well as on glucose and lipid metabolism are discussed to address three main questions: 1. Can ARB effects on blood vessels, heart, kidney and metabolic function be explained by blood pressure lowering alone or are they additionally directly related to blockade of the renin-angiotensin system? 2. Are they shared by other inhibitors of the renin-angiotensin system, e.g. angiotensin converting enzyme inhibitors? 3. Are some effects specific for one or more compounds within the ARB class? Taken together these data profile ARBs as a drug class with unique properties that have beneficial effects far beyond those on blood pressure reduction and, in some cases distinct from those of angiotensin converting enzyme inhibitors. The clinical relevance of angiotensin receptor-independent effects of some ARBs remains to be determined.

The harmful effects of subarachnoid hemorrhage on extracerebral organs

Subarachnoid hemorrhage (SAH) is a devastating neurological disorder. Patients with aneurysmal SAH develop secondary complications that are important causes of morbidity and mortality. Aside from secondary neurological injuries, SAH has been associated with nonneurologic medical complications, such as neurocardiogenic injury, neurogenic pulmonary edema, hyperglycemia, and electrolyte imbalance, of which cardiac and pulmonary complications are most common. The related mechanisms include activation of the sympathetic nervous system, release of catecholamines and other hormones, and inflammatory responses. Extracerebral complications are directly related to the severity of SAH-induced brain injury and indicate the clinical outcome in patients. This review provides an overview of the extracerebral complications after SAH. We also aim to describe the manifestations, underlying mechanisms, and the effects of those extracerebral complications on outcome following SAH.

Relationship between sympathetic nervous activity and inflammatory response after subarachnoid hemorrhage in a perforating canine model

The objective of the present study was to evaluate the correlation between sympathetic nerve activation and inflammatory response in the acute stage of subarachnoid hemorrhage (SAH) in a canine perforating model. SAH was induced by perforation of the basilar artery with the use of a microcatheter via the femoral artery in 20 mongrel dogs. Hemodynamic parameters and intracranial pressure were recorded, and blood sample for C3a, C5b-9, IL-6, IL-8 and noradrenaline kinetic determination were measured at 0, 5, 15, 30, 60, 120, and 180 min after SAH. Noradrenaline (pg/mL) increased abruptly from 104+/-59 to 2010+/-918 at 5 min after SAH. C3a and C5b-9 reached peak values at 15 min and IL-6 and IL-8 reached peak values at 30 min after SAH, respectively. The peak values of C3a and C5b-9 correlated positively with the peak value of noradrenaline (r=0.743 and r=0.753, respectively). The peak values of IL-6 and IL-8 also correlated positively with the peak values of noradrenaline (r=0.603 and r=0.681, respectively).These results suggest that a pronounced activation of the sympathetic nervous system and the inflammatory response occurs in acute stage of SAH. Significant association between the rate of spillover of norepinephrine to plasma and the plasma levels of inflammatory markers indicates that the two processes, sympathetic activation and immune response are quantitatively linked in early stage after SAH. The exact mechanisms underlying this phenomenon deserved further investigations.

Effects of common medications on cerebral vasospasm after subarachnoid haemorrhage

Cerebral vasospasm is a common and serious complication of aneurysmal subarachnoid haemorrhage (SAH). At present, no consistently effective preventative and therapeutic measures are available, perhaps because of incomplete understanding of the pathogenesis of vasospasm. Experimental studies provide evidence that the incidence and severity of vasospasm after SAH can be modulated by drugs that affect neurotransmitter levels, intracellular signalling mechanisms, vascular smooth muscle function, inflammation and cellular proliferation, and the concentration of 'spasmogenic' factors. Preliminary clinical studies indicate that some illicit drugs and common prescription medications can have similar effects in humans. Recognition of these pharmacological effects is important because medications that can worsen or alleviate vasospasm are frequently administered to SAH patients to treat coincident medical problems.

Systemic up-regulation of angiotensin II type 1 receptor in cardiac donors with spontaneous intracerebral hemorrhage

Donor spontaneous intracerebral hemorrhage (ICH) is a potential risk factor for morbidity and mortality after cardiac transplantation. We hypothesized that donor ICH is associated with systemic up-regulation of angiotensin II receptor type 1 (AT1R). We evaluated mRNA expression of AT1R and AT2R in donor spleen lymphocytes and in heart biopsies from 20 recipients of hearts from donors with spontaneous ICH which were compared with 20 recipients from trauma donors. Heart biopsies showed 4.7-fold increased mRNA expression of AT1R (p < 0.0001) in the ICH group compared with the Trauma group. The ICH group also showed 2.6-fold (p < 0.01) increased mRNA expression of AT1R in the donor spleen lymphocytes, suggesting the presence of systemic activation before transplantation. At 1 year, the ICH group had increased coronary vasculopathy by vascular ultrasound. Using multivariate regression analysis, mRNA expression of AT1R in the donor spleen lymphocytes was found to be a strong independent predictor of transplant vasculopathy (odds ratio = 4.397, CI = 1.243-15.553, adjusted p = 0.02). This is the first report to describe splenic up-regulation of AT1R in the presence of spontaneous ICH and its association with subsequent development of transplant vasculopathy.