Angiotensin converting enzyme inhibition and the upper limit of cerebral blood flow autoregulation: effect of sympathetic stimulation

The history of Danish neuroscience

The history of Danish neuroscience starts with an account of impressive contributions made at the 17th century. Thomas Bartholin was the first Danish neuroscientist, and his disciple Nicolaus Steno became internationally one of the most prominent neuroscientists in this period. From the start, Danish neuroscience was linked to clinical disciplines. This continued in the 19th and first half of the 20th centuries with new initiatives linking basic neuroscience to clinical neurology and psychiatry in the same scientific environment. Subsequently, from the middle of the 20th century, basic neuroscience was developing rapidly within the preclinical university sector. Clinical neuroscience continued and was even reinforced during this period with important translational research and a close co-operation between basic and clinical neuroscience. To distinguish 'history' from 'present time' is not easy, as many historical events continue in present time. Therefore, we decided to consider 'History' as new major scientific developments in Denmark, which were launched before the end of the 20th century. With this aim, scientists mentioned will have been born, with a few exceptions, no later than the early 1960s. However, we often refer to more recent publications in documenting the developments of initiatives launched before the end of the last century. In addition, several scientists have moved to Denmark after the beginning of the present century, and they certainly are contributing to the present status of Danish neuroscience-but, again, this is not the History of Danish neuroscience.

No effect of the angiotensin receptor blocker candesartan on cerebrovascular autoregulation in rats during very high and low sodium intake

Autoregulation of cerebral blood flow (CBF) denotes that CBF is constant despite fluctuation of blood pressure within wide limits. Inhibition of the renin–angiotensin system (RAS) is known to decrease the lower and upper limits of CBF autoregulation. We have previously shown that this includes inhibition by the angiotensin receptor blocker (ARB) candesartan. In the present study we investigated the influence of the ARB candesartan on the lower limit of CBF autoregulation in two groups of Sprague-Dawley rats, on high (4.0% Na⁺) and low (0.004% Na⁺) sodium diet, respectively. Control animals were given the same diet, but no ARB. CBF was studied with the laser Doppler method. Blood pressure was lowered by controlled bleeding. Results revealed that both high and low sodium diet with low and high renin levels respectively block the influence of candesartan on CBF autoregulation. This was expected in rats on a high salt diet with a low renin level, but unexpected in rats with a low salt intake with a high renin level.

Vascular Gap Junctions Contribute to Forepaw Stimulation-Induced Vasodilation Differentially in the Pial and Penetrating Arteries in Isoflurane-Anesthetized Rats

Somatosensory stimulation causes dilation of the pial and penetrating arteries and an increase in cerebral blood flow (CBF) in the representative region of the somatosensory cortex. As an underlying mechanism for such stimulation-induced increases in CBF, cerebral artery dilation has been thought to propagate in the vascular endothelium from the parenchyma to the brain surface. Vascular gap junctions may propagate vasodilation. However, the contribution of vascular gap junctions to cerebrovascular regulation induced by somatosensory stimulation is largely unknown. The aim of the present study was to investigate the contribution of vascular gap junctions to the regulation of the pial and penetrating arteries during neuronal activity attributed to somatosensory stimulation. Experiments were performed on male Wistar rats (age: 7-10 weeks) with artificial ventilation under isoflurane anesthesia. For somatosensory stimulation, the left forepaw was electrically stimulated (1.5 mA, 0.5 ms and 10 Hz, for 5 s). The artery in the forelimb area of the right somatosensory cortex was imaged through a cranial window using a two-photon microscope and the diameter was measured. Carbenoxolone (CBX) was intravenously (i.v.) administered, at a dose of 100 mg/kg, to block vascular gap junctions. The forepaw electrical stimulation increased the diameter of the pial and penetrating arteries by 7.0% and 5.0% of the pre-stimulus diameter, respectively, without changing the arterial pressure. After CBX administration, the change in pial artery diameter during forepaw stimulation was attenuated to 3.2%. However, changes in the penetrating artery were not significantly affected. CBF was measured using a laser speckle flowmeter, together with somatosensory-evoked potential (SEP) recorded in the somatosensory cortex. The extent of CBF increase (by 24.1% of the pre-stimulus level) and amplitude of SEP were not affected by CBX administration. The present results suggest that vascular gap junctions, possibly on the endothelium, contribute to pial artery dilation during neuronal activity induced by somatosensory stimulation.

Posterior reversible encephalopathy in the intensive care unit

Posterior reversible encephalopathy syndrome (PRES) is increasingly diagnosed in the emergency department, and medical and surgical intensive care units. PRES is characterized by acute onset of neurologic symptoms in the setting of blood pressure fluctuations, eclampsia, autoimmune disease, transplantation, renal failure, or exposure to immunosuppressive or cytotoxic drugs, triggers known to admit patients to the intensive care unit (ICU). Although the exact pathophysiology remains unknown, there is growing consensus that PRES results from endothelial dysfunction. Because of the heterogeneous nature of the disorder, it is probable that different mechanisms of endothelial injury are etiologically important in different clinical situations. The presence of bilateral vasogenic edema on brain imaging, particularly in parieto-occipital regions, is of great diagnostic utility but PRES remains a clinical diagnosis. Although largely reversible, PRES can result in irreversible neurologic injury and even death. The range of clinical and radiographic manifestations of the syndrome is probably broader than previously thought, and it is imperative that clinicians become familiar with the full spectrum of the disorder, as prompt recognition and elimination of an inciting factor improve outcome. PRES may be the most frequent toxic-metabolic encephalopathy seen in the ICU.

Management of Brain Edema Complicating Stroke

Ischemic brain edema, the accumulation of fluid within the brain parenchyma following stroke, is a predictable consequence of both ischemic and hemorrhagic strokes. Its development is the result of injury to both brain parenchyma and the blood vessels supplying the parenchyma. Ischemic stroke produces both cytotoxic (intracellular) edema, which develops when cells are damaged, and vasogenic (extracellular) edema, which arises from injury to structures essential to blood-brain barrier integrity. An understanding of the distinction between cytotoxic and vasogenic edema is essential in preventing secondary brain injury, since the treatments for the two entities differ. The development of new brain imaging technologies has advanced our understanding of brain edema. Both computed tomography (CT) and magnetic resonance imaging (MRI) can detect edema. Specific MRI sequences such as diffusion-weighted imaging can distinguish cytotoxic and vasogenic subtypes, and thereby detect ischemic cell injury within minutes of the onset of symptoms.

Brain edema causes neurologic deterioration predominantly through its mass effect, which leads to distortion of the intracranial contents and impairment of both regional and global cerebral blood flow (CBF). Edema may also cause local tissue dysfunction. Management of the intracranial hypertension and tissue shifts caused by ischemic brain swelling is based on the fundamental relationship between pressure, flow, and resistance. Interventions are directed at preserving CBF and preventing secondary brain injury. Strategies include reducing intracranial blood volume with hypocapnia, reducing brain volume with osmotic agents, reducing cerebral metabolism with hypothermia and barbiturates, reducing resistance with rheologic agents, increasing blood pressure with vasoconstrictors, and expanding the cranial vault with decompressive surgery. All individual therapies must be used as part of a structured approach that involves frequent serial neurologic assessments, quantitative measures of pressure, flow, and resistance, and prespecified protocols for intervention.

Sympathetic nerve activity in the superior cervical ganglia increases in response to imposed increases in arterial pressure

Sympathetic vasoconstriction of cerebral vessels has been proposed to be a protective mechanism for the brain, limiting cerebral perfusion and microcirculatory pressure during transient increases in arterial pressure. To furnish direct neural evidence for this proposition, we aimed to develop a method for recording cerebral sympathetic nerve activity (SNA) from the superior cervical ganglion (SCG). We hypothesized that SNA recorded from the SCG increases during imposed hypertension, but not during hypotension. Lambs (n = 11) were anesthetized (alpha-chloralose, 20 mg.kg(-1).h(-1)) and ventilated. SNA was measured using 25-microm tungsten microelectrodes inserted into the SCG. Arterial blood pressure (AP) was pharmacologically raised (adrenaline, phenylephrine, or ANG II, 1-50 microg/kg iv), mechanically raised (intravascular balloon in the thoracic aorta), or lowered (sodium nitroprusside, 1-50 microg/kg iv). In response to adrenaline (n = 10), mean AP increased 135 +/- 10% from baseline (mean +/- SE), and the RMS value of SNA (Square Root of the Mean of the Squares, SNA(RMS)) increased 255 +/- 120%. In response to mechanically induced hypertension, mean AP increased 43 +/- 3%, and SNA(RMS) increased 53 +/- 13%. Generally, (9 of 10 animals), SNA(RMS) did not increase, as AP was lowered with sodium nitroprusside. Using a new model for direct recording of cerebral SNA from the SCG, we have demonstrated that SNA increases in response to large induced rises, but not falls, in AP. These findings furnish direct support for the proposed protective role for sympathetic nerves in the cerebral circulation.

Effect of nephrectomy and captopril on autoregulation of cerebral blood flow in rats

The present study investigated the effect of circulating versus locally present renin on cerebral blood flow (CBF) and its autoregulation in rats. CBF was measured repetitively with the intracarotid 133Xe injection method, whereas blood pressure was lowered to determine the lower limit of autoregulation. To remove renin from the blood, rats were bilaterally nephrectomized and kept alive with peritoneal dialysis for 48 h. Five groups of animals were studied: 1) nephrectomized dialyzed rats, 2) nephrectomized dialyzed rats given a single intravenous dose of the angiotensin-converting enzyme inhibitor captopril (10 mg/kg), 3) sham nephrectomized and dialyzed rats, 4) rats receiving drugs as dialyzed rats but no surgery, and 5) rats given the same diet as the other groups but no drugs and no surgery. Baseline blood pressure was significantly lower in nephrectomized rats compared with controls. Nephrectomy, captopril, sham operation, or dialysis did not influence baseline CBF. The lower limit of CBF autoregulation was significantly lower in nephrectomized (53 +/- 4 mmHg) and sham-operated (58 +/- 4 mmHg) rats compared with diet control rats (78 +/- 3 mmHg). Captopril significantly decreased the lower limit in nephrectomized rats (35 +/- 2 mmHg). Thus removal of circulating renin caused no change in the lower limit of autoregulation. By contrast, captopril lowered the lower limit even in the absence of circulating renin and hence appeared to exert its effect on components of the renin-angiotensin system in the cerebral resistance vessel walls.

JG. Effect of angiotensin inhibition on the coronary artery lower pressure limit in anesthetized dogs

The renin-angiotensin system plays a critical role in regulating vasoconstriction and vasodilatation that can influence myocardial blood flow and its transmural distribution. We tested the hypothesis that angiotensin inhibition can induce a leftward shift of the coronary autoregulatory pressure-flow relation and preserve distribution of myocardial blood flow at lower coronary perfusion pressures. We established circumflex artery pressure-flow relations under baseline conditions and after intracoronary enalaprilat or losartan potassium. Thereafter, transmural myocardial blood flow was measured at baseline and at the lower coronary pressure limit (LPL). With enalaprilat, the LPL was shifted leftward from 48 ± 6 mmHg at baseline to 43 ± 3 mmHg (P = 0.026); with losartan, the LPL was shifted leftward from 48 ± 10 mmHg at baseline to 41 ± 5 mmHg (P = 0.027). The leftward shift occurred while cardiac hemodynamics and M[Formula: see text]O2 were maintained at control levels. These results indicate that angiotensin inhibition extends the range of coronary autoregulation to lower LPL while preserving myocardial blood flow distribution, a physiologic effect that might explain the lower incidence of coronary events in treated patients.Key words: angiotensin, microcirculation, microspheres, myocardium.

Mathematical considerations for modeling cerebral blood flow autoregulation to systemic arterial pressure

The shape of the autoregulation curve for cerebral blood flow (CBF) vs. pressure is depicted in a variety of ways to fit experimentally derived data. However, there is no general empirical description to reproduce CBF changes resulting from systemic arterial pressure variations that is consistent with the reported data. We analyzed previously reported experimental data used to construct autoregulation curves. To improve on existing portrayals of the fitting of the observed data, a compartmental model was developed for synthesis of the autoregulation curve. The resistive arterial and arteriolar network was simplified as an autoregulation device (ARD), which consists of four compartments in series controlling CBF. Each compartment consists of a group of identical vessels in parallel. The response of each vessel category to changes in perfusion pressure was simulated using reported experimental data. The CBF-pressure curve was calculated from the resistance of the ARD. The predicted autoregulation curve was consistent with reported experimental data. The lower and upper limits of autoregulation (LLA and ULA) were predicted as 69 and 153 mmHg, respectively. The average value of the slope of the CBF-pressure curve below LLA and beyond ULA was predicted as 1.3 and 3.3% change in CBF per mmHg, respectively. Our four-compartment ARD model, which simulated small arteries and arterioles, predicted an autoregulation function similar to experimental data with respect to the LLA, ULA, and average slopes of the autoregulation curve below LLA and above ULA.

Cerebral blood flow autoregulation is absent in rats with thioacetamide-induced hepatic failure

Cerebral blood flow normally remains constant within a wide range of mean arterial blood pressure values. In fulminant hepatic failure, however, it is not known whether autoregulation of cerebral blood flow is maintained. In the present study, cerebral blood flow autoregulation was investigated in rats 3 days after induction of fulminant hepatic failure. Wistar rats were given intraperitoneal thioacetamide or saline injections. The mean arterial blood pressure was varied by means of norepinephrine infusion or venesection, respectively. As mean arterial blood pressure declined, repeated cerebral blood flow measurements were performed by the intracarotid 113Xenon injection method. The relation between mean arterial blood pressure and cerebral blood flow was examined by statistical regression analysis, and the lower limit of autoregulation was determined in each rat. Cerebral blood flow baseline values were unaltered in liver failure compared to the control group (73 (36-92) vs. 79 (57-87) ml.100 g-1.min-1, median and range). Baseline mean arterial blood pressure was also similar in the two groups (90 (75-113) vs. 95 (70-112)). Mean arterial blood pressure varied between 40 (35-50) and 110 (90-135) mmHg in the control rats and between 50 (45-68) and 110 (95-126) mmHg in the rats with liver failure. A lower limit of autoregulation was identified in all control rats at a mean arterial blood pressure of 67 (55-78) mmHg. Below this limit, cerebral blood flow declined in parallel with mean arterial blood pressure. None of the rats with liver failure exhibited autoregulation, and cerebral blood flow changed in parallel with mean arterial blood pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

Angiotensin II AT2 receptor stimulation increases cerebrovascular resistance during hemorrhagic hypotension in rats

The effects of the angiotensin II (ANG II) AT2 ligand PD 123319 and the AT1 antagonist losartan on cerebral blood flow (CBF) were studied during hemorrhagic hypotension in anesthetized rats using laser-Doppler flowmetry. In the control group CBF remained stable when mean arterial blood pressure (MABP) was lowered from 84 mmHg (baseline) to 45 mmHg, whereafter there was a pressure dependent decrease in CBF indicating inadequacy of autoregulation. Cerebrovascular resistance (CVR) was reduced until MABP 40 mmHg, where a maximum dilation was reached. PD 123319 dose-dependently (3-30 mg/kg i.v.) increased CVR through all blood pressures. Losartan 3 mg/kg i.v. had an effect similar to PD 123319. Selective stimulation of AT2 receptors with intravenous ANG II infusion, in the presence of AT1 receptor blockade by losartan, also increased CVR. As a result, reduced CBF was seen in the treatment groups. The effects of ANG II and PD 123319 30 mg/kg were antagonized by the nonselective ANG II antagonist Sar1,Ile8-ANG II (4 micrograms/kg/min i.v.). None of the treatments affected baseline CBF. The results confirm that ANG II contributes to cerebrovascular resistance and participates in the regulation of CBF apparently through AT2 receptors.

Angiotensin II AT2 receptor stimulation extends the upper limit of cerebral blood flow autoregulation: agonist effects of CGP 42112 and PD 123319

The effects of the angiotensin II AT2 receptor ligands CGP 42112 and PD 123319, the AT1 antagonist losartan, and the nonselective angiotensin II antagonist Sar1,Ile8-angiotensin II on the upper limit of CBF autoregulation were studied in pentobarbital-anesthetized rats. Blood pressure was increased by intravenous phenylephrine infusion, while CBF was measured continuously from the parietal cortex by laser-Doppler flowmetry. Intravenous infusions of CGP 42112 (0.1 and 1 mg kg-1 min-1) and PD 123319 (0.36 and 1 mg kg-1 min-1) shifted the upper limit of CBF autoregulation toward higher blood pressures without affecting baseline CBF. Sar1,Ile8-angiotensin II (4 micrograms kg-1 min-1) had no effect on baseline CBF or CBF autoregulation but antagonized the effect of CGP 42112 and PD 123319. Losartan (10 mg/kg i.v. bolus) reduced baseline blood pressure and CBF and shifted the autoregulation curve toward higher blood pressures. Sar1,Ile8-angiotensin II blocked the effect of losartan on baseline CBF but not on CBF autoregulation. These results suggest that both CGP 42112 and PD 123319 exert their effects on CBF autoregulation through stimulation of angiotensin II AT2 receptors. The mechanism by which losartan affects CBF remains unclear.

Cerebrovascular regulation and neonatal brain injury

Neuropathology occurring as a result of hemodynamic injury occurs in up to 25% of preterm newborns of less than 1,500 gm birth weight and in a much smaller, but nonetheless meaningful, proportion of more mature infants. Abnormalities in cerebrovascular regulation have been proposed as major contributing factors to both ischemic and hemorrhagic injuries in the newborn brain. In this review we explore several factors that play a role in cerebrovascular regulation in the immature brain and relate them to what is known about vascular regulation in the mature brain and to the types of pathology that occur in the newborn brain. One goal in this "decade of the brain" should be to increase our basic and clinical knowledge about the cerebrovasculature of the newborn in order to enhance our ability to predict and prevent perinatal brain injury.

Acute sympathetic denervation does not eliminate the effect of angiotensin converting enzyme inhibition on CBF autoregulation in spontaneously hypertensive rats

The effect of angiotensin converting enzyme inhibition with captopril (10 mg/kg i.v.) on CBF autoregulation was studied in 16 spontaneously hypertensive rats (8 control and 8 treated with captopril) subjected to acute cervical sympathectomy. CBF was measured repetitively by the intra-arterial 133Xe injection method, during the manipulation of MABP by norepinephrine or hemorrhagic hypotension. Prior to the administration of drugs, baseline MABP was 112 +/- 10 mm Hg in the control group and 119 +/- 11 mm Hg in the captopril group. Baseline CBF was 99 +/- 19 ml/100 g/min, with no difference in the two groups. In agreement with previous findings in rats with intact sympathetic nerves, the lower limit of CBF autoregulation was reduced from the MABP interval of 70-89 to 50-69 mm Hg by captopril.

Angiotensin converting enzyme inhibition and cerebral circulation--a review

1. The identification of a vascular wall renin angiotensin system and of angiotensin converting enzyme on the luminal surface of the endothelium in many tissues, including the brain, has stimulated research on the influence of the renin angiotensin system on regional blood flows. 2. In experimental studies inhibition of the angiotensin converting enzyme shifts the limits of cerebral blood flow autoregulation towards lower blood pressure values. 3. In patients with chronic arterial hypertension and in patients with chronic heart failure cerebral blood flow is not changed by acute or chronic angiotensin converting enzyme inhibition, despite in some cases pronounced reductions in the mean arterial blood pressure. Angiotensin converting enzyme inhibition does not change ischaemic regional cerebral blood flow in acute stroke. 4. It is concluded that following angiotensin converting inhibition cerebral blood flow is maintained at an unchanged level. The mechanism may include inhibition of locally produced angiotensin II leading to a selective dilation of larger cerebral arteries with a compensatory constriction of the smaller cerebral arteries.